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Stress and stability comparison between different systems for high tibial osteotomies
© Luo et al.; licensee BioMed Central Ltd. 2013
Received: 16 October 2012
Accepted: 14 March 2013
Published: 25 March 2013
High tibial osteotomy (HTO) with a medial opening wedge has been used to treat medial compartment osteoarthritis. However, this makes the proximal tibia a highly unstable structure and causes plate and screws to be the potentials sources for mechanical failure. Consequently, proper design and use of the fixation device are essential to the HTO especially for overweight or full weight-bearing patients.
Based on the CT-based images, a tibial finite-element model with medial opening was simulated and instrumented with one-leg and two-leg plate systems. The construct was subjected to physiological and surgical loads. Construct stresses and wedge micromotions were chosen as the comparison indices.
The use of locking screws can stabilize the construct and decrease the implant and bone stresses. Comparatively, the two-leg design provides a wider load-sharing base to form a force-couple mechanism that effectively reduces construct stresses and wedge micromotions. However, the incision size, muscular stripping, and structural rigidity are the major concerns of using the two-leg systems. The one-leg plates behave as the fulcrum of the leverage system and make the wedge tip the zone of tension and thus have been reported to negatively affect the callus formation.
The choice of the HTO plates involved the trade-off between surgical convenience, construct stability, and stress-shielding effect. If the stability of the medial opening is the major concern, the two-leg system is suggested for the patients with heavy load demands and greater proximal tibial size. The one-leg system with locking screws can be used for the majority of the patients without heavy bodyweight and poor bone quality.
High tibial osteotomy is a surgical treatment for the correction of medial compartment osteoarthritis. In the literature, there are two HTO types, namely laterally closing and medially opening osteotomies [1–4]. The medially opening HTO has become more popular with its better outcome and numerous complications can be avoided [2, 5, 6]. From the biomechanical viewpoint, the wedge makes the proximal tibia highly unstable, and structural stiffness of the HTO fixation has been reported to be closely related to the postoperative outcome [2, 7, 8].
Historically, some internal and external fixation devices were used to maintain the instrumented graft in place and stabilize the osteotomized construct [2, 5, 8, 9]. Among the various devices, the Puddu and TomoFix plate systems were the most common type specifically designed for the HTO procedure [1, 6, 7, 10]. However, recent reports have shown that some shorter systems (e.g. Puddu system) result in graft nonunion and implant failure [5, 11–13]. The clinical study by Nelissen et al. revealed that a shorter plate and nonlocking screws provide less ability to stabilize the osteotomized construct. For the TomoFix system with a longer plate, Kolb et al. revealed that the use of the locking screws would yield better short-term results. Although a high rate of satisfactory outcome was reported, the improper selection and use of the implant often induced nonunion and even fracture of the HTO construct, especially for overweight or full weight-bearing patients [14–18].
There were two types of the HTO plate systems used in this study (Figure 1). The first type was the one-leg design: the TomoFix plate and the conventional T plate (Figure 1a and 1d). The second type was the two-leg design: the hybrid use of two conventional T and I plates and the π-shaped plate (Figure 1b and 1c). The π-shaped plate slightly extended the medioposterior area to stabilize the opening in that region. Below and above the opening site, the numbers of the screws to fix the four plates are shown in Figure 1. All plates and screws were developed using the software SolidWorks Version 2010 (SolidWorks Corporation, Concord, MA, USA). The aim of this study was investigate the effect of plate design on construct behavior, thus modeling of the screw threads was omitted to simulate a rigid-bond at the tibia-screw interface.
The constitutive laws for the bones (cortical and cancellous) were assumed to be linearly elastic, homogeneous, and isotropic. The simulation of bone qualities were base on a middle-aged population, and the values of Young’s modulus and Poisson’s ratio were 17 GPa and 0.33 for cortical bone and 5 GPa and 0.33 for trabecular bone, respectively [21, 22]. Except for the titanium-based TomoFix system, the material of all plate systems were surgical stainless steel (AISI 316 L) with homogeneous and linear properties (elastic modulus = 210 GPa, yielding strength = 750 MPa, and Poisson’s ratio = 0.3). The length, width, and thickness of the plates are about 115 × 35 × 3 mm for the TomoFix plate, 115 × 34 × 3 mm for the T plate, 90 × 11 × 3 mm for the I system plate, and 120 × 45 × 3 mm for the π-shape plate, respectively. In this study, the bone graft was neglected in the finite-element analysis to simulate the worst-case scenario of implant loading. Both tibia-plate and plate-screw interfaces were modeled as surface-surface contact elements which allow for separation and slippage. The tangential friction law was based on Coulomb’s criterion, ignoring any friction from adherence with the friction coefficient assumed to be 0.3.
This study employed an automatic mesh generation algorithm with Simulation Version 2010 software (SolidWorks Corporation, Concord, MA, USA), which provides a special element density at plate-screw junctions three times that of the remainder of the model with the overall average element size of 2 mm. The meshing strategy was designed for curved element boundaries, thus there were no sharp discontinuities to induce an unrealistically high stress concentration. By using the aspect ratio and Jacobian checks, all elements were within acceptable distortion limits to maximize the result accuracy. The model was meshed by ten-node tetrahedral solid elements. On average, the final meshes consisted of 110,000 elements and 155,000 nodes for four finite-element models. The mesh refinement at the plate-screw interfaces was executed for modeling accuracy until excellent monotonic convergence behavior with < 1% difference in the total strain energy was achieved.
Four indices were chosen to compare the difference in stress and micromotion between the four plate variations. The first three were the indices of implant and bone failure that were calculated in terms of maximal von Mises stresses at screws, plates, and surrounding bone. The last was the index of construct stability for measuring the change in height at edges aa, bb, and cc of the opening (Figure 2b). The load ratio through the posterior to the anterior leg at the section plane AA was used to provide information of the load-transferring mechanism at the distal legs of the T+I and π plates (Figure 1c).
Concentrated stresses of screw, plate, and bone
For the plate stresses, the T construct was still the most stressed and was followed by the TomoFix. The plate stresses of the other constructs were comparable and 67.5% less than that of the TomoFix construct. Similar to the screw stress, the use of the I plate can decrease 56.6% of the plate stress. For the T-shaped design, the use of locking screws and titanium alloy made the plate stress of the TomoFix system 31.9% less than that of the T plate. The difference in bone stress between the four plate-bone constructs was similar to that of the differences between the stresses in the plates. The surrounding bone of the T plate was the highest stressed and the stress values were 55.3%, 68.0%, and 128.7% higher than the TomoFix, T+I, and π constructs, respectively. For the one-leg systems (i.e. TomoFix and T), the screw, plate, and bone stresses concentrated at the interfaces, and the most proximal screw below the opening, was the most stressed. The posterior leg of the two-leg system (i.e. T+I and π) was more loaded and also stressed highest at the same region as the one-leg design.
Height change and load ratio
The knee is one of the most heavily loaded joints of the human body during daily activities. For the HTO surgery, the medial opening is an extremely unstable condition for the proximal tibia, and the fixation device is used to stabilize the opening and enhance bone union. This study used the one- and two-leg plates to evaluate the effects of locking screw and plate leg on the construct stress and wedge micromotion. For the one-leg design, the screw, plate, and bone stresses of the TomoFix construct were respectively 42.2%, 31.9%, and 35.6% less than those of the T construct (Figure 5). For the two-leg design, the aforementioned stresses of the π plate can be reduced by 20.3%, 11.9%, and 26.5% as compared with the T+I construct. This indicated that the use of locking screws can significantly reduce the mechanical demands of the implants and surrounding bone.
For the two-leg systems, the load ratio of the posterior to anterior legs can provide biomechanical information of the load-transferring mechanism. If the tops of both T and I plates were linked, the value of the load ratio could be decreased from 4.7 to 3.9. This indicated that the anterior leg of the π plate could share the loads of the posterior leg to reduce the mechanical failure at that location. Meanwhile, the results of the load ratio demonstrated the effective distribution of the enormous loads from the knee joint to the distal tibia.
The height changes in the edges aa, bb, and cc were regarded as the indices of the construct stability. Except for the π plate, the micromotions of the other systems consistently exceeded the reported maximum value (> 100 μm) of the allowable movement for the bone union . However, some studies have shown that the highly rigid fixation may cause osteoporosis due to the stress-shielding effect [30, 31]. Moreover, adequate micromotion of fracture interfaces can enhance the callus formation [32, 33]. Historically, the trade-off between rigid fixation and interfacial micromotion is still unknown .
For the one-leg systems, the wedge micromotions were significantly higher that the counterparts and were contributed to the bone-screw loosening and construct instability [2, 35]. At the wedge tip (edge aa) of the one-leg systems, Figure 6 showed the occurrence of bone separation that results from the leverage effect and negatively affects the callus formation . In the literature, some researchers proposed the anteromedial side as the optimal position of the one-leg plate to stabilize an osteotomized tibia [2, 37]. On the other hand, follow-up and cadaveric studies of the opponents found that the anteromedial placement induces posterior tilt of the tibial plateau and construct instability [38, 39]. Comparatively, the two-leg plate provides a greater area to cover the circumferential sides of the medial opening (Figure 1). This can deter the placement-induced effects of the one-leg plate on the construct responses and ensure more stable support to the opening.
There are some limitations inherent in this study. The use of both physiological and surgical loads may be overestimated for the early stage of the HTO healing. After surgery, however, the treated leg was often partially loaded by using a crutch or a walker . If shorter bed-resting time was desired, this study can provide the worst-case comparison of the different HTO plate systems in the situation of the full body-weight . From the biomechanical viewpoint, both bone variations and wedge sizes might alter the stabilizing effect of those HTO plates. However, these were not included in this study and the experimental validation should be performed.
The optimal use of the HTO plates involves the trade-off between surgical convenience, construct stability, and the effects of stress-shielding. The incision size of a one-leg plate can be less than that of its counterpart. If the plate thickness was appropriately designed, the one-leg plate can shield the bony loads less than the two-leg system. However, surgical planning for plate placement is necessary due to the smaller size of the one-leg system. The two-leg plate can provide a stable plate-leg base and form a force-couple mechanism to effectively resist the applied loads. If the construct stability is the major concern, the two-leg system can be used for patients with a heavy load demand and greater proximal tibial size. Except for the larger incision size, however, the two-leg system may result in stress-shielding in the region around the wedge and smaller micromotion across the opening gap. It suggested the one-leg system with locking screws can be used for the majority of the patients without heavy bodyweight and poor bone quality. Alternatively, the two-leg plate is recommended to be used for the heavy (i.e. higher implant stress) and osteoporotic (i.e. weaker bone strength) patients.
Special thanks to Yu-Lin Chen for proving encouragement and data collection in developing this manuscript.
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