| | Biomechanical evaluation of different fixation plates in medial opening upper tibial osteotomyReceived 28 April 2006; received in revised form 11 August 2006; accepted 9 October 2006. published online 18 November 2006. Abstract In this biomechanical study, 25 in vitro calf tibial models were used in order to compare the stability of the plates under axial compression loading. A 10-mm medial opening gap was stabilized in each of the five calf tibial models either with four or two-holed rectangular shaped plates with wedges, with four-holed reversed L-shaped plates with wedges, with the combination of these two types of plates, or with six-holed anatomical T-plates. The compression behavior of the model was tested by using a universal mechanical testing system. The specimens fixed with the combination of plates with the four-holed reversed L-shaped and with two-holed rectangular shaped; or with six-holed anatomical T-plates, showed significantly better stability than those of others. Four different kinds of failure (slippage of wedge, lateral cortex fracture, damage and/or loosening of screws, and bending of plates) were observed on the models. When the average value of force loading on the plates that were designed by the first author was considered, the plates were stable and the average force values at these points were higher than the loading force on a knee during the normal paced walking or running conditions. 1. Introduction  Proximal tibial osteotomy (PTO) was proven to be an effective treatment method for medial compartment osteoarthritis of the knee for young and more active elderly patients. Although unicompartmental and total knee arthroplasty cannot offer the high functional level of desired activity in young patients, PTO can offer high activity levels for the patients [1], [2], [3] Various techniques with stable osteosynthesis have been described in the literature [1], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Therefore, a stable fixation at the time of osseous consolidation of the PTO is a prerequisite for a satisfactory result. However, biomechanical experimental studies comparing the fixation techniques of PTO remain insufficient [2], [7], [16], [17], [18], [19], [20], [21]. The aim of this study was to compare the stability of the commonly used anatomical “T” plates with the three different plates supported with wedges (and their combinations) designed by the first author [5], [6]. 2. Materials and methods An in vitro calf tibial model was used to compare the stability of the plates under axial compression loading. The specimens, which were extracted from lower legs of domestic 1.5 years old male calves, were used for the experimental tests. The legs were carefully removed from the calves weighted on average 600 kg by a butcher. After the plates were mounted the specimens were kept in a fridge medium of − 20 °C. Regarding the specimen preparation, the removed tibia was shortened to a length of 25 cm in order to study it easily, and to prevent the effects of the buckling on the calf tibia specimens. Distal tibial cut was perpendicular to the anatomical axis of tibia and was parallel to transfers axis. The specimens were thawed at room temperature for 12 h before the tests. A standardized osteotomy was performed starting at the medial cortex 4 cm distal to the medial tibial articular surface, and running through to the point which was 1 cm distal to the lateral tibial articular surface and 1 cm medial to the lateral tibial cortex. An approximately 15° (13°–17°) osteotomy angle which was suitable for the medial opening wedge osteotomy, was distracted to 10 mm by using the angle scale distractor which was developed by the first author and reported in detail elsewhere [22]. Consequently, osteotomy was fixed by three types of plates and with their four types of combination which were supported by wedges with the height of 10 mm (TR-2002 02021Y-Hipokrat Corp., Izmir/Turkey) [5], [6] and six holed anatomical “T” plates without wedge (Hipokrat Corp., Izmir/Turkey) as shown in Fig. 1. Plates have wedge-shaped triangular parts with a 4-mm depth to internally support the osteotomy surfaces. The plates were manufactured according to the anatomical inclination of the medial surface of the proximal tibia. As an example, a rectangular, four-holed plate with a wedge height of 10 mm was manufactured in 2-mm thickness, 46.5-mm length, and 25-mm width, whereas the rectangular, two-holed plate was manufactured in 2-mm thickness, 43.3-mm length, and 12.3-mm width [5], [6]. All plates and screws were made of stainless steel. The fixation was performed to the five distinct groups of the total of 25 calf tibias with a wedge supported rectangular four-holed plate (Group A), wedge supported double rectangular two-holed plates (Group B), a wedge supported reversed “L” shaped four-holed plate (Group C), an anatomical six-holed “T” plate without wedge support (Group D) and a combination of a rectangular two-holed plate with reversed “L” shaped four-holed plate (Group E) (Fig. 2). In order to minimize the effect of screws and to obtain a homogenous distribution, plate fixations were done by using standard 60×6.5 mm fully threaded cancellous screws for the proximal fixation and 55×6.5 mm fully threaded cancellous screws for the distal fixation. The anatomical axis was drawn on the bone, and proximal and distal cuts were placed vertical to the anatomical axis. After osteosynthesis; anteroposterior and lateral radiographs of the same tibia of all specimens were taken in order to control the osteotomy line and the position of all implants and to determine the possible infractions of the bone. Before performing the mechanical tests, the specimens were mounted proximally and distally to a rigid polymer cylinder which was a polyester base material containing 99% polyester resin liquid solution, 0.5% polyester accelerator, and 0.5% polyester hardener. A rigid and aligned contact between the specimen and the anvil head of the compression test machine was maintained. Axial stability and stiffness of the plates, that were implanted similarly, were compared to each other by using a universal testing machine, Hounsfield H50KM. Axial alignment of the compression load applied by the testing machine was parallel to the anatomical axis of calf tibia and carefully moved a little medially towards the plates with the help of a balance device and lathe. All specimens were positioned vertically in the testing machine. A single load up to the catastrophic failure point was applied on the specimens. The test was stopped when the maximum axial compression load that causes failure of the system was achieved. The load applied to the specimens, in Newton, and displacement, in millimetre, was recorded digitally at the testing machine and moreover the load–displacement curve was recorded by using an X–Y recorder. All alterations on the tibial models during the tests were observed and recorded simultaneously by three different observers and they were justified by the characteristics of the load–displacement curve. The mean slope was obtained to compare each plate system by using the maximal axial compression loading points of the load–displacement curves. Finally, the optical and radiological controls for all specimens were arranged to determine the forms and causes of the failure. 2.1. Statistical analyses The data were analysed by using a commercially available statistics software package SPSS. Distribution of the groups were analysed with one sampled Kolmogrov–Smirnov test. All groups demonstrated the normal distribution, so that the parametric statistical methods were used to analyse the data. One-way ANOVA test was performed and Post Hoc multiple comparisons were done with LSD. Results were presented as means and standard deviation (SD). p values less than 0.05 were regarded as statistically significant. 3. Results Four different kinds of failure, either single or in combination of those four, have been observed in the in vitro calf tibial model while the axial compression loading is applied up to the catastrophic failure point; slippage of wedge, lateral cortex fracture, damage and/or loosening of screws and bending of plates (Fig. 3). Average axial compression force at first damage in anatomical “T” plates (8693 N) and the combination of wedge supported rectangular two-holed plates and reversed “L” shaped four-holed plates (8130 N) were significantly different (significantly bigger) from the wedge supported rectangular four-holed plate (3390 N) and two rectangular two-holed plates (3766 N) (p<0.05). It was observed that the first damage to the model was the slippage of the proximal parts of the wedge blocks from the supported cortical bone and/or fracture of lateral cortex. The failure has been monitored by the close examination during the test. Furthermore, the breaks on the load–displacement curve also confirmed this result. The stability was scored due to the loading force occurring at the first damage and scaled as A, B, C, E and D in increasing order (Fig. 4-A) (Table 1). | | |  | Groups | Average slopeat insufficiency point(N/mm)—SD | Average loading at insufficiency point (N) — SD | Average loading at first damage(N) — SD | |
|---|
| A | 820.5 | 61.7 | 6443.3 | 1383.5 | 3390.0 | 1454.8 | | | B | 1143.3 | 153.8 | 8460.0 | 1756.5 | 3766.7 | 1154.7 | | | C | 986.4 | 283 | 7963.0 | 1797.9 | 5530.0 | 144.2 | | | D | 1254.2 | 97.0 | 10,993.3 | 2190.9 | 8693.3 | 724.0 | | | E | 1214.5 | 298.5 | 13,320.0 | 3544.7 | 8130.0 | 3784.9 | | | | |
Average ultimate axial compression load for the combination of wedges supported by the rectangular two-holed, and the reversed “L” shaped, four-holed plates (13,320 N) was significantly different (significantly bigger) from the wedge supported by one rectangular four-holed plates (6443 N), one reversed “L” shaped four-holed plates (7963 N) and two rectangular two-holed plates (8460 N) (p<0.05). At the same point, anatomical “T” plates (10,993 N) were statistically different only from the rectangular four-holed plates that were supported by wedge (p< 0.05). The stability was scored according to the ultimate compression force and scaled as A, C, B, D and E in increasing order (Fig. 4-B) (Table 1). Starting point of the third region, which was evaluated as plastic deformation considering the slope of the curve on the load–displacement graphic; anatomical “T” plate (1254 N/mm) and the combination of wedge supported rectangular two-holed plate with reversed “L” shaped, four-holed plate (1214 N/mm) were significantly different from the rectangular four-holed plates that were supported by the wedge (820 N/mm) (p<0.05). The stability was scored according to the mean value obtained from the curve of graphic by dividing the maximal compression load to displacement at this point and scaled as A, C, B, E and D in increasing order (Table 1). A randomly selected set of five different load–displacement curves, which were obtained from five different groups, are shown in Fig. 5. The first two parts showed reversing tendency, whereas the last part was irreversible responding to the intermittent unloading. Furthermore, the failure types were detected along the third part. In the first and second step of the curves, stiffness of the system did not change for various plate types. However, later development of the curve attempts to change its slope or its stiffness and A < D < B < C < E (in increasing order) was observed regarding the stiffness degree of the third step. It was demonstrated that, the other kinds of damages had already occurred before the failure of the model. 4. Discussion In this study, we used the calf tibial model. Calf tibias are relatively reliable for rigidity, have low price, and are easily available in large amounts compared to the human cadaver tibiae. A possible problem in our study was the difference in the dimensions and cortical thickness of the calf tibia compared to those of humans. Since this study aimed to investigate the relatively biomechanical properties of different plate systems, we believed that this difference was insignificant. Spahn et al. have used lower leg specimens from domestic pigs for their two different experiments [16], [17]. The alternative use of human cadaver specimens as described by Flamne et al. [7], and Zhim et al. [21] also includes some possible problems such as the differences in bone quality and size, difficulties of obtaining human cadaver specimens and influence of age and sex on specimens. The mineral bone density of human cadaver tibia is usually low and has other pathologies due to the fact that they are mostly provided from elderly patients [7], [17]. The alternative use of composite tibia specimens as described by Agneskirchner et al. [18], Hartford et al. [2], and Stoffel et al. [19] also includes some possible problems. The stability of fixation decreased significantly in the models that had lateral hinges already damaged before the study. Hence, it is important to realize that the stability is dependent largely on an intact lateral hinge [2], [5], [6], [7], [13], [17], [19], [22]. The lateral cortex was seen radiologically intact in the specimens before the experiments. In our study, the axial compression force applied by the testing machine was parallel to the anatomical axis of the calf tibial model, but a little bit medial towards the plate in order to get standardized. In this way, we hoped that the damage occurred at the failure point was the damage of plates and screw rather than the damage of the intact lateral cortex. These results demonstrated that the first damage has incurred as fracture of the tibial lateral cortex and pull-out of the wedge of plates rather than the damage of the plates. However, the damage of screws and plates occurred at the last failure point. These results showed the stability and resistance of our plates. Stuart et al. [20] evaluated the static response of Puddu plates to compression loads in a human cadaver model. They demonstrated that Puddu plate construction appeared marginally strong enough to withstand the estimated axial load on the proximal tibia during gait [20]. Spahn and Wittig [17] compared the stability of the tibial plate with a mobile spacer (C-plate), Puddu plate and standard AO tibial plate in a laboratory study with lower leg specimens from animal cadaver models. They showed that C-plate was the most resistant to the axial compression force. Its stability was significantly higher than that of other implants examined. Moreover, they also showed that stability of Puddu plate was higher than that of AO tibial plate. Metal block spacer was better at removing the axial compression force. The cause of the failure for Puddu plate was mainly a displacement or inflection of the implant. However, for the stiffer C-plate a fracture of the lateral cortex always occurred [17]. However, considering the number of the screws, C and AO plates have five screws, three of which are proximal and two of which are distal. Furthermore, considering their sizes, C and AO plates are longer and thicker than Puddu plate. The increased strength of the system might be contributed by the length and thickness of the plate and by the number of screws. We used in the present study 2-mm thickness plates. We did not observe any damage to the plate before the fracture of lateral cortex and sleeve of screw and wedge of plate occurred. Thus, these results proved that the plate used in the present study was strong enough mechanically. Stuart et al. [20] demonstrated that the axial compression loading resulted in failure at a mean of 1810 N due to bone collapse, fracture, or translation in Puddu plates. Spahn and Wittig [17] found axial stress resistances for the Puddu and C-plates of 1678 N and 2042 N respectively. Stoffel et al. [19] demonstrated that the axial compression force causing failure was 2537 N in Puddu plate and 2904 N in TomoFix plate. However, according to our results, almost 6000 N was observed during the failure. The results may be due to the higher strength of calf tibia than that of the composite one and pig tibia. Also, note that, closer axial compression loading force existed in our model. The limitation of our study was lack of rotational and cyclic loading comparisons with more tibial models. Our plates were undamaged under almost 10,000 N (range; 5600 N to 14,900 N) axial compression force. However, they may exert stronger fixation and stability in human beings. In this study, it is clearly seen that our different designed plates were able to maintain the correction for a load of more than 5600 N, which is close to the axial compressive load in the knee of an adult during single limb stance of fast walking [23] and exceeds that of normal walking [24]. After the evaluation of the graphics, we observed that the first damage were lateral cortical fracture and taking the wedge out of the plates at the supporting cortical bone. The damage of plates and screws occurred at insufficiency points. The loading force, which caused the first damage and failure at groups E and D, was significantly higher than that of groups C, B and especially that of group A. However, we believe that the stability of our different designed wedge supported two-holed and four-holed rectangular shaped and four-holed reversed “L” shaped plates that were used clinically, is high enough for the maximal loading force applied on the knees during walking and running. Acknowledgements The authors would like to thank to HIPOKRAT Corp.-Izmir-TURKEY for the preparation and the availability of these special plates. References [1]. [1]Hanssen A. Osteotomy about the knee: American perspective. In: Insall JM, Scott NM editor. Surgery of the knee. 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a Inonu University, School of Medicine, Department of Orthopaedics and Traumatology, 44065, Malatya, Turkey b Firat University, School of Eng., Department of Metallurgical and Materials, Eng., Elazig, Turkey c Inonu University, School of Medicine, Department of Physiology, Malatya, Turkey  Corresponding author. Tel.: +90 422 3218555; fax: +90 422 3258283.
PII: S0968-0160(06)00166-9 doi:10.1016/j.knee.2006.10.003 © 2006 Elsevier B.V. All rights reserved. | |
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