Rare Muscular Variations Identified in a Single Cadaveric Upper Limb: A Four‐headed Biceps Brachii and Coracobrachialis Brevis Muscle Variant
Bibliographic record
Abstract
Introduction Supernumerary, or accessory, heads of the biceps brachii are variations associated with the biceps brachii and can vary in number and location in the arm. Variations in other arm muscles, such as the coracobrachialis, can accompany variations in supernumerary biceps brachii musculature. Testut (1899) described two classes of anatomical variations associated with the coracobrachialis muscle: the coracobrachialis longus and coracobrachialis brevis. While they share a common origin at the coracoid process, the coracobrachialis longus and brevis are differentiated by their insertions at the medial epicondyle, and the surgical neck of the humerus, respectively. The objective of this study is to provide a comprehensive description of two rare variant muscles identified in a single cadaveric upper limb: a four‐headed biceps brachii and the tendinous elevator of the latissimus dorsi, a rare coracobrachialis brevis variant. Methods The anatomical variations were identified during routine dissection of the brachial plexus of a left upper limb from an 83‐year old male cadaver. Use of cadaveric material for research purposes was approved by the local research ethics council in the Department of Anatomy & Cell Biology at the University of Western Ontario (REF#: 21082015). Results We observed a four‐headed biceps brachii muscle consisting of both an infero‐medial and infero‐lateral head both originating from the humeral surface. The humeral heads shared a common insertion with the biceps brachii tendon. The tendinous elevator of the latissimus dorsi originated from the coracoid process and inserted on the anterior surface of the latissimus dorsi tendon. A thick fascial structure, considered to be an ancestral remnant of the dorsal epitrochlearis muscle, extended from the latissimus dorsi tendon to the fascia covering the long head of the biceps brachii. Furthermore, an accessory brachialis muscle was also identified in this cadaveric specimen. A variant innervation pattern consisting of a communicating branch between the median and musculocutaneous nerves was also observed. Conclusion These anatomical variations are clinically relevant to surgeons and radiologists as they have potential to compress neurovasculature in the shoulder and arm. Supernumerary heads of the biceps brachii could complicate bone fracture injuries and, subsequent, surgical interventions by causing unusual fracture patterns of the humerus due to their size, orientation and forces produced by these muscles. Furthermore, coracobrachialis brevis variants have been associated with chronic shoulder pain in some individuals. The variants identified here are considered rare, and their co‐occurrence in a single upper limb has not been documented in the literature to our knowledge. Support or Funding Information Supported by NSERC
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.002 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".