Cervical facet joint angle and deep extensor degeneration are correlated with laterally deviated type ossification of the posterior longitudinal ligament

This study aimed to determine if the characteristics of the facet joints, muscle morphology, and segmental range of motion (ROM) are correlated with the ossification pattern of the posterior longitudinal ligament (OPLL). According to the axial ossified pattern of OPLL, patients were classified into central type group and lateral deviated type group. The sides in the lateral deviated type group were categorized according to their status as the OPLL side or the non-OPLL side. Segmental ROM, muscle degeneration (bilateral deep extensors DE, deep flexors DF, superficial flexors SF), and facet joint characteristics (facet joint angle FA and cervical facet joint degeneration CFD) were compared between the central type group and the lateral deviated type group. Furthermore, these parameters were also compared between the bilateral side within each group. A total of 326 consecutive OPLL patients were enrolled in this study, and a final cohort of 208 patients was analyzed. The adjusted functional cross-sectional area (afCSA) of DE was significantly smaller in the central type group compared to the laterally deviated type group. In the lateral deviated type group, the FA, as well as the adjusted cross-sectional area (aCSA), afCSA, and fat infiltration ratio (FI%) of the DE, were significantly smaller on the OPLL side compared to the non-OPLL side. The present study is the first to assess the association between facet joint characteristics, muscle morphology, segmental ROM and axial ossified pattern of OPLL. In the laterally deviated type group, the OPLL side exhibited significantly smaller values in FA, as well as the aCSA, afCSA, and FI% of the DE compared to the non-OPLL side, suggesting a potential asymmetric biomechanical load underlying disease progression.