Lumbar ossification of the posterior longitudinal ligament as a distinct phenotype of diffuse spinal ligament ossification¹.

Abstract

Lumbar ossification of the posterior longitudinal ligament (L-OPLL) has been underrecognized and remains poorly characterized clinically. We hypothesized that L-OPLL constitutes a distinct phenotype within the broader OPLL spectrum, sharing features of obesity and diffuse spinal ligament ossification. To evaluate the clinical and radiographic features of L-OPLL and assess their relationship with diffuse spinal ligament ossification and obesity-related factors. Cross-sectional study with a replication cohort. A total of 186 patients with OPLL were diagnosed using whole-spine computed tomography (CT) at a regional spine center in Japan (2007-2024). Additionally, 75 asymptomatic individuals with OPLL from a population-based health screening cohort comprised the replication cohort. Patient background, including BMI, was assessed. Spinal ligament ossification was evaluated using whole-spine CT. The severity of ossification was scored for four ligaments-OPLL, OALL (ossification of the anterior longitudinal ligament), OLF (ossification of the ligamentum flavum), and ossification of the supra/interspinous ligament-and summed to define the ossification index (OS index). Regional scores from the cervical, thoracic, and lumbar spine were combined to calculate the total index. In the primary analysis, patients were classified into L-OPLL and non-L-OPLL groups, and their clinical and radiographic features were compared. Multiple linear regression analysis was used to assess the independent association between L-OPLL and OS index. In the secondary analysis, patients were classified into three groups: localized cervical OPLL (C-OPLL), thoracic OPLL (T-OPLL), and L-OPLL groups, and comparisons were made between the localized C-OPLL group and the T- and L-OPLL groups. The L-OPLL group had a significantly higher BMI (median 27.5 vs. 26.0 kg/m², p=0.003) and greater prevalence of obesity than the non-L-OPLL group, along with significantly elevated thoracic OPLL and OLF indices. Multiple linear regression analysis confirmed that L-OPLL was independently associated with a higher OS index (regression coefficient: 0.448, 95% confidence interval: 0.162 to 0.735, p=0.002). The L-OPLL group also exhibited significantly higher BMI and OS index than the localized C-OPLL group, primarily driven by increased thoracic and lumbar OPLL and OLF. The replication cohort results were consistent with an association between L-OPLL, obesity, and diffuse ligament ossification. L-OPLL is rarely an isolated lumbar lesion; instead, it commonly coexists with extensive spinal ligament ossification and marked obesity. Its distinct clinical and radiographic features support classification as a separate entity within the broader OPLL spectrum.