Optimal intramuscular needle-penetration depth.

OBJECTIVE: The US Centers for Disease Control and Prevention has needle-length recommendations for intramuscular vaccinations in the thigh and shoulder on the basis of the age of the child. Underpenetration of the intramuscular layer with short needles has been documented; however, few studies have focused on the risk for overpenetration of the intramuscular level with needles that are too long. The purpose of this study was to determine the optimal needle length for intramuscular vaccination of children of various ages and sizes at the shoulder and thigh levels by using MRI and computed tomography scan measurements.
METHODS: A total of 250 MRI and computed tomography scans of shoulders and thighs of children who were 2 months to 18 years of age at a large children's hospital were reviewed. The thicknesses of the subcutaneous fat tissue and muscle layers were measured. Measurements were correlated with age and weight, and regression analysis was performed.
RESULTS: Use of the Centers for Disease Control and Prevention's recommended 1- and 1 (1/4)-in needles for intramuscular vaccination in the thigh of children >/=1 year of age would result in 11% (11 of 100) and 39% (34 of 88) overpenetration, respectively, with a minimal risk for underpenetration at 2% (2 of 100). Patients with vaccinations in the shoulder with the Centers for Disease Control and Prevention-recommended 58-, 78-, and 1-in needles would experience 11% (16 of 150), 55% (83 of 150), and 61% (92 of 150) overpenetration, respectively.
CONCLUSIONS: There is a substantial risk for overpenetration of the intramuscular layer when using current Centers for Disease Control and Prevention recommendations for vaccination needle lengths. We recommend a revision of the needle-length guidelines for thigh and shoulder injections to minimize the risk for needle overpenetration on the basis of the variability observed in the fat thickness.