Incidental posterior rib hyperostosis on chest CT: incidence and etiology

Objective: Incidental rib hyperostosis is defined as asymmetric cortical thickening and sclerosis of the medial posterior ribs and is important because it may lead to unnecessary laboratory testing, additional imaging and occasionally biopsy. The purpose of this study is to identify the incidence of rib hyperostosis within different patient groups known to have an increased propensity towards osteophyte formation and ankylosis.

Methods: This study was a retrospective cohort study reviewing chest CT examinations in a control group of patients over 50 years old and three different patient populations: psoriatic arthritis, ankylosing spondylitis, and diffuse idiopathic skeletal hyperostosis (DISH). Each CT was evaluated by an attending musculoskeletal and cardiothoracic radiologist to identify rib hyperostosis, osteophyte formation, ankylosis, and spinal curvature. Two senior radiology residents also performed consensus reads and inter-reader reliability was calculated between the two groups.

Results: Two hundred eighty-two individuals were included in the study of which 38 (13.5%) had at least one hyperostotic rib. The ankylosing spondylitis population and the DISH population had the highest incidence of rib hyperostosis with a relative risk of 5.6 (p = 0.012) and 5.3 (p = < 0.001) when compared to the control group. There was good inter-reader reliability for the presence of rib hyperostosis with a kappa estimate of 0.739.

Conclusion: Incidentally detected rib hyperostosis is most likely the sequela of abnormal stress on the ribs secondary to rib hypomobility from fusion at the costovertebral joint. The incidence of rib hyperostosis is markedly increased in the DISH and ankylosis spondylitis study populations.