Risk of radial nerve injury during deltoid intramuscular injection in shoulder internal rotation and abduction...

Radial nerve injury following intramuscular injection into the upper arm is rare but clinically significant. Although injection-site selection is important, the effect of shoulder position, particularly internal rotation, on radial nerve location remains unclear. Because the radial nerve courses within the triceps brachii and pierces the lateral intermuscular septum close to the humerus, we hypothesized that shoulder abduction combined with internal rotation would decrease the distance from the acromion to the radial nerve relative to the neutral arm-at-side position. In this cadaveric anatomical study, six fresh-frozen upper limbs (n = 6) were examined in four shoulder positions combining 0° or 80° of internal rotation with 0° or 40° of abduction. The high-risk zone was defined as the region where the radial nerve coursed relatively close to the skin surface while running along the surface of the humerus. The proximal border of this zone was located 18.5 ± 1.5 cm below the acromion in the neutral arm-at-side position and shifted proximally to 13.5 ± 1.4 cm with combined abduction and internal rotation. Shoulder position significantly affected the distance from the acromion to the proximal border of the high-risk zone (p < .001). This proximal shift narrowed the anatomical safety margin between the acromion and the radial nerve. It may therefore increase the risk of radial nerve injury during deltoid intramuscular injection in the hand-on-ipsilateral-hip posture.