Anatomy of the distal tendinous structure of the triceps brachii: implications for the role of the triceps brachii to resist valgus elbow forces during baseball pitching.

Background: Biomechanical studies suggest that the triceps brachii muscle generates resistive force against valgus stress on the elbow during baseball pitching. However, given the parallel fiber orientation in the distal tendinous structure of the triceps brachii, the mechanism behind this anti-valgus force remains unclear. In the present study, we aimed to examine the anatomy of the distal tendinous structure of the triceps brachii using bony morphological, macroscopic, and histological methods.

Methods: We analyzed 25 elbow specimens from 18 cadavers, all of Japanese ethnicity, using micro-computed tomography (micro-CT). Four specimens were excluded due to severe osteophytes, leaving 21 specimens randomly allocated to three groups: 13, 4, and 4 specimens to the macroscopic examination group, the histological examination group, and the chemically débrided bone examination group, respectively. In 6 of the 13 specimens analyzed macroscopically, we quantitatively measured the local thickness of the triceps tendon using micro-CT.

Results: The distal tendinous portion of the triceps brachii was comprised of an intramuscular tendon and a superficial aponeurosis. The intramuscular tendon, located between the long and medial heads of the triceps, curved medially to laterally and inserted broadly onto the proximal and lateral facets via fibrocartilage. The superficial aponeurosis, attached to the lateral and medial heads, had fibers that ran straight and merged distally with the aponeurosis of the anconeus. The intramuscular tendon (2.2 ± 0.4 mm) was significantly thicker than the superficial aponeurosis (0.9 ± 0.2 mm, P < 0.001).

Conclusion: The current study revealed a nonuniform distal tendinous structure of the triceps brachii, with a thick intramuscular tendon and a thin superficial aponeurosis. In particular, the intramuscular tendon was curved from the medial-to-lateral direction and had a firm insertion. These findings suggest that the intramuscular tendon can contribute force to prevent valgus stress on the elbow as a dynamic stabilizer during the pitching motion.