A 35-year-old woman presents with progressive weakness of wrist and finger extension over 3 weeks, following a mid-shaft humeral fracture treated conservatively 4 weeks ago. Examination shows wrist drop and loss of finger extension, with intact sensation over the dorsal first web space. Radiographs show callus formation at the fracture site. EMG shows denervation potentials in extensor carpi radialis longus and extensor digitorum communis, with no motor units recruited on volitional effort. What is the most appropriate next step in management?