Fractures in the ankylosed spine are associated with poor bone quality and lower hounsfield units

Study design: Retrospective study Introduction: Patients with ankylosing spinal disorders have a higher risk of fractures, highlighting the need for bone health surveillance. Bone assessment by dual energy x-ray absorptiometry (DXA) is challenging due to abnormal bone formation but measurements by quantitative computed tomography (qCT) have demonstrated higher sensitivity and specificity. However, no studies have analyzed bone quality using qCT in the ankylosed spine population to assess three-column fracture characteristics and subsequent outcomes. Methods: 106 patients with 115 three-column fractures were identified from 1999 to 2020. Patient demographics, Charlson comorbidity index, and injury severity score were extracted. Bone quality measured in Hounsfield units (HU), fracture characteristics, neurologic injury, and mortality were obtained. Results: Most injuries occurred in the thoracic spine (70.4%) following a ground level fall (60.5%). HU adjacent to the fracture (127 HU) was significantly lower than the mobile segments (173 HU) (p < 0.001). Fracture adjacent HU was significantly lower in AS patients compared to DISH (109 vs 150 HU, p = 0.02, respectively) and were lower in fractures that resulted in a non-union or revision surgery (88 vs 137 HU, p = 0.04). Patients with longer fused segments were associated with multilevel and displaced fractures. Conclusions: Fracture adjacent HUs within the autofused segments were significantly lower than in the mobile segments, and longer fusion segments were associated with displaced, multilevel fractures. This study reinforces the importance of assessing patients for decreased HUs as well as better understand how the length of fused segments is associated with displaced, multilevel fractures. Level of evidence: Level III.