Study Design: Retrospective cohort study.
Objective: Mono-segmental thoracic and lumbar nonfusion (MTLN) can occur during spinal fusion in patients with ankylosing spondylitis (AS). This study aimed to summarize the clinical characteristics of AS-MTLNs, identify correlative variables for their occurrence, and propose a new classification system.
Methods: We retrospectively reviewed the clinical materials of patients with AS diagnosed between 2008 and 2023 in our institution, and 803 patients were included. The patients were divided into an AS-MTLN group (n = 155, 19.3%) and a non-AS-MTLN group (n = 648, 80.7%). Fifteen variables were collected, and the differences between groups were compared. The proposed new classification system included three main types and seven subtypes, including noninflammatory nonfusion (Type 1), inflammatory nonfusion without intervertebral destruction (Type 2), and inflammatory nonfusion with intervertebral destruction (Type 3).
Results: The prevalence of AS-MTLNs in descending order was lower thoracic spine (n = 84, 54.2%), lumbar spine (n = 62, 40.0%), and upper thoracic spine (n = 9, 5.8%). The computed tomography (CT) Hounsfield unit (HU) of the L1 vertebra was the only variable that showed a significant difference between two groups (p = 0.007), and the binary logistic regression model further confirmed its correlation with the occurrence of AS-MTLN (p = 0.005, odds ratio = 0.993, 95%). Regarding the new classification system, the AS-MTLN numbers of three types were (1) Type 1: 53 cases, 34.2%; (2) Type 2: 31 cases, 20%; (3) Type 3: 71, 45.8%. Type 1 was more common in lumbar spine (33 cases, 62.3%), Type 2 was more common in lower thoracic spine (20 cases, 64.5%), and Type 3 was also more common in lower thoracic spine (52 cases, 73.2%).
Conclusion: This study systematically described the clinical characteristics of AS-MTLNs and recognized that the CT HU of the L1 vertebra was correlated with the occurrence of AS-MTLN. The newly proposed classification system includes all types of AS-MTLNs, with value for clinical applications and popularization.
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