Appropriate needle penetration depth for intramuscular injections depends on the injection site, recipient arm position, and injection technique: A randomized crossover study

Objective: Various medications, such as COVID-19 vaccines, are administered via intramuscular (IM) injection into the deltoid muscle. Although the depth of needle insertion into the deltoid muscle may vary depending on the injection site, recipient arm position, and the injection technique that bundles or stretches the tissue, no research has validated these observations. Therefore, this study aimed to investigate the effects of different injection sites, arm positions, and injection techniques on subcutaneous tissue, muscle thickness, and skin-to-bone depth in order to enhance the safety of IM injection practices. Methods: Thirty healthy young adults were enrolled in this randomized crossover study. An ultrasound device evaluated the effects of two injection sites, two recipient arm positions, and three injection techniques on subcutaneous tissue thickness, muscle thickness, and skin-to-bone depth. Results: The subcutaneous tissue exhibited increased thickness at the injection site distal to the deltoid muscle. In addition, muscle thickness was greater on the distal side of the deltoid when the recipient's arm was extended, while it was greater on the proximal side when the arm flexed. The injection technique involving muscle bunching exclusively thickened the muscle layer without changing subcutaneous tissue thickness and simultaneously increased the depth from the skin to the bone. Conclusions: The safe insertion depth of the needle for IM injection into the deltoid muscle varied depending on the injection site, recipient arm position, and the injection technique employed. Trial Registration: UMIN-CTR. Clinical Trial: UMIN000048289.