Impact of BMI, Osteoporosis, and Disc Degeneration on Post-UBE Lumbar Stability: A Finite Element Analysis of Nonlinear Synergistic Effects

Jingbo Ma^1,2Tusheng Li^1,3Rigbat Rozi¹Jiaheng Han^1,4Qiang Jiang¹Hanshuo Zhang¹Xuyan Song¹Guotong Zhao¹YU DING^1,2*

• ¹Sixth Medical Center of PLA General Hospital, Beijing, China
• ²Anhui Medical University, Hefei, China
• ³Beijing Chaoyang Hospital Affiliated to Capital Medical University, Beijing, China
• ⁴South China University of Technology School of Medicine, Guangzhou, China

Objective: This study aimed to quantify the independent and combined biomechanical effects of increased body mass index (BMI), osteoporosis, and disc degeneration on lumbar stability after unilateral biportal endoscopic (UBE) decompression, providing evidence for preoperative risk stratification and postoperative rehabilitation strategies. Methods: A validated 3D finite-element model of L3–S1 was developed from CT data of a healthy 31-year-old male in ANSYS APDL 13.0. UBE decompression was simulated at L4–L5. Four BMI levels (22.86, 26.12, 29.39, 32.65 kg/m²), two bone-quality states (normal vs. osteoporotic), and two degeneration grades (mild vs. severe) were modeled, yielding 24 combinations. Axial compressive loads proportional to body weight (457–653 N) and ±10 N·m pure moments were applied. Outcome metrics included segmental range of motion (ROM), intradiscal pressure (IDP), and facet-joint von Mises stress, validated against published experimental benchmarks. Results: 1.Single-factor effects: Increasing BMI raised IDP by ~9–12% in non-degenerated discs but shifted load posteriorly in degenerated discs, elevating annular and facet stresses. Severe osteoporosis increased vertebral axial displacement by ~55% and facet stress by ~48%, reflecting reduced structural stiffness. Progressive disc degeneration reduced IDP markedly in axial rotation (~70%), followed by flexion (~65%) and lateral bending (~63%), with the smallest decrease in extension (~12%). 2.Synergistic effects: When high BMI coexisted with severe osteoporosis and degeneration, facet-joint stress rose from 1.02 to 2.47 MPa, exceeding additive expectations. Load concentration along the cranio-caudal axis was observed, with annular stress peaking at ≈1.90 MPa in the lower lumbar segments. 3.Pseudo-stability: Severe degeneration with osteoporosis reduced axial-rotation ROM by ~18% (mechanical locking) while internal stresses remained high (≈2.5 MPa), indicating hidden overload despite reduced gross motion. Conclusion: Coexisting high BMI, osteoporosis, and disc degeneration significantly increase posterior-element and disc stresses after UBE. While ROM changes are modest or reduced, internal overload persists, creating a pseudo-stability state. These findings underscore the need for integrated assessment of weight, bone quality, and disc health, and support stress-based evaluations as more reliable indicators of postoperative instability risk than motion alone.