Age-related changes in biomechanical characteristics of the spine related to sagittal alignments: finite element model analysis

PURPOSE: Age-related changes in spine-pelvis sagittal alignment significantly impact spinal degeneration, yet their biomechanical correlations remain poorly understood. This study investigated whether spinal sagittal alignment exhibits distinct mechanical characteristics across different age groups. METHODS: We developed parametric finite element models for five age groups (40-60 s, 60-65 s, 65-70 s, 70-75 s, and 75 over years) using radiological data from 625 adult volunteers. Models simulated upright posture based on trunk mass distribution. RESULTS: Overall sagittal range of motion (ROM) of the spine remained < 5° across all ages, transitioning from extension to flexion after 65 years. Vertebral ROM alternated between flexion (-1.46°) and extension (4.02°). Distal lumbar segments shifted from extension to flexion after 60, while pelvic rotation increased rearward with age. The trend in maximum matrix stress of the annulus fibrosus paralleled that of IDPs (0.09-0.31 MPa), while maximal fiber stress within the annulus fibrosus consistently increased with age across the five age models. CONCLUSION: Our study reveals that after 65, the spine's overall sagittal ROM shifts from extension to flexion with age: age-related increases in IDPs, annulus matrix, and fiber stress lead distal lumbar rotation to transition from extension to flexion after 60 (initiating lumbar lordosis loss), while thoracic and pelvic rearward extension/rotation later sustain sagittal balance. Excluding age-related degenerative parameters may slightly underestimate inter-group stress differences, but does not alter the observed alignment-biomechanics trend, given the focus on sagittal alignment findings, enhancing understanding of age-related spinal biomechanics, and informing clinical management of degenerative spine conditions.