Association of Inflammation, Ectopic Bone Formation, and Sacroiliac Joint Variation in Ossification of the Posterior Longitudinal Ligament

Ossification of the posterior longitudinal ligament (OPLL) is considered a multifactorial condition characterized by ectopic new bone formation in the spinal ligament. Recently, its connections with inflammation as well as sacroiliac (SI) joint ankylosis have been discussed. Nevertheless, whether inflammation, spinal ligament ossification, and SI joint changes are linked in OPLL has never been investigated. In this study, whole-spinal computed tomography and serum high-sensitive C-reactive protein (hs-CRP) levels were obtained in 162 patients with cervical OPLL. Ossification lesions were categorized as plateau and hill shapes. Accordingly, patients were divided into plateau-shaped (51 males and 33 females; mean age: 67.7 years) and hill-shaped (50 males and 28 females; mean age: 67.2 years) groups. SI joint changes were classified into four types and three subtypes, as previously described. Interactions among ossification shapes, hs-CRP levels, and morphological changes in the SI joint were investigated. The plateau shape was more common in the vertebral segments (59.5%), compared to the hill shape, which was predominant in the intervertebral regions (65.4%). Serum hs-CRP levels in the plateau-shaped group (0.11 ± 0.10 mg/dL) were significantly higher than those in the hill-shaped group (0.07 ± 0.08 mg/dL). SI joint intra-articular fusion was the main finding in the plateau-shaped group and showed significantly higher hs-CRP levels compared to the anterior para-articular bridging, which more frequently occurred in the hill-shaped group. Our findings suggested a possible inflammation mechanism that might contribute to the new bone formation in OPLL, particularly the plateau shape.