Considering skin-to-muscle depth for successful intramuscular injections in an increasingly obese population

Intramuscular (IM) injection practice is an essential nursing skill. Current practice relies on clinical judgement to determine needle length (unless specified in the medicine's product licence). Obesity is increasing in the global population, but guidelines have largely ignored how to select needle length to meet individual patient need.

The aim of this review was to systematically review the skin-to-muscle depth required to achieve injection into muscle in adults. The objectives were to identify any implications of obesity status when selecting an appropriate needle length and site in clinical practice. Search and review methodology: Studies of subjects above the age of 18 years using observational or experimental designs where the distance from the skin to muscle had been measured at any IM injection site, and obesity status was reported, were included in the search strategy. The primary outcome of interest was the distance from skin surface to muscle penetration.

14 studies were identified that investigated the dorsogluteal, ventrogluteal, deltoid and vastus lateralis sites, all used cross-sectional observational designs. Ten used ultrasound, three used computed tomography (CT) and one used magnetic resonance imaging. Obesity status was reported as BMI or hip-to-waist ratio. In all studies there was a correlation between obesity status and the distance from skin surface to muscle. In females this exceeded 37 mm at both gluteal sites, independent of obesity status.

There should be an assessment of obesity status before selecting needle length for IM injections in both genders. Needles longer than the standard 37 mm are recommended for all females, whatever their obesity status, for any gluteal site. Injections into gluteal sites should be avoided in females who are obese. Deltoid injections are more likely to achieve muscle penetration in both genders, and in patients who are overweight or obese. Further research is required.