Ultrasonographic examination of the safe zone for medial needle approach towards the median-innervated flexor digitorum profundus muscle: Effect of changes in position of the forearm.

The flexor digitorum profundus (FDP) is a forearm flexor muscle. Certain cases require the needle to be inserted accurately and safely into the deep, lateral portion of the FDP, which is innervated by the anterior interosseous nerve. In this study, we compared 2 techniques for approaching the median-innervated FDP (MFDP) medially, each according to the position of the forearm, supinated or pronated. The forearms of healthy volunteers without any musculoskeletal problems of the upper extremities were examined. Using high-resolution ultrasonography, the medial aspects of the forearms were scanned with elbows flexed at 90°. Using the images obtained, several parameters for distance and angle were measured in 2 different positions: forearm-supinated and forearm-pronated. Thirty-seven forearms from the volunteers were subject to examination. The angle α, which is the valid angle of insertion when approaching with the needle towards the deeply located MFDP, slightly increased from 22.89° to 23.41° when the forearm was pronated from the supinated position; however, this increase was not statistically significant. In contrast, the angle β, which is the safe angle of insertion when approaching with the needle towards the MFDP without contacting the ulnar nerve, was significantly increased from 41.40° to 46.80° upon forearm pronation. Because the safe angle of insertion of the needle medially into the MFDP increases with forearm pronation, the forearm-pronated position is recommended, instead of the forearm-supinated position, when inserting a needle into the MFDP in the medial aspect of the forearm.