What is the loss of function associated with a posterior interosseous nerve injury?

The loss of function associated with a posterior interosseous nerve injury primarily results in a loss of extension. The posterior interosseous nerve, which is a branch of the radial nerve, innervates the extensor muscles located in the forearm. These muscles are responsible for extending the wrist and fingers.

When this nerve is injured, the motor function of these extensors is compromised, leading to difficulty or inability to extend the wrist and fingers properly. This presents as a characteristic wrist drop, where the individual is unable to maintain extension of the wrist and fingers, thus affecting their ability to perform activities that require grip and extension strength.

Understanding this specific motor loss helps clarify why the other options are not associated with a posterior interosseous nerve injury. For example, forearm supination primarily involves the biceps brachii and supinator muscles, which are not innervated by the posterior interosseous nerve. Similarly, thumb opposition is facilitated by the opponens pollicis, and wrist flexion primarily involves the flexor muscles, which are not affected by an injury to the posterior interosseous nerve.