Sacroiliac Joint Variation in Patients With Ossification of the Posterior Longitudinal Ligament

Study Design: Retrospective Cohort Study. Objectives: Ossification of the posterior longitudinal ligament (OPLL) reveals heterotopic ossification in the spinal ligament. OPLL also tends to ossify ligaments and entheses throughout the body. However, hallmarks of sacroiliac (SI) joint ossification and its variation in OPLL have not been clarified. Here, we investigated the morphological changes in SI joints in individuals with and without OPLL. Methods: We included 240 age- and sex-matched patients (OPLL+, 120; OPLL−, 120) in the study. SI joint variations were classified into 4 types: Type 1, normal or small peripheral bone irregularity; Type 2, subchondral bone sclerosis and osteophyte formation; Type 3, vacuum phenomenon; and Type 4, bridging osteophyte and bony fusion. Type 4 was further divided into 3 subgroups as previously described. Interactions between the ossified spinal region in OPLL and morphological changes in the SI joint were evaluated. Results: SI joint ankylosis occurs more frequently in patients with OPLL (51.7%) than in those without (non-OPLL) (33.3%). The SI joint vacuum phenomenon (49.2%) was the main finding in non-OPLL. SI joint ankylosis in OPLL was characterized by anterior bridging and intra-articular fusion. OPLL patients with multilevel ossification tend to develop degeneration and ankylosis of the SI joints. Conclusions: OPLL conferred a high risk of SI joint ossification compared with non-OPLL, and patients with extensive ossification had a higher rate of SI joint ankylosis. Understanding SI joint variation could help elucidate OPLL etiology and clarify the phenotypic differences in the SI joint between OPLL and other spinal disorders.