Ultrasonographic evaluation of safer deltoid intramuscular injection sites to reduce the risk of SIRVA

Intramuscular injection into the deltoid muscle has increased in Japan following coronavirus disease 2019 (COVID-19) vaccination programs and the introduction of vaccines such as Shingrix™. Limited experience with this technique may increase the risk of shoulder injury related to vaccine administration (SIRVA). Landmark-based techniques have not been sufficiently validated using ultrasonography. Thirty-four healthy volunteers were enrolled. Using ultrasound, we measured the distances and depths from the conventional injection site-defined as the intersection of a vertical line from the acromion and the axillary horizontal line (AHL)-to the axillary nerve, radial nerve, and subdeltoid bursa. The axillary nerve/posterior circumflex humeral artery was located 1.8 ± 1.2 cm from the AHL at a depth of 2.6 ± 1.2 cm. The inferior margin of the subdeltoid bursa was located - 0.3 ± 1.2 cm at a depth of 2.5 ± 0.4 cm. The radial nerve was located 9.4 ± 2.0 cm caudally. There were no differences between left and right. An injection site located 3.0-3.5 cm below AHL, with a needle depth < 1.7 cm, may represent a safer approach for injection. These findings provide anatomical insights that may help optimize injection techniques and potentially reduce the risk of SIRVA. Clinical trial registration: UMIN000048777.