**4. SC thickness**

The SC tissue is the target for insulin. Injections must reach the SC tissue, but not go deeper into the muscle fascia or the muscle itself. Therefore, the thickness of the SC is critical in determining the desired length of the needle as well as the injection technique (e.g., lifting a skin fold or not). SC tissue thickness varies widely depending on gender, site of injection, and BMI. Women, on average, have approximately 5-mm thicker SC fat than men, when one controls for BMI and body site. Truncal sites (abdomen and buttocks) have more SC fat than limbs (arm and thigh), in the same patient. The higher the BMI, the thicker the SC fat. Studies within the last decade have used precision US to determine the SC tissue thickness in a diverse group of adults [7, 9, 15, 16], adolescents, and children [13, 14].


\* Assumes injection straight at 90° without pinch-up (the table adapted from Hirsch [16]).

With kind permission from Hirsch L et al. [16]. Intramuscular risk at insulin injection sites—measurement of the distance from the skin to the muscle and rationale for shorter-length needles for subcutaneous insulin therapy.

**Table 1.** Estimated IM injection risk in adults, by body site\* .


**Table 2.** Calculated risk of IM injection in children and adolescents as a function of injection site, age, and needle length\* . Babies have more SC tissue than preschool children. Children from 2 to 6 years have very little SC tissue regardless of gender. Children from 7 to 13 years gain SC tissue slowly but SC tissue thickness is almost the same in both genders until puberty. At puberty, girls increase their SC tissue more rapidly than boys as a result of hormonal differences.

SC tissue thickness when combined with the currently available needle lengths yields a relatively clear indication of the risk of IM injection. **Tables 1** and **2** show the risks for adult and pediatric persons with diabetes, respectively. It is clear from these data that the shorter the needle, the lower the risk of IM injection.
