Subscapularis muscle enervation: the effect of arm position.

Extensive subscapularis muscle release is common in major shoulder reconstructive surgery, and manipulation of the muscle's anterior surface may damage its enervation. The subscapularis nerves and their points of entry into the muscle have rarely been described. The objective of this study was to define these points of entry into the subscapularis muscle and the possible modifications that may occur in such points according to the common positions in which the arm is placed during shoulder surgery, that is, internal, neutral, and external rotation. Twenty-five cadaver's shoulders were studied by an extensive anterior approach. The nerves were dissected and marked at their points of entry into the muscle. Measurements were taken of the distances of these nerves in the three previously mentioned arm positions. The points of entry of the subscapularis nerves into the muscle may be very close to the surgical field (as close as 1 cm from the glenoid border). Therefore extensive release of the anterior surface of the subscapularis muscle may cause damage to the subscapularis nerves. The risk can be potentially increased by the position of the arm, especially for the upper nerve when the arm is in external rotation.