Finite element analysis of short-segment fixation combined with expandable polyetheretherketone vertebral body replacement in osteoporotic vertebrae.

backgroundCombined anterior-posterior fixation offers significant advantages in reconstructing spinal stability, correcting kyphotic deformities, and achieving neural decompression. However, osteoporosis increases the surgical risks associated with this technique, and relevant studies remain limited. This study aimed to analyze the biomechanical characteristics of posterior short-segment fixation combined with expandable polyetheretherketone (PEEK) vertebral body replacement (VBR) in osteoporotic spines, with the goal of optimizing surgical strategies and material selection.

methodsFinite element analysis (FEA) was conducted to evaluate the biomechanical performance of T12 vertebral body replacement. CT images of the T10-L2 segment were imported into Geomagic Warp to construct a digital model. Four fixation models were developed using SolidWorks based on different posterior fixation methods and VBR materials. M1: Intermediate bilateral screws combined with titanium alloy VBR. M2: Intermediate bilateral screws combined with PEEK VBR. M3: Cement-augmented screws spanning the fractured vertebra combined with PEEK VBR. M4: Intermediate bilateral screws and cement-augmented screws combined with PEEK VBR. The models were imported into ANSYS Workbench to evaluate the range of motion (ROM), as well as the maximum von Mises stress and strain in the fixation system and adjacent endplates under four loading conditions (flexion, extension, lateral bending, and axial rotation).

results① Compared with M1, the ROM values of M2 were higher under all four motion conditions. The largest ROM occurred during left rotation, with values of 0.83° for M1 and 0.86° for M2. The von Mises stress on the VBR and adjacent segment endplates were lower in M2 than in M1, with all values remaining below the material fatigue thresholds. ② Among the M2, M3, and M4, the M4 exhibited the smallest ROM under all four motion conditions, while M2 showed the largest ROM during extension, left bending, and left rotation. However, the ROM differences among these three models did not exceed 0.1°. The internal fixation system in M4 demonstrated the lowest stress and strain values. The maximum von Mises stress on pedicle screws in M3 reached 97.9 MPa during left bending, which was 62.3% higher than that in M4. Nonetheless, the stress and strain levels in all fixation systems remained below the fatigue limits of their respective materials, and the differences in VBR and endplates stress among the three models were all within 5.0%.

conclusionThis study demonstrates that in osteoporotic finite element models, the biomechanical performance of PEEK VBR is comparable to that of titanium alloy VBR and may be superior in reducing endplate stress and mitigating the risk of implant subsidence. Posterior short-segment fixation combined with PEEK VBR provides sufficient biomechanical stability. Among various configurations, intermediate bilateral screws and cement-augmented screws offer the best biomechanical performance. In contrast, cement-augmented screws spanning the fractured vertebra may increase the risk of screw fatigue failure. Intermediate bilateral screws without cement augmentation may serve as a viable alternative in selected cases.