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Shoes for the Insensitive Foot: The Effect of a ''Rocker Bottom'' Shoe Modification on Plantar Pressure Distribution Peter S. Schaff and Peter R. Cavanagh Foot Ankle Int 1990 11: 129 DOI: 10.1177/107110079001100303 The online version of this article can be found at: http://fai.sagepub.com/content/11/3/129
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0198-0211/90/1103-0129$03.00/0 FOOT8 ANKLE Copyright 0 1990 by the American Orthopaedic Foot and Ankle Society, Inc.
Shoes for the Insensitive Foot: The Effect of a “Rocker Bottom’’ Shoe Modification on Plantar Pressure Distribution Peter S. Schaff* and Peter R. Cavanaght Munich, Federal Republic of Germany and University Park, Pennsylvania
shown to be particularly at risk for ulceration of the plantar surface of the great toe,’s5 and the use of a rocker bottom shoe to reduce hallux pressures has been suggested for such patients.” This device, as used by Brand,’ is characterized by a totally rigid sole which is angled sharply up in the forefoot. Although extension cannot occur at the metatarsophalangeal joints, walking is possible because the shoe “tips” forward when the center of pressure moves distal to the rocker fulcrum (Fig. 1). An alternative design, usually referred to as a “roller sole,” is progressively contoured towards the toe18 in a manner that is supposed to mimic the motion of the talocrural joint during walking. It is assumed, in both shoe and cast situations, that the combination of a rigid platform for the foot and the rocker provides a “more even” distribution of plantar pressures during walking than would occur if the foot were allowed to extend at the metatarsophalangeal joints. Such an assumption appears to be reasonable, and previous studies, discussed below, using discrete transducers at various locations on the plantar surface of the foot have suggested that up to 50% reduction in pressure can be achieved at certain sites depending upon the exact configuration of the rocker and hoe.'^,'',*^ The present authors have not been able to
ABSTRACT The purpose of this study was to examine the effects on plantar pressure of the rocker bottom shoe, which is a frequently used intervention for the ulcerated diabetic foot. In-shoe pressure distribution was recorded during walking in a conventional extra-depth shoe which was then modified into a rocker bottom configuration with a 2 4 O rocker, and the experiment was repeated. Peak pressures in the rocker shoe were reduced by -30% compared to the conventional shoe in the medial and central forefoot and in the toe regions, but pressures were elevated in the heel, the midfoot, and in the lateral forefoot regions. These experiments suggest that a correctly designed rocker bottom shoe may reduce the risk of ulceration in certain areas of the foot. However, since pressures in some regions can also be elevated by this type of footwear, attention to individual design is critical.
INTRODUCTION
There is growing interest in the use of footwear as a therapeutic modality for the insensitive foot with a history of ~lceration,~ yet little scientific evidence exists to support or refute interventions that have become commonplace in clinical practice. Typically, an extradepth shoe with a custom molded inlay is a first line of defense when a patient with diabetes or Hansen’s disease is thought to be at risk for ulceration.” If there has been recurrent ulceration in the forefoot or if there is significant bone injury or limitation of motion, many experts recommend that the patient, once healed, should be placed in a rocker bottom shoe.’ A rigid wooden sole and a rocker are also usually applied to the plantar surface of a total contact cast for ambulation during plantar ulcer healing.13,22 Patients with loss of sensation and hallux limitus have been
STE
Institute for Biomechanical Analysis in Sports and Interdisciplinary Studies (BASIS), Ridlerstrasse31, D-8000 Munich 2, F.R.G. t To whom all correspondence should be addressed at the Center for Locomotion Studies, Penn State University, University Park, PA 16802.
PIVOT
Fig. 1. Schematic diagram of a rocker bottom shoe. A steel shank is inserted between midsole layers and the rear part of the shoe is built up to allow the angle in front of the rocker axis to be achieved.
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locate any studies where the plantar pressure between the foot and a rocker bottom shoe were examined at more than four sites simultaneously. The purpose of the present paper is therefore to examine the effect of a rocker bottom modification to the shoe on plantar pressure during walking using a 72-element pressure sensitive insole in order to get a more complete picture of plantar pressure distribution. REVIEW OF LITERATURE
The use of rigid footwear for the insensitive foot has its roots in the methods pioneered in the 1950s and 60s by Brand,’ Price,” Ross,’~and Ward“ for the care for Hansen’s disease patients with plantar ulceration. While the rigid sole is common to all authors, there continues to be some debate over the placement and orientation of the rocker axis. Among the placements suggested are under the metatarsal proximal to the metatarsal heads,’6~’8forward of the midpoint of the hoe,'^,'^ at the midpoint of the shoe,I6and “as far back as possible.28Proposed placements in relation to the metatarsal heads are complicated by the fact that the heads do not generally lie on a straight line. Although Rossz4had used a Harris mat to record inshoe pressures, the first landmark quantitative study of footwear for insensitive feet was performed by Bauman, et al. in 1963.3 Using discrete capacitance transducers described by Bauman and Brand,* “normal” values for sensate and insensate feet were first defined during barefoot ~ a l k i n gFor . ~ patients without deformity, no deviation from “normal” pressure values were found in the insensate feet. These authors then measured pressures inside a conventional Oxford shoe and six different show variations, including a rocker in two different positions: with the axis “centered under the metatarsal heads” and “1.7 cm behind the metatarsal heads.” The subjects walked in time to a metronome at 100 steps/min. Other experiments, not discussed here, made similar studies in patients with shortened feet and pressure measurements inside a total contact cast. In shoes without an insole, the rocker shoes yielded the lowest pressures under the hallux (maximum relief of approximately 12% with the anteriorly placed rocker) and second metatarsal heads (maximum relief of 30% with the posteriorly placed rocker). Pressures under the heel were approximately 40% higher in all rocker conditions compared to the conventional shoe. Bauman, et aL3 also demonstrated the consequences of inadequate rocker height and showed a recording of high forefoot pressures when the tip of the shoe came into contact with the ground. These authors discussed
the design dilemma of placing the rocker axis far enough anteriorly to ensure that there was no toe contact, yet far enough posteriorly to relieve metatarsal head pressures. In 1983, Pollard, et al.” developed a new transducer to measure shear forces and used it, together with a discrete vertical pressure transducer, to record the changes in interface conditions using a number of types of footwear, including a rocker shoe with “a deep rocker behind the metatarsal heads.” They reported no significant alteration in the vertical pressure at any site when the rocker shoe was compared to “conventional leather shoes,” but noted a “small” reduction in a single transducer under the heads of the second and third metatarsals. However they did find significant reductions in the longitudinal shear pressure in the rocker shoe under all the metatarsal heads except the first.” The relief of shear pressure under the central metatarsal heads was over 70%; a change of this magnitude suggests that this aspect of interface pressure is in need of more attention. Unfortunately these authors do not appear to have published on the topic since 1983, and no other investigators have used the technique. In 1985, Birke et al.’ measured plantar pressures at four sites inside a conventional padded cast, a total contact cast, and a standard shoe in six normal subjects. Walking heels were placed on the casts, centered at 60% of foot length from the toe and extending approximately 2 inches in each direction from this point. They found significant reductions in pressure in the casts compared to the shoe during walking of 84% at the first and 75% at the third metatarsal head. The pressures under the heel and head of the fifth metatarsal were not significantly different between cast and shoe conditions (although the trend was towards a reduction), and the two types of casts were not different from each other at any location. In a more comprehensive, but similar, study to that performed by Bauman, et al.,3 Coleman” studied 15 subjects in six different shoe modifications using four discrete pressure transducers. His modifications included rigid soles with no contour, two types of “roller” sole with a continuously curved outsole, and three rocker soles with angles varying from 15’ to 30”.In every case the outsole modification started “just behind the fifth metatarsal head.”” The subjects walked overground at 80 steps/min in all conditions in time to a metronome. Coleman found no significant differences between any shoe condition in the heel, but all modifications showed significant pressure relief compared to the unaltered shoe in the hallux, second metatarsal head, and the fourth metatarsal head. In general, the pressure relief at all forefoot sites was greatest in the
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two rockers with the highest angle, less in the low rocker and the roller soles, and least in the rigid shoe with no outsole modification. A mean relief of approximately 50% was obtained at the second and fourth metatarsal head locations. Sims and Birke2’ extended this work by using similar discrete transducers under the first, third, and fifth metatarsal heads and heel of symptom free subjects who walked in a normal shoe and four different rocker shoes in which the rocker axis was progressively moved anterior and posterior to the first metatarsal head. The orientation of the rocker axis was stated as 30’. They found that axis placement had no effect on pressures at the fifth metatarsal head where pressures were not significantly different from the normal shoe. At the first and third metatarsal heads a posterior placement tended to relieve plantar pressures whereas anterior placement showed a significantly increased pressure at the first metatarsal head. No absolute values for changes in plantar pressure achieved by the various designs were given. Schaff, et al.26used a 15element measuring insole to analyze pressure distribution in 13 different shoes and reported considerable variation in the distribution in the forefoot region. Nawoczenski et al.” examined the effect of six different designs of rocker and roller shoes using discrete pressure transducers taped to four locations on the feet of symptom free individuals. They reported pressure reduction at all sites tested (hallux, first, third, and fifth metatarsal heads) when the experimental shoes were compared to a standard Oxford shoe. A maximum pressure relief of 31O/O was found underneath the hallux using a shoe similar to that shown in Figure 1. In all other designs, maximum pressure relief was achieved under the third metatarsal head. In summarizing the related literature, Milgram’s statement that “prescription shoe fitting and shoe correcting are more an art than a science”’* seems to apply well to considerations of design and function of the rocker bottom shoe. This type of shoe has been widely recommended on a clinical basis for pressure relief in feet with a previous history of ulceration and a lack of sensation. Despite the apparent success of this intervention, there appears to be no controlled prospective studies of healing, and no authoritative sources which describe the optimum combination of design parameters for a particular pattern of pathology. Studies with discrete transducers have produced a variety of results when compared to conventional footwear, ranging from over 50% reductions at some sites to no significant differences or actual increases at other locations.
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METHODS Subjects
Eight nondiabetic subjects who were free of pathology in the lower extremity volunteered to participate in this study. Their average age was 31 years, average body mass 69 kg, and average height 1.76 m. Because only one insole was available the subjects were also selected based upon their shoe size, which was US men’s size 9 in every case. The same size 9 pair of shoes was used for all subjects in all experiments. Shoe Specifications
Data were first collected as the subjects walked in an extra-depth shoe with no insole (Fig. 2A). After all subjects had completed their trials in this “normal” shoe condition, the shoe was modified to produce the rocker configuration shown in Figure 29. The outsole was tapered anteriorly and a rigid metal corset stay was attached to the exposed surface of the remaining outsole. Sufficient crepe material was then added to the
A
Fig. 2. A, The extra depth shoe used for the experiments. B, The shoe after modifications had been made to produce the rocker sole.
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outsole so that it was possible to grind the sole under the forepart of the shoe to produce a 24' angle between the rear and fore parts of the outsole. The resulting total material thickness was 5.5 cm under the heel and 3.5 cm under the ball of the foot just proximal to the rocker, and this gave a heel-ball height differential of 2 cm. To allow a reasonably natural heel strike, the rear border of the midsole was beveled to a 44' angle starting 3 cm anterior to the posterior border of the heel. The heel-ball height differential was identical to the unmodifiedshoe which had 0.8 cm of material under the forefoot and 2.8 cm under the heel. The geometry of the rocker axis produced by the shoe modification is shown in Figure 3. The axis formed an angle of 90' with the midline of the posterior one half of the shoe. This midline formed an angle of 7.5' with the longitudinal axis of the shoe (a line drawn through the anterior and posterior shoe midpoints). The location of the rocker in relation to the subjects' anatomy is also shown in Figure 3 and will be discussed below. A
The flexible pressure measuring insole, which was worn in the right shoe only, consisted of 72 active elements arranged in a contiguous matrix sandwiched between layers of surgical rubber (Fig. 4). Most of the elements were 1 x 1.4 cm and operated as air capacitors as has previously been d e ~ c r i b e d . ' ~Details , ~ ~ , of ~~ the frequency, hysteresis, cross sensitivity between adjacent active and inactive elements, and linearity of the device are given in Schaff and Ha~ser.'~ We performed our own calibration of the insole by recording the electrical output while a pressure vessel positioned over the insole was relaxed from various pressure values. This process gave individual calibration factors for each element derived from linear least squares equations which, in general, provided a satisfactory fit such that higher order approximations were not needed. Force platform measurements of ground reaction forces and centers of pressure were also made but these results are outside the scope of the present paper. Relationship of Shoe and Pressure Insole Geometry to Anatomy
Outsole outline
339
In order to define the relation of the shoe and insole geometry to the anatomy of the subjects' feet, the approach taken in Figure 5 was used. An exact outline in the dimensions of the footbed of the shoe was cut by a jig saw from a wooden panel using the vinyl insole liner of the shoe as a guide. A transparency showing
/
I
67%
Instrumentation
- + / - L o n g i t u d of i n shoe a l axis
r---
I
--&
Start of heel bevel
B Fig. 3. The geometry of the rocker modification made to the shoe showing the metatarsal head locations during standing with respect to the rocker axis for each of the eight subjects. The shaded area represents the difference between the insole outline (the entire area in contact with the foot) and the outsole. See text for further discussion.
Fig. 4. The actual EMED pressure measuring insole used for the experiments.
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less than complete coverage. This was a result of the need to run leads from the rows and columns to the electronics unit. Treadmill Protocol
During the collection of pressure data, subjects walked at a freely chosen cadence on a treadmill set to a speed of 0.83 m.sec-’. This is a slow walking speed (1.9 mph or 3km/hr) typical of that which older patients might use during unpaced ambulation. Although all the subjects were accustomed to treadmill locomotion, they walked for 15 min on the treadmill in each shoe before measurements were taken. In the initial experiments, cadence was freely chosen by the subjects and determined from measurement of the time required for 30 contacts of the right foot. Since the treadmill speed was accurately known, step length could also be calculated from this measurement. Three separate trials for each condition were collected and at least three foot contacts of the right foot were available for pressure analysis from each trial. Between each trial the treadmill was stopped and the subject rested for approximately 3 min. The only instructions that the subjects were given in the use of the rocker bottom shoes was to walk “so that the toes do not touch the ground.” These instructions, similar to those that would be given to a patient when the shoes are dispensed, encouraged the wearer not to defeat the object of the rocker by using that part of the outsole anterior to the rocker axis as a major weight bearing surface. ANALYSIS OF DATA
Fig. 5. A weight-bearing photograph taken from underneath a wooden form in the shape of the insole outline. The transparency shows the location of each one of the 72 measuring elements and the rocker axis. The heads of the metatarsals (located by palpation) are marked with black circles.
the location of each active measuring element was then positioned in exact relation to the shoe outline, and the location of the rocker axis was also marked. By palpation, the heads of all five metatarsals were also identified and a photograph was taken from underneath as the subject stood with all his weight on the right foot (Fig. 5). This enabled the locations shown in Figure 3 to be constructed where the positions of the metatarsal heads of each subject are shown in relation to the rocker axis. It is apparent from Figure 5 that certain regions of the foot could not be monitored with the insole used in this study. In particular, a band in the midfoot, the lateral aspect of the forefoot and toes, and the medial aspects of the heel and great toe received
The peak pressure, defined as the largest pressure recorded from the insole regardless of its location, was determined at each instant of time by software and a peak pressure versus time array was assembled. This curve was automatically scanned to divide the four second data collection period into individual steps. The peak pressure exerted on each element during each step (regardless of the time of occurrence) was then determined and a mean “peak pressure array” was assembled by taking the average of the peak pressure for each element during the nine separate steps available for each condition. A scheme for regional analysis was also devised from the photographs of the plantar surface in relation to the shoe and insole geometry. This scheme, shown in Figure 6, involved a combination of elements into 10 anatomical areas in order to simplify the analysis. Although all subjects were comfortable in the size 9 shoe, both the length and proportions of the foot varied between individuals, and this led to some variation in the anatomical structures which were included in the
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B.
A.
1 Medial Toes 2 Lateral Toes 3 Medial Forefoot 4 Central Forefoot 5 Lateral Forefoot 6 Medial Midfoot 7 Lateral Midfoot 8 Medial Heel 9 Central Heel 10 Lateral Heel
nominally labelled regions. The rather general anatomical terms, such as “central forefoot,” have been deliberately used because of this variability and because of the fact that individual anatomical structures frequently overlap two defined regions. This overlap was principally in the mediolateral direction, and there was never a case where, for example, a metatarsal head was incorrectly located in a toe or midfoot region. By further combination of the 10 regions described above, more global statements about pressure distribution could be made. For example “peak toe pressure” was obtained by identifying the largest value from regions 1 and 2, “peak forefoot pressure” from regions 3 through 5, “peak midfoot pressure” from regions 6 and 7, and “peak heel pressure” from regions 8, 9, and 10. Peak pressure versus time curves could also be generated for all regions. To estimate the net load bearing in a given region, the instantaneous force on a region was calculated as follows: x = n
fit =
C Px(ax) x = 1
where: Fit is the net force acting at time t on the ith region at a given time n is the number of elements in the ith region Px is the pressure acting on element x a, is the functional element x.
MIDFOOT
Fig. 6. A , The 10 regions that were generated by a combination of individual elements. The regions are labelled in relation to the anatomical structures that they measure. 8,Regions generated by the pooling of smaller regions. For example, a single heel region was formed by a combination of regions 8, 9, and 10 and a single forefoot region from regions 3. 4, and 5 .
The estimate of force is subject to some inaccuracy due to the integration of residual noise on elements which are not heavily loaded. The value of f i could also be plotted as a function of time and the area under this curve (the force time integral) indicates the total loading of the region during ground contact. All statistical analyses were conducted using a twotailed Wilcoxon signed ranks test with a probability level P c .05. This nonparametric paired t-test was used due to the likelihood of violating the requirement of normal distribution with the small sample size in the present experiment (n = 8). RESULTS
Step Parameters
The mean step lengths (SL) and step frequencies (SF) used by the subjects while walking in the two shoe types are shown in Table 1. The subjects took 8 cm shorter steps at a rate of approximately 6/min more when wearing the rocker bottom shoes compared to walking with the normal shoes. Both SL and SF were significantly different between the two shoe conditions (P < .05). These differences reflect readily observable changes in the lower extremity movement patterns that occurred when subjects were walking in the rocker bottom shoes. The measurement of the exact kinematic
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TABLE 1 Means and Standard Deviations for Step Length and Step Frequency ( n = 8) during Treadmill Walking at 0.83 msec-' in the Two Shoe Types
Step Length (SF) (m) Normal shoe Rocker shoe Difference (rocker - normal)
1.19 (0.034) 1.11 (0.049)
-0.08"
Step Frequency (SF) (min-')
83.8(2.4) 90.0(4.0) 6.2"
A step is defined as the distance between successive contacts of the right and left feet. ' Significant difference (P < 0.05).
differences in segment and joint angles was beyond the scope of the present study. Pressure
Peak pressure pictures for one subject who showed changes in the same direction as the group means are shown in Figure 7. These diagrams show the peak pressure at each measuring point without regard to the time of occurrence of that pressure maximum. The intersection of lines in the grid in Figure 7 represent the position of active elements, and the height of the display at a given point is proportional to the pressure at that location. The vertical edges on the lateral border are regions where a boundary element has a nonzero pressure value and this should be compared with Figure 5 where, as mentioned earlier, it is apparent that not all weight-bearing areas of the foot are monitored with the insole used. If there were more lateral elements in the insole, they might be expected to show values closer to zero which would result in a smoother contour of the display. The most noticeable feature of Figure 7 is the marked reduction in peak pressure on the medial and middle metatarsal heads in the rocker bottom shoe condition. However, it is clear that pressure is not lower in all regions of the rocker shoe, but appears to be somewhat elevated in the heel, the midfoot and the lateral forefoot. This impression is confirmed by the results of the gross regional analysis presented in Table 2, where the foot was divided into four large regions in a proximal to distal manner. These data indicate that the rocker shoe significantly reduced peak pressure in the forefoot and toes by approximately 30%, but also resulted in significantly increased peak pressures of approximately 20% in the heel. The large mean increase in the midfoot was not statistically significant. The impulse data show most clearly the manner in which the rocker bottom shoe changed the function of the foot during walking. In the normal shoe, the ratio of heel region impulse to forefoot region impulse was 0.85, reflecting the dominance of the forefoot in the load-
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bearing process. The same ratio in the rocker shoe was 1.42, a change caused by both a significant 35% reduction in forefoot impulse and by a significant 42% increase in heel impulse. The over 100% increase in the midfoot impulse for the rocker shoe condition was also statistically significant, but there was no significant change in toe impulse. Examples of regional force-time curves for one subject, typical of those from which the above impulse values were calculated, are shown in Figure 8. This figure shows the higher forces applied for a longer period of relative time in the heel and midfoot, the lower force for approximately the same period of relative time in the forefoot, and the lower force applied for a longer period of relative time in the toes. The time base in Figure 8 is percentage of the support phase. Since the majority of lesions in the diabetic foot occur in the toe and forefoot regions,' particular attention was given to the footwear-induced changes in pressure distribution within the large forefoot and toe regions discussed above. Table 3 shows peak pressure and impulse results for the smaller regions 1 through 5 which comprised the toe and metatarsal head areas (Fig. 6). It is apparent from Table 3 that the rocker bottom shoe caused a statistically significant reduction in peak pressure in the forefoot that was fairly uniform as far as percentage was concerned (approximately 30% reduction) in all regions except the lateral forefoot where the peak pressure was greater in the rocker shoe by an amount that failed to achieve statistical significance. The impulse results, however, only showed significant differences in the medial and central metatarsal heads where there were 53% and 35% reductions in the impulse in the rocker shoe compared to the normal shoe. DISCUSSION Generalizability of the Results
It should be stated at the outset that the results from this study cannot be generalized to all rocker-bottom shoe modifications. As was apparent from the earlier review of the literature, there is no consensus of opinion concerning such important variables as the angle of the rocker, the anteroposterior position of the rocker axis, and the orientation of the axis with respect to the shoe midline. The particular shoe used in these experiments was modified by a physical therapist who had extensive experience in the design of footwear for insensitive feet. His design was a function of the standard of practice that he had learned during his professional experience and a response to the need to produce a single modification that would be worn by all eight subjects. The resulting shoe was just one example of
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A
I
B
300 kPa
Fig. 7. Peak pressure pictures for the subject closest to the mean response in the normal shoe ( A ) and the rocker bottom shoe ( B ) . Note the reduction in pressure achieved by the rocker shoe over the medial and central regions of the forefoot. Pressure is. however, slightly increased in the heel and the lateral midfoot regions. The scale bar shown is 300 kPa. TABLE 2 Mean Values from all Eight Subjects for Regional Peak Pressure and Impulse with the Foot Divided on a Proximal to Distal Basis
Heel
Midfoot
Forefoot
Toes
Peak pressure Normal shoe Rocker shoe Difference(rocker - normal)
176 209 33 (18.7%)”
59 95 36 (61.O?’o)
21 9 155 -64 (-29.2%)”
194 131 -63 (-32.5%)”
Impulse Normal shoe Rocker shoe Difference(rocker - normal)
144 205 61 (42.2%)”
13 26 13 (102.3%)8
169 111 -58 (-34.7%)8
41 33 -8 (-18.7%)
The peak pressure is in kiloPascals (kPa) and the impulse is in newton seconds (Ns). Values in the “difference” category represent (Rocker shoe) - (Normal shoe) and are therefore negative when the rocker provides a relief of pressure or a reduction in impulse and positive when the rocker shoe results in an increase. Percentage changes are shown in parentheses following the difference in absolute units. Note: lOOkPa = 14.1pounds per square inch (psi) = 10 N/cmZ. Denotes a significant difference between normal and rocker shoe conditions P < 0.05.
an infinite number of possible rocker shoe designs where the position and orientation of the axis could be altered and the composition and stiffness of midsole changed. In the absence of definitive experimental evidence on the optimum design for a rocker shoe, the opinion of an experienced clinician seemed an appropriate starting point for the investigation of alterations the shoe may cause. We also decided not to provide a custom-molded insole for either shoe since it would have been difficult to separate the effects of outsole geometry from those due to insole composition on any results obtained. In practice, an insole of some description is invariably used. The pressure measurements in the present experiments were conducted on a motorized treadmill and thus it is posible, although unlikely, that the effects during overground walking would be different.
Step Parameters
Since all subjects walked with smaller step lengths in the rocker shoe, a finding which confirms the observations of Ward2* and E d w a r d ~ , ’it~is reasonable to hypothesize that the reduction in pressure could have simply been a function of the smaller step length and not a footwear effect at all. To investigate this explanation, the experiment using the normal shoe was repeated, but subjects were given an auditory cue from an electronic metronome. The device was set individually for each subject to produce a tone at the same interval as the step frequency which that subject used during his walk in the rocker bottom shoe. Since the walking velocity was fixed by treadmill speed, an increased cadence would cause a shorter step length and vice versa.
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MIDFOOT
o
FOREFOOT
40
20
80
60
inn
TIME ( % contact)
TIME ( % contact)
I
400
TIME (%contact)
Fig. 8 . Regional force time curves for one subject showing the higher forces applied for a longer period of relative time in the heel and midfoot, the lower force for approximately the same period of relative time in the forefoot, and the lower force applied for a longer period of relative time in the toes. The time base is 100% of the support phase. TABLE 3 Mean Values from all Eight Subjects for Regional Peak Pressure and Impulse in the Toe and Metatarsal Head Regions Peak Pressure Normal shoe Rocker shoe Difference (rocker - normal) Impulse Normal shoe Rocker shoe Difference (rocker - normal)
Medial toes
Lateral toes
194" 131 -63 (-32.3%)
104" 78 -26 (-25.3%)
22.7 21.4 -1.3 (-6.0%)
18.0 11.7 -6.3 (-34.7%)
Medial forefoot
Central forefoot
219" 155 -64 (-29.1%)
213" 143 -70 (-32.9%)
62.9" 29.5 -33.4 (-53.1 Yo)
85.5" 55.1 -30.4 (-35.5%)
Lateral forefoot 123 138 15 (12.2%) 20.5 25.6 5.1 (25.0%)
The peak pressure is in kiloPascals (kPa) and the impulse is in newton seconds (Ns). Values in the "difference" category are calculated in a similar manner to that described in Table 2. Significantly greater than the corresponding value in the rocker shoe (P< 0.05).
Subjects found this "forced cadence" experiment fairly easy to perform, and three, 4-sec trials were collected yielding a total of nine separate contact phases for analysis. The results, shown in Table 4, indicated that walking at the faster cadence, with its accompanying shorter step length, .actually caused a significant increase in pressures in three of the ten anatomical regions, when compared to the pressure in
the same shoe during walking at a freely chosen cadence. In every other region except the medial heel, there was a nonsignificant increase in the mean peak pressure when walking at the faster cadence. Only one impulse value, that in the medial toes, was significantly different during walking at the faster cadence. The interaction of shorter contact times in the forced cadence condition with the higher pressures probably
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TABLE 4 Mean Peak Regional Pressure (kPa) and Impulse (Ns) for all Subjects during Treadmill Walking
Medial toes
Lateral toes
Medial forefoot
Central forefoot
Peak pressure Chosen Forced Difference O/O Difference
194 243 49” (20.1)
104 119 15 (12.6)
219 260 41 (15.9)
213 249 36” (14.6)
Impulse Chosen Forced Difference OO / Difference
22.7 31.3 8.4” (26.9)
18.0 17.7 -0.2 (-1.4)
62.9 60.9 -2.0 (-3.3)
85.5 99.2 13.7 (13.9)
Lateral forefoot 123 140 18 (12.5) 20.5 18.6 -1.9 (-10.1)
Medial midfoot
Lateral midfoot
Medial heel
Central heel
Lateral heel
14 26 13 (47.6)
59 68 9 (13.0)
174 166 -8 (-4.5)
176 179 3 (1.4)
159 178 19” (10.6)
0.6 0.7 0.1 (16.7)
12.2 13.1 0.9 (6.7)
51.1 47.4 -3.7 (-7.9)
41.5 40.6 -0.9 (-2.2)
51.9 60.9 9.0 (14.8)
Subject walked at a freely chosen cadence and a forced cadence in the normal shoe. The forced cadence was set individually to the value used by the subject during walking in the rocker bottom shoe, and was in every case greater (see Table 1). * Significant difference between forced and chosen cadence conditions.
tended to produce similar values for impulse. It would seem that, in experiments with a rocker bottom shoe, it is more reasonable to allow subjects to chose their own cadence rather than to impose a cadence as Bauman et aL3 and Colemani2 did. The change to a different type of footwear seems to carry with it an inevitable change in cadence and this is part of the effect that is achieved with the rocker footwear. Despite this change, it appears from our forced cadence experiments that an increase in cadence can be discounted as a mechanism for the reduced pressures that were encountered in the rocker bottom shoe. There is, however, a possibility that the increases in heel pressure found in the rocker shoe were a result of increased cadence. Pressure
The most important observation that can be made from the pressure results of the present experiment is that the particular rocker bottom modification used resulted in both increases and decreases in the applied pressure in different anatomical regions. This finding is somewhat similar to the results of Sims and Birke” who found that the pressure on the lateral aspect of the foot was not significantly altered by their footwear interventions and to the results of Bauman et aL3 who found increased heel pressure in rocker shoes. Nawoczenski et al.” also reported significant decreases with a shoe similar to ours at all sites except the fifth metatarsal head. However our results appear to be in conflict with those of Coleman’* who found no significant changes in the heel but reduction at three other forefoot and toe sites. Differences in shoe design could account for the observed differences since our lateral forefoot region appears to include the fourth metatarsal head, which was the site of Coleman’s most lateral sensor. Another
possible explanation is a difference in the active area of the sensors between the two studies which may have resulted in averaging of pressures by Coleman’s larger transducers (2.2 cm2 versus 1.4 cm2 in our device). The mixed pattern of pressure relief, no significant change, and significant increase in pressure found in our experiments, together with the conflict between our results and some previously published work has important implications for shoe prescription and design that will be discussed in the next section. Although it is not well stated in the literature, most clinicians believe that the mechanism of unloading in the rocker shoe is some combination of the following effects. 1. A redistribution of the load over a larger area. 2. An increase in the loading time for the regions of the foot in contact with the rigid shoe. 3. A change in the function of the foot, due to the restriction of motion, particularly at the metatarsophalangeal joints. 4. A change in the patterns of motion of the lower extremity due to the altered geometry and rigidity of the shoe. 5. A reduction in shear pressure on the plantar surf ace.
The present results provide a perspective on the validity of some of these spectulations. The increases in impulse in the heel and midfoot in the rocker bottom shoe are approximately twice as great as the increases in pressure (Table 2). If one makes the assumption that the single peak pressure value is representative of changes in average pressure in the region, this suggests that an increased duration of loading is more important in these regions than an increase in pressure (although both impulse and peak pressure are significantly higher). However, in the forefoot, the reduction
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in peak pressure and impulse are approximately the same, suggesting that pressure reduction is the key element. Since no metatarsophalangeal joint extension was possible in the rocker shoe, it seems reasonable to suggest that this restriction was responsible for the reduction in peak plantar pressure of approximately 30% that was found in the toes, and the medial and central forefoot areas in the rocker shoe. The hypothesis that metatarsophalangeal joint extension causes the metatarsal heads to be driven plantarwards, resulting in increased plantar pressures appears to be partially supported. A more complete test of this hypothesis would require the progressive restriction of metatarsophalangeal joint motion without the other changes induced by the rocker shoe. Since the hallux is the site of many lesions which result in the prescription of rocker bottom shoes, the results in the medial toe region are of particular interest. As shown in Table 3, there was a significant decrease in peak pressure in this region, but the impulse was not different between the two shoe conditions. Since it is well known anecdotally that many lesions of the hallux can be prevented from recurrence by a rocker bottom shoe, this finding would tend to suggest that peak pressure rather than impulse might be the critical factor in lesion development. Such an observation is, however, highly speculative and needs to be confirmed by experiments with patients who have had prior lesions of the hallux. There is still no clear evidence as to which of the various parameters that can be derived from pressure distribution measurement are important in the etiology of plantar lesions in the insensitive foot. Shear pressure is an extremely difficult quantity to measure and the present experiments provide no insight into the possible modification of shear pressure by the rocker shoe. Implications for the Design and Prescription of Footwear for Insensitive Feet
None of the subjects in the present study had a major foot deformity or focal concentrations of high pressure and it remains to be seen if the reductions in pressure that were found in these symptom-free subjects will be similar in patients who do exhibit such abnormalities. It is clear from the present results that the design of rocker bottom shoe used in this experiment would be contraindicated for patients with a history of lesions in the heel. Pressure and impulse were both significantly increased and this could be expected to cause a recurrence of symptoms. The increase in pressure seen under the lateral margin of the foot, although not statistically significant, is worthy of discussion since the increases were substantial in some individuals. Since
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patients with lesions in the forefoot, including those under the fifth metatarsal head, frequently receive a rocker bottom shoe of similar design to the current shoe, it is likely that some shoe prescriptions have actually increased pressures in regions that they were designed to relieve. An examination of Figure 3 shows that the head of the fifth metatarsal was either on, or proximal to, the rocker axis in every subject, whereas the head of the first, for example, was distal to the rocker in every case. Although the fifth metatarsal head itself was not monitored in most subjects due to the configuration of the insole (Figure 5), the four active elements in this region still showed a tendency towards increased pressures. It is possible that if the axis of the rocker had been placed individually such that it was proximal to the heads of all the metatarsals, a reduction in pressure on all of the forefoot regions may have occurred. However, the orientation of the axis (its angle in relation to the shoe midline) is another important variable which probably affects pressure distribution and no statements about this factor can be made as a result of the present experiments. This variable is certainly in need of further investigation in the future. The rather varied changes in pressure distribution that have been found in the present experiment emphasize that pressure relief by modification of footwear is a considerably more complex issue than might have been supposed. It is likely that this particular shoe modification has actually exacerbated plantar lesions, or perhaps caused a recurrence of healed lesions in some patients. Of course, a far greater number of lesions are likely to have been resolved or prevented since significant reductions in pressure and/or impulse occurred in four out of five forefoot regions. The rocker bottom shoe is just one of many shoe interventions with a poorly understood effect. Footwear prescription is, at the present time, an extremely empirical process. Typically, the physician specifies the type of shoe or insole to be used and the process of finding a footwear solution to the problem of the particular foot is frequently done in a custom shoe store by a pedorthist. The physician may only become involved in the loop again if a major mismatch occurs and the foot is threatened. As new techniques become available to evaluate the effects of shoe intervention on the mechanical conditions at the foot-shoe interface, it is vital that the process of shoe prescription be examined more critically. Ideally, the results of a given intervention would be measured individually on each patient before the patient is allowed to progress to normal ambulation wearing shoes. More realistically, it should now be possible to establish some general patterns concerning the effect of placement of the rocker axis in relation to
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the anatomy on pressure distribution. If such general guidelines were available, it would allow prescription to be much more specific and would probably increase the frequency with which a prescription actually achieves its desired effect. It is likely that, in the near future, we will look back with considerable bemusement on the rather haphazard process by which shoes are presently prescribed for a particular pathology. ACKNOWLEDGMENTS
The authors are grateful to Novel Inc. which loaned the insole used in the experiments. David Sims modified the shoe used in the experiments and Jan Ulbrecht provided the encouragement needed to complete the preparation of this manuscript. Sims and Ulbrecht also provided useful comments on an earlier draft of the manuscript.
REFERENCES
1. Barrett, J.P., and Mooney, V.: Neuropathic and diabetic pressure lesions. Orthop. Clin. North America, 4(1):43-47,1973. 2. Bauman, J.H., and Brand, P.W.: Measurement of pressure between foot and shoe. The Lancet, 23:629-632,1963. 3. Bauman, J.H., Girling, J.P., and Brand, P.W.: Plantar pressures and trophic ulceration: An evaluation of footwear. J. Bone Joint Surg., 458(4):652-673,1963. 4. Bild, D.E., Selby, J.V., Sinnock, P., Browner, W.S., Braveman, P., and Showstack, J.A.: Lower-extremity amputation in people with diabetes. Diabetes Care, 12(1):24-31,1989. 5. Birke, J.A., Cornwall, M.A., and Jackson, M.: Relationship between hallux limitus and ulceration of the great toe. J. Orthop. Sports Physical Ther., 10(5):172-176,1988. 6. Birke, J.A., and Sims, D.S.: Plantar sensory threshold in the ulcerative foot. Leprosy Reviews, 57:261-267,1986. 7. Birke, J.A., Sims, D.S.,and Buford, W.L.: Walking casts: effect of plantar foot pressures. J. Rehab. Res. Dev., 22(3):18-22,
1985. 8. Brand, P.W.: Addendum to Chapter XXII. In Leprosy in Theory and Practice. Cochrane, R.G. (ed), Bristol, England: John Wright and Sons, 1959. 9. Brand, P.W.: The diabetic foot. In: Diabetes Mellitus. Theory and Practise. Ellenberg, M., Rifkin, H. (eds), 3rd ed. Vol 2.New Hyde Park, N.Y.: Medical Examination Publishing; 1983,pp. 829-850. 10. Cavanagh, P.R., and Ulbrecht, J.S.: Pressure measurement in the diabetic foot. In: Sammarco, J. The Diabetic Foot. Philadelphia: W.B. Saunders, 1990.
11. Coleman, W.C.: Footwear in a management program of injury prevention. In: The Diabetic Foot. Levin, M.E., O'Neal, L.W. (eds.), 4th ed. St. Louis: C.V. Mosby, 1988,pp. 293-309. 12. Coleman, W.C.: The relief of pressures using outer shoe sole modifications. In: Proceedings of the International Conference on Biomechanics and Clinical Kinesiology of the Hand and Foot. MothiramiPatil, K., Srinivasa, H. (eds), Madras, India: Indian Institute of Technology, 1985,pp. 29-31. 13. Coleman, W.C., Brand, P.W., and Birke, J.A.: The total contact cast: a therapy for plantar ulceration on insensitive feet. J. Am. Podiatry Assoc., 74(11):584-552,1984. 14. Edwards, C.A.: Orthopedic Shoe Technology for the Orthopedic Shoe Technician. Muncie, Indiana: Precision Printing Co., 1981, p. 246. 15. Enna, C.D., Brand, P.W., Reed, J.K., and Welch, D.: Orthotic care of the denervated foot in Hansen's disease. Orthot. Prosthetics, 30(1):33-39,1976. 16. Hampton, G.H.: Therapeutic footwear for the insensitive foot. Physical Therapy, 59(1):23-29,1979. 17. Marquart, W.: Orthopaedishe Schuhe und Einlagen: Begriffsbestimmung. Orthopaedie; 1979,p. 8. 18. Milgram, J.E., and Jacobson, M.A.: Footgear:therapeutic modificationsof the sole and heel. Orthop. Rev., VII(11):57-62,1978. 19. Nawoczenski, D.A., Birke, J.A., and Coleman, W.C.: Effect of rocker sole design on plantar forefoot pressures. J. Am. Podiatric Med. Assoc., 78(9):455-460,1988. 20. Nicol, K., and Hennig, E.M.: Measurement of pressure distribution by means of a flexible large surface mat. In: Biomechanics VI-A. Asmussen, E., Jorgenson, K., (eds.), Baltimore: University Park Press, 1978,pp. 374-380. 21. Pollard, J.P., Le Quesne, L.P., and Tappin, J.W.: Forces under the foot. J. Biomed. Engin. 5:37-40,1983. 22. Price, E.W.: Studies on plantar ulceration in leprosy. VI. The management of plantar ulcers. Leprosy Review, 31(3):159-171,
1960. 23. Ross, W.F.: Etiology and treatment of plantar ulcers. Leprosy Review, 31:25-40,1960. 24. Ross, W.F.: Footwear and the prevention of ulcers in leprosy. Leprosy Review, 33:202-206,1962. 25. Schaff, P., and Hauser, W.: Dynamische Druckverteilungsmessung mit flexiblen Messmatten - ein Innovatives Messverfahren in der Sportorthopaedie und Traumatologie. SportverletzungSportschaden,4:185-222,1987. 26. Schaff, P., Kirsch, W., Hauser, W., and Mehnert, H.: Eine Geraeteentwicklungzur Messung der Druckverteilungunter der Fussohle im Schuh und deren Anwendbarkeit in der Diabetologie. Akt. Endokrin. Stoffw.. 7(3),1986. 27. Sims, D.S., and Blrke, J.A.: Effect of rocker sole placement on plantar pressures (Abstract). In: Proceedings of the 20th Annual Meeting of the USPHS Professional Association. Atlanta, 1985, p. 53. 28. Ward, D.: Footwear in leprosy. Leprosy Review, 3494-105,
1962.
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Shoes for the Insensitive Foot: The Effect of a ''Rocker Bottom'' Shoe Modification on Plantar Pressure Distribution Peter S. Schaff and Peter R. Cavanagh Foot Ankle Int 1990 11: 129 DOI: 10.1177/107110079001100303 The online version of this article can be found at: http://fai.sagepub.com/content/11/3/129
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0198-0211/90/1103-0129$03.00/0 FOOT8 ANKLE Copyright 0 1990 by the American Orthopaedic Foot and Ankle Society, Inc.
Shoes for the Insensitive Foot: The Effect of a “Rocker Bottom’’ Shoe Modification on Plantar Pressure Distribution Peter S. Schaff* and Peter R. Cavanaght Munich, Federal Republic of Germany and University Park, Pennsylvania
shown to be particularly at risk for ulceration of the plantar surface of the great toe,’s5 and the use of a rocker bottom shoe to reduce hallux pressures has been suggested for such patients.” This device, as used by Brand,’ is characterized by a totally rigid sole which is angled sharply up in the forefoot. Although extension cannot occur at the metatarsophalangeal joints, walking is possible because the shoe “tips” forward when the center of pressure moves distal to the rocker fulcrum (Fig. 1). An alternative design, usually referred to as a “roller sole,” is progressively contoured towards the toe18 in a manner that is supposed to mimic the motion of the talocrural joint during walking. It is assumed, in both shoe and cast situations, that the combination of a rigid platform for the foot and the rocker provides a “more even” distribution of plantar pressures during walking than would occur if the foot were allowed to extend at the metatarsophalangeal joints. Such an assumption appears to be reasonable, and previous studies, discussed below, using discrete transducers at various locations on the plantar surface of the foot have suggested that up to 50% reduction in pressure can be achieved at certain sites depending upon the exact configuration of the rocker and hoe.'^,'',*^ The present authors have not been able to
ABSTRACT The purpose of this study was to examine the effects on plantar pressure of the rocker bottom shoe, which is a frequently used intervention for the ulcerated diabetic foot. In-shoe pressure distribution was recorded during walking in a conventional extra-depth shoe which was then modified into a rocker bottom configuration with a 2 4 O rocker, and the experiment was repeated. Peak pressures in the rocker shoe were reduced by -30% compared to the conventional shoe in the medial and central forefoot and in the toe regions, but pressures were elevated in the heel, the midfoot, and in the lateral forefoot regions. These experiments suggest that a correctly designed rocker bottom shoe may reduce the risk of ulceration in certain areas of the foot. However, since pressures in some regions can also be elevated by this type of footwear, attention to individual design is critical.
INTRODUCTION
There is growing interest in the use of footwear as a therapeutic modality for the insensitive foot with a history of ~lceration,~ yet little scientific evidence exists to support or refute interventions that have become commonplace in clinical practice. Typically, an extradepth shoe with a custom molded inlay is a first line of defense when a patient with diabetes or Hansen’s disease is thought to be at risk for ulceration.” If there has been recurrent ulceration in the forefoot or if there is significant bone injury or limitation of motion, many experts recommend that the patient, once healed, should be placed in a rocker bottom shoe.’ A rigid wooden sole and a rocker are also usually applied to the plantar surface of a total contact cast for ambulation during plantar ulcer healing.13,22 Patients with loss of sensation and hallux limitus have been
STE
Institute for Biomechanical Analysis in Sports and Interdisciplinary Studies (BASIS), Ridlerstrasse31, D-8000 Munich 2, F.R.G. t To whom all correspondence should be addressed at the Center for Locomotion Studies, Penn State University, University Park, PA 16802.
PIVOT
Fig. 1. Schematic diagram of a rocker bottom shoe. A steel shank is inserted between midsole layers and the rear part of the shoe is built up to allow the angle in front of the rocker axis to be achieved.
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locate any studies where the plantar pressure between the foot and a rocker bottom shoe were examined at more than four sites simultaneously. The purpose of the present paper is therefore to examine the effect of a rocker bottom modification to the shoe on plantar pressure during walking using a 72-element pressure sensitive insole in order to get a more complete picture of plantar pressure distribution. REVIEW OF LITERATURE
The use of rigid footwear for the insensitive foot has its roots in the methods pioneered in the 1950s and 60s by Brand,’ Price,” Ross,’~and Ward“ for the care for Hansen’s disease patients with plantar ulceration. While the rigid sole is common to all authors, there continues to be some debate over the placement and orientation of the rocker axis. Among the placements suggested are under the metatarsal proximal to the metatarsal heads,’6~’8forward of the midpoint of the hoe,'^,'^ at the midpoint of the shoe,I6and “as far back as possible.28Proposed placements in relation to the metatarsal heads are complicated by the fact that the heads do not generally lie on a straight line. Although Rossz4had used a Harris mat to record inshoe pressures, the first landmark quantitative study of footwear for insensitive feet was performed by Bauman, et al. in 1963.3 Using discrete capacitance transducers described by Bauman and Brand,* “normal” values for sensate and insensate feet were first defined during barefoot ~ a l k i n gFor . ~ patients without deformity, no deviation from “normal” pressure values were found in the insensate feet. These authors then measured pressures inside a conventional Oxford shoe and six different show variations, including a rocker in two different positions: with the axis “centered under the metatarsal heads” and “1.7 cm behind the metatarsal heads.” The subjects walked in time to a metronome at 100 steps/min. Other experiments, not discussed here, made similar studies in patients with shortened feet and pressure measurements inside a total contact cast. In shoes without an insole, the rocker shoes yielded the lowest pressures under the hallux (maximum relief of approximately 12% with the anteriorly placed rocker) and second metatarsal heads (maximum relief of 30% with the posteriorly placed rocker). Pressures under the heel were approximately 40% higher in all rocker conditions compared to the conventional shoe. Bauman, et aL3 also demonstrated the consequences of inadequate rocker height and showed a recording of high forefoot pressures when the tip of the shoe came into contact with the ground. These authors discussed
the design dilemma of placing the rocker axis far enough anteriorly to ensure that there was no toe contact, yet far enough posteriorly to relieve metatarsal head pressures. In 1983, Pollard, et al.” developed a new transducer to measure shear forces and used it, together with a discrete vertical pressure transducer, to record the changes in interface conditions using a number of types of footwear, including a rocker shoe with “a deep rocker behind the metatarsal heads.” They reported no significant alteration in the vertical pressure at any site when the rocker shoe was compared to “conventional leather shoes,” but noted a “small” reduction in a single transducer under the heads of the second and third metatarsals. However they did find significant reductions in the longitudinal shear pressure in the rocker shoe under all the metatarsal heads except the first.” The relief of shear pressure under the central metatarsal heads was over 70%; a change of this magnitude suggests that this aspect of interface pressure is in need of more attention. Unfortunately these authors do not appear to have published on the topic since 1983, and no other investigators have used the technique. In 1985, Birke et al.’ measured plantar pressures at four sites inside a conventional padded cast, a total contact cast, and a standard shoe in six normal subjects. Walking heels were placed on the casts, centered at 60% of foot length from the toe and extending approximately 2 inches in each direction from this point. They found significant reductions in pressure in the casts compared to the shoe during walking of 84% at the first and 75% at the third metatarsal head. The pressures under the heel and head of the fifth metatarsal were not significantly different between cast and shoe conditions (although the trend was towards a reduction), and the two types of casts were not different from each other at any location. In a more comprehensive, but similar, study to that performed by Bauman, et al.,3 Coleman” studied 15 subjects in six different shoe modifications using four discrete pressure transducers. His modifications included rigid soles with no contour, two types of “roller” sole with a continuously curved outsole, and three rocker soles with angles varying from 15’ to 30”.In every case the outsole modification started “just behind the fifth metatarsal head.”” The subjects walked overground at 80 steps/min in all conditions in time to a metronome. Coleman found no significant differences between any shoe condition in the heel, but all modifications showed significant pressure relief compared to the unaltered shoe in the hallux, second metatarsal head, and the fourth metatarsal head. In general, the pressure relief at all forefoot sites was greatest in the
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two rockers with the highest angle, less in the low rocker and the roller soles, and least in the rigid shoe with no outsole modification. A mean relief of approximately 50% was obtained at the second and fourth metatarsal head locations. Sims and Birke2’ extended this work by using similar discrete transducers under the first, third, and fifth metatarsal heads and heel of symptom free subjects who walked in a normal shoe and four different rocker shoes in which the rocker axis was progressively moved anterior and posterior to the first metatarsal head. The orientation of the rocker axis was stated as 30’. They found that axis placement had no effect on pressures at the fifth metatarsal head where pressures were not significantly different from the normal shoe. At the first and third metatarsal heads a posterior placement tended to relieve plantar pressures whereas anterior placement showed a significantly increased pressure at the first metatarsal head. No absolute values for changes in plantar pressure achieved by the various designs were given. Schaff, et al.26used a 15element measuring insole to analyze pressure distribution in 13 different shoes and reported considerable variation in the distribution in the forefoot region. Nawoczenski et al.” examined the effect of six different designs of rocker and roller shoes using discrete pressure transducers taped to four locations on the feet of symptom free individuals. They reported pressure reduction at all sites tested (hallux, first, third, and fifth metatarsal heads) when the experimental shoes were compared to a standard Oxford shoe. A maximum pressure relief of 31O/O was found underneath the hallux using a shoe similar to that shown in Figure 1. In all other designs, maximum pressure relief was achieved under the third metatarsal head. In summarizing the related literature, Milgram’s statement that “prescription shoe fitting and shoe correcting are more an art than a science”’* seems to apply well to considerations of design and function of the rocker bottom shoe. This type of shoe has been widely recommended on a clinical basis for pressure relief in feet with a previous history of ulceration and a lack of sensation. Despite the apparent success of this intervention, there appears to be no controlled prospective studies of healing, and no authoritative sources which describe the optimum combination of design parameters for a particular pattern of pathology. Studies with discrete transducers have produced a variety of results when compared to conventional footwear, ranging from over 50% reductions at some sites to no significant differences or actual increases at other locations.
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METHODS Subjects
Eight nondiabetic subjects who were free of pathology in the lower extremity volunteered to participate in this study. Their average age was 31 years, average body mass 69 kg, and average height 1.76 m. Because only one insole was available the subjects were also selected based upon their shoe size, which was US men’s size 9 in every case. The same size 9 pair of shoes was used for all subjects in all experiments. Shoe Specifications
Data were first collected as the subjects walked in an extra-depth shoe with no insole (Fig. 2A). After all subjects had completed their trials in this “normal” shoe condition, the shoe was modified to produce the rocker configuration shown in Figure 29. The outsole was tapered anteriorly and a rigid metal corset stay was attached to the exposed surface of the remaining outsole. Sufficient crepe material was then added to the
A
Fig. 2. A, The extra depth shoe used for the experiments. B, The shoe after modifications had been made to produce the rocker sole.
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outsole so that it was possible to grind the sole under the forepart of the shoe to produce a 24' angle between the rear and fore parts of the outsole. The resulting total material thickness was 5.5 cm under the heel and 3.5 cm under the ball of the foot just proximal to the rocker, and this gave a heel-ball height differential of 2 cm. To allow a reasonably natural heel strike, the rear border of the midsole was beveled to a 44' angle starting 3 cm anterior to the posterior border of the heel. The heel-ball height differential was identical to the unmodifiedshoe which had 0.8 cm of material under the forefoot and 2.8 cm under the heel. The geometry of the rocker axis produced by the shoe modification is shown in Figure 3. The axis formed an angle of 90' with the midline of the posterior one half of the shoe. This midline formed an angle of 7.5' with the longitudinal axis of the shoe (a line drawn through the anterior and posterior shoe midpoints). The location of the rocker in relation to the subjects' anatomy is also shown in Figure 3 and will be discussed below. A
The flexible pressure measuring insole, which was worn in the right shoe only, consisted of 72 active elements arranged in a contiguous matrix sandwiched between layers of surgical rubber (Fig. 4). Most of the elements were 1 x 1.4 cm and operated as air capacitors as has previously been d e ~ c r i b e d . ' ~Details , ~ ~ , of ~~ the frequency, hysteresis, cross sensitivity between adjacent active and inactive elements, and linearity of the device are given in Schaff and Ha~ser.'~ We performed our own calibration of the insole by recording the electrical output while a pressure vessel positioned over the insole was relaxed from various pressure values. This process gave individual calibration factors for each element derived from linear least squares equations which, in general, provided a satisfactory fit such that higher order approximations were not needed. Force platform measurements of ground reaction forces and centers of pressure were also made but these results are outside the scope of the present paper. Relationship of Shoe and Pressure Insole Geometry to Anatomy
Outsole outline
339
In order to define the relation of the shoe and insole geometry to the anatomy of the subjects' feet, the approach taken in Figure 5 was used. An exact outline in the dimensions of the footbed of the shoe was cut by a jig saw from a wooden panel using the vinyl insole liner of the shoe as a guide. A transparency showing
/
I
67%
Instrumentation
- + / - L o n g i t u d of i n shoe a l axis
r---
I
--&
Start of heel bevel
B Fig. 3. The geometry of the rocker modification made to the shoe showing the metatarsal head locations during standing with respect to the rocker axis for each of the eight subjects. The shaded area represents the difference between the insole outline (the entire area in contact with the foot) and the outsole. See text for further discussion.
Fig. 4. The actual EMED pressure measuring insole used for the experiments.
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less than complete coverage. This was a result of the need to run leads from the rows and columns to the electronics unit. Treadmill Protocol
During the collection of pressure data, subjects walked at a freely chosen cadence on a treadmill set to a speed of 0.83 m.sec-’. This is a slow walking speed (1.9 mph or 3km/hr) typical of that which older patients might use during unpaced ambulation. Although all the subjects were accustomed to treadmill locomotion, they walked for 15 min on the treadmill in each shoe before measurements were taken. In the initial experiments, cadence was freely chosen by the subjects and determined from measurement of the time required for 30 contacts of the right foot. Since the treadmill speed was accurately known, step length could also be calculated from this measurement. Three separate trials for each condition were collected and at least three foot contacts of the right foot were available for pressure analysis from each trial. Between each trial the treadmill was stopped and the subject rested for approximately 3 min. The only instructions that the subjects were given in the use of the rocker bottom shoes was to walk “so that the toes do not touch the ground.” These instructions, similar to those that would be given to a patient when the shoes are dispensed, encouraged the wearer not to defeat the object of the rocker by using that part of the outsole anterior to the rocker axis as a major weight bearing surface. ANALYSIS OF DATA
Fig. 5. A weight-bearing photograph taken from underneath a wooden form in the shape of the insole outline. The transparency shows the location of each one of the 72 measuring elements and the rocker axis. The heads of the metatarsals (located by palpation) are marked with black circles.
the location of each active measuring element was then positioned in exact relation to the shoe outline, and the location of the rocker axis was also marked. By palpation, the heads of all five metatarsals were also identified and a photograph was taken from underneath as the subject stood with all his weight on the right foot (Fig. 5). This enabled the locations shown in Figure 3 to be constructed where the positions of the metatarsal heads of each subject are shown in relation to the rocker axis. It is apparent from Figure 5 that certain regions of the foot could not be monitored with the insole used in this study. In particular, a band in the midfoot, the lateral aspect of the forefoot and toes, and the medial aspects of the heel and great toe received
The peak pressure, defined as the largest pressure recorded from the insole regardless of its location, was determined at each instant of time by software and a peak pressure versus time array was assembled. This curve was automatically scanned to divide the four second data collection period into individual steps. The peak pressure exerted on each element during each step (regardless of the time of occurrence) was then determined and a mean “peak pressure array” was assembled by taking the average of the peak pressure for each element during the nine separate steps available for each condition. A scheme for regional analysis was also devised from the photographs of the plantar surface in relation to the shoe and insole geometry. This scheme, shown in Figure 6, involved a combination of elements into 10 anatomical areas in order to simplify the analysis. Although all subjects were comfortable in the size 9 shoe, both the length and proportions of the foot varied between individuals, and this led to some variation in the anatomical structures which were included in the
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B.
A.
1 Medial Toes 2 Lateral Toes 3 Medial Forefoot 4 Central Forefoot 5 Lateral Forefoot 6 Medial Midfoot 7 Lateral Midfoot 8 Medial Heel 9 Central Heel 10 Lateral Heel
nominally labelled regions. The rather general anatomical terms, such as “central forefoot,” have been deliberately used because of this variability and because of the fact that individual anatomical structures frequently overlap two defined regions. This overlap was principally in the mediolateral direction, and there was never a case where, for example, a metatarsal head was incorrectly located in a toe or midfoot region. By further combination of the 10 regions described above, more global statements about pressure distribution could be made. For example “peak toe pressure” was obtained by identifying the largest value from regions 1 and 2, “peak forefoot pressure” from regions 3 through 5, “peak midfoot pressure” from regions 6 and 7, and “peak heel pressure” from regions 8, 9, and 10. Peak pressure versus time curves could also be generated for all regions. To estimate the net load bearing in a given region, the instantaneous force on a region was calculated as follows: x = n
fit =
C Px(ax) x = 1
where: Fit is the net force acting at time t on the ith region at a given time n is the number of elements in the ith region Px is the pressure acting on element x a, is the functional element x.
MIDFOOT
Fig. 6. A , The 10 regions that were generated by a combination of individual elements. The regions are labelled in relation to the anatomical structures that they measure. 8,Regions generated by the pooling of smaller regions. For example, a single heel region was formed by a combination of regions 8, 9, and 10 and a single forefoot region from regions 3. 4, and 5 .
The estimate of force is subject to some inaccuracy due to the integration of residual noise on elements which are not heavily loaded. The value of f i could also be plotted as a function of time and the area under this curve (the force time integral) indicates the total loading of the region during ground contact. All statistical analyses were conducted using a twotailed Wilcoxon signed ranks test with a probability level P c .05. This nonparametric paired t-test was used due to the likelihood of violating the requirement of normal distribution with the small sample size in the present experiment (n = 8). RESULTS
Step Parameters
The mean step lengths (SL) and step frequencies (SF) used by the subjects while walking in the two shoe types are shown in Table 1. The subjects took 8 cm shorter steps at a rate of approximately 6/min more when wearing the rocker bottom shoes compared to walking with the normal shoes. Both SL and SF were significantly different between the two shoe conditions (P < .05). These differences reflect readily observable changes in the lower extremity movement patterns that occurred when subjects were walking in the rocker bottom shoes. The measurement of the exact kinematic
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TABLE 1 Means and Standard Deviations for Step Length and Step Frequency ( n = 8) during Treadmill Walking at 0.83 msec-' in the Two Shoe Types
Step Length (SF) (m) Normal shoe Rocker shoe Difference (rocker - normal)
1.19 (0.034) 1.11 (0.049)
-0.08"
Step Frequency (SF) (min-')
83.8(2.4) 90.0(4.0) 6.2"
A step is defined as the distance between successive contacts of the right and left feet. ' Significant difference (P < 0.05).
differences in segment and joint angles was beyond the scope of the present study. Pressure
Peak pressure pictures for one subject who showed changes in the same direction as the group means are shown in Figure 7. These diagrams show the peak pressure at each measuring point without regard to the time of occurrence of that pressure maximum. The intersection of lines in the grid in Figure 7 represent the position of active elements, and the height of the display at a given point is proportional to the pressure at that location. The vertical edges on the lateral border are regions where a boundary element has a nonzero pressure value and this should be compared with Figure 5 where, as mentioned earlier, it is apparent that not all weight-bearing areas of the foot are monitored with the insole used. If there were more lateral elements in the insole, they might be expected to show values closer to zero which would result in a smoother contour of the display. The most noticeable feature of Figure 7 is the marked reduction in peak pressure on the medial and middle metatarsal heads in the rocker bottom shoe condition. However, it is clear that pressure is not lower in all regions of the rocker shoe, but appears to be somewhat elevated in the heel, the midfoot and the lateral forefoot. This impression is confirmed by the results of the gross regional analysis presented in Table 2, where the foot was divided into four large regions in a proximal to distal manner. These data indicate that the rocker shoe significantly reduced peak pressure in the forefoot and toes by approximately 30%, but also resulted in significantly increased peak pressures of approximately 20% in the heel. The large mean increase in the midfoot was not statistically significant. The impulse data show most clearly the manner in which the rocker bottom shoe changed the function of the foot during walking. In the normal shoe, the ratio of heel region impulse to forefoot region impulse was 0.85, reflecting the dominance of the forefoot in the load-
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bearing process. The same ratio in the rocker shoe was 1.42, a change caused by both a significant 35% reduction in forefoot impulse and by a significant 42% increase in heel impulse. The over 100% increase in the midfoot impulse for the rocker shoe condition was also statistically significant, but there was no significant change in toe impulse. Examples of regional force-time curves for one subject, typical of those from which the above impulse values were calculated, are shown in Figure 8. This figure shows the higher forces applied for a longer period of relative time in the heel and midfoot, the lower force for approximately the same period of relative time in the forefoot, and the lower force applied for a longer period of relative time in the toes. The time base in Figure 8 is percentage of the support phase. Since the majority of lesions in the diabetic foot occur in the toe and forefoot regions,' particular attention was given to the footwear-induced changes in pressure distribution within the large forefoot and toe regions discussed above. Table 3 shows peak pressure and impulse results for the smaller regions 1 through 5 which comprised the toe and metatarsal head areas (Fig. 6). It is apparent from Table 3 that the rocker bottom shoe caused a statistically significant reduction in peak pressure in the forefoot that was fairly uniform as far as percentage was concerned (approximately 30% reduction) in all regions except the lateral forefoot where the peak pressure was greater in the rocker shoe by an amount that failed to achieve statistical significance. The impulse results, however, only showed significant differences in the medial and central metatarsal heads where there were 53% and 35% reductions in the impulse in the rocker shoe compared to the normal shoe. DISCUSSION Generalizability of the Results
It should be stated at the outset that the results from this study cannot be generalized to all rocker-bottom shoe modifications. As was apparent from the earlier review of the literature, there is no consensus of opinion concerning such important variables as the angle of the rocker, the anteroposterior position of the rocker axis, and the orientation of the axis with respect to the shoe midline. The particular shoe used in these experiments was modified by a physical therapist who had extensive experience in the design of footwear for insensitive feet. His design was a function of the standard of practice that he had learned during his professional experience and a response to the need to produce a single modification that would be worn by all eight subjects. The resulting shoe was just one example of
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A
I
B
300 kPa
Fig. 7. Peak pressure pictures for the subject closest to the mean response in the normal shoe ( A ) and the rocker bottom shoe ( B ) . Note the reduction in pressure achieved by the rocker shoe over the medial and central regions of the forefoot. Pressure is. however, slightly increased in the heel and the lateral midfoot regions. The scale bar shown is 300 kPa. TABLE 2 Mean Values from all Eight Subjects for Regional Peak Pressure and Impulse with the Foot Divided on a Proximal to Distal Basis
Heel
Midfoot
Forefoot
Toes
Peak pressure Normal shoe Rocker shoe Difference(rocker - normal)
176 209 33 (18.7%)”
59 95 36 (61.O?’o)
21 9 155 -64 (-29.2%)”
194 131 -63 (-32.5%)”
Impulse Normal shoe Rocker shoe Difference(rocker - normal)
144 205 61 (42.2%)”
13 26 13 (102.3%)8
169 111 -58 (-34.7%)8
41 33 -8 (-18.7%)
The peak pressure is in kiloPascals (kPa) and the impulse is in newton seconds (Ns). Values in the “difference” category represent (Rocker shoe) - (Normal shoe) and are therefore negative when the rocker provides a relief of pressure or a reduction in impulse and positive when the rocker shoe results in an increase. Percentage changes are shown in parentheses following the difference in absolute units. Note: lOOkPa = 14.1pounds per square inch (psi) = 10 N/cmZ. Denotes a significant difference between normal and rocker shoe conditions P < 0.05.
an infinite number of possible rocker shoe designs where the position and orientation of the axis could be altered and the composition and stiffness of midsole changed. In the absence of definitive experimental evidence on the optimum design for a rocker shoe, the opinion of an experienced clinician seemed an appropriate starting point for the investigation of alterations the shoe may cause. We also decided not to provide a custom-molded insole for either shoe since it would have been difficult to separate the effects of outsole geometry from those due to insole composition on any results obtained. In practice, an insole of some description is invariably used. The pressure measurements in the present experiments were conducted on a motorized treadmill and thus it is posible, although unlikely, that the effects during overground walking would be different.
Step Parameters
Since all subjects walked with smaller step lengths in the rocker shoe, a finding which confirms the observations of Ward2* and E d w a r d ~ , ’it~is reasonable to hypothesize that the reduction in pressure could have simply been a function of the smaller step length and not a footwear effect at all. To investigate this explanation, the experiment using the normal shoe was repeated, but subjects were given an auditory cue from an electronic metronome. The device was set individually for each subject to produce a tone at the same interval as the step frequency which that subject used during his walk in the rocker bottom shoe. Since the walking velocity was fixed by treadmill speed, an increased cadence would cause a shorter step length and vice versa.
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MIDFOOT
o
FOREFOOT
40
20
80
60
inn
TIME ( % contact)
TIME ( % contact)
I
400
TIME (%contact)
Fig. 8 . Regional force time curves for one subject showing the higher forces applied for a longer period of relative time in the heel and midfoot, the lower force for approximately the same period of relative time in the forefoot, and the lower force applied for a longer period of relative time in the toes. The time base is 100% of the support phase. TABLE 3 Mean Values from all Eight Subjects for Regional Peak Pressure and Impulse in the Toe and Metatarsal Head Regions Peak Pressure Normal shoe Rocker shoe Difference (rocker - normal) Impulse Normal shoe Rocker shoe Difference (rocker - normal)
Medial toes
Lateral toes
194" 131 -63 (-32.3%)
104" 78 -26 (-25.3%)
22.7 21.4 -1.3 (-6.0%)
18.0 11.7 -6.3 (-34.7%)
Medial forefoot
Central forefoot
219" 155 -64 (-29.1%)
213" 143 -70 (-32.9%)
62.9" 29.5 -33.4 (-53.1 Yo)
85.5" 55.1 -30.4 (-35.5%)
Lateral forefoot 123 138 15 (12.2%) 20.5 25.6 5.1 (25.0%)
The peak pressure is in kiloPascals (kPa) and the impulse is in newton seconds (Ns). Values in the "difference" category are calculated in a similar manner to that described in Table 2. Significantly greater than the corresponding value in the rocker shoe (P< 0.05).
Subjects found this "forced cadence" experiment fairly easy to perform, and three, 4-sec trials were collected yielding a total of nine separate contact phases for analysis. The results, shown in Table 4, indicated that walking at the faster cadence, with its accompanying shorter step length, .actually caused a significant increase in pressures in three of the ten anatomical regions, when compared to the pressure in
the same shoe during walking at a freely chosen cadence. In every other region except the medial heel, there was a nonsignificant increase in the mean peak pressure when walking at the faster cadence. Only one impulse value, that in the medial toes, was significantly different during walking at the faster cadence. The interaction of shorter contact times in the forced cadence condition with the higher pressures probably
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TABLE 4 Mean Peak Regional Pressure (kPa) and Impulse (Ns) for all Subjects during Treadmill Walking
Medial toes
Lateral toes
Medial forefoot
Central forefoot
Peak pressure Chosen Forced Difference O/O Difference
194 243 49” (20.1)
104 119 15 (12.6)
219 260 41 (15.9)
213 249 36” (14.6)
Impulse Chosen Forced Difference OO / Difference
22.7 31.3 8.4” (26.9)
18.0 17.7 -0.2 (-1.4)
62.9 60.9 -2.0 (-3.3)
85.5 99.2 13.7 (13.9)
Lateral forefoot 123 140 18 (12.5) 20.5 18.6 -1.9 (-10.1)
Medial midfoot
Lateral midfoot
Medial heel
Central heel
Lateral heel
14 26 13 (47.6)
59 68 9 (13.0)
174 166 -8 (-4.5)
176 179 3 (1.4)
159 178 19” (10.6)
0.6 0.7 0.1 (16.7)
12.2 13.1 0.9 (6.7)
51.1 47.4 -3.7 (-7.9)
41.5 40.6 -0.9 (-2.2)
51.9 60.9 9.0 (14.8)
Subject walked at a freely chosen cadence and a forced cadence in the normal shoe. The forced cadence was set individually to the value used by the subject during walking in the rocker bottom shoe, and was in every case greater (see Table 1). * Significant difference between forced and chosen cadence conditions.
tended to produce similar values for impulse. It would seem that, in experiments with a rocker bottom shoe, it is more reasonable to allow subjects to chose their own cadence rather than to impose a cadence as Bauman et aL3 and Colemani2 did. The change to a different type of footwear seems to carry with it an inevitable change in cadence and this is part of the effect that is achieved with the rocker footwear. Despite this change, it appears from our forced cadence experiments that an increase in cadence can be discounted as a mechanism for the reduced pressures that were encountered in the rocker bottom shoe. There is, however, a possibility that the increases in heel pressure found in the rocker shoe were a result of increased cadence. Pressure
The most important observation that can be made from the pressure results of the present experiment is that the particular rocker bottom modification used resulted in both increases and decreases in the applied pressure in different anatomical regions. This finding is somewhat similar to the results of Sims and Birke” who found that the pressure on the lateral aspect of the foot was not significantly altered by their footwear interventions and to the results of Bauman et aL3 who found increased heel pressure in rocker shoes. Nawoczenski et al.” also reported significant decreases with a shoe similar to ours at all sites except the fifth metatarsal head. However our results appear to be in conflict with those of Coleman’* who found no significant changes in the heel but reduction at three other forefoot and toe sites. Differences in shoe design could account for the observed differences since our lateral forefoot region appears to include the fourth metatarsal head, which was the site of Coleman’s most lateral sensor. Another
possible explanation is a difference in the active area of the sensors between the two studies which may have resulted in averaging of pressures by Coleman’s larger transducers (2.2 cm2 versus 1.4 cm2 in our device). The mixed pattern of pressure relief, no significant change, and significant increase in pressure found in our experiments, together with the conflict between our results and some previously published work has important implications for shoe prescription and design that will be discussed in the next section. Although it is not well stated in the literature, most clinicians believe that the mechanism of unloading in the rocker shoe is some combination of the following effects. 1. A redistribution of the load over a larger area. 2. An increase in the loading time for the regions of the foot in contact with the rigid shoe. 3. A change in the function of the foot, due to the restriction of motion, particularly at the metatarsophalangeal joints. 4. A change in the patterns of motion of the lower extremity due to the altered geometry and rigidity of the shoe. 5. A reduction in shear pressure on the plantar surf ace.
The present results provide a perspective on the validity of some of these spectulations. The increases in impulse in the heel and midfoot in the rocker bottom shoe are approximately twice as great as the increases in pressure (Table 2). If one makes the assumption that the single peak pressure value is representative of changes in average pressure in the region, this suggests that an increased duration of loading is more important in these regions than an increase in pressure (although both impulse and peak pressure are significantly higher). However, in the forefoot, the reduction
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in peak pressure and impulse are approximately the same, suggesting that pressure reduction is the key element. Since no metatarsophalangeal joint extension was possible in the rocker shoe, it seems reasonable to suggest that this restriction was responsible for the reduction in peak plantar pressure of approximately 30% that was found in the toes, and the medial and central forefoot areas in the rocker shoe. The hypothesis that metatarsophalangeal joint extension causes the metatarsal heads to be driven plantarwards, resulting in increased plantar pressures appears to be partially supported. A more complete test of this hypothesis would require the progressive restriction of metatarsophalangeal joint motion without the other changes induced by the rocker shoe. Since the hallux is the site of many lesions which result in the prescription of rocker bottom shoes, the results in the medial toe region are of particular interest. As shown in Table 3, there was a significant decrease in peak pressure in this region, but the impulse was not different between the two shoe conditions. Since it is well known anecdotally that many lesions of the hallux can be prevented from recurrence by a rocker bottom shoe, this finding would tend to suggest that peak pressure rather than impulse might be the critical factor in lesion development. Such an observation is, however, highly speculative and needs to be confirmed by experiments with patients who have had prior lesions of the hallux. There is still no clear evidence as to which of the various parameters that can be derived from pressure distribution measurement are important in the etiology of plantar lesions in the insensitive foot. Shear pressure is an extremely difficult quantity to measure and the present experiments provide no insight into the possible modification of shear pressure by the rocker shoe. Implications for the Design and Prescription of Footwear for Insensitive Feet
None of the subjects in the present study had a major foot deformity or focal concentrations of high pressure and it remains to be seen if the reductions in pressure that were found in these symptom-free subjects will be similar in patients who do exhibit such abnormalities. It is clear from the present results that the design of rocker bottom shoe used in this experiment would be contraindicated for patients with a history of lesions in the heel. Pressure and impulse were both significantly increased and this could be expected to cause a recurrence of symptoms. The increase in pressure seen under the lateral margin of the foot, although not statistically significant, is worthy of discussion since the increases were substantial in some individuals. Since
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patients with lesions in the forefoot, including those under the fifth metatarsal head, frequently receive a rocker bottom shoe of similar design to the current shoe, it is likely that some shoe prescriptions have actually increased pressures in regions that they were designed to relieve. An examination of Figure 3 shows that the head of the fifth metatarsal was either on, or proximal to, the rocker axis in every subject, whereas the head of the first, for example, was distal to the rocker in every case. Although the fifth metatarsal head itself was not monitored in most subjects due to the configuration of the insole (Figure 5), the four active elements in this region still showed a tendency towards increased pressures. It is possible that if the axis of the rocker had been placed individually such that it was proximal to the heads of all the metatarsals, a reduction in pressure on all of the forefoot regions may have occurred. However, the orientation of the axis (its angle in relation to the shoe midline) is another important variable which probably affects pressure distribution and no statements about this factor can be made as a result of the present experiments. This variable is certainly in need of further investigation in the future. The rather varied changes in pressure distribution that have been found in the present experiment emphasize that pressure relief by modification of footwear is a considerably more complex issue than might have been supposed. It is likely that this particular shoe modification has actually exacerbated plantar lesions, or perhaps caused a recurrence of healed lesions in some patients. Of course, a far greater number of lesions are likely to have been resolved or prevented since significant reductions in pressure and/or impulse occurred in four out of five forefoot regions. The rocker bottom shoe is just one of many shoe interventions with a poorly understood effect. Footwear prescription is, at the present time, an extremely empirical process. Typically, the physician specifies the type of shoe or insole to be used and the process of finding a footwear solution to the problem of the particular foot is frequently done in a custom shoe store by a pedorthist. The physician may only become involved in the loop again if a major mismatch occurs and the foot is threatened. As new techniques become available to evaluate the effects of shoe intervention on the mechanical conditions at the foot-shoe interface, it is vital that the process of shoe prescription be examined more critically. Ideally, the results of a given intervention would be measured individually on each patient before the patient is allowed to progress to normal ambulation wearing shoes. More realistically, it should now be possible to establish some general patterns concerning the effect of placement of the rocker axis in relation to
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the anatomy on pressure distribution. If such general guidelines were available, it would allow prescription to be much more specific and would probably increase the frequency with which a prescription actually achieves its desired effect. It is likely that, in the near future, we will look back with considerable bemusement on the rather haphazard process by which shoes are presently prescribed for a particular pathology. ACKNOWLEDGMENTS
The authors are grateful to Novel Inc. which loaned the insole used in the experiments. David Sims modified the shoe used in the experiments and Jan Ulbrecht provided the encouragement needed to complete the preparation of this manuscript. Sims and Ulbrecht also provided useful comments on an earlier draft of the manuscript.
REFERENCES
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11. Coleman, W.C.: Footwear in a management program of injury prevention. In: The Diabetic Foot. Levin, M.E., O'Neal, L.W. (eds.), 4th ed. St. Louis: C.V. Mosby, 1988,pp. 293-309. 12. Coleman, W.C.: The relief of pressures using outer shoe sole modifications. In: Proceedings of the International Conference on Biomechanics and Clinical Kinesiology of the Hand and Foot. MothiramiPatil, K., Srinivasa, H. (eds), Madras, India: Indian Institute of Technology, 1985,pp. 29-31. 13. Coleman, W.C., Brand, P.W., and Birke, J.A.: The total contact cast: a therapy for plantar ulceration on insensitive feet. J. Am. Podiatry Assoc., 74(11):584-552,1984. 14. Edwards, C.A.: Orthopedic Shoe Technology for the Orthopedic Shoe Technician. Muncie, Indiana: Precision Printing Co., 1981, p. 246. 15. Enna, C.D., Brand, P.W., Reed, J.K., and Welch, D.: Orthotic care of the denervated foot in Hansen's disease. Orthot. Prosthetics, 30(1):33-39,1976. 16. Hampton, G.H.: Therapeutic footwear for the insensitive foot. Physical Therapy, 59(1):23-29,1979. 17. Marquart, W.: Orthopaedishe Schuhe und Einlagen: Begriffsbestimmung. Orthopaedie; 1979,p. 8. 18. Milgram, J.E., and Jacobson, M.A.: Footgear:therapeutic modificationsof the sole and heel. Orthop. Rev., VII(11):57-62,1978. 19. Nawoczenski, D.A., Birke, J.A., and Coleman, W.C.: Effect of rocker sole design on plantar forefoot pressures. J. Am. Podiatric Med. Assoc., 78(9):455-460,1988. 20. Nicol, K., and Hennig, E.M.: Measurement of pressure distribution by means of a flexible large surface mat. In: Biomechanics VI-A. Asmussen, E., Jorgenson, K., (eds.), Baltimore: University Park Press, 1978,pp. 374-380. 21. Pollard, J.P., Le Quesne, L.P., and Tappin, J.W.: Forces under the foot. J. Biomed. Engin. 5:37-40,1983. 22. Price, E.W.: Studies on plantar ulceration in leprosy. VI. The management of plantar ulcers. Leprosy Review, 31(3):159-171,
1960. 23. Ross, W.F.: Etiology and treatment of plantar ulcers. Leprosy Review, 31:25-40,1960. 24. Ross, W.F.: Footwear and the prevention of ulcers in leprosy. Leprosy Review, 33:202-206,1962. 25. Schaff, P., and Hauser, W.: Dynamische Druckverteilungsmessung mit flexiblen Messmatten - ein Innovatives Messverfahren in der Sportorthopaedie und Traumatologie. SportverletzungSportschaden,4:185-222,1987. 26. Schaff, P., Kirsch, W., Hauser, W., and Mehnert, H.: Eine Geraeteentwicklungzur Messung der Druckverteilungunter der Fussohle im Schuh und deren Anwendbarkeit in der Diabetologie. Akt. Endokrin. Stoffw.. 7(3),1986. 27. Sims, D.S., and Blrke, J.A.: Effect of rocker sole placement on plantar pressures (Abstract). In: Proceedings of the 20th Annual Meeting of the USPHS Professional Association. Atlanta, 1985, p. 53. 28. Ward, D.: Footwear in leprosy. Leprosy Review, 3494-105,
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