Brachial Plexus Variants – Roots, Trunks and Cords Meta-analysis

The brachial plexus is formed predominantly by the ventral rami of C5–T1 and classically organised into roots, three trunks, six divisions and three cords around the axillary artery, but cadaveric and imaging studies show that this “standard” pattern is present in only a proportion of specimens.

Variability begins at the level of root contributions, with prefixed (C4–C8) and postfixed (C6–T2) plexuses, and extends through trunk formation, cord morphology and the origin of major terminal branches such as the musculocutaneous nerve, median nerve, ulnar nerve and radial nerve.^{1, 8} These variants can change the relationship of neural elements to the subclavian and axillary vessels and modify the effect of regional blocks or nerve transfers.

Recent meta-analyses by Benes and colleagues, together with focused cadaveric series by Han, Pandey and others, provide pooled prevalence estimates for regular and variant patterns at each hierarchical level of the plexus and describe clinically important cord-level anomalies and communicating branches.^{1, 3} This article summarises those data with a specific focus on variants of roots, trunks and cords and their implications for surgery and regional anesthesia.

This meta-analytic narrative is anchored on the work of Benes et al., who synthesised anatomical variability of the brachial plexus in a series of studies covering supraclavicular organisation (roots, trunks, divisions and cords) and infraclavicular branching patterns.^{1, 5} Part I pooled data from 40 cadaveric studies (3055 limbs) reporting root contributions, trunk formation, cord configuration and their relationship to the axillary artery, using random-effects models to estimate prevalence and heterogeneity.

Part III extended the analysis to 75 anatomical studies (4772 limbs) describing infraclavicular branching of the lateral, medial and posterior cords and the origins of major terminal branches.⁵ To provide specimen-level context, these pooled estimates are integrated with a 60-cadaver series that classified variations by segment and documented the frequency of communicating branches, and with a larger cadaveric study detailing abnormal cord positions and absence of the posterior cord.^{3, 4}

Additional review and clinical papers on brachial plexus anesthesia, clinically relevant variations and fetal development were used to interpret the developmental and procedural significance of the pooled prevalence figures.^{6, 9} Percentages in text and charts are rounded to the nearest whole number for clarity while preserving the direction and magnitude of the original data.

Roots and Trunk

In the supraclavicular meta-analysis, a regular C5–T1 plexus with typical root contributions and three canonical trunks was present in approximately 84% of upper limbs, confirming that the textbook pattern is common but not universal.¹ Prefixed plexuses, in which C4 contributes significantly and T1 is reduced, occurred in about 11% of limbs, whereas postfixed plexuses with a prominent T2 contribution were identified in around 1%.

Pellerin et al. reviewed earlier series and showed broadly similar magnitudes, emphasising that variable definitions of prefixed and postfixed plexuses contributed to heterogeneity in historic data.² Fetal work by Uysal et al. demonstrated that atypical C4 and T2 contributions are already established before birth, supporting a developmental basis for these patterns rather than purely adaptive remodelling.⁹ Figure 1 summarises the pooled prevalence of regular, prefixed and postfixed configurations.

Trunk formation and supraclavicular organisation

Despite root-level variability, trunk formation showed relative robustness: in the same meta-analysis, the classic three-trunk pattern arising from C5–C6, C7 and C8–T1 was again observed in roughly 84% of limbs, closely mirroring the proportion of regular root contributions.¹ Variant patterns included superior trunks incorporating C4 in prefixed plexuses, inferior trunks receiving fibres from C7–T1 or C8–T2 in postfixed plexuses, and occasional additional or split trunks that did not fit the three-trunk model.

A regional cadaveric study in Sri Lankan adults reported comparable dominance of the three-trunk arrangement but documented a small subset of plexuses with merged trunks or asymmetrical formation, illustrating how population-specific data sit within the global meta-analytic range.¹⁰ Figure 2 contrasts regular supraclavicular organisation with the pooled proportion of plexuses showing any root or trunk-level variant.

Cord Morphology and Localization

Pandey and Shukla examined 172 cadavers and identified cord or median nerve variations in about 13% of limbs, including abnormal cord position relative to the axillary artery, absence of the posterior cord and atypical median nerve formation.⁴ Absence of the posterior cord, with its usual branches arising instead from the lateral and medial cords and sometimes crossing anterior to the axillary artery, was seen in roughly 3–4% of limbs, while abnormal cord position around the artery was documented in a smaller subset. Figure 4 summarises the relative frequency of normal cord position, abnormal position and absence of the posterior cord in this series.

In the supraclavicular meta-analysis, the regular relationship of cords to the second part of the axillary artery was present in about 96% of limbs, corroborating that gross displacement of cords is uncommon but clinically significant when it occurs.¹ In the infraclavicular meta-analysis, Benes et al. found a dominant “typical” pattern of branching from the lateral, medial and posterior cords but confirmed non-trivial frequencies of atypical origins, such as medial pectoral nerves or musculocutaneous–median communications arising from unexpected cords or from multiple cords.^{5, 6}

Together, the meta-analytic and cadaveric datasets show that the brachial plexus combines a remarkably stable macro-architecture with substantial micro-level variation. At the macro level, most limbs display a regular C5–T1 plexus forming three trunks, six divisions and three cords positioned predictably around the axillary artery.¹ At the micro level, however, root shifts in prefixed and postfixed plexuses, accessory connections between divisions and cords, and variable origins or communications of major branches are common, particularly for the medial and lateral cords.^{3, 6}

This duality explains why surgeons and anesthetists frequently encounter unexpected branching or mixed territories despite learning a single “standard” diagram: the gross scaffold is usually intact, but the detailed routeing of fibres often differs from textbook expectations.^{6, 8} For regional anesthesia, prefixed and postfixed plexuses alter the position of root and trunk bundles between the scalene muscles or above the first rib, potentially narrowing the ultrasound safety window and increasing the risk of incomplete block or phrenic nerve involvement.

For trauma reconstruction and nerve transfers, knowledge of how often the median nerve receives multiple roots, how frequently the musculocutaneous and median nerves communicate, and how branches arise from unexpected cords helps avoid misidentification of donor and recipient fascicles and improves interpretation of intraoperative neurophysiology.^{4, 6} Prefixed and postfixed plexuses and subtle cord displacements also influence thoracic outlet decompressions and may underlie some cases of otherwise unexplained neurogenic symptoms, although high-quality outcome data remain limited.²

Fetal studies indicate that many of these patterns are present in utero, suggesting that variation reflects developmental patterning of ventral rami and limb bud innervation rather than later remodelling.⁹ From an educational perspective, presenting a single “classic” diagram without quantified alternatives is misleading; integrating simple bar and donut charts that show how often key variants occur aligns teaching more closely with what clinicians encounter in the operating room and regional anesthesia practice.

Current evidence indicates that a regular C5–T1 brachial plexus with standard trunk and cord formation is present in most individuals, yet approximately one in nine shows a prefixed plexus and about one in a hundred a postfixed plexus.^{1, 2} At the level of divisions and cords, the gross three-cord scaffold is macroscopically regular in about 96% of limbs, but accessory communications and atypical branch origins are frequent and can significantly alter the functional map of nerve territories.^{1, 3}

Uncommon but critical cord-level anomalies—such as abnormal cord position relative to the axillary artery or absence of the posterior cord—directly affect surgical corridors and the safety profile of infraclavicular and axillary blocks.^{4, 5} Integrating quantified prevalence data and typical variant patterns into preoperative planning, ultrasound protocols and anatomy teaching should reduce iatrogenic nerve injury and improve outcomes in brachial plexus surgery, trauma reconstruction and regional anesthesia.^{6, 7}