Clinical Assessment of Anatomical Landmarks of the Shoulder for Safe Intramuscular Injections. A Cross-Sectional Study among Adult Patients

Shoulder Anatomical Landmarks for Safe IM Injections

Abstract

Background: Standard deltoid intramuscular injection landmarks may not account for sex-specific anatomical variations, increasing the risk of subcutaneous injection, periosteal contact, or neurovascular injury.

Objectives: To quantify deltoid region anthropometry and tissue-depth biomarkers in Pakistani adults and assess sex differences to improve injection safety.

Methods: This cross-sectional study (June 2023–November 2024) enrolled 100 outpatients (50 men, 50 women; aged 18–65 years) at Nawaz Sharif Medical College and Aziz Bhatti Shaheed DHQ Hospital, Gujrat, following ethical approval and informed consent. Acromion–tuberosity distance and upper arm circumference were measured with a flexible tape; triceps skinfold thickness with Harpenden calipers; and deltoid muscle thickness using a 7.5 MHz ultrasound probe. Data were analysed with independent-samples t-tests and Pearson correlation in SPSS v26 (p < 0.05).

Results: Mean acromion–tuberosity distance was 53.2 ± 4.8 mm, higher in men (55.6 ± 4.2 mm) than women (50.8 ± 4.1 mm; p < 0.001). Upper arm circumference averaged 28.5 ± 2.9 cm (men: 30.2 ± 2.7 cm; women: 26.8 ± 2.3 cm; p < 0.001). Triceps skinfold thickness was greater in women (13.4 ± 4.0 mm) than men (11.8 ± 3.5 mm; p = 0.014), while deltoid muscle thickness was higher in men (19.2 ± 3.1 mm) than women (17.6 ± 3.0 mm; p = 0.022). Triceps skinfold showed strong correlation with BMI (r = 0.65; p < 0.001), and deltoid muscle thickness showed moderate correlation (r = 0.42; p = 0.005).

Conclusion: Although the 50 mm landmark generally identifies deltoid muscle, significant sex differences in muscle and fat warrant individualised needle selection using anthropometric or ultrasound guidance to minimise injection-related complications.