Congenital Anatomical Variations of the Posterior Cord of the Brachial Plexus in Human Fetuses: Branching Patterns and Surgical Implications

Background: The posterior cord of the brachial plexus, formed by the posterior divisions of the upper, middle, and lower trunks (C5-T1), gives rise to the upper subscapular, thoracodorsal, lower subscapular, and axillary nerves and continues as the radial nerve. Variations in its formation and branching are clinically important for regional anesthesia (infraclavicular and axillary blocks), axillary surgery, and nerve-transfer procedures in brachial plexus palsy. Fetal dissection can reveal congenital patterns before postnatal or post-mortem distortion. Objective: The single combined objective was split into two discrete, explicitly stated aims: (i) to document the prevalence and morphological types of variation in posterior cord formation and branching; and (ii) to test associations of these variations with gestational age, fetal sex, and side. Materials and methods: Twenty-three formalin-preserved aborted human fetuses (gestational age 20-40 weeks; 46 upper limbs) were dissected bilaterally in the Department of Anatomy, SMBT Institute of Medical Sciences and Research Centre, Nashik, India. The pectoral, axillary, and arm regions were exposed to trace the roots, trunks, divisions, posterior cord, and its branches. Variations were classified by root contribution, posterior cord formation, and branching pattern. Associations with gestational age, fetal sex, and side were tested using the chi-square and Fisher’s exact tests (IBM SPSS Statistics for Windows, Version 25 (Released 2017; IBM Corp., Armonk, New York, United States); P<0.05 considered significant); odds ratios (ORs) with 95% confidence intervals (CIs) were calculated where appropriate. Results: Posterior cord variations were present in 28 of 46 limbs (60.87%). The classical three-posterior-division pattern was seen in 37 limbs (80.43%), whereas variant formations (one, two, three (united distally), or four posterior divisions) occurred in nine limbs (19.57%). Prefixed roots (C4-T1 or C3-T1) were present in six limbs (13.04%). Branching was classical in 21 limbs (45.65%); the commonest anomaly was a common trunk for the axillary and lower subscapular nerves (10 limbs, 21.74%). Variations were significantly more frequent on the right side (18/23, 78.26%) than the left (10/23, 43.48%; χ2=5.84, P=0.016; OR 4.68, 95% CI 1.29-16.98) but were not associated with gestational age or fetal sex. Conclusions: The posterior cord shows substantial congenital variability in human fetuses, with a right-sided predominance. Awareness of these patterns may inform surgical planning, neurotization, and regional anesthesia, with the potential to reduce the risks of incomplete blockade and iatrogenic nerve injury.