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5. Mechanical properties of the Pinless extemal fixator on human tibiae

kRRemiger,IVID ASIF Research Institute Clavadelerstrasse CH-7270 Davos Platz

5.1 Ah&act In the treatment of either acute severe open tibia1 fractures or their sequelae, a convenient external furator is desirable. The conventional transosseous fixation with pins entering the medullaxy cavity is associated with problems such as pin loosening and pin track infection. Due to the bacterial contamination of the medullary space via the pin track the change of treatment from primary external fixation to secondary medullary nailing is an infection risk. In order to minimize these problems an external clamp fixator, the Pinless, was created. Medullaly penetration is avoided by substitution of the conventional pins with clamps. The latter are inserted hy hand (removable handles) and anchored only in the bone cortex. The medullary cavity stays intact. But is this clamp futation stable enough for clinical use ? Material and Methods: On paired human cadaver tibiae, we compared the mechanical properties of the experimental Pinless, the conventional AO-tubular futator and the Ultra-X furator. Clamps differing in size (small/large) and material (steel/titanium) were used and compared to Schanz screws (steel, 5.0 mm diameter). We measured the stiffness of comparable configurations (1 or 2 bars) under axial compression, four-point-bending in two planes, and torsion. The pull-out force of the different clamps in relation to the bone diameter and number of rocking movements during insertion was also determined. Results: The Pinless configurations with small clamps and 1 bar showed stiffness values as follows (as a percentage of corresponding AO-tubular !ixator): 42J36 % the (steel/titanium clamp) axial stiffness, 61/43 % bending stiffness perpendicular to the reference plane, 78/79 % bending stiffness parallel to the reference plane, and 9OP.5 % torsional stiffness. The corresponding Ultra-X device was not as stiff as the Pinless. The use of two longitudinal rods increased the relative stiffness only under axial compression. The mean pull-out force on the proximal tibia was 1011 N for the small steel clamp, 717 N for the large steel clamp, 681 N for the small and 777 N for the large titanium clamp. At the lowest tibia1 diameter the values were reduced by 10 to 43 %. The rocking movements doubled the pull-out force, e.g. there was a pull-out force for the large clamp of 600 N

with five rocking movements compared to 310 N without. Discussion: The Pinless was not as stiff as the conventional AO-tubular device but stiffer than the clinically used Ultra-X, especially in sagittal bending, the main load on a tibia1 fracture in the first weeks after trauma. A certain number of rocking movements improve the anchorage of the clamps in the cortex tremendously. In clinical trials the Pinless appeared to be mechanically sufficient for temporaly tibia1 fracture stabilisation in patients not bearing weight and for support of the lower leg during management of soft tissue traumata (bum injury, compartment syndrome, soft tissue reconstruction). Furthermore, the clamp proved ideal as a traction device.

5.2 Intiction

Treatment of severe tibia1 fractws (II and III degree open, according to Gustilo and Anderson, 1976, or closed tibia1 fractures grade GIII according to Tscheme and Oestem, 1982) is still a problem and a challenge in clinical orthopaedics. Common primary treatment starts with the stabilisation of the fracture using an external fixator and, if necessary, the debridement of avascular and damaged soft and bony tissue. Secondary procedures are still under discussion. A number of authors leave the fiiator in situ until bone healing (Aho, 1983; Edwards, 1983, 1988, Etter, 1983; Karlstroem, 1983; De Bastiani, 1984, Behrens, 1986, Rommens, 1986). However, the longterm use of fiiators gives rise to complications such as pin track infection with subsequent pin loosening and malalignment of the fragments (Green, 1983). Delayed fracturr healing caused by distraction and static fixation using an extnzmely stiff external fiiator has also been described (Aho, 1983; Wu, 1984, Hammer, 1985). These difficulties plus the long healing time of severe tibiil fractures treated by external fixation resulted in the idea of temporay external fixation only. Due to the complications described above, secondary operative procedures such as additional fiiation, change of pins, osteotomy of the fibula, or bone g-mfting must be carried out. Another option is to change the treatment modality from external to internal fixation. With the revival of unreamed medullary nailing with its reduced risk of infection in severe fractures, the convertion from primary external fixation to secondary medullary nailing

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Mwhanikal properties

has become more widespread. The underlying principle is to stab&e the main fragments by an external fiator until soft tissue healing and to change to an internal fixation before any of the complications normally described may arise. With an unreamed medullary nail, adequate anatomical reduction and improved patient comfort can be achieved with less aggressive surgery since there is no additional damage to the periosteal blood supply. The endosteal damage to the perfusion by the central artery is only temporary. The vessels recover with 3-6 weeks after nail insertion (G. Oedekoven, 1992). However, the high rate of infection after secondary nailing preceded by any external fixator is still a centre of debate. Reports have shown that the incidence of infection can be as high as 70 % (Karlstroem, 1975, 1983; Bone, 1986, McGraw, 1988, Fischer, X991). A clear relationship exists between these deep infections and previous pin track infections or partially healed soft tissue cover (Maurer, 1989; McGraw, 1988; Tomqvist, 1990). Perioperative antibiotics and an implant-free interval (days or months) after removal of the fixator does not really improve the situation (Puno, 1986, Bone, 1986, Johnson, 1987; Blachut, 1990, McGraw, 1988, Tijmqvist, 1990). The origin of these pin track infections lies in the conventional tmnsosse~~~~ pi” fixation which perforates the bony cortex and exposes the medullary cavity to bacterial invasion. A reduced infection rate may be achieved by changing either the medullary implant (reamed to unreamed) or the external fiiator. One solution is to alter the way in which the external stabiliser is fixed to the bone. The basic idea of the Pinless fixator was to produce a for severe stable, temporary, minimally invasive fiator tibia1 fractures which guarantees a safer conversion to the medullary nail (Swiontkowski, Frigg, perscorn.). The principle of Pinless fixation is the avoidance of transosseous pin fixation which necessitates exposure of the medullary cavity (Fig. 1). The conventional pins or wires are substituted by clamps anchored only intracortically. The bone is only grasped from outside by two trocar points of a clamp similar to the ice tongs used to lift heavy blocks of ice.

S 29

the dorsomedial edge of the tibia. The muscle compartments should remain intact. The clamps are in the socalled “safe corridor” (Behrens, 1983). Removable handles are squeezed together whilst rocking the symmetrical clamps around their trocar point axis. The asymmetrical clamp has a bifurcated tip on one arm and a single trocar point on the other. Hence, rocking around the trocar axis is not possible because the bifurcated tip would create a groove in the bone with resultant tilting and instability of the clamp. The asymmetrical clamps are simply pressed onto the bone. By these means, the tips are fiiy anchored in the bony cortex (pig. 2). The depth of insertion of the tip depends on the bone’s density, the number of rocking movements, the squeezing force, and whether the clamp is mounted on the diaphysis or metaphysis. In the latter case, the cortex is rather thin, even in the bone of a young person, and the risk of perforation is high (Remiger, unpublished data). Histological investigation of tibiae of different age, sex and bone density has shown that some experience and practice is necessary to insert the clamp to the correct depth. The preloaded clamp is secured by means of a nut on the central hinge (Frigg, 1992).

Fig. 2: The “rocking” movement: Moving the clamps back and forth around the trocar point axis while squeezing the handles together. This anchors the tips in the bony cortex. Another position of the clamp on the tibiil bone is its fixation with one trocar in the medial and the other in the lateral tibia1 surface, ie. in the sagittal position. Similar to conventional bilateral external fiation, the border of the anterior muscle compartment is ignored. The advantage of this application is that the Pinless clamp comes to lie in the sagittal plane, i.e. the plane of maximal load in the lower leg in the fit few weeks after the fracture (Behrens, 1983). Due to the acute or chronic functional loss by transfixation of the muscles and the risk of pin track infection, the application of the clamp on the medial aspect (safe corridor) is to be preferred.

Fig. 1: Pinless clamp (small) with removable handles on the tibia1 diaphysis (cross section). The clamps are inserted on the medial aspect of the tibia with one trocar point in the anterior crest and the other in the dorsal medial edge. Rocking movements around the trocar tip axis drive the tips into the cortex. These clamps are placed onto the medial aspect of the tibia by two transverse stab incisions. One trocar point is positioned on the anterior crest or surface, the other on

5.5Aimnoflhesludy The aim of the present study is to determine the basic mechanical properties of the new Pinless external clamp fixation in comparison to clinically established fiiators using transosseous pins. A comparison of the stiffness values should answer the question of whether the Pinless clamp is firm enough to stabilise tibiil fractures temporarily. The stiffness values measured should forestall overtaxing this

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new device in clinical practice and restrict its use to application below the critical threshold of stability. Hand in hand with clinical studies, these results help to define clear indications and applications of the Pinless device. Therefore, the test setup, especially the positioning of the clamps on the bone and loading conditions, was designed to simulate clinical conditions as closely as possible. Pullout forces greater than the weight of the lower leg might extend the possible applications of the Pinless fixator to include its use as a support and even as a traction device with a minimum number of clamps. The hypothesis is that the conventional transosseous pin fixation is stiffer than intracortical clamp fixation only. A statistical analysis with several multiple range variance tests was performed to accept or reject this hypothesis and was evaluated graphically with the RS1 software programme. We performed a one-way analysis of variance to determine the contribution of individual factors to the total variation within a sample (P S 0.05). In a multiple comparisons graph, the function of the Bonferroni 95 % confidence interval for all paired differences is included. The Student-Newman-Keuls and the Duncan’s multiple range tests were performed to compare each group with every other group.

The original clamps were made of stainless steel with an arm diameter of 5 mm and in two sizes (large and small). The second generation of clamps which are now under clinical testing were made of commercially pure titanium (cpTi) and have an arm diameter of 5 mm (small clamps) or 6 mm (large clamps and asymmetrical small clamps). The clamps were different in size (large and small) and in their geometry (small symmetrical and small asymmetrical) (Pig. 4). The idea of the asymmetrical clamp was to prevent the pressure on the soft tissue (skin and muscle) created by the curvature of the symmetrical clamps. On the medial face of the proximal half of the lower leg, the gastmcnemius and soleus muscles were irritated by the curvature of the large symmetrical clamps. This resulted in superficial “clamp track” infections with serous discharge, inflammation and pain. Straightening the curvature solved these problems. The asymmetrical- clamps reduced the pressure on the soft tissue to a minimum (Remiger, Claudi, 1991, unpublished data). A total of five different clamps were tested.

5.4 Materials and methods We compared the mechanical properties of the experimental Pinless fixator, the conventional AO-tubular fixator and the Ultra-X fixator. The last two devices are in clinical use. Comparisons of the stiffness values of the complete frames were undertaken. Finally, the pull-out force of single clamps was measured.

Pinless lldemal

Fiator (PEP) (Raron CaLtd)

Fig. 3: A Piess frame with four small titanium clamps (two symmetrical and two asymmetrical) and one bar. The Pinless frames tested in this study were all four clamp constructions (Fig. 3). A clamp consists of a central hinge with two arms and a small, adjustable connecting rod, two removable handles to open and close the clamp, and two nuts on the central hinge to secure the arm and rod in position. The single clamps are joined to the connecting bar (stainless steel tube with an 11 mm diameter) by means of single adjustable clamps (Synthes Co. Ltd.).

Fig. 4: The Piess clamps differing in size and geometry: From left to right: large (steel and titanium); small symmetrical (steel and titanium); small asymmetrical clamps (titanium only). 70 mm and 45 mm are the bonebar distances also for the correspondiig AO-tubular and Ultra-X fiators. The trocar tips of the small symmetrical and asymmetrical clamps are reinforced by widening the base of the trocar tip to prevent failure due to shearing. In earlier tests, the slim trocar tip of the first titanium clamps failed several times because the tip broke off at the base (Fig. 5). As described above, the asymmetrical clamps incorporate two different tip designs. The bifurcated tip should be mounted on the anterior tibiil crest.

Fig. 5: The different tips: Left: the original slim base of the trocar, middle: the new broad based trocar tip right: the bifurcated tip of the asymmetrical clamp; The plateau at the base of the trocar points should prevent perforation of the cortex. Positioning the connecting bar as close as possible to the top of the hinge means that the bone-to-bar distance will be 45 mm for the small clamp and 70 mm for the large clamp. The same bone-to-bar distances were used for the

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Mechanical properties

other two fixator devices (with reference to the inner bar if more than one bar was used).

AO-tubular fixator (Synthes CaLtd.) Four long threaded 5 mm Schanz screws, single adjustable clamps, and one or two anteromedially positioned stainless steel tubes (11 mm dia.) were used for a complete frame. The bar was mounted at a distance of 45 mm and 70 mm from the bone for small and large Pinless clamps respectively. The frame is fixed in the sagittal plane of the tibia anteromedially (Fig. 6) (Behrens, 1983, 1986, 198% Miiller et al., 1991).

Ultra-X Fiiator (Jrbwmedica CaLtdJ

Between the two centre clamps an osteotomy with a middiaphyseal gap of 20 mm was created. The centre clamps were placed 15 mm from the gap. The distance between the clamps on any one fragment was 80 mm. The 11 mm diameter bar (tube) was placed anteromedially as close as possible to the clamp hinge. In one test series with the large clamps, the bar was placed below the hinge in order to reduce the bone-to-bar distance to a minimum (Fig. 7). One and two bar Pinless configur&ons were tested. Usually the second bar was placed above the fit bar in the same plane, but on the large steel clamps this bar may be attached to the side of the arm by means of the single adjustable clamps. The curvati of the arms of the small clamps prohibits side attachment of the bar by means of single adjustable clamps as does an arm diameter larger than 5 mm (Fig. 7). The clamps are only made to fii pins or screws with a maximum diameter of 5 mm.

The same type of Schanz screws were used as for the AOtubular device. The ball joints and connecting bar were made of carbon fibre in contrast to the stainless steel tube construction of the AO. Only a one-bar construction was tested. The screws were inserted into the tibia in the same manner as for the AO-tubular device.

Mechanical testing of the fixator frames was carried out on four pairs of human cadaver tibiae. Another four pairs of tibiae had to be used for the pull-out tests. Three bones failed and were substituted by three further pairs of human tibiae of the same sex, nearly the same age, and radiologically similar bone density. In total, 11 pairs of human cadaver tibiae with an average age of 62 years (2973 years) were used. The explanted bones were deep frozen and thawed in 20° Ringer solution at room temperature as required. The proximal and distal ends were embedded in polymethylmethacrylate (PMMA) in a special mould. During testing the bones were kept moist. Radiographs were taken of all intact bones for an analysis of bone density by X-ray densitometry using the step-wedge (Matter, 1977).

Fig. 6: Test specimens: top: conventional transosseous fiiatar, bottom: Pinless clamp fiiator. Note the same dimensions (in cm).

Test machines The specimens were tested on an Instron 4302 testing machine (axial compression, bending, pull-out force) and on a custom-made torsional testing rig connected to an x/y-plotter (Graphtec Co. Ltd.). Methods The Pinless frame consisting of four clamps was tested on the bone first because it causes less destruction than the Schanz screws. The application method for the clamps (i.e. rocking movements, medial aspect) was described above. The steel and large titanium clamps were mounted on the medial side of the tibia, the small titanium clamps in the sag&al position. The AO-tubular and the Ultra-X frame could be placed onto the same tibia once the Pinless clamp had been tested and removed. All fmmes were mounted in the same dimensions for accurate comparison (Fig. 6).

Fig. 7: Bar positioning: Left: uual position (above the hinge); Middle: below the hinge; Right: one bar above the hinge and one side bar. The double and single stacked unilateral frames (A0 tubular and Ultra-X) were applied in a similar fashion and in the same dimensions. The Schanz screws were inserted according to the A0 method (Miiller et al., 1977).

Loading amditionB In OUTstudy, we used the following loading conditions for the complete frames with four pins/clamps and one or two bars: axial compassion, bending in two planes and

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torsion. After completion of the frame tests, the pull-out force of single clamps was measured.

The axial compression was performed on the INSTRON 4302. The PMMA embedded ends of the tibia were fixed in the testing machine with a ball joint to the load cell and the basement respectively. The fixator frame was loaded eccentrically (Fig. 8).

Fig. 9: Set-up for four-point bending, shown here parallel to the reference plane. The bottom alumuu ’ ‘um bar is fixed to the base of the INSTRON, the top bar to the load cell. The latter can be moved upwards. The four support bars are hinged onto the brackets. Four-point-bending results in a constant bending moment between the two fixation brackets.

Fig. 8: Set-up for axial compression: eccentric axial loading of the fiiator as is present in the clinical situation involving a bone defect. The entire axial load must be carried by the fixator. This is the worst mechanical situation.

The reference plane of the bending was the area between the longitudinal axis of the tibia and the bar. A bending moment of 23 Nm was applied parallel and perpendicular to this plane (Fig. 10).

The force versus displacement curve was recorded, the gradient of which indicates the axial stiffness (N/mm). The maximum force applied to the specimen was 1000 N, the maximum displacement of the load cell was 12 mm.

The bending tests were performed in a four-point-bending mode in two planes also on the INSTRON. The embedded bone ends were fixed with special brackets on the fourpoint-bending device (Fig. 9).

Fig. 10: The reference plane of the bending

tests.

The stiffness parallel to this plane represents the maximum bending stiffness, the stiffness perpendicular to that plane represents the minimum bending stiffness of the Pinless fiiator. Values for the sag&al bending stiffness of the steel clamps and the large titanium clamps lie between the bending stiffnesses recorded in this study because these clamps are mounted medially on the tibia. The small titanium clamps were mounted in the sag&al position. The gradient of the curve of the bending moment versus displacement of the load cell is the bending stiffness.

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hkchanikal

Torsion The torsion testing was performed on a special custommade torsional rig up to a maximum torque of 44 Nm and a maximum rotational angle of 60°. The proximal fragment was twisted against the fixed distal end. The gradient of the torque versus the angle of displacement curve was documented as the torsional stiffness. The torsion testing was done by hand and recorded on an xlyplotter (Graphtec Ltd.) (Fig. 11).

Fig. 11: Custom-made torsional rig with a lever arm for manual application of torque. The proximal end yright) is twisted towards the fixed distal bone fragment (left). The torque versus angulation curve is recorded.

Pull-out

s 33

pm~rties

teat

The pull-out force depends on the bone diameter, the bone density, the number of rocking movements and the properties of the clamps. Hence, the clamps were fixed on the tibia in three positions of different bone diameter and cortical thickness. The proximal clamp was applied at the junction between the metaphysis and the diaphysis just below the tuberositas tibiae (i.e. Dmax). The distance between the trocar tips of the inserted clamp was 35 mm on average. The second clamp was positioned at the minimum tibia1 diameter (i.e. Dmin) in the distal half of the bone with a distance of 22 mm on average between the tips. The third position represented the sagittal application of the clamps with one trocar in the medial and the other in the lateral cortex (i.e. Dsag). The average distance between the tips was 16 mm (Fig. 12).

Fig. 12: The position of the clamps for the pull-out test: Dmax = junction between the metaphysis and the diaphysis, Dmin = minimum tibia1 diameter, Dsag = sag&al clamp position in the distal half of the tibia. The pull-out testing was also performed on the INSTRON with a 10 kN crosshead and a velocity of 5 m&mm. The

force was applied perpendicular to the longitudinal axis of the tibia. The force versus crosshead displacement curve and the peak force (i.e. pull-out force) we-m recorded (pig. 13).

Fig. 13: Set-up for the pull-out testing. The aluminium frame is fixed to the INSTRON, the bone, held only by the clamp, is pulled towards the frame. The clamp is pulled upwards by means of the small connection rod fixed to the crosshead of the testing machine.

55 Results The transosseous pin fixation with the AO-tubular fiator was stiffer than the intracortical clamp fixation with the new Pinless fixator. Compared to the Ultra-X fixator with its carbon fibre rods and ball joints, the Pinless device was as stiff or even stiffer irrespective of the construction or loading condition. Comparing the fit generation of steel clamps to the second generation made of titanium with partially reinforced arm diameters (6 mm instead of 5 mm), no difference in stiffness could be demonstrated. Using two bars instead of one increased the stiffness of the Pinless frames by 4 “/o(torsional stiffness) and 26 % (axial stiffness) with the small stainless steel clamps, and by 2 % (torsional stiffness) and 28 % (axial stiffness) using the small titanium clamps. In comparison, the stiffness under torsion of the AO-tubular device was increased by a second bar by 6 % only, the axial stiffness by 48 %. For the large clamps and the large bone-bar distance, the maximum stiffness increase occurred under axial compression (+ 18 % for the steel clamp, + 24 % for the titanium, + 34 % for the AO-tubular fiator). Comparing the small clamps to the large clamps, the former are nearly twice as stiff as the latter. The axial stiBness was the weak point of the Pinless fixator independent of the material, the size or the geometry of the clamp. The stiffest Pinless configuration with small clamps and one bar showed axial stiffness values as follows (in percentage of the corresponding AOtubular one rod frame): 25 f 4 N/mm (42 %) for the steel clamp, 21 f 3 N/mm (36 %) for the symmetrical and asymmetrical titanium clamps (no statistical difference,

534

P > 0.05). There is no statistical difference in the axial stiffness of the Ultra-X and the Pinless clamp whether with small or large clamps (one bar construction). The large clamps showed nearly the same percentages of stiffness of the corresponding AO-tubular frames. A second bar increased the axial stiffness maximally by 26 ‘$I (21 N/mm for small titanium clamps with one bar, 29 N/mm for the same clamps with two bars). However, the percentage of the AO-tubular device remained nearly the same. The exception was the side-bar construction with large steel clamps which improve the stiffness values from 14 f 1 N/mm to 43 N/mm, which is 91 % of the axial stiffness of the two bar AO-tubular device (Figs 14 and 15).

1000

force tN1 ,

mean

+/-lSD(n*B)

800 800 400 200 0 0

1

2

3

4

6

dirplacement

8

7

crorshead

8

0

10

11

12

Fig. 16: Axial stiffness (N/mm) of all two bar constructions (box plot): 1 = Pinless large steel clamp/one bar above the hinge and a side bar, 2 = Pinless large titanium clamp; 3 = Pinless large steel clamp; 4 = Pinless small steel 5 = Pinless asymmetrical titanium clamp; clamp; 6 = Pinless small symmetrical titanium clamp, 7 = AOtubular with a bone-bar distance of 70 mm; 8 = AOtubular with a bone-bar distance of 45 mm. Note the different scale in comparison to Fig. 15.

[mm1

Fig. 14: Axiil stiffness of all one and two bar frames with a bone-bar distance of 45 mm: 1 = AO-tubular/two bars; 2 = AO-tubular/one bar, 3 = Pinless steel/two bars; 4 = Pinless titanium/two bars; 5 = Pinless steel/one bar, 6=Pinless titanium/one bar, 7=Pinless asymmetricaj/one bar, 8 = Ultra-X/one bar. Under axial compression the Pinless frames were considerably weaker than the corresponding AO-tubular devices, but were as stiff as the Ultra-X fixator.

Fig. 15: Axial stiffness (N/mm) of all one bar constructions (box plot): 1 = Ultra-x with a bone-bar distance of 70 ITU~; 2 = Ultra-X with a bone-bar distance of 45 mm; 3 = Pinless large steel clamp; 4 = Pinless small steel clamp; 5 = Pinless large steel clamp with a bar below the small asymmetrical titanium clamp; hinge; 6 = Piiess 7 = Pinless small symmetrical titanium clamp; 8 = AOtubular with a bone-bar distance of 70 mm; 9 = AOtubular with a bone-bar distance of 45 nun; 10 = Pinless large titanium clamp; 11 = Pinless large titanium clamp with a bar below the hinge.

There is no difference within the group of either the large or the small clamps. The only exception is the side bar construction (column 1 in Fig. 16) which is comparable to the AO-tubular two bar construction with a 70 mm bonebar distance (column 7 in Fig. 16) (pig. 16). The overall percentage for the stiffness of the Pinless clamp in respect to the AO-tubular device was improved for the bending tests (40 - 79 %). The bending stiffness perpendicular to the reference plane is lower than the bending stiffness parallel to the reference plane. Under bending perpendicular to the reference plane the Pinless configuration with small clamps (steel and titanium) has stiffness values as follows: 2.7 f 0.6 Nm/mm (61% stiffness of the corresponding AOtubular fiiator) for the steel clamp with one bar, 2.9 f 0.4 Nm/mm (56%) for the steel clamp with two bars, 1.9 f 0.3 Nm/mm (50%) for the symmetrical titanium clamps with one bar and 2.1 f 0.2 Nm/mm (40%) for the symmetrical titanium clamps with two bars, and 2.2 f 0.5 Nm/mm (43%) for the asymmetrical titanium clamps with one bar (pigs 17-19). The Ultra-X with one bar was significantly (P 5 0.05) weaker than the Pinless clamp (1.8 f 0.6 Nm/mm for the small bone-bar distance of 45 mm and 1.2 f 0.4 N&nm for the large bone-bar distance of 70 mm) (pigs 18 and 19). The large clamp frames (1 or 2 bars) showed the same percentages of stiffness of the corresponding AO-tubular devices. The stiffness values were significantly lower (3050%) than the small clamps, e.g. small titanium clamp device (1 bar) with 1.9 i 0.3 Nm/mm compared to 1.2 f 0.2 Nm/mm of the large titanium clamp device with 1 bar. A second bar in the normal position gave only a slight increase in bending stiffness perpendicular to the reference plane either in the small Pinless clamp or in the cooresponding AO-tubular construction (P s 0.05) (pig. On large clamps the second bar above the hinge improved the stiffness of the titanium clamps by 14%, the steel clamps by 24% and the AO-tubular device up to 27%. These percentages are the differences in the stiffness

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Mechanical p~~periies

s 35

between the one and two-bar constructions. Fixing the second bar on the arms of the large steel clamps (side bar construction) was the stiffest frame under bending perpendicular to the reference plane. The side bar construction was even stiffer than the AO-tubular device (3.3 + 0.5 Nm/mm versus 4.8 f 1.6 Nm/mm of the side bar frame !). With that frame several failures occurred. In most cases, the two inner clamps were pulled out with the tmcar points of one cortex. The elasticity of one arm of the clamp was blocked by a stiff steel bar. The displacement of the fragments in the gap was much larger than on the bone ends. So the clamps “reinforced” by a side bar could onlv be deformed to a limited extent. Above a displacement threshold dependent on the clamp the trocar points were each bone, position on continuously pulled out of the cortex. The complete frame was still stable on the bone under this loading. However, clinically the lower leg is not only loaded unilaterally. There are different loading conditions simultaneously. Thus, one might have an unstable frame in another plane.

3. bend. moment INml

mean +/-lSD(n=8)

I

I

-r

Fig. 18a and b: Bending stiffness (NmAnm) perpendicular to the reference plane for all two bar frames with a bonebar distance of 45 mm/ small clamps (Fig. Ma (top)) and 70 mm/ large clamps (Pig. 18b (bottom)): 1 = Pinless large steel; 2 = Pinless large titanium; 3 = Pinless large steel with a side bar, 4 = Pinless small steel; 5 = Pinless small titanium, 6 = Pinless small titanium, asymmetrical; symmetrical; 7 = AO-tubular 70 mrq 8 = AO-tubular 45 mm. The Pinless steel clamp with a side bar is even stiffer than the AO-tubular two bars (70 mm bone-bar distance).

_. 0

12

3

4

5

6

displacement

7

3

la

0

1111

[mm]

Fig. 17: Bending stiffness perpendicular to the reference plane @n/mm) of all frames, with all one and two bar frames with a bone-bar distance of 45 mm: 1 = AOtubular/two bars; 2 = AO-tubular/one bar, 3 = Pinless steel/one bar, 4 = Pinless steel/two bars; 5 = Pinless asymmetrical titanium/one bar, 6 = Pinless titanium/one bar, 7 = Pinless titanium/two bar, 8 = Ultra-X/one bar. There was a slight difference (P ZG0.05) between the one and two bar constructions of the Pinless steel clamp and the AO-tubular fixator. The bar was not the weakest part of the construction, but the clamps or the Schanz screws.

4..

6--

1

I--

z--

0

Fig. 18az

j&dyq

Q I

3

I

nxAm*

T

a+

6 5 IIXATOR TYPE

B

TYPE

Fig. 19: Bending stiffness perpendicular to the reference plane of all one bar frames with a bone-bar distance of 45 mm/ small clamps (Fig. 19a (top)) and 70 mm/ large clamps (Fig. 19b (bottom)): 1 = Ultra-X 70 ITUTL; 2 = Ll’ltraX 45 ITUIL;3 = Pinless large steel; 4 = Pinless large steel, bar below the hinge; 5 = Pinless large titanium; 6 = Pinless large titanium, bar below the hinge; 7 = Pinless small steel; 8 = Pinless small titanium, asymmetrical; 9 = Pinless small titanium, symmetrical; 10 = AO-tubular 70 mm; 11 = AO-tubular 45 mm. The below the hinge construction of the Pinless clamp improved the stiffness under this loading condition. The shorter bone-bar distance reduced the lever arm between the bone and bar and thereby increased the stiffness.

S36

plane, there was only IAbendingpan&Itothell?ference a slight difference (P I; 0.05) between the transosseous pin fixation (AO-tubular) and the clamp fixation with the Pinless fiiator. The clamp fixation had stiffness values as a percentage of the corresponding AO-tubular device as follows: 78% and 73% for the small steel clamps with one and two bars, 79% and 69% for the small symmetrical titanium clamps with one and two bars, 71% and 61% for the small titanium asymmetrical clamps, 67% and 56% for the large steel clamps with one or two bars, 81% for the side bar construction (see Fig. 7), and finally 59% and 47% for the large titanium construction with one and two bars. The Ultra-X was much weaker either for the small bonebar distance (3.4 f 1.6 Nm/mm = 38% of the AO-tubular) or for the large bone-bar distance (3.8 f 1.1 Nm/mm = 43%). Under this loading condition the bone-bar distance had no great influence on the stiffness values. The bar was applied exactly in the loading plane which meant that the stiffness of the frame was at a maximum. stiffness second bar increased the from A 6.9 f 0.8 Nm/mm to 8.3 f 1.9 Nn&un for the small steel clamps and from 7.0 f 0.8 Nm/mm to 7.9 ?: 1.3 Nm/mm for the small titanium clamps. For the large clamps, the second bar increased the stiffness values from 5.9 f 0.6 N&run to 6.7 f 0.9 Nm/mm for the large steel clamps (7.1 + 1.5 Nm/mm for the side bar construction), and from 5.2 f 0.4 Nm/mm to 5.6 5 1.1 NmAnm for the titanium clamps (P 2 0.05). In the latter case, the weak part of the frame was the titanium clamps and not the bar. The side bar construction was of no advantage under these loading conditions. The Ultra-X failed several times due to the gliding of the ball joints fied to the Schanz screws and bar. This resulted in malalignment of the fragments (Figs 20-22). so bend.moment INml

mean +I-lSD(n=8)

I

0

1

2

3

4

5

displacement

6

7

3

s

lo

11

& .

Fig. 21: Bending stiffness parallel to the reference plane of one bar frames with a bone-bar distance of 45 mm/ small clamps (Fig. 21a (top)) and 70 mm/ large clamps (Fig. 21b (bottom)): 1 = Ultra-X 70 mm; 2 = Ultra-X 45 mm; 3 = Pinless large steel; 4 = Pinless large steel, bar below the hinge; 5 = Pinless large titanium; 6 = Pinless large titanium, bar below the hinge; 7 = Pinless small steel clamp; 8 = Pinless small titanium clamp, asymmetricak 9 = Pinless small titanium clamp, symmetrical; 10 = AOtubular 70 mm; 11 = AO-tubular 45 mm. Under these loading conditions, there is only a slight difference between the different bone-bar distances.

12

crosshead [mm]

Fig. 20: Bending stiffness parallel to the reference plane of all one and two bar frames with a bone-bar distance of 45 mm: 1 = AO-tubular/two bars; 2 = Pinless steel/two bars; 3 = AO-tubular/one bar, 4 = Pinless symmetrical titanitrm/two bars; 5 = Pinless steel/one bar, 6 = Pinless titanium/one bar, 7 = Pinless asymmetrical titanium/one bar, 8 = Ultra-X/one bar. The differences between the Pinless frames and corresponding AO-tubular devices decreased under this loading condition. The Ultra-X was much weaker than all other frames. It failed several times due to slippage of the carbon fibre rod in the carbon fibre ball joint.

e .

Fig. 22a:

s 37

Rem&er: Merhanikal properties

small titanium clamp; 1 or 2 = 1 or 2 bars; AOTub = AO-tubular device; f SD = f 1 standard deviation. Them is no statistical difference between the different fixatom with small bone-bar distance (45 mm). torclional stiffnew

INm /degl

1 0.8

d

7

3

2

I

0.6 0.4

Fig. 22: Bending stiffness parallel to the reference plane of alI two bar frames with a bone-bar distance of 45 mm/ small clamps (Fig. 22a (top)) and 70 mm/ large clamps (Fig. 22b (bottom)): 1 = Pinless large steeb 2 = Pinless large titanium; 3 = Pinless large steel, with a side bar, 4 = Pinless small steel; 5 = Pinless small titanium, asymmetrical; 6 = Pinless small titanium, symmetrical; 7 = AO-tubular 70 mm; 8 = AO-tubular 45 mm. The two bar frames were only slightly stiffer than the one bar construction. The side bar (3) was of no significant advantage. In torGonal stiffness, the AO-tubular frame was superior to all other frames, except with the two bar construction with a bone-bar distance of 45 mm (small). There was no difference at all between the AO-tubular fiiator and the Pinless steel or titanium constructions. The Ultra-X was as stiff as the small Pinless frames and even stiffer with a large bone-bar distance (70 mm). Between the steel and titanium clamps (small and large), there was no difference in torsional stiffness. Neither was there any change in the stiffness values by using different bar positions (above versus below the hinge) nor one or two bars (Figs 23 and 24). Under torsional loading, the side bar was of no advantage compared to all the other frames. All torsion tests ended in failure of the fixator: Either at least two Schanz screws were plastically deformed (bent) after testing or the Pinless clamp had an increased distance of the two trocar points after removing them from the bone. Hence the arms were plastically deformed. The Pinks failed due to clamp loosening by glidiig of the trocar points on one fragment only. There was statistically no preference for the proximal or distal fragment in terms of where the failure occurred. Two tibiae failed in fracture and were replaced by other bones of comparable sex, age and bone density.

0,2 0 Btd

ateel

var.

0

l/-

CfJTi

AO-tub.

1SD

0

torrional

CpTi var.

ultra-x

rtiffnerr

Fig. 24: Torsional stiffness of one bar frames with a bonebar distance of 70 mm or large clamps: steel var. = Pinless large steel clamps with the bar below the hinge; titanium var. = Pinless large titanium clamps with the bar below the hinge. The AO-tubular and the Ultra-X are both stiffer than the Pinless constructions. The position of the bar (above or below the hinge) made no difference to the stiffness.

Pulhlt

force

The pull-out force was measured in respect to different bone diameters (Dmax, Dsag, Dmin) and an increasing number of rocking movements (0, 2, 5, 10 and u) rocking movements). The small clamps showed no difference between Dmax and Dmin, but the clamp in the sag&al position (Dsag) was always much weaker than in the other two positions. Even with rocking movements, the small clamps in the sagittal position reached no higher pull-out force than in the other two positions without any rocking (Fig. 25). Rocking movements also improved the stability on the small bone diameters. The smaller the diameter, the lower the pull-out force even with a rising number of rocking movements, e.g. Dsag compared to Dmin or Dmax (Figs 25 and 26).

torsional stiffness INm /degl 1.4,

no rocking

”

Pinl.Sl

AO-Tub1 0

Pinl.92 +I-

lSD

AO-Tub2 EEj

PinLTl

Pinl.TO

torrion*l

atiffn*s*

PinLAl

Ultra

X

Fig. 23: Torsional stiffness (Nm/deg) of all one and two bar frames with a bone-bar distance of 45 mm: Pinl. = Pinks; S = steel; T = titanium; A = asymmetrical

movement

20

rockinq

movements

Fig. 25: Pull-out force of the small symmetrical titanium clamp mounted on three different bone diameters. There was a huge increase of the pull-out force when using 20 rocking movements compared to no rocking movements. The clamp in the sagittal position @sag) is much weaker than other positions even with a large number of rocking movements.

S38

Force IN1 ‘*O”l

800 800

0 max/r8t

minlr8t

0

maxll8t

min/lSt

mrx/lTi

+18EM

(n-8)

m

minllli

Pull-out

maxhli

min/eTi

force

Fig. 26~ Pull-out force of the small and large clamps in relation to different bone diameters (max. and min.) after 20 rocking movements: max. = large bone diameter at the junction between the metaphysis and the diiphysis; bone diameter, s = small clamp; mUI.= minimum Ti = titanium; clamp; St = steel; 1 = large SEM = standard error of the mean. steel clamps (left two columns in Fig. 26) and the small symmetrical titanium clamps (right two columns in Fig. 26) showed no difference between the two positions Dmax and Dmin. Using large clamps at the minimum tibia1 diameter reduced the pull-out force tremendously (4th and 6th columns in Fig. 26). Hence in diaphyseal areas with small bone diameters, small clamps have to be applied to reduce the risk of failure. The small

Comparing all clamps on the large bone diameter (Dmax) with a different number of rocking movements showed that the small asymmetrical clamp was the most stable one without any rocking and the small steel clamp was more stable than all the other clamps using 10 or 20 rocking movements. At 5 rocking movements, all curves were levelling off, but there was still a statistical difference between the pull-out forces at 5 and 20 rocking movements. With a high number of rocking movements, the curves of the symmetrical small and large clamps were close together. Only the small steel clamp was significantly (P zz 0.05) more stable (Fig. 27).

, 2ooForce INI

mean +/-lSEM(r4

200 0 0

2

4

8

8

10

12

U

b

18

20

No. of rocking movements

Fig. 27: Pull-out force of all five clamps in relation to the number of rocking movements. Only a few rocking movements (n = 5) nearly double the pull-out force of all symmetrical clamps. The asymmetrical small titanium clamp reached a higher pull-out force compared to all other clamps without any rocking movement. This clamp is squeezed onto the bone only.

Pull-out occurred most frequently at the trocar on the dorsomedial edge. This trocar point slid on the medial aspect of the tibii until the clamp snapped off the bone. Whilst sliding the force decreased. There was no fracture of the tibia, only some bone chips were carved out of the outer shell of the bone by the sliding trocar tip. With the slim based trocar tip (Fig. 5) on the large titanium clamps, two trocars sheared off at the base. The new reinforced base showed no failure. All titanium trocars were bent, blunt or otherwise defective after several uses (more than three). The bifurcated tip of the asymmetrical titanium clamp (pig. 5) was extremely vulnerable to deformation. After using them twice, all tips were bent divergently. The steel clamps could be used repeatedly until the tips became blunt

5.6 Discumion and ccmdusiuiu~ The external fixator in general has proved particularly valuable for second and third degree open tibiil fractures. In these cases, it is used either as a temporary stabiir or as the final fiiator for the whole period of healing. The problem of external fixation is still that of pin track infection and pin loosening and the difficulty of secondary fiiation using the medullary nail. To avoid the potential risk of medullary cavity contamination and subsequent infection, nailing takes place some time after removal of the fiiator (Johnson, 1990). A further difficulty is related to drilling before insertion of the pins. The medullary cavity is opened and the bone weakened (Bumtein, 1972) - the number of wrong holes and the possibility of correction is limited. Additional thermal necroses may develop, possibly leading to ring sequestm. A pin tmck infection may originate here. Anchoring the implant without the use of transosseous fixation may help to avoid this complication. The new fiiator is pressed onto the bone by hand without drilling. The axis of the fixator is not determined by the position of the fit clamps; a second ventml bar (above the hinge) can be mounted after insertion of all the clamps. With the short connection rods (moveable), the adaptation of the second bar is easy. Secondary fiiation using the medullary nail can be carried out with the Pinless clamp in place because the’ medullary cavity is still intact anyway. This integrity of the medullary cavity should considerably reduce the rate of infection after secondary nailing (McGraw, 1989). The quality of a fixator is often judged on the basii of its stiffness. For many years, maximal stiffness was considered essential. However, the positive experience of recent years with so-called “bio-logical” fixation has shown that for some indications a more flexible implant is more advantageous for fracture healing (Mast, 1989). The decisive factor is the atmumatic OR technique and the preservation of the blood supply to the fracture fragments as opposed to exact reduction and stabiition using stiff implants. Of course, the fixator must keep the reduced fragments in position even during normal loading of the fractured tibia and of the fiiator. Isolated or combined soft tissue injuries do not necessitate the use of a fiiator of maximal stiffness. On the other hand, the successful fiiation of chronically infected pseudarthmses demands maximal stabiity at the fracture gap. The basic idea of the Pinless fiiator is its use as a temporary stabiliser for open tibiil fractures until closed soft tissue consolidation Fixation with the medullary nail follows. The patient is able to bear weight Furthermore,

Rem&r:

s 39

Mechanical pxvperties

the Pinless clamp is intended as a temporary support for the lower limb in cases of soft tissue trauma and reconstruction surgery (muscle flap grafting), burn injury, and compartment syndrome. The tests described in this paper were designed to simulate the load on a segmental fracture of the tibia of a patient not bearing weight. The pull out tests simulate the support of the lower leg and the traction on the tibia or calcaneus by clamps. It is important to know what kind of loads must be borne by the injured limb under these conditions. The main load is a bending one on the fracture when the leg is supported. The bending moment may be roughly calculated from the length of the limb to the fracture and its weight. Furthermore, the moments of bending and torque due to the musculature cannot be neglected. There are also the additional forces exerted by nursing staff and physiotherapist to be considered. As the examples show, the bending load in the sag&al plane plays the most important role in th e primary phase of fracture healing. Secondarily, frontal bending, torque and, with increasing weight-bearing axial compression play a role (Behrens, 1986). If these conditions are transferred to the test situation, the Pinless fiiator shows its disadvantages under axial compression, the tubular fixator its advantages. Our tests were carried out on a transverse osteotomy with a defect. This is the worst possible situation as far as stability is concerned. Axial load transfer only takes place via the fixator. If there is contact between the two main fragments, then the bone can be at least partially loaded and the fixator compensates for less axial forces and moments. The bending stiffness of the small Pinless fixator is 60 - 80 ‘$J that of the AO-tubular fiiator. The Pmless fiiator becomes stiffer, the more congruence there is between the plane of loading and the area between the longitudinal axis of the tibia and the axis of the tube. The small titanium clamps were mounted in the sag&J plane (one tip each on the medial and lateral surface of the tibia) and tested in sag&al and frontal bending. The sag&al stiffness was acceptable and was in the range of 79 $G of the one bar tubular fixator and 69 % of the two bar frame. The Ultra-X in clinical use has a sagittal stiffness of 38 %. This fixator is designed in this form for temporary use only. Are the applied forces and moments logical range of loading ?

within the physio-

1000 N axial compression corresponds to a body weight of 100 kg. A fresh fracture of the tibia is unIikely to be loaded fully in the first weeks after internal fiiation. In the fit six weeks, the patient usually only applies a partial load of approx. 15 kg and increases this load gradually in steps of 10 - 15 kg. This takes place under regular clinical and radiological supervision at 2 to 4 week intervals. The passive and active mobiition of adjacent joints occurs as soon as possible after operation. Chao (1989) reported that the axial stiffness of most fractures is somewhere between 200 and 400 N/mm. This means that a partial load of 20 kg leads to a cyclical axial movement at the fracture gap of 0.5 to 1 mm (Chao, 1989). During active and passive mobiksation of the joints, the stabilised fracture is subjected to a variety of bending moments. Kempson and Campbell justified their use of 28 N bending load (2.85 kg) as being the weight of the foot and the lower leg (according to Braune and Fischer,

1889, 1985). In simplified terms, the weight and the lever arm (approx. 30 cm for a tibia1 shaft fracture) affecting the fracture result in a bending moment of 9 Nm on midshaft &ii fracture (Kempson and Campbell, 1981). If the torque of 40 Nm used in our tests is compared to reported values, then we are using a much higher maximal torque. Finlay (1987) and Moroz (1989) used a torque of up to 25 Nm for mechanical tests on the AOtubular fixator and the Hoffmann-Vidal device. McCoy (1983) used 17 Nm and Beluens (1986) only 4.5 Nm. The torsional stiffness of frequently used fixators lies between 1.1 Nm/deg (Brooker, 1983; Behrens, 1983; Briggs, 1982; Churches, 1985; Claes, 1981; McCoy, 1983; Fiiy, 1987) and 4 Nrn/deg (Kempson and Campbell, 1981). Almost as significant is the speed of the torque: in our study, the speed was not measurable and was purely rnanuaL No differences between manually applied low and high speeds were observed. Other authors used automatic devices and could keep the speed constant and regulate it: Claes applied a torque of 10 Nm and 18 degree&in to the specimen (Claes, 1981); Briggs and Brooker used 0.5 Nn+ec to test the Hoffmann-VidaI fiiator (Briggs, 1982; Brooker, 1983); Momz and Finlay used the unit of degrees&c: the torque is applied at a rate of 0.15 degree&c (= 9 degree&&) to the fiiator, Le. at half the speed used by Claes (Pinlay, 1987; Momz, 1989). The torsional stiffness of the Pinless clamp (small) is comparable to the AO-tubular and Ultra-X, so there is no limitation to its use. The methods of pull-out testing were adopted from previous tests on Schanz screws (Hecht, 1990). The relationship of the measurements to the number of rocking movements, the diameter of the bone, and the size of the clamp were adapted to suit the Pinless fixator and its function and can be applied to variables of insertion techniques and clamp position. The direction of pull-out perpendicular to the tibia1 axis and the axis of, the Pinless fiator was chosen with reference to clinical relevance in terms of adequate stability when supporting the lower leg with the fiiator. We do not expect any alteration to the results when pulling perpendicular to the longitudinal axis of the tibia. A force of over 1000 N is greater than that of the lower leg and the traction weights. During traction of the lower leg in an axial direction, no axial gliding of the clamps on the bone occurred. A second bar above the hinge can easily be mounted on the Pinless fixator, but it is not always necessary. The short adjustable connecting rods make it possible to mount an additional bar independent of the position of the clamps and without moving them. A second bar leads to a 25 % improvement in axial stiffness when using the small steel clamps. For the other loading conditions, it only leads to a slight improvement in stiffness. For the large clamps, a second bar can double the stiffness values of the Pinless fixator, depending on the loading condition. The combination of a bar above the hinge and a side bar with large steel clamps increases stabiity considerably (Fig. 28). This configuration is similar to the unilatemL biplanar tent construction of the tubular systems (Behrens, 1989). The additional side bars proved to be valuable stabiiing factors in terms of axial compression and bending perpendicular to the reference plane. Nonetheless, they had to be

s40

mounted so that the wound remained accessible and so that all clamp arms were the same distance from the side bar. The clamps must be absolutely parallel which is not easy to achieve on the irregularly shaped tibia. If congruence is not achieved, the incongruent clamp will pull out of the bone when the adjustable clamp is tightened. This is easy to see on cadaver bones, but during surgery the bone is covered by the soft tissue. The use of side bars on the titanium clamps is impossible anyway because the diameter of the clamp arms is too large (6 mm) to accommodate the clamps (Fig. 28).

Fig. 28: Pinless large steel clamp construction with a bar above the hinge and a side bar. The side bar is fixed to the clamps by single adjustable clamps. If the clamps are not aligned exactly, then the trocar point of the misaligned clamp witl be pulled out of the cortex when the adjustable clamp is tightened. This leads to an unstable situation and the risk of failure is high. There are references in the literature to numerous test procedures for various fiiatom. Unfortunately, different methods and parameters are used nearly every time. This makes a comparison of our results with those of other workers almost impossible. Behrens and Johnson tested an AO-tubular fixator on linen models. The fixator frames were similar but the bone-bar distances were different (25 mm and 80 mm). The pins in the fragment were 90 mm ,apart. The one-bar version (bone-bar distance = 25 mm). had the following stiffness values: axial stiffness = 252 N/mm; sag&al bending stiffness = 16 Nm/deg.; frontal bending stiffness = 8.7 Nm/deg.; torsional stiffness = 1.3 Nm/deg. The values for the same fiiator with a bone-bar distance of 88 mm were: 31 N/mm; 16 Nm/deg.; 3.8 Nm/deg.; 0.9 Nm/deg. The two-bar system had signifi364 N/mm, 20.7 Nm/deg.; 10.6 cantly higher values: Nmfdeg.; 1.4 Nmldeg. (Behrens and Johnson, 1989). The axial stiffness in our tests on the one-bar AO-tubular fiator (bone-bar distance of 70 mm) is comparable with the one-bar system with a bone-bar distance of 80 mm. In other ways the results of Behrens are considerably higher. There was no difference between the values for torsional stiffness and the relationship between sag&al and frontal bending stiffness for the one-bar system. It would be interesting to compare the AO-tubular system with the Oxford fixator. Churches calculated stiffness on the basis of mathematical models and confirmed his results in mechanical tests. For four pins (6 mm dia.) and a bone-bar distance of 55 mm stiffness was as follows: axial = 129 N/mm; sag&al = 16 Nm/mm; frontal = 8 Nm/mm and torsional = 1.6 Nddeg. (Churches, 1985). On the whole, the Oxford fiiator is stiffer (screw dia. = 6 mm) but the reiationship of sag&al and frontal stiffness is comparable. Evans and Kenwright did pull-out tests on, 6 mm Schanz

screws and achieved a pull-out force of 1800 N (Evans, 1979). Hecht recorded an average pull-out force of 3500 N for 5 mm Schanz screws in tests done on human cadaver tibiae at the A0 Research Institute. The pull-out forces of the Pinless fixator are comparatively small (zG loo0 N). In clinical application, the weaknesses of the Pinless fixator will restrict its use. The conventional fiiators are not only used for temporary stabiition of open fractures but also for a multiplicity of other indications, e.g. permanent fracture fiiation, arthrodesis, interfragmentary compression of pseudarthrosis, lengthening osteotomy and segment transport. In such cases, the fixator must hold for several months. High stability and stiffness level are often pmrequisites for the success of these treatments. What are the limitations of the Pinless fiiator ? The main limitation is its low stiffness under different loading conditions, e.g. axial compression. The Pinless fixator cannot be compared with its stiffer, conventional counterparts with regard to indications. If this implant is used for the wrong indications, complications will arise and it will be discredited. The basic idea of the Pinless fixator is its use as a temporary fracture stabiliser for open tibia1 fractures until the soft tissue has healed. A secondary change of procedure to the medullary nail is then planned and has to be carried out (Swiontkowski 1992). The low axial stiffness means that patients cannot weightbear particularly early, but it does not imply immobiition. If ‘the patient is able to walk on crutches, then the injured limb can be loaded in the sense that there is floor contact (Claudi, 1992, perscorn.). A partial load of 15 kg leads to axial movements in the fracture gap of a bony defect of 5-6 mm. The eccentric application of force on the fiiator gives rise to additional bending moments which affect the fracture. The axial stiffness can be increased slightly by the addition of a second rod (the above-mentioned axial movements are reduced to 4 mm). The bending stiffness is greatest in the fiiator plane. The surgeon can also influence the stability and stiffness by placing the clamps in the main plane of loading. This principle is utilised in the case of the AO-tubular fiiator. The sagittal plane is the main plane of loading for the tibia (Behrens, 1986). If the clamps are mounted on the medial tibia1 surface, then the sag&al bending stiffness lies between 40 % and 80 % of that of the AO-tubular fixator. The wide range is due to the multiplicity of the clamps and configurations. For a sagittal bending moment of 9 Nm under static loading of the tibia (Braune, Fischer, 1889, Kempson and Campbell, 1981), there would be a dii placement of approx. 1.5 deg. of the fragments for the small Pinless fiiator. The effect of the muscles has not been taken into account here. The stability of the Pinless fiator depends on its exact implantation in the bone. The stability of the individual clamps depends on the specific properties of the implant such as clamp width, material and diameter of the arms plus the properties of the bone (diameter, thickness) and the insertion technique. The clamp should always be mounted at the point of maximum diameter at all levels of the tibia (between the ventral and domomedial tibiil crest) in order to achieve adequate preload of the clamp. The trocar tips should be inserted almost perpendicularly into the cortex. The more acute the angle to the bone surface, the weaker the anchorage in the bone. Pull-out tests showed the influence of rocking movements on the anchorage in the cortex: only a few rocking movements double the pull-out force of the symmetrical clamps, as

Remiger

Mechanical pqmties

s 41

opposed to squeezing the clamps without rocking. The lower pull-out force of the large clamps on the smallest tibia1 diameter indicates the necessity of clamp preload. An average bone diameter of 24 mm produced too little preload to make use of the large clamp effective. Here, a small clamp would be stable (pull-out force of > 90 kg). The thinner the bone, the more important it is to use a small clamp for the fixation. The two incisions per clamp seem to be a return to bilateral fiiators. Exact positioning of the clamp in the socalled “safe corridor” over the medial aspect of the tibia avoids soft tissue damage. This corridor covers an anteromedial sector of the tibia of 2200 proximally and 1200 distally (Green, 1981, Behrens, 1986). During initial clinical application in the proximal third of the tibia, a bend in one of the large clamps caused pressure irritation of the gastrocnemius and the soleus muscles. The skin and insertion site became inflamed and was accompanied by serous discharge and local pain. The shape of the clamp has been changed to the asymmetrical form. For certain indications, the principle of Pinless fixation is definitely an alternative to conventional fixation. A very stiff fiiator is not necessary for the application of a temporary implant for fracture stabilisation when the patient is not bearing weight or for support of the limb. The mechanical tests have shown that the Pinless fiiator is stable enough for clinical use, however, clinical trials should confirm this. The test results provide essential details of the properties of the Pinless fiiator and the use of various configurations.

Fig. 29: The Pinless device as a support for the lower leg after soft tissue reconstruction. According to the results of the pull-out tests, two clamps are enough to keep the lower leg elevated. A one clamp construction was used for traction on the calcaneus, on the tibia1 head and on the distal femur (BClaudi, 1991, perscorn.). It was proven to be mechanically sufficient either in longer traction periods (in bed) or for only a short time on the extension table (P.Matter, 1992, perscorn.) (Fig. 30).

The initial application of the Pinless system in the clinic as a temporary stabiliser for non-weight-bearing patients has proved very promising. Its stability is adequate to support the leg and allow the patient to do physiotherapy and to be cared for normally. Partial loading of an unstable fracture should be restricted to 15 kg. Furthermore, the Pinless fixator is an ideal instrument for supporting the leg. A two clamp and one-bar construction served as an ideal support device in soft tissue injuries such as in compartment syndrome, burn injuries and soft tissue defects. The prerequisite for the use of two clamps only is unfractured bone or an already fixed fracture (internal osteosynthesis). In soft tissue reconstruction with or without muscle flap grafting, the advantages of the clamps were their easy handling and the possible change of their position on the bone. As opposed to drilling and screw insertion, there is no damage to the bone. Therefore, the tibia is not weakened by a change of the clamp position as it would be after several changes of pins (But-stein, 1972) (Fig. 29).

Fig. 30: The Pinless clamp used as a traction device on the calcaneus. The pull-out force of the small and even of the large clamps is much higher than the weights used for traction of the lower leg (up to one tenth of body weight). The one clamp traction system could also be used in the supracondylar femoral metaphysis and on the tibia1 head (large clamp) (courtesy of P.Matter, Davos Hospital, Switzerland). A longterm use of the Pinless clamp is possible but some problems occurred in our fit applications. The most important one is the appearance of severe osteolysis beneath the trocar points. After 6 - 8 weeks of clamp implantation, this bone loss was noticed in nearly all patients with a longterm use of the Pinless fixator. The fixator was used for permanent fracture healing, for longterm soft tissue reconstruction or for segmental bone transport over a medullary nail (“Monorail” procedure) (Claudi, 1991; Raschke, 1992; Remiger, unpublished data). This osteolysis might lead to infections comparable to pin track infections (i.e. clamp track infection). Due to these problems a temporary use of the Pinless external fiiator is to be preferred. If the Pinless clamp is not overtaxed, it does offer an

S 42

alternative to the conventional external fiiators for special indications. These applications should be worked out in a prospective multicentre study as soon as possible. Acknowleclgementsz My thanks goes to Prof.S.M.Perren and to all the People working at the A0 Research Institute in Davos for their support and criticism in this project and for their hospitality. Special thanks go to USchlegel for his help and support in critical situations, to Dr.J.Cordey for carrying out the statistical analysis, to Prof.B.Claudi for his clinical ideas and kind encouragement, to my friend Piet Imken for his excellent drawings, to CGiintensperger and E.Omerbegovic for their prompt assistance with photographic work, and to R.Frigg and his fantastic team (GScandella, P.D&cher, RAmbiihl) for their ideas, help and teaching in mechanics. Last but not least, my thanks go to Joy Buchanan for her personal commitment and help without which this Paper would not have been possible. Thank you.

Burstein AH, Currey I, Fmnkel VH (1972) Bone strength: The effect of screw holes. J. Bone Joint Surg. 54A:1143-1156 Chao EYS, Am I-IT, Lewallen DG, Kelly PJ (1989) The effect of rigidity on fracture healing in external fiiation. Clin. Orthop. Rel. Res. 2412435 Churches AE, Tanner KE, Harris JD (1985) The Oxford external fiiator: Fixator stiffness and the effects of bone pin loosening. MEP Ltd 14(1):311 Claes L, Burri H, Gemgross H (1981) Vergleichende Stabilit%suntersuchungen an symmetrischen und einseitig ventromedialen Fixateur-exteme-Osteosynthesen an der Tibia. Unfallchirurgie 7~194-197 Claudi B, Oedekoven G (1991) Biologische Osteosynthesen. Der Chimrg, 62367-377

5.7 Reiferencm

Edwards CC (1983) Staged reconstruction of complex open tibia1 fractures using Hoffmann external fixation. Clin. Orthop. 178:130-161

Aho AJ, Nieminen SJ, Nylamo EI (1983) External fixation by Hoffmann-Vidal-Adrey osteotaxis for severe tibia1 fractures. Clin. Orthop. 181:154-X4

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