Chance-Type Fracture of the Sacrum in an Elderly Patient: A Case Report.

Chance-type fractures involving the sacrum have not been previously reported to the best of our knowledge. We describe a rare case in a 79-year-old man who fell into deep snow, the pelvis became effectively fixed, and the trunk flexed forward, creating a seat-belt-like flexion-distraction mechanism, resulting in low back and left leg pain. Initial plain radiographs were normal, and the patient was managed conservatively for a presumed sacral fracture. Progressive bilateral buttock and leg pain prompted magnetic resonance imaging (MRI) at four weeks, which demonstrated marrow edema in S1, fracture of the L5 spinous process, and posterior ligamentous complex (PLC) disruption, although the PLC injury was initially unrecognized. Persistent pain led to computed tomography (CT) at 18 weeks, revealing a horizontal fracture across L5-S1 with nonunion, clarifying the diagnosis and demonstrating mechanical instability. Because nonunion and PLC rupture predicted failure of further conservative care, we performed posterior fixation from L4 to S2 combined with vertebroplasty of S1. The procedure immediately relieved pain; CT confirmed solid union at one year, and the patient regained independent ambulation and full activities of daily living at two years. Although the sacrum comprises fused sacral vertebrae, flexion forces can concentrate at S1 owing to L5-S1 mobility, permitting Chance-type injury. Such fractures are likely underdiagnosed when assessment relies only on plain radiographs. This diagnostic sequence-normal radiographs, abnormal MRI, and confirmatory CT, highlights the importance of early cross-sectional imaging. In elderly patients with suspected sacral fractures, early MRI to assess PLC integrity and timely stabilization when instability is present should be considered.