Universal joint slippage as a cause of Hofmann half-frame external fixator failure F.L.I.P. Drijber and J.B. Finlay Orthopaedic Research Laboratory, University Hospital, P.O. Box 5339, London, Ontario, Canada, MA 5A5
University
of Western
Ontario,
Received April 1991, accepted December 1991
Slippage of th universal joints of externalJixation devices is known to occur but its significance or incidence is often overlooked.In this study, controlledexperiments were used to determine the relationshi$ betweenjoint slipPageand the maximum loao3 a single ha&r ame could bear for the Hofiann device. The experiments showed that: (a) the joints slipped at minimal loadsand (3)fi amef ai‘1ure, i.e. lossof initial alignment of thefiame components,was determined by joint sliflage. 7% importance of the controlof slippage cannot be overstated;the orthopaedic&ommuni~ must educate itself and itspatients and guard against the problem in order to avoid complicationssecondary to slippage. Keywords: Biomechanics, half-frame, fixator, universal joint, slippage
INTRODUCIION The use of external fixation devices for the treatment of long bone fractures, especial1 complicated fractures of the tibia, is a well-establis h ed techniqueiA. In recent ears the trend has been to use half-frame devices L . Reasons such as the decreased risk of pin, related complications, less bulk, frame ‘elasticity’ and easier ambulation have all been advanced for this change in philosophy. All have merit, but there is an increased risk of at least one complication associated with the use of half-frames: slippage of the universal joints of the fixator. The universal joints are used to maintain the rigid link between the bone pins and sidebars (Figure 7); if they fail to maintain that rigidity the whole frame assembly will collapse. As the frames decrease in size, the number of joints available to bear the loads to which the fixator is subject also decreases. This means each ‘oint bears a greater load in a half-frame assem b ly. Joint slippage in half-frames has been noted by a number of authors. Finlay referred to joints as ‘the weak link in flexible external fixation’ and noted in one study that half-frames would fail due to joint slippage under loads as low as 1Okg vertical forceg. Laboratory biomechanical evaluations of nearly every fixator have noted that slippage at the joints can occur*G13. Clinically, Behrens reports at least one case of obvious slippage in a series of fractures’. DeBastiani also notes in one of his studies that some frames were unstable due to weakness of the ‘articuSuch loss of frame alignment lating components’7. could, of course, lead to fracture malunion, a most unfortunate complication. Correspondence and reprint requests to: J.B. Finlay, University Hospital, P.O. Box 5339, London, Ontario, Canada, N6A 5A5 0 1992 Butterwortl-Heinemann 0141-5425/92/06509-07
A review of the literature will show that malunion often occurs following treatment with external fixation devices. Studies of fractures treated with halfframe fixators have shown an incidence of malunion ranging from 7%14 to 64Y0’~. It is interesting to note that in Benum’s study, where a comparison of the incidence of malunion was made between fractures treated with half-frame fixation and with the quadrilateral frame, malunion was found to be nearly twice as high (64% ve~su.r36%) in those treated with the half-frame configuration. Such clinical evidence would seem to be a compelling reason to determine whether or not slippage really occurs with increased frequency in half-frames and its overall effect on frame stability. However, certain authors, notabl Behrens, suggest that joints only slip at loads far ac ove what would be encountered clinically’3, and that the malunion seen is more likely to be due to poor reduction at the time of surgery8 or other frame weaknesses such as insufficiently rigid corn onentsi6. The role of fracture in fin s ae tiolo clinical outcome cannot be gY ignored 7, ** . Problem fractures, such as those for which external fixation is a primary indication, are often less likely to heal in perfect anatomic alignment. Such thoughtful arguments notwithstanding, it is unlikely that these factors can account for all malunion. It was therefore felt worthwhile to conduct an investigation into the role of joint slippage in overall half-frame stability.
MA-S
ANTI METHODS
For this study, the Hofmann device was chosen due to its opularity and the fact that its propensity to slip ha B been noted in several studie& Jr, 15.
for BES J. Biomed. Eng. 1992, Vol. 14, November
509
Relating Hofiann
faator failure and joint slippage: F.L.I.P. Drijber andJ.B. Finlny
Bone pin
Joint .-b I
Sidebar -b Figure 3 Test apparatus used to test the slippage resistance of the rod/clip torsional interface
Figure 1 Schematic diagram of a half-frame fixator showing its major components
The study was carried out in two steps. The first was to establish the moments at which the various interfaces (rod/& torsional, cheek/clip, cheek/ bowl) (see Fi w 2P of the Hofmann universal joint would slip w8”en the joints were tested in isolation from the frame. The second was to assemble these same joints into half-frame assemblies (Figure 7) and determine the loads that had to be applied to the frame to cause frame failure. These loads could then be converted into equivalent moments at the ‘oint interfaces through simple biomechanical ana r’ysis. These two sets of slippage moments could then be compared to establish the exact role of joint slippage with respect to overall half-frame stability.
Wing-nut
Initially a sample of 15 (n = 15) different universal joints was prepared for the study. The joints were selected according to the following protocol. A number of universal joints were disassembled, washed and autoclaved according to standard surgical procedure, and then reassembled. Each joint was assembled in a random manner from the available parts. After assembly each joint was tested for binding in all ranges of motion and those exhibiting binding were rejected. The sample of 15 was taken from those joints that were not rejected. The joints were tested in an MTS Bionix testing machine (Minneapolis, MN, USA), special fixtures being developed to allow testing of the individual interfaces as shown in Figures3-5. All testing was done in Displacement control. This form of testing was chosen because it offered several advantages: first there would be no preload on the joint; second, the MTS would deliver whatever force was necessary to cause the preset displacement and so there would be no possibili of the slippage resistance being out of control; third, slippage range for tx e force-servo would be virtually guaranteed in every test provided that the displacement was large enough. The joints were tested with a range of different tightening torques (6, 8, 10, and 12Nm) on the clamp. These values were chosen because they
Cheeks
,Clip
Pin clamp Bowl
Figure 2 Diagram of a Hoffmann universal joint showing its major components
510
J. Biomed. Eng. 1992, Vol. 14, November
Figure 4 Test apparatus used to test the slippage resistance of the cheek/clip interface
Relating HoJhann fkator failure and joint srippagc F.L.I.P. DrajbcradJ.B. Finlay
Figure 5 Test apparatus used to test the slippage resistance of the cheek/bowl interface
represented the range found clinically1Q~2”;torques were applied in ascending increments of 2Nm to avoid bias resulting from damage as the torque was increased12. The testing encompassed this range of torques to ensure the joints responded to increased torque with an increased resistance to slippage, as had been seen in several other studiesliTW21,22. Since the joints used in this study were obtained from reviously used hospital stock it was possible they had 1 een damaged during prior use; this test was used to ensure the joints were still in ood condition. A calibrated torque wrench was usef to ensure accuracy and consistency in tightening during all tests. Each joint was tested three times at each clamp torque for each interface to obtain an average value for the resistance to slippage. Between each test, that is each test of resistance to slippage at each clamp torque, the clamp was loosened and the joint tested in all ranges of motion to ensure no binding was occurring. If binding occurred the ‘oint would not be tested further. The test rate was arL itrarily chosen to be 10°sec-l to minimize ex erimental time. The data were recorde B on a Hewlett Packard 100 MHz digitizing oscilloscope (Model No 545OlA)
with a second-order, low-pass filter with break frequencies of 48 and 72Hz. The oscilloscope was chosen because in preliminary tests it was found that the maximum slewing rate of the pen of the X-Y plotter was exceeded. The oscillosco e output was recorded as a voltageversus-time grap! on a printer with the voltage representing the torque applied by the MTS and time representing displacement. Slippage was defined as the oint of deviation from original linearity (Figure 6).h e average values were then tabulated. After these slippage values had been obtained the second part of the experiment was conducted. Ten of the joints from the first experiment were used for these trials; they were used in the assembly of five single half-frame configurations (two joints er halfframe) mounted on simulated fracture mo Bels construtted as shown in Figure7. Acrylic rods were used for mounting due to their consistent material properties during testing”. The frames were assembled with the gap between the modelled fracture fragments to ensure only the frame would be resisting any external deforming forces applied to the fracture model. The ten joints were selectively assigned into groups of two. The grouping sought to pair joints so that three different groups were represented. These included a
46.2
Point of slippage
7 $ $
Y
b Time (displacement)
Figure 6 Pictorial representation of the determination of slippage values from the oscilloscope-generated graphs
25 1111111) Figure 7 Guidelines employed for the construction of the halfframes used in testing (all measurements in mm)
J. Biomed. Eng. 1992, Vol. 14, November
511
Relating Hojkann jhor
fGire
and joint s&kge:
F.L.(.P. Dnjh
andJ3, +&y
Figure 8 Test apparatus used for testing the half-frame assemblies in axial compression
Figure 10 Test apparatus used for testing the half-frame assemblies in medial-lateral four-point bending
pair with wide1 different slippage values, a pair with equally high srippage values, and a pair with low slippage values. It was felt the first pair would allow for easy prediction and demonstration of which joint should slip first and the latter two would more clearly illustrate the dependence of frame stability on the sli page resistance of the joints. Pairing of the joints diEered depending on the interface being tested. rations were tested on The single half-frame co& the MTS using specially CY esigned fixtures. Four modes of testing were used. These modes included axial compression, torsional and two types of fouroint bending: anterior-posterior (AP) and medialPateral (ML), as shown in Figures 8- 7 7. Each mode chosen tested a different joint interface, with the cheek/bowl interface being tested twice (mediallateral bending and axial compression). The frames were tested in Dis lacement control for the same reasons as the joints. & e failure loads were again read from the oscilloscope-generated graphs as in Figure 6 and converted by static anal sis techniques into e uivalent loads for the joints. E ach frame was tested 9, ree times at each interface to obtain an average value for the slippage load. Between each test the clamp was loosened and the frame reassembled according to the configuration requirement shown in
Figure 7. The testing was carried out with a tightening torque of 10 Nm on the clamp. The displacement limits for each test mode were set so as to obtain the same IO”of movement at the joint interface as in the original joint tests. The limits used were as follows:
Figure 9 torsion
512
Test apparatus used for testing the half-frame assemblies in
J. Biomed. Eng. 1992, Vol. 14, November
8 mm displacement in axial compression; 13.8mm displacement in four-point bending; and 10” rotation in torsion. Only one clamp torque was used as the objective of this experiment was to determine how well the joint slippage values of individual joints translated into frame performance. It is already known from the work of the authors and others that clamp torque has an effect on slippage values. This investigation sought solely to examine whether joint slippage had a dominant effect in determining actual frame efficiency. Before testing, a mark was inscribed on the interface of interest on both joints to facilitate resolution of which joint actually failed. It was assumed that only one joint would slip. This assumption was based on the remise that during testing a pure moment would g e generated. Since a pure moment, by definition, is equal throughout a rigid
Figure 11 Test apparatus used for testing the half-frame assemblies in anterior-posterior four-point bending
Relding Ho&uannJGa&or failure and joint slippage: F.L.I.P. Dnjber andJ.B. Finlay
bod , then both joints would experience the same lo J . On ‘oint slippage, the frame would fail, that is, start to tal e on a permanent deformation due to the slippage at the joint. As the frame was no longer able to resist the deforming forces (due to this same slippage and subsequent frame deformation) no net increase in bending moment would occur and only the joint that slipped first would show signs of deformation. Two forms of setting were used for analysis. The first test involved comparing the mean slippage values of each of the five frames at failure with the mean slipp e value for the group of ten individual joints for eat“a interface tested. The second test sou ht to compare the slipp e value of the joint that faiPed within a frame with a;Be slip age value it had when tested in isolation. Statisticap testing (P< 0.05) was done by independent and paired t-tests respectively.
AC
I
Rod/clip
I
I
Cheek/clip Joint
I
Cheek/bowl
interface
Figure 13 Graphical illustration comparing the mean slippage values of the various interfaces of the Hoffmann universal joints when tested within half-frame assemblies and when tested in isolation from the frame. 0 Tested in isolation; 0 tested in hame. AC=axial compression test; ML = medial-lateral bending test
RE!WLTS From the original 15 joints tested, only ten were used for the final study. Four joints were discarded as they exhibited no consistent or marked change in slippage values with increased clamp torque. Thus, according to the testing protocol, they were considered ineligible for further testing. The other was discarded as it showed binding on one interface. The graphical results for each interface, showing the average slippage values plotted against clamp tor ue, are shown in Figure 12. 8 ne minor problem was encountered in the analysis of data for the frames. In the four-point bending tests, it was often found that both joints had slipped. This, of course, was reasonable as each joint was essentially tested in isolation. Unlike the torsion and axial compression tests, where a single moment was exerted throughout the frame, in these tests two identical moments were generated on each modelled fracture fragment. Although both ‘oints were noted to slip, one general1 displace d more than the other; this was assumeJ to be the one that sli ped first. The fact that one joint slipped and then tl!e other resulted in an initially linear, then curvilinear, the oscilloscope. The value for theT s ippage 0 thefrom first Of *afh 16 14 1
12-
0
I
I
I
I
I
6
8
10
12
14
Wing-nut
torque
(Nm)
Figure 12 Graphical illustration of the mean slippage resistance values for the various interfaces tested of the Hoffmann universal joints. 0 Rod/&p torsional interface; 0 cheek/bowl interface; A cheek/clip interface
joint was therefore taken to be the original point of deviation from linearity in the oscilloscope-generated gra h. Por the first test, corn axing converted frame failure values and the mean sPippage values of the group of joints, correlation was found between the mean e values of the joints when mounted in a frame slip andPX e mean slippage values of the sample of individual joints (see Figure 73). This correlation was significant (P < 0.05) and consistent for all three interfaces of the joint. For the second test, comparing the individual converted frame failure values with the slippage values of the individual joints that slipped, results also showed correlation, althou not so impressively. For the rod/clip torsional inteP ace, correlation existed for only 40% of the joints examined. A correlation was seen for the clip/cheek interface in only 60% of cases and a correlation for the cheek/bowl interface in 90% of cases. All correlation results were significant at the P < 0.05 level. DISCUSSION The correlation of the mean slippage values of joints within a frame and joints in isolation for all joint interfaces tested shows conclusively that half-frame stability is dependent on joint stability. A single halfframe can withstand no greater load than a joint can resist. Moreover, these results give strong backing to the proposal that joint sli page can lead to frame failure and possible secon Bary fracture malunion. The fact that a lower ercentage of cases showed fiositive correlation for sPippage values of individual joints within and apart from the frame is not too discouraging. A number of possible explanations exist for the discrepancies, some of which will be discussed below. First, the motion that occurred at the joint interface was often minimal, making it difficult to determine which joint slipped. This was especial1 true of the torsion test. Second, it was mentioned x at the frame testing sought to test frames with joints of either widely different or similar slippage values. It must be
J. Biomed. Eng. 1992, Vol. 14, November
513
RelatingHo&mn fiator failure andjoint sli@ge: F.L.I.P. DrijberandJB. Finkzy
remembered that these values, which were used to pair the joints, were average values. These average values were obtained in the first part of the experiment based on the three tests of each joint for each interface at each torque. Thus, for those joints with similar average slippage values, the within joint (intrajoint) variation of the slip age values was often greater than the between ‘oint Q interjoint) variation of their average slippage v a!ues. Therefore, it would be difficult to predict reliably which joint would slip first. Third, the geometry that the frame assumed during testing may have negated the likelihood of ap lyin a pure moment. This was robably best exemp Pified t y the correlation for the cl! eek/bowl interface for 100% of the cases tested when tested in four-point bending (which almost ensures a pure moment13), while only 80% of joints tested showed correlation when testing this same interface in axial compression. This same lack of a pure moment may explain (a) why the slippage values were generally higher in torsional testing and (b) the corresponding low correlation between joints within and separate from the frame in this form of testing. It is also interesting to note that the loads which caused failure were not extreme. The vertical component for the fixator model used in this study, for example, would only have to be a maximum of about 17 kg. Cunningham et al. 22 found in their study that the vertical loads exerted on a fixator could easily reach 20 kg. Similarly Chao23 presented data for one atient walking on a fixated limb and showed the Poads to be approximately 45 kg vertically, 88 N in the AP plane and 27N in the ML plane. Again, the maximum loads that this stud showed the modelled fixator could withstand woulcr be about 17 kg vertically and 50N in the ML and AP planes. Clearly all except the ML forces would exceed the resistive capacity of the Hoffinann half-frames tested in this stud OE vlousl * this speculation does not allow for any art of the Ioad to be borne by the healing fracture. e ndoubtedly some of the load is distributed through the fracture once healing starts. Until such healing starts, however, the frame alone bears all loads acting on the limb. Frame failure could therefore very easily ensue from even minimal loads during the early healing process. Likewise this study did not consider the effect of adding a second sidebar to the half-frame contiguration. Such an addition would provide a second set of joints over which to distribute the load. Such configurations are biomechanically more rigid93l1 and perhaps less prone to slippage. Assumin the loads are distributed evenly over the joints, t!ley would ideally be twice as resistive to joint slippage. Yet, even if the frames were twice as resistive, they could not protect against the vertical load found in Chao’s study. Further, it is quite possible the frame does not distribute the load e ually over both joints and that the failure loads willa e closer to that of the weakest joint in the system. Su portive evidence for such a proposition is found in g inlay’s studyg. Testing of the quadrilateral frame (with two joints and two articulating components resisting slippage at each end of the fixator) revealed that it evidenced slippage in the AP
514 J. Biomed. Eng. 1992, Vol.
14, November
plane at on1 13 Nm torque. This is far less than the 16 Nm whit K would be the predicted failure moment if the resistive capacity of just two universal joints alone was considered. The current trend appears to be one of using the least number of components. Therefore it is quite likely that some sur eons may try to mana e a fracture with a single a alf-frame. On that basis s ose earlier speculations regarding possible frame failure may accurately represent what could be encountered clinically. It must be remembered also that this fault of slipping joints is not restricted to the Hoffmann device; slippage has been documented in nearly all types of fmators. Certainly the loads necessary to cause slippage vary ” . Indeed, the Hoffmann device was chosen because it is known to be prone to slip at minimal loads. This does not change the fact that this roblem is a potential complication of the use of any rixator and, as such, joint slippage ,must be considered a risk for every fracture treated with any fixator. Great care must be taken to prevent loss of reduction of the fracture when using Hoffmann halfframe fixators, or indeed any half-frame fixator. Only minimal weight-bearing should be permitted until some bony continui exists (as bony continuity will probably decrease 3: e load borne by the fixator). Only in this way can the risk of frame failure secondary to joint slippage be effectively reduced. CONCLUSION The resistance to slippage of the Hoffmann universal ‘oint directly determines, in vitro, the magnitude of i oad a single half-frame configuration can withstand. ACKNOWLEDGEMENTS This work was sup orted through the Medical Research Council oP Canada (MT-10835) and a Studentshi for the rimary author. I would also like to thank I! rs Ian I? uerden, Bob Hardie and Tack Lam, and Andrew Dempsey for all their kind and helpful assistance with this work. REFERENCES 1. Ordway CB. Application of external fixation to the tibia. In: Pope MH, Seligson D, eds. Conq%s in External Fkation. New York: Gmne & Stratton, 1982, 219-45. 2. Edwards CC, Simmons SC, Browner BD, Weigel MC. Severe open tibial fractures: results treating 202 injuries with external fixation. Clin Orthqb 1988; 230: 98-115. 3. Karlstrom G, Olerud S. External fixation of severe open tibial fractures with the Hoffmann frame. Clin Orthop 1983; 180: 68-77. 4. Behrens F. Unilateral external fmation for severe lower extremity lesions: experience with the ASIF (AO) tubular frame. In: Pope MH, Seligson D, eds. Conceptsin External Fixation. New York: Grune & Stratton, 1982, 279-91. 5. Kenwright J, Harris JD, Evans M. External fracture fixation - the analysis and design of a new system. Eng Med 1979; 8: 138-42. 6. Schmidt A, Rorabeck CH. Fractures of the tibia treated by flexible external fixation. Clin Orthop 1983; 178: 162-72.
Relating Hoffmann fixator failure andjoint sliflage: F.L.I.P. Drijber andJ.B. Finlay
7. DeBastiani G, Aldegheri R, Brivio LT. The treatment of fractures with a dynamic axial fixator. J Bone Joint Surg
1984; 66-B: 538-45. 8. Behrens F, Comfort TH, Searls K, Denis F, Young JT.
9.
10.
11.
12. 13.
14.
15.
Unilateral external fixation for severe open tibia1 fractures: preliminary report of a prospective study. Clin Orthop 1983; 178: 11 l-20. Finlay JB, Moroz TK, Rorabeck CH, Davey JR, Boume RB. Stability of ten configurations of the Hoffmann external fixation frame. JBone Joint Surg 1987; 69-A: 734-44. Churches AE, Tanner KE, Evans M, Gwillim J. Fracture healing assessment with external fixation. Eng Med 1985; 14: 13-20. Moroz TK, Finlay JB, Rorabeck CH, Boume RB. External skeletal fixation: choosing a system based on biomechanical stability. J Orthop Trauma 1989, 2: 284-96. Chao EYS, Hein TJ. Mechanical performance of the standard Orthohx external fixator. Orthopedics 1988; 11: 1057-69. Behrens F, Johnson WD, Koch TW, Kovacevic N. Bending stiffness of unilateral and bilateral hxator frames. Clin Orthop 1983; 178: 103-10. Burny FL. Elastic external fixation of tibia1 fractures: study of 1421 cases. In: Brooker AF, Edwards CC, eds. &tern& Fkation: the Current State of the Art. Baltimore: Williams and Wilkins, 1979, 55-73. Benum P, Svenningsen S. Tibia1 fractures treated with
16. 17.
18. 19.
20. 2 1.
22.
23.
Hoffmann’s external fixation: a comparative analysis of Hoffmann bilateral frames and the Vidal-Adrey double frame modification. Acta Orthop &and 1982; 53: 471-6. Behrens F, Searls K. External fixation of the tibia: basic concepts and prospective evaluation. J Bone Joint Surg 1986; 68-B: 246-54. Karlstrom G, Olerud S. Fractures of the tibial shaft: a critical review of treatment alternatives. Clin Orthop 1974; 105: 82- 115. Austin RT. Fractures of the tibia1 shaft: is medical audit possible? InjuT 1977; 9: 93-101. Shiba R, Chao EYS, Kasman R. Fatigue properties of the Hoffmann-Vidal external fixation apparatus. Orthopedics 1984; 7: 443-56. Vossoughi J, Youm Y, Bosse M, Burgess AR, Poka A. Structural stiffness of the Hollinann simple anterior tibia1 external fixation frame. Ann Biomed Eng 1989; 17: 127-41. Drijber FLIP, Finlay JB, Moroz TK., Rorabeck CH. Source of the slippage in the universal joints of the Hoffmann external fixator. Med Biol Eng Gomput 1990; 28: 8- 14. Cunningham JL, Evans M, Kenwright J. Measurement of fracture movement in patients treated with unilateral external skeletal fixation. J Biomed Eng 1989; 11: 118-22. Chao EYS, Pope MH. The mechanical basis of external fixation. In: Seligson D, Pope MH, eds. Concepts in External Fixation. New York: Grune and Stratton, 1982, 13-39.
J. Biomed. Eng. 1992, Vol. 14, November
515
University
of Western
Ontario,
Received April 1991, accepted December 1991
Slippage of th universal joints of externalJixation devices is known to occur but its significance or incidence is often overlooked.In this study, controlledexperiments were used to determine the relationshi$ betweenjoint slipPageand the maximum loao3 a single ha&r ame could bear for the Hofiann device. The experiments showed that: (a) the joints slipped at minimal loadsand (3)fi amef ai‘1ure, i.e. lossof initial alignment of thefiame components,was determined by joint sliflage. 7% importance of the controlof slippage cannot be overstated;the orthopaedic&ommuni~ must educate itself and itspatients and guard against the problem in order to avoid complicationssecondary to slippage. Keywords: Biomechanics, half-frame, fixator, universal joint, slippage
INTRODUCIION The use of external fixation devices for the treatment of long bone fractures, especial1 complicated fractures of the tibia, is a well-establis h ed techniqueiA. In recent ears the trend has been to use half-frame devices L . Reasons such as the decreased risk of pin, related complications, less bulk, frame ‘elasticity’ and easier ambulation have all been advanced for this change in philosophy. All have merit, but there is an increased risk of at least one complication associated with the use of half-frames: slippage of the universal joints of the fixator. The universal joints are used to maintain the rigid link between the bone pins and sidebars (Figure 7); if they fail to maintain that rigidity the whole frame assembly will collapse. As the frames decrease in size, the number of joints available to bear the loads to which the fixator is subject also decreases. This means each ‘oint bears a greater load in a half-frame assem b ly. Joint slippage in half-frames has been noted by a number of authors. Finlay referred to joints as ‘the weak link in flexible external fixation’ and noted in one study that half-frames would fail due to joint slippage under loads as low as 1Okg vertical forceg. Laboratory biomechanical evaluations of nearly every fixator have noted that slippage at the joints can occur*G13. Clinically, Behrens reports at least one case of obvious slippage in a series of fractures’. DeBastiani also notes in one of his studies that some frames were unstable due to weakness of the ‘articuSuch loss of frame alignment lating components’7. could, of course, lead to fracture malunion, a most unfortunate complication. Correspondence and reprint requests to: J.B. Finlay, University Hospital, P.O. Box 5339, London, Ontario, Canada, N6A 5A5 0 1992 Butterwortl-Heinemann 0141-5425/92/06509-07
A review of the literature will show that malunion often occurs following treatment with external fixation devices. Studies of fractures treated with halfframe fixators have shown an incidence of malunion ranging from 7%14 to 64Y0’~. It is interesting to note that in Benum’s study, where a comparison of the incidence of malunion was made between fractures treated with half-frame fixation and with the quadrilateral frame, malunion was found to be nearly twice as high (64% ve~su.r36%) in those treated with the half-frame configuration. Such clinical evidence would seem to be a compelling reason to determine whether or not slippage really occurs with increased frequency in half-frames and its overall effect on frame stability. However, certain authors, notabl Behrens, suggest that joints only slip at loads far ac ove what would be encountered clinically’3, and that the malunion seen is more likely to be due to poor reduction at the time of surgery8 or other frame weaknesses such as insufficiently rigid corn onentsi6. The role of fracture in fin s ae tiolo clinical outcome cannot be gY ignored 7, ** . Problem fractures, such as those for which external fixation is a primary indication, are often less likely to heal in perfect anatomic alignment. Such thoughtful arguments notwithstanding, it is unlikely that these factors can account for all malunion. It was therefore felt worthwhile to conduct an investigation into the role of joint slippage in overall half-frame stability.
MA-S
ANTI METHODS
For this study, the Hofmann device was chosen due to its opularity and the fact that its propensity to slip ha B been noted in several studie& Jr, 15.
for BES J. Biomed. Eng. 1992, Vol. 14, November
509
Relating Hofiann
faator failure and joint slippage: F.L.I.P. Drijber andJ.B. Finlny
Bone pin
Joint .-b I
Sidebar -b Figure 3 Test apparatus used to test the slippage resistance of the rod/clip torsional interface
Figure 1 Schematic diagram of a half-frame fixator showing its major components
The study was carried out in two steps. The first was to establish the moments at which the various interfaces (rod/& torsional, cheek/clip, cheek/ bowl) (see Fi w 2P of the Hofmann universal joint would slip w8”en the joints were tested in isolation from the frame. The second was to assemble these same joints into half-frame assemblies (Figure 7) and determine the loads that had to be applied to the frame to cause frame failure. These loads could then be converted into equivalent moments at the ‘oint interfaces through simple biomechanical ana r’ysis. These two sets of slippage moments could then be compared to establish the exact role of joint slippage with respect to overall half-frame stability.
Wing-nut
Initially a sample of 15 (n = 15) different universal joints was prepared for the study. The joints were selected according to the following protocol. A number of universal joints were disassembled, washed and autoclaved according to standard surgical procedure, and then reassembled. Each joint was assembled in a random manner from the available parts. After assembly each joint was tested for binding in all ranges of motion and those exhibiting binding were rejected. The sample of 15 was taken from those joints that were not rejected. The joints were tested in an MTS Bionix testing machine (Minneapolis, MN, USA), special fixtures being developed to allow testing of the individual interfaces as shown in Figures3-5. All testing was done in Displacement control. This form of testing was chosen because it offered several advantages: first there would be no preload on the joint; second, the MTS would deliver whatever force was necessary to cause the preset displacement and so there would be no possibili of the slippage resistance being out of control; third, slippage range for tx e force-servo would be virtually guaranteed in every test provided that the displacement was large enough. The joints were tested with a range of different tightening torques (6, 8, 10, and 12Nm) on the clamp. These values were chosen because they
Cheeks
,Clip
Pin clamp Bowl
Figure 2 Diagram of a Hoffmann universal joint showing its major components
510
J. Biomed. Eng. 1992, Vol. 14, November
Figure 4 Test apparatus used to test the slippage resistance of the cheek/clip interface
Relating HoJhann fkator failure and joint srippagc F.L.I.P. DrajbcradJ.B. Finlay
Figure 5 Test apparatus used to test the slippage resistance of the cheek/bowl interface
represented the range found clinically1Q~2”;torques were applied in ascending increments of 2Nm to avoid bias resulting from damage as the torque was increased12. The testing encompassed this range of torques to ensure the joints responded to increased torque with an increased resistance to slippage, as had been seen in several other studiesliTW21,22. Since the joints used in this study were obtained from reviously used hospital stock it was possible they had 1 een damaged during prior use; this test was used to ensure the joints were still in ood condition. A calibrated torque wrench was usef to ensure accuracy and consistency in tightening during all tests. Each joint was tested three times at each clamp torque for each interface to obtain an average value for the resistance to slippage. Between each test, that is each test of resistance to slippage at each clamp torque, the clamp was loosened and the joint tested in all ranges of motion to ensure no binding was occurring. If binding occurred the ‘oint would not be tested further. The test rate was arL itrarily chosen to be 10°sec-l to minimize ex erimental time. The data were recorde B on a Hewlett Packard 100 MHz digitizing oscilloscope (Model No 545OlA)
with a second-order, low-pass filter with break frequencies of 48 and 72Hz. The oscilloscope was chosen because in preliminary tests it was found that the maximum slewing rate of the pen of the X-Y plotter was exceeded. The oscillosco e output was recorded as a voltageversus-time grap! on a printer with the voltage representing the torque applied by the MTS and time representing displacement. Slippage was defined as the oint of deviation from original linearity (Figure 6).h e average values were then tabulated. After these slippage values had been obtained the second part of the experiment was conducted. Ten of the joints from the first experiment were used for these trials; they were used in the assembly of five single half-frame configurations (two joints er halfframe) mounted on simulated fracture mo Bels construtted as shown in Figure7. Acrylic rods were used for mounting due to their consistent material properties during testing”. The frames were assembled with the gap between the modelled fracture fragments to ensure only the frame would be resisting any external deforming forces applied to the fracture model. The ten joints were selectively assigned into groups of two. The grouping sought to pair joints so that three different groups were represented. These included a
46.2
Point of slippage
7 $ $
Y
b Time (displacement)
Figure 6 Pictorial representation of the determination of slippage values from the oscilloscope-generated graphs
25 1111111) Figure 7 Guidelines employed for the construction of the halfframes used in testing (all measurements in mm)
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Relating Hojkann jhor
fGire
and joint s&kge:
F.L.(.P. Dnjh
andJ3, +&y
Figure 8 Test apparatus used for testing the half-frame assemblies in axial compression
Figure 10 Test apparatus used for testing the half-frame assemblies in medial-lateral four-point bending
pair with wide1 different slippage values, a pair with equally high srippage values, and a pair with low slippage values. It was felt the first pair would allow for easy prediction and demonstration of which joint should slip first and the latter two would more clearly illustrate the dependence of frame stability on the sli page resistance of the joints. Pairing of the joints diEered depending on the interface being tested. rations were tested on The single half-frame co& the MTS using specially CY esigned fixtures. Four modes of testing were used. These modes included axial compression, torsional and two types of fouroint bending: anterior-posterior (AP) and medialPateral (ML), as shown in Figures 8- 7 7. Each mode chosen tested a different joint interface, with the cheek/bowl interface being tested twice (mediallateral bending and axial compression). The frames were tested in Dis lacement control for the same reasons as the joints. & e failure loads were again read from the oscilloscope-generated graphs as in Figure 6 and converted by static anal sis techniques into e uivalent loads for the joints. E ach frame was tested 9, ree times at each interface to obtain an average value for the slippage load. Between each test the clamp was loosened and the frame reassembled according to the configuration requirement shown in
Figure 7. The testing was carried out with a tightening torque of 10 Nm on the clamp. The displacement limits for each test mode were set so as to obtain the same IO”of movement at the joint interface as in the original joint tests. The limits used were as follows:
Figure 9 torsion
512
Test apparatus used for testing the half-frame assemblies in
J. Biomed. Eng. 1992, Vol. 14, November
8 mm displacement in axial compression; 13.8mm displacement in four-point bending; and 10” rotation in torsion. Only one clamp torque was used as the objective of this experiment was to determine how well the joint slippage values of individual joints translated into frame performance. It is already known from the work of the authors and others that clamp torque has an effect on slippage values. This investigation sought solely to examine whether joint slippage had a dominant effect in determining actual frame efficiency. Before testing, a mark was inscribed on the interface of interest on both joints to facilitate resolution of which joint actually failed. It was assumed that only one joint would slip. This assumption was based on the remise that during testing a pure moment would g e generated. Since a pure moment, by definition, is equal throughout a rigid
Figure 11 Test apparatus used for testing the half-frame assemblies in anterior-posterior four-point bending
Relding Ho&uannJGa&or failure and joint slippage: F.L.I.P. Dnjber andJ.B. Finlay
bod , then both joints would experience the same lo J . On ‘oint slippage, the frame would fail, that is, start to tal e on a permanent deformation due to the slippage at the joint. As the frame was no longer able to resist the deforming forces (due to this same slippage and subsequent frame deformation) no net increase in bending moment would occur and only the joint that slipped first would show signs of deformation. Two forms of setting were used for analysis. The first test involved comparing the mean slippage values of each of the five frames at failure with the mean slipp e value for the group of ten individual joints for eat“a interface tested. The second test sou ht to compare the slipp e value of the joint that faiPed within a frame with a;Be slip age value it had when tested in isolation. Statisticap testing (P< 0.05) was done by independent and paired t-tests respectively.
AC
I
Rod/clip
I
I
Cheek/clip Joint
I
Cheek/bowl
interface
Figure 13 Graphical illustration comparing the mean slippage values of the various interfaces of the Hoffmann universal joints when tested within half-frame assemblies and when tested in isolation from the frame. 0 Tested in isolation; 0 tested in hame. AC=axial compression test; ML = medial-lateral bending test
RE!WLTS From the original 15 joints tested, only ten were used for the final study. Four joints were discarded as they exhibited no consistent or marked change in slippage values with increased clamp torque. Thus, according to the testing protocol, they were considered ineligible for further testing. The other was discarded as it showed binding on one interface. The graphical results for each interface, showing the average slippage values plotted against clamp tor ue, are shown in Figure 12. 8 ne minor problem was encountered in the analysis of data for the frames. In the four-point bending tests, it was often found that both joints had slipped. This, of course, was reasonable as each joint was essentially tested in isolation. Unlike the torsion and axial compression tests, where a single moment was exerted throughout the frame, in these tests two identical moments were generated on each modelled fracture fragment. Although both ‘oints were noted to slip, one general1 displace d more than the other; this was assumeJ to be the one that sli ped first. The fact that one joint slipped and then tl!e other resulted in an initially linear, then curvilinear, the oscilloscope. The value for theT s ippage 0 thefrom first Of *afh 16 14 1
12-
0
I
I
I
I
I
6
8
10
12
14
Wing-nut
torque
(Nm)
Figure 12 Graphical illustration of the mean slippage resistance values for the various interfaces tested of the Hoffmann universal joints. 0 Rod/&p torsional interface; 0 cheek/bowl interface; A cheek/clip interface
joint was therefore taken to be the original point of deviation from linearity in the oscilloscope-generated gra h. Por the first test, corn axing converted frame failure values and the mean sPippage values of the group of joints, correlation was found between the mean e values of the joints when mounted in a frame slip andPX e mean slippage values of the sample of individual joints (see Figure 73). This correlation was significant (P < 0.05) and consistent for all three interfaces of the joint. For the second test, comparing the individual converted frame failure values with the slippage values of the individual joints that slipped, results also showed correlation, althou not so impressively. For the rod/clip torsional inteP ace, correlation existed for only 40% of the joints examined. A correlation was seen for the clip/cheek interface in only 60% of cases and a correlation for the cheek/bowl interface in 90% of cases. All correlation results were significant at the P < 0.05 level. DISCUSSION The correlation of the mean slippage values of joints within a frame and joints in isolation for all joint interfaces tested shows conclusively that half-frame stability is dependent on joint stability. A single halfframe can withstand no greater load than a joint can resist. Moreover, these results give strong backing to the proposal that joint sli page can lead to frame failure and possible secon Bary fracture malunion. The fact that a lower ercentage of cases showed fiositive correlation for sPippage values of individual joints within and apart from the frame is not too discouraging. A number of possible explanations exist for the discrepancies, some of which will be discussed below. First, the motion that occurred at the joint interface was often minimal, making it difficult to determine which joint slipped. This was especial1 true of the torsion test. Second, it was mentioned x at the frame testing sought to test frames with joints of either widely different or similar slippage values. It must be
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RelatingHo&mn fiator failure andjoint sli@ge: F.L.I.P. DrijberandJB. Finkzy
remembered that these values, which were used to pair the joints, were average values. These average values were obtained in the first part of the experiment based on the three tests of each joint for each interface at each torque. Thus, for those joints with similar average slippage values, the within joint (intrajoint) variation of the slip age values was often greater than the between ‘oint Q interjoint) variation of their average slippage v a!ues. Therefore, it would be difficult to predict reliably which joint would slip first. Third, the geometry that the frame assumed during testing may have negated the likelihood of ap lyin a pure moment. This was robably best exemp Pified t y the correlation for the cl! eek/bowl interface for 100% of the cases tested when tested in four-point bending (which almost ensures a pure moment13), while only 80% of joints tested showed correlation when testing this same interface in axial compression. This same lack of a pure moment may explain (a) why the slippage values were generally higher in torsional testing and (b) the corresponding low correlation between joints within and separate from the frame in this form of testing. It is also interesting to note that the loads which caused failure were not extreme. The vertical component for the fixator model used in this study, for example, would only have to be a maximum of about 17 kg. Cunningham et al. 22 found in their study that the vertical loads exerted on a fixator could easily reach 20 kg. Similarly Chao23 presented data for one atient walking on a fixated limb and showed the Poads to be approximately 45 kg vertically, 88 N in the AP plane and 27N in the ML plane. Again, the maximum loads that this stud showed the modelled fixator could withstand woulcr be about 17 kg vertically and 50N in the ML and AP planes. Clearly all except the ML forces would exceed the resistive capacity of the Hoffinann half-frames tested in this stud OE vlousl * this speculation does not allow for any art of the Ioad to be borne by the healing fracture. e ndoubtedly some of the load is distributed through the fracture once healing starts. Until such healing starts, however, the frame alone bears all loads acting on the limb. Frame failure could therefore very easily ensue from even minimal loads during the early healing process. Likewise this study did not consider the effect of adding a second sidebar to the half-frame contiguration. Such an addition would provide a second set of joints over which to distribute the load. Such configurations are biomechanically more rigid93l1 and perhaps less prone to slippage. Assumin the loads are distributed evenly over the joints, t!ley would ideally be twice as resistive to joint slippage. Yet, even if the frames were twice as resistive, they could not protect against the vertical load found in Chao’s study. Further, it is quite possible the frame does not distribute the load e ually over both joints and that the failure loads willa e closer to that of the weakest joint in the system. Su portive evidence for such a proposition is found in g inlay’s studyg. Testing of the quadrilateral frame (with two joints and two articulating components resisting slippage at each end of the fixator) revealed that it evidenced slippage in the AP
514 J. Biomed. Eng. 1992, Vol.
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plane at on1 13 Nm torque. This is far less than the 16 Nm whit K would be the predicted failure moment if the resistive capacity of just two universal joints alone was considered. The current trend appears to be one of using the least number of components. Therefore it is quite likely that some sur eons may try to mana e a fracture with a single a alf-frame. On that basis s ose earlier speculations regarding possible frame failure may accurately represent what could be encountered clinically. It must be remembered also that this fault of slipping joints is not restricted to the Hoffmann device; slippage has been documented in nearly all types of fmators. Certainly the loads necessary to cause slippage vary ” . Indeed, the Hoffmann device was chosen because it is known to be prone to slip at minimal loads. This does not change the fact that this roblem is a potential complication of the use of any rixator and, as such, joint slippage ,must be considered a risk for every fracture treated with any fixator. Great care must be taken to prevent loss of reduction of the fracture when using Hoffmann halfframe fixators, or indeed any half-frame fixator. Only minimal weight-bearing should be permitted until some bony continui exists (as bony continuity will probably decrease 3: e load borne by the fixator). Only in this way can the risk of frame failure secondary to joint slippage be effectively reduced. CONCLUSION The resistance to slippage of the Hoffmann universal ‘oint directly determines, in vitro, the magnitude of i oad a single half-frame configuration can withstand. ACKNOWLEDGEMENTS This work was sup orted through the Medical Research Council oP Canada (MT-10835) and a Studentshi for the rimary author. I would also like to thank I! rs Ian I? uerden, Bob Hardie and Tack Lam, and Andrew Dempsey for all their kind and helpful assistance with this work. REFERENCES 1. Ordway CB. Application of external fixation to the tibia. In: Pope MH, Seligson D, eds. Conq%s in External Fkation. New York: Gmne & Stratton, 1982, 219-45. 2. Edwards CC, Simmons SC, Browner BD, Weigel MC. Severe open tibial fractures: results treating 202 injuries with external fixation. Clin Orthqb 1988; 230: 98-115. 3. Karlstrom G, Olerud S. External fixation of severe open tibial fractures with the Hoffmann frame. Clin Orthop 1983; 180: 68-77. 4. Behrens F. Unilateral external fmation for severe lower extremity lesions: experience with the ASIF (AO) tubular frame. In: Pope MH, Seligson D, eds. Conceptsin External Fixation. New York: Grune & Stratton, 1982, 279-91. 5. Kenwright J, Harris JD, Evans M. External fracture fixation - the analysis and design of a new system. Eng Med 1979; 8: 138-42. 6. Schmidt A, Rorabeck CH. Fractures of the tibia treated by flexible external fixation. Clin Orthop 1983; 178: 162-72.
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7. DeBastiani G, Aldegheri R, Brivio LT. The treatment of fractures with a dynamic axial fixator. J Bone Joint Surg
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Unilateral external fixation for severe open tibia1 fractures: preliminary report of a prospective study. Clin Orthop 1983; 178: 11 l-20. Finlay JB, Moroz TK, Rorabeck CH, Davey JR, Boume RB. Stability of ten configurations of the Hoffmann external fixation frame. JBone Joint Surg 1987; 69-A: 734-44. Churches AE, Tanner KE, Evans M, Gwillim J. Fracture healing assessment with external fixation. Eng Med 1985; 14: 13-20. Moroz TK, Finlay JB, Rorabeck CH, Boume RB. External skeletal fixation: choosing a system based on biomechanical stability. J Orthop Trauma 1989, 2: 284-96. Chao EYS, Hein TJ. Mechanical performance of the standard Orthohx external fixator. Orthopedics 1988; 11: 1057-69. Behrens F, Johnson WD, Koch TW, Kovacevic N. Bending stiffness of unilateral and bilateral hxator frames. Clin Orthop 1983; 178: 103-10. Burny FL. Elastic external fixation of tibia1 fractures: study of 1421 cases. In: Brooker AF, Edwards CC, eds. &tern& Fkation: the Current State of the Art. Baltimore: Williams and Wilkins, 1979, 55-73. Benum P, Svenningsen S. Tibia1 fractures treated with
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Hoffmann’s external fixation: a comparative analysis of Hoffmann bilateral frames and the Vidal-Adrey double frame modification. Acta Orthop &and 1982; 53: 471-6. Behrens F, Searls K. External fixation of the tibia: basic concepts and prospective evaluation. J Bone Joint Surg 1986; 68-B: 246-54. Karlstrom G, Olerud S. Fractures of the tibial shaft: a critical review of treatment alternatives. Clin Orthop 1974; 105: 82- 115. Austin RT. Fractures of the tibia1 shaft: is medical audit possible? InjuT 1977; 9: 93-101. Shiba R, Chao EYS, Kasman R. Fatigue properties of the Hoffmann-Vidal external fixation apparatus. Orthopedics 1984; 7: 443-56. Vossoughi J, Youm Y, Bosse M, Burgess AR, Poka A. Structural stiffness of the Hollinann simple anterior tibia1 external fixation frame. Ann Biomed Eng 1989; 17: 127-41. Drijber FLIP, Finlay JB, Moroz TK., Rorabeck CH. Source of the slippage in the universal joints of the Hoffmann external fixator. Med Biol Eng Gomput 1990; 28: 8- 14. Cunningham JL, Evans M, Kenwright J. Measurement of fracture movement in patients treated with unilateral external skeletal fixation. J Biomed Eng 1989; 11: 118-22. Chao EYS, Pope MH. The mechanical basis of external fixation. In: Seligson D, Pope MH, eds. Concepts in External Fixation. New York: Grune and Stratton, 1982, 13-39.
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