Changes in facet orientation may simultaneously affect the potential risk of annulus tears and zygapophyseal joint osteoarthritis: a clinical review and biomechanical simulation.

Background: The lumbar motion segment, comprising the intervertebral disc and bilateral articular processes, functions as a cohesive unit for load transmission. Morphological alterations in these components can influence local stress distribution, potentially contributing to the development of degenerative changes. Research indicates that sagittally oriented articular processes may reduce the incidence of severe zygapophyseal joint osteoarthritis (ZJOA) by lowering the load transmission ratio. However, this alignment may also heighten the risk of annular tears by intensifying stress concentration.

Methods: A retrospective analysis of clinical data from 111 patients was conducted to test this hypothesis. Facet orientations and ZJOA grades were assessed using MRI and CT imaging, while annular integrity was evaluated with diffusion tensor fibre tractography. Regression analyses identified independent predictors for ZJOA severity and annular tears. Furthermore, lumbo-sacral numerical models with varying facet orientations in the L45 motion segment were developed to compute stress distributions on the annulus and facet cartilage, elucidating the biomechanical mechanisms behind the observed clinical phenomena.

Results: Consistent with existing literature, sagittally oriented articular processes were identified as an independent predictor of severe ZJOA. In contrast, coronally oriented processes were associated with a higher incidence of mild ZJOA and a greater likelihood of annular tears. Numerical simulations demonstrated that coronally oriented articular processes shift load transmission anteriorly, thereby increasing stress on the annulus while reducing stress on the facet cartilage, and vice versa.

Conclusions: Facet orientation influences local load transmission. Coronally oriented articular processes may reduce ZJOA severity while increasing the risk of annular tears by redistributing load anteriorly. Conversely, sagittally oriented articular processes likely exert the opposite effect on both ZJOA progression and annular integrity, driven by the same biomechanical principles.