Severe Unilateral Iliopsoas Atrophy After Dysplastic Hip Arthroplasty Leading to Recurrent L4/L5 Disc Degeneration: A Report of a Biomechanical Case.

The coordinated function of the hip, pelvis and lumbar spine is essential for maintaining mechanical stability, and the iliopsoas muscle contributes both to primary hip flexion and anterior lumbar stabilisation. Dysfunction of this muscle after dysplastic total hip arthroplasty (THA) may significantly alter load transmission and predispose to lumbar degeneration. We present the case of a 45-year-old male patient with a history of Perthes disease who underwent dysplastic THA complicated by peroneal nerve palsy, persistent gait asymmetry and pelvic imbalance. Several years later, he developed right-sided L4/L5 radiculopathy treated with full-endoscopic transforaminal discectomy, with only temporary improvement. Quantitative MRI demonstrated marked unilateral iliopsoas atrophy (57-85% CSA reduction), mild multifidus asymmetry and gluteus medius/minimus atrophy. A persistent right-sided L4/L5 foraminal/lateral disc protrusion correlated with recurrent symptoms. The combination of severe iliopsoas degeneration, hip abductor insufficiency and compensatory overload of the multifidus created a biomechanically vulnerable environment in which loss of anterior stabilisation increased shear forces at L4/L5, contributing to recurrent disc failure. This case underscores a muscular mechanism within the hip-spine relationship rarely described in the literature and highlights the importance of assessing psoas and gluteal musculature in patients presenting with lumbar symptoms following THA.