Which nerve injury is indicated by weakness of finger extensors while triceps and supinator function normally?

The scenario described points to an injury of the posterior interosseous nerve. This particular nerve is a branch of the radial nerve and is responsible for innervating the extensor muscles in the forearm, which control finger extension.

In this case, the weakness of finger extensors indicates that the neural supply to these specific muscles has been compromised. However, the fact that triceps and supinator function normally suggests that the main trunk of the radial nerve is intact, as these muscles are innervated directly by the radial nerve before it branches into the posterior interosseous nerve.

This differentiation is important; if there were a complete radial nerve injury, one would expect both the triceps and the finger extensors to be affected, leading to more severe motor deficits. Instead, the preservation of triceps and supinator function while experiencing deficits in finger extension strongly supports an isolated injury to the posterior interosseous nerve. This injury typically leads to a clinical condition known as "wrist drop," characterized by an inability to extend the wrist and fingers, while still maintaining elbow extension and forearm supination.