Anatomically safe sites for intramuscular injections: a cross-sectional study on young adults and cadavers with a focus on the thigh

The anatomical safety of intramuscular injections at the deltoid and ventrogluteal sites has been investigated; however, the anatomical relationship between intramuscular injection sites in the thigh and major blood vessels and nerves remains unclear. We aimed to compare intramuscular injection sites in the rectus femoris and vastus lateralis with those at the deltoid and ventrogluteal sites and identify safe intramuscular injection sites in the thigh. Twenty-seven young adult volunteers were recruited, and the thicknesses of subcutaneous tissue and muscle as well as the number of blood vessels present were evaluated at two sites on the deltoid, ventrogluteal, and thigh using ultrasound equipment. The right thighs of 24 cadavers were used, and the thickness of muscle, number of blood vessels or nerves present, and the distance between each examined site and major blood vessels or nerves were evaluated in the rectus femoris and vastus lateralis. A major blood vessel was observed in the middle of the rectus femoris in young adults. In cadavers, the descending branch of the lateral circumflex femoral artery and muscle branch of the femoral nerves to the vastus lateralis were observed at the middle point, distal two-thirds point, and middle point between the middle and distal two-thirds points of the rectus femoris, but not at the middle of the vastus lateralis. The middle of the vastus lateralis is an appropriate site for intramuscular injections because of the low risk of vascular or nerve damage. The present results support good practices for site selection for intramuscular injections.