A 48-year-old man sustained a cut on his right forearm after falling into a glass door. An emergency doctor performed the clinical examination and sutured the skin. However, on the following day, he was unable to dorsiflex his wrist and was referred to our department. We observed a 5-cm sutured wound on the volar aspect of the right proximal forearm and wrist drop on the affected side, with no sensory disturbance. During surgery, we observed a 50% brachioradialis tear, and complete tear of both the extensor carpi radialis longus and brevis and the posterior interosseous nerve（PIN）. Microscopic neurorrhaphy and myorrhaphy were performed. The patient was able to dorsiflex his wrist normally five months after surgery and extend his fingers after one year. There are only six previous reports of isolated PIN tears due to non-iatrogenic penetrating trauma. PIN tears are usually initially overlooked during primary assessment; therefore, non-specialists have to perform the clinical examination carefully. However, even if a specialist performs the primary assessment, the muscle tear may initially be more conspicuous. This case illustrates the importance of careful neurological evaluation and sufficient anatomical knowledge for predicting the site of traumatic injury.
