Radiologic and Pathological Investigation of Pseudarthrosis in Ankylosing Spondylitis: Distinguishing Between Inflammatory and Traumatic Etiology

Objective. To investigate the pathogenesis of pseudarthrosis in ankylosing spondylitis (AS) based on the pathological analysis of specimens harvested during surgery. Methods. Radiographic and clinical data for 17 consecutive AS patients with pseudarthrosis were retrospectively analyzed. Meanwhile, the pathological analysis of specimens obtained during surgery was also performed. Results. In total, 18 extensive Andersson lesions were included. Pseudarthrosis located at the apical region were noted in 12 patients. Complete ossified anterior longitudinal ligaments above or below pseudarthrosis and fracture through posterior elements or facet joints were observed in 7 and 6 lesions, respectively. The most definitive pathological characteristic in all cases was proliferating hypovascular edematous fibrous tissue involving disc, bone-disc border, and vertebral body. Fibrinoid necrosis, necrotic bone fragments, hemosiderin deposits, and active subchondral osteogenesis were found, indicating trauma process. Mild perivascular collections of inflammatory cells were detected in only 2 cases. Conclusion. AS-related pseudarthrosis is more likely to originate from mechanical trauma than inflammation. The above-mentioned radiological and histological findings showed that multiple mechanisms lead to the formation of pseudarthrosis. These mechanisms include excessive stress, insufficiency fracture, and an acute fracture involving a 3-column structure.