Background <p>Anterior hip dislocation after internal fixation of a proximal femoral fracture is extremely rare. Previously reported cases have mainly involved trochanteric fractures treated with a dynamic hip screw or sliding hip screw [1–3]. To our knowledge, atraumatic anterior hip dislocation after cephalomedullary nailing for an intertrochanteric femoral fracture has not been previously reported.</p> Case presentation <p>An 87-year-old woman sustained a left intertrochanteric femoral fracture (AO/OTA 31-A3) after a fall in her residential facility. Before injury, she ambulated with a walker. Her medical history included osteoporosis and vertebral compression fractures. The Beighton score was 1/9. The fracture was treated with a long TFNA nail (130°, 320&#xa0;mm × 10&#xa0;mm) with 2.5&#xa0;cc of cement augmentation. Weight-bearing training was started on the first postoperative day, and she was transferred to another facility 1 month after surgery. Four months after surgery, she was found to have an anterior hip dislocation without any definite traumatic episode. Closed reduction under general anesthesia was unsuccessful, and conversion to total hip arthroplasty was subsequently performed 1 week later.</p> Results <p>Femoral anteversion was assessed on computed tomography using the posterior condylar axis of the knee as the reference and was approximately 31°. Retrospective computed tomography review demonstrated progressive telescoping of 8.96&#xa0;mm, varus collapse, and lack of medial cortical healing at the intertrochanteric fracture site. These findings suggested persistent mechanical instability at the fracture site, with secondary loss of offset and periarticular soft-tissue tension around the hip. Total hip arthroplasty was performed using the AMIS-K anterior approach. A 32-mm ceramic head on highly cross-linked polyethylene was used, and cementation was performed with Simplex bone cement (Stryker). No redislocation or other complication was observed during 1 year of follow-up.</p> Conclusions <p>This case highlights not only the rarity of anterior hip dislocation after cephalomedullary nailing for an intertrochanteric fracture, but also the importance of targeting positive medial cortical support rather than purely anatomical reduction at the index surgery, and preventing excessive postoperative telescoping. Residual malreduction, insufficient medial support, and progressive shortening may result in an atypical failure pattern, including hip instability, rather than more familiar complications such as cut-out or nonunion.</p>