Spontaneous L4/L5 Dislocation Associated With Diffuse Idiopathic Skeletal Hyperostosis in a Young Patient With Severe...

Diffuse idiopathic skeletal hyperostosis (DISH) is characterized by extensive ossification of the spinal ligaments and entheses, resulting in a rigid spine that behaves biomechanically like a long bone. Although spinal fractures and dislocations after minor trauma are well recognized in ankylosed spines, non-traumatic spontaneous instability is rare. We report a rare case of spontaneous L4/L5 dislocation in a young patient with severe obesity and DISH. A 40-year-old man presented with progressive low back pain, bilateral lower extremity weakness, and bladder-bowel dysfunction. He had initially been treated at another hospital for septic shock due to urinary tract infection, but his neurological deficits persisted after improvement of the infection. On transfer to our institution, he was 168 cm tall, weighed 132 kg, and had a body mass index of 46.7 kg/m². Neurological examination showed severe bilateral lower extremity weakness and sensory disturbance below the L1 level. Computed tomography demonstrated extensive thoracolumbar ankylosis consistent with DISH, with the L4/L5 segment representing the only remaining mobile level. An L5 superior articular process fracture, facet joint separation, and spontaneous L4/L5 dislocation were identified. Magnetic resonance imaging showed cauda equina compression at L4/L5. Because the lesion was considered highly unstable, staged circumferential reconstruction was performed. Initial percutaneous posterior fixation from L2 to S1 was carried out in the lateral decubitus position to avoid aggravation of the dislocation that might have occurred in the prone position. Pelvic fixation with S2 alar-iliac screws was then added in a second stage after repositioning the patient prone, because screw insertion was technically difficult in the lateral position. Anterior lumbar interbody fusion at L4/L5 with iliac bone grafting was subsequently performed. At six months postoperatively, low back pain had improved, and local stability was maintained, although severe motor deficits and bladder-bowel dysfunction persisted. In this patient, severe obesity and extensive ankylosis likely concentrated chronic mechanical stress at the L4/L5 segment, which functioned as the last mobile segment. This resulted in progressive failure culminating in spontaneous dislocation. Clinicians should recognize that, even without trauma, the last mobile segment in DISH may fail catastrophically. In such cases, rigid circumferential stabilization with long-segment fixation should be considered.