Anatomical analysis of the posterior elbow joint capsule and small muscle bundle deep to the triceps brachii: a combined cadaveric and in vivo study

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Abstract
Background: Restricted elbow flexion sometimes persists even after stretching the triceps brachii. Although prior descriptions have noted that a small muscle bundle is located between the posterior elbow joint capsule and the triceps brachii, its anatomical relationship with surrounding muscles and dynamic changes during forearm rotation remain unclear. We hypothesized that the small muscle bundle separates from the triceps and contributes to forearm rotation. This study aimed to clarify the anatomical structure of the small muscle bundle and its dynamic changes during forearm rotation. Methods: We analyzed 19 elbow specimens from 16 Japanese cadavers, donated to the Department of Anatomy, Institute of Science Tokyo, to identify the small muscle bundle covering the joint capsule. One specimen was excluded because of severe osteophytes, leaving 18 specimens randomly allocated to three groups (10, 3, and 5 specimens in the macroscopic examination, intramuscular nerve distribution examination, and histological examination groups, respectively). Additionally, we investigated 10 elbows of five healthy volunteers to evaluate dynamic changes in the small muscle bundle and joint capsule using ultrasonography during forearm rotation. Results: Anatomically, deep to the triceps brachii, a small muscle bundle covering the posterior elbow joint capsule was identified. The small muscle bundle was continuous laterally with the anconeus muscle and was separated from the triceps by loose connective tissue. The small muscle bundle and the anconeus were innervated by the same branches of the radial nerve. Ultrasonically, the small muscle bundle was found to be significantly thicker during forearm supination (6.0 ± 1.4 mm) than pronation (4.1 ± 1.0 mm; p = 0.002), and the distance between the olecranon fossa and the posterior edge of the joint capsule was significantly shorter during supination (8.9 ± 1.1 mm) than pronation (9.6 ± 1.1 mm; p = 0.023). Conclusions: The small muscle bundle formed a muscular complex with the anconeus and was distinct from the triceps. The small muscle bundle deep to the triceps may act during forearm rotation, affecting the posterior elbow joint capsule. These anatomical and ultrasonographic findings may contribute to a better understanding of the pathophysiology of restricted elbow flexion.
Funding Information
  • JA Kyosai Research Institute
  • Japan Society for the Promotion of Science (JP 25K10141)

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