Sagittal-oriented articular processes and intervertebral disc degeneration phenotypes can concurrently lead to an increased presence of annulus tears in the cranial adjacent motion segment : a clinical review and biomechanical simulation.

Aims: Intervertebral disc degeneration (IDD) and sagittal-oriented articular processes can restrict motility and increase stiffness of the motion segment, potentially causing compensatory stress and higher motility in adjacent segments. It is unclear if these factors trigger IDD progression in adjacent segments. This study aimed to elucidate this using functional MRI, and identify biomechanical mechanisms with a validated numerical model. Methods: Clinical data from 95 patients were retrospectively collected from January 2022 to April 2023. Disc collapse and fibrosis were assessed by disc height ratio and fractional anisotropy (FA) values in the L4-L5 segment. The orientation of articular facet processes in L4-L5 was examined. Annulus fibrosus integrity was investigated using diffusion tensor fibre tractography in cranial (L3-L4) and caudal (L5-S1) segments. Statistical analyses determined differences between patients with and without annulus tears, and regression analyses identified predictors of annulus tears. Numerical models of L3-S1 motion segment were developed, incorporating variations in disc collapse, fibrosis, and facet orientation angles in L4-L5. Stress distribution on cranial and caudal discs was calculated under various loading conditions. Results: Compared to patients with intact annulus at the cranial segment (L3-L4), those with annulus tears show reduced facet orientation angles and disc height ratios, and elevated FA values. These parameters are independent risk factors for cranial annulus tears, not observed on the caudal side. Models with sagittal-oriented articular processes (facet orientation = 35°), disc collapse, and fibrosis show higher stress on the cranial disc, particularly within the annulus, compared to models with coronal-oriented processes (facet orientation = 65°) and healthy discs. Conclusion: Sagittal orientation of articular processes and IDD phenotypes may increase the risk of annulus tears in the cranial adjacent segment by compromising the biomechanical environment. This offers a novel perspective for understanding biomechanical interactions in adjacent segments during IDD progression.