MSS012: MIDAS-MILITARY INTRAMUSCULAR DEPTH AND AUTO-INJECTOR EFFECTIVENESS VIA SONOGRAPHY: ASSESSING SEX-BASED DISPARITIES IN INTRAMUSCULAR INJECTIONS AMONG ACTIVE DUTY SERVICE MEMBERS
Abigail Kelly, MD; Brooklyn Williams, DO; Jonathan Lutgens, MD; Zachary Riggenbach, MD; Jacob Glaser, MD; Michael Loughren, PhD, CRNA; Madigan Army Medical Center
Introduction: Intramuscular injection (IMI) is a cornerstone of medication delivery in both civilian and military medicine, enabling rapid systemic administration of lifesaving drugs. Tactical Combat Casualty Care (TCCC) guidelines recommend IMI for battlefield use of antibiotics, ketamine, and ondansetron, while warfighters also rely on autoinjectors containing agents such as naloxone, epinephrine, and atropine. The success of these interventions hinges on achieving intramuscular penetration. Civilian studies have demonstrated that females face a two- to five-fold higher risk of subcutaneous injection (SQI) compared with males, attributable to greater skin-to-muscle depth (STMD). In emergencies, failed IM delivery may lead to catastrophic outcomes. This study evaluated whether such sex-based differences in STMD exist in an active-duty population with strict fitness and body composition standards.
Methods: A cross-sectional study was conducted at Madigan Army Medical Center using ultrasound to measure STMD at four standard IMI sites—deltoid, ventrogluteal (VG), dorsogluteal (DG), and vastus lateralis (VL). Bilateral measurements were obtained in relaxed and probe-compressed states to simulate autoinjector use. Demographic and body composition data, including age, body mass index (BMI), body fat percentage (BFP), and Occupational Physical Assessment Test (OPAT) classification, were collected. A total of 101 active-duty service members (51 males, 50 females) were enrolled. Mean STMD values were compared between sexes overall and across body composition subgroups.
Results: Females exhibited greater STMD at all sites in both relaxed and compressed states, with statistically significant differences at the VL (relaxed p<0.001; compressed p<0.001), VG (relaxed p=0.004; compressed p<0.001), and DG (relaxed p<0.001; compressed p<0.001). At the VL compressed site, used as a surrogate for autoinjector performance, mean STMD was 7.2 ± 2.4 mm in males versus 12.3 ± 4.6 mm in females, a 5.1 mm disparity. Notably, females demonstrated significantly greater STMD even within equivalent BMI and BFP classes. Based on standard autoinjector needle lengths, 40% of females and 8% of males were at risk of SQI.
Conclusions: Active-duty females have consistently greater STMD compared to males, creating a higher risk of failed IM delivery with current autoinjector designs. These findings underscore a sex-based vulnerability with critical implications for combat readiness and survival. Device redesign or tailored deployment strategies may be necessary to ensure effective medication delivery across all service members.