To describe the effectiveness of a combined surgical approach—humeral rotational osteotomy and lower trapezius transfer—in improving shoulder function in an adult patient with long-standing obstetric brachial plexus palsy.

The case of a 27-year-old male with sequelae of Erb’s palsy (Narakas type II), initially treated in infancy with nerve reconstruction and subsequently lost to follow-up is presented. At re-evaluation, the patient exhibited severe limitation of shoulder external rotation and abduction. Following multidisciplinary assessment, he underwent external rotational osteotomy of the humerus combined with lower trapezius transfer using a bone-to-bone fixation technique. Clinical and radiographic follow-up was performed up to 12 months postoperatively.

The postoperative course was uneventful, with no complications. At 6 months, the patient was able to bring his hand to his mouth and demonstrated recovery of active abduction. At 12 months, he achieved 90 ° of abduction and 90 ° of external rotation, with significant functional improvement (Mallet score improved to III–IV) and the ability to perform movements such as placing the hand behind the head without compensatory mechanisms.

The combination of humeral rotational osteotomy and lower trapezius transfer represents an effective surgical option for adult patients with long-standing obstetric brachial plexus palsy. This approach improves both passive alignment and active shoulder function, enabling meaningful functional recovery even decades after the initial injury. Further studies with larger cohorts are needed to validate these findings.