Biomechanical Effects of Osteoporosis on the Sacroiliac Joint After Lumbosacral Fusion Surgery

Objective: Lumbar/lumbosacral fusion is a common surgical approach for treating lumbar degenerative disc disease, a condition that disproportionately affects a large portion of elderly population. Osteoporosis, another age-related disease, is also prevalent among the elderly. Previous studies have elucidated the biomechanical effects of osteoporosis on the postoperative lumbar spine. However, little attention has been paid to adjacent sacroiliac joints (SIJs), and it remains unclear whether or how osteoporosis impacts the SIJ biomechanics. The aim of this study was to determine the effects of osteoporosis on the range of motion (ROM) and stress distribution within the SIJs following lumbosacral fusion, and to identify potential implications for postoperative outcomes. Methods: The previously validated, normal human lumbo-pelvic finite-element model was used to simulate the interbody fusion procedures at L5–S1, and an osteoporotic condition was simulated by reducing bone mechanical properties in the model. The ROM and stress distribution within the left and right SIJs were compared between the surgical models with and without osteoporosis in flexion, extension, lateral bending, and axial rotation motions. Results: The presence of osteoporosis led to a significant increase in SIJ ROM: 38.9% and 36.5% in flexion, 19.5% and 15.9% in extension, 33.3% and 40.0% in lateral bending, and 26.38% and 20.0% in axial rotation, respectively, for left and right SIJs. Moreover, higher stress concentrations were observed at the SIJs in the osteoporotic model. Conclusion: Osteoporosis exacerbates the SIJ motion and stress after lumbosacral fusion, indicating a potential increased risk of SIJ instability and degeneration among affected patients. The findings underscore the importance of considering osteoporosis in preoperative planning and postoperative management strategies for lumbosacral fusion procedures.