The authors report an isolated case of right sacrospinous ligament ossification causing sciatic pain because of compression of the proximal sciatic nerve. The initial conservative treatment of injections in the conflict zone and the intervertebral joints was insufficient. Surgical exploration was undertaken via a posterior approach, demonstrating the conflict between the nerve and the ossified ligament. Resection of the ossified ligament eliminated the symptoms at the cost of transitory buttock hypesthesia. The anatomic and pathologic exam identified simple osseous metaplasia. Lumbar-pelvic coxometry analysis showed sagittalization of the acetabular roofs, excessive bilateral femoral torsion, and a step-off attenuation at the anterior cervicocephalic junction (low anterior cervico-cephalic femoral offset). In addition, reduced lumbar lordosis and accentuated sacral slope were noted, factors that could be related to modifications in the static balance of the lumbar-pelvic complex. This imbalance could be correlated to an increase in the forces applied to the pelvic ligament layers. The etiological hypothesis retained was that this osseous metaplasia was a reaction to excessive stresses on the sacrospinous ligament.