Finite element analysis of novel medial opening-wedge high tibial osteotomy plate: “High tibial osteotomy 7-plate”

INTRODUCTION

High tibial osteotomy (HTO) is a surgical technique primarily used to correct varus deformities of the knee and to treat medial compartment osteoarthritis. This procedure aims to realign the mechanical axis of the lower limb, thereby shifting load from the damaged medial compartment to the lateral compartment, ultimately reducing pain and potentially delaying total knee arthroplasty.^[1,2] While effective, traditional plate designs used for HTO frequently encounter challenges, such as when HTO is needed in combination with anterior cruciate ligament (ACL) reconstruction. Several previous designs employed a T-shaped plate configuration, which may leave limited space on the anterior side for ACL reconstruction.^[3,4] The conditions may alter ACL graft fixation and graft healing due to limited bone stock.

Before clinical application, it is important to evaluate the plate’s mechanical behavior and performance to prevent complications in the clinical setting.^[5,6] This study focused on the mechanical behavior and performance of the novel HTO 7-plate, which was analyzed using finite element analysis (FEA). The newly developed design, incorporating an Indonesian-manufactured HTO 7-plate, provides a wide area on the anterior aspect of the tibia for ACL reconstruction [Figure 1]. This study aimed to thoroughly evaluate the design’s performance under both tensile (axial) and compressive physiological loading conditions that mimic the forces experienced by the implant during typical activities. The goal of this investigation was to determine whether the innovative HTO 7-plate design effectively distributes mechanical loads throughout its structure without exceeding the critical yield stress of the selected biomaterial, SS316L. The SS 316L was selected as it is a cost-effective and widely available material for orthopedic implants.

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Figure 1:

Design of high tibial osteotomy 7-plate. (a) anterior view, (b) lateral view.

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MATERIALS AND METHODS

A precise three-dimensional model of the novel HTO 7-plate was meticulously developed using advanced computer-aided design software.^[7] This detailed digital model served as the foundation for the subsequent mechanical analysis. The analysis was then conducted rigorously in the SolidWorks simulation module, a widely used FEA software in engineering and biomedical fields for its robust simulation capabilities.^[8]

The material designated for the implant model was 316L stainless steel (SS316L). SS316L is a widely used biomaterial in orthopedic applications due to its excellent biocompatibility, corrosion resistance, and high mechanical properties, particularly its high yield strength of approximately 300 MPa. This yield strength serves as a critical benchmark against which the simulated stresses are compared to ensure the implant’s structural integrity.

Two distinct and critical physiological loading conditions were simulated to comprehensively evaluate the implant’s mechanical response under realistic scenarios. The first scenario is the axial load (tensile). The axial load forces representing tensile loads were applied in both the positive and negative Y-axis directions. These forces were systematically varied in increments of 250 N, 500 N, and 750 N. This simulation aimed to assess the implant’s resistance to pulling or distracting forces that might occur across the osteotomy gap. The second scenario is the compression load. The compression forces representing compressive loads were applied along the same Y-axis and within the identical magnitude range of 250 N, 500 N, and 750 N. This simulation was designed to evaluate the implant’s ability to withstand compressive forces that simulate weight-bearing activities. The simulated forces used in this study were decided based on the predicted clinical setting.

The key output parameters meticulously analyzed from these finite element simulations include maximum displacement (mm), maximum equivalent strain, and maximum von Mises stress (MPa). The maximum displacement (mm) is the parameter that quantifies the maximum deformation or movement observed at any point on the implant under the applied load. Minimal displacement indicates high stiffness and stability.^[7,8] The maximum equivalent strain, also known as the von Mises strain, represents the overall deformation of the material and provides insight into regions where it undergoes the greatest shape change. Maximum von Mises stress (MPa) is a critical parameter that predicts the yielding or failure of ductile materials such as SS316L. It represents a single, equivalent stress value that combines the effects of all principal stresses, providing a comprehensive measure of the stress state within the implant. Comparing this value to the material’s yield strength is essential for assessing mechanical safety.

RESULTS

The FEA provided an extensive, highly detailed dataset on the mechanical response of the HTO 7-plate implant under axial (tensile) and compressive loading conditions. The simulated physiological loads enabled a comprehensive evaluation of key mechanical parameters, including maximum displacement, maximum equivalent strain, and maximum von Mises stress obtained from the finite element models of the HTO 7-plate implant.

The simulation results for axial loading distinctly revealed a progressive increase in von Mises stress with increasing applied force [Table 1]. Specifically, the maximum von Mises stress was 25.2 MPa at 250 N load, increasing to 49.9 MPa at 500 N, and reaching 76.6 MPa at 750 N axial load. It is important to note that these stress values, even at the highest applied load of 750 N, remained significantly below the approximately 300 MPa yield strength of SS316L. Visual representations of the implant’s response under axial (tensile) loading are presented comprehensively in a series of figures. Figure 2a illustrates the displacement patterns, showing how the deformation occurs under an axial load of 750 N. Similarly, Figure 2b depicts the equivalent strain distributions, highlighting regions of material deformation under an axial load of 750 N. The von Mises stress distributions under an axial load of 750 N are clearly illustrated in Figure 2c, providing visual insight into where stresses concentrate within the implant.

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Figure 2:

Axial load (tensile) of implant at 750N (a) displacement, (b) strain equivalent, (c) von Mises stress distribution.

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The analysis of critical stress points derived from the simulation results consistently showed a similar pattern across all three axial load conditions. As vividly depicted in Figure 3, these critical stress points, where the highest stresses are concentrated, were primarily located at the bottom hole of the implant and the two holes in the upper row, specifically at the left and right ends of the plate.

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Figure 3:

Critical point (a) under axial loading (b) under compression load.

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Table 2 presents a comprehensive overview of the maximum displacement, maximum equivalent strain, and maximum von Mises stress values obtained from finite element models of the HTO implant under various compression loading conditions. These values are essential for understanding the implant’s behavior under compressive forces, which are frequently encountered in weight-bearing scenarios, and are crucial for postoperative recovery. Under compressive loading, the maximum von Mises stress in the implant was 6.38 MPa at 250 N, increasing to 12.76 MPa at 500 N, and reaching 19.14 MPa at 750 N. Figure 4 provides detailed visual representations of the implant’s behavior under compressive loads. Figure 4a illustrates the displacement patterns under 750 N of compressive load of 750 N, demonstrating how the implant resists deformation. Figure 4b depicts the equivalent strain distributions under a 750 N compressive load, highlighting areas of localized strain. The von Mises stress distributions and the compression loads are clearly shown in Figure 4c, providing a visual interpretation of stress concentration within the implant under a 750 N compression load.

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Figure 4:

Compression load of implant at 750N (a) displacement, (b) strain equivalent, (c) von Mises stress distribution.

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DISCUSSION

A previous study compared various plate designs for opening-wedge HTO. The study found that the rigid long medial plate (T-plate) showed minimal displacement at the osteotomy site compared with short-spacer type plates.^[9,10] Although a long, rigid T-plate appears to provide the best mechanical stability, it can cause problems with bone space in cases of combined HTO and ACL reconstruction. Therefore, we propose a novel design of a long, rigid HTO-7 plate with a wider bone space at the anterior site. The larger bone stock at the anterior site may possibly improve the stability of ACL graft fixation and graft healing.

The FEA simulation results clearly demonstrate that the novel HTO 7-plate design effectively withstands both tensile (axial) and compressive forces. Notably, even the highest stress values observed – 76.6 MPa under axial tension at a load of 750 N and 19.14 MPa under compression at the same load – are significantly below the approximately 300 MPa yield limit of SS316L material. This substantial safety margin highlights the implant’s ability to resist deformation and failure under expected in vivo forces, consistent with the material’s properties suitable for orthopedic applications. Consistent with the axial load results, these compressive stress values are substantially below the yield strength of SS316L material (approximately ± 300 MPa).

The observed displacement values under compression were very small, further indicating the design’s robustness and its ability to maintain structural integrity with minimal deformation.

The analysis revealed that axial loads induced higher stress in the implant than compressive loads. Axial tension produced higher stresses than compression, with critical stress concentrations observed around the screw holes (particularly, the distal hole and the two proximal-row holes). This pattern is consistent with classical plate biomechanics, in which geometric discontinuities act as stress risers; similar stress localization around fixation holes and at plate ends has been documented in both FEA and in several studies of HTO plates.^[11-13]

As the findings were consistent with previous HTO plates, it is predicted that there will be no difference in the clinical setting regarding rehabilitation protocol after applying the HTO 7-plate. The HTO 7-plate design effectively minimizes loading in the central region while shifting stress peaks to predictable, reinforced areas around the screw holes, thereby aligning with these principles. In addition, compression modeling studies suggest that constructs designed to promote controlled compression across an osteotomy reduce plate stresses.^[9,10] This supports our finding that compression loads are far less demanding than axial tension. The FEA results suggest that the HTO-7 plate design can serve as a mechanically stable platform for medial opening-wedge correction in procedures addressing medial knee osteoarthritis.

Because this study was based on computerized simulations, its findings may differ from actual conditions. It is therefore essential to emphasize that while FEA offers valuable insights into mechanical behavior, further in vitro biomechanical testing and rigorous in vivo clinical trials are necessary to comprehensively confirm the implant’s long-term safety, durability, and functional efficacy in a complex biological environment.

CONCLUSION

The results of the FEA suggest that the novel HTO 7-plate design has adequate mechanical strength and exhibits a favorable stress distribution under both simulated axial and compressive loads. The implant demonstrated consistent structural soundness, with all calculated stress values remaining safely below the material’s yield strength across the simulated physiological force range.