The equine bicipital apparatus ‐ review of anatomy, function, diagnostic investigative techniques and clinical conditions

Abstract

The equine biceps brachii (bicipital apparatus) is a complex structure that has 2 functions: contributing to the forelimb passive stay apparatus and assisting in forelimb protraction (Gyuru and Zajer 1982; Hermanson et al. 1991). The bicipital apparatus spans the scapulohumeral (shoulder) and elbow joints and this permits the extension of the shoulder and flexion of the elbow. The biceps brachii originates from the supraglenoid tubercle of the scapula. In the proximal area it is entirely tendonous and has a bilobed structure with an isthmus connecting the 2 lobes (Sisson 1975; Reef 1998). This bilobed structure conforms to the ‘M shape of the cranial humerus (Fig 1). The lateral lobe of the tendon is ‘tear drop’-shaped and the medial lobe has an elongated rectangular shape (Gillis 1996). Interposed between the tendon and the humerus is the intertubercular (bicipital) bursa. Considering the embryonic development and function, the intertubercular bursa resembles a tendon sheath (Dyson 1991). The humeral surface of the bursa is covered with fibrocartilage but its composition does not appear to have been described. The tendon o f the biceps brachii muscle is bound down to the proximal humerus by a tendonous band of the pectoralis ascendens muscle that extends between the lesser tubercle and the cranial part of the greater tubercle of the humerus (Sisson 1975). The tendon continues through the more distal biceps brachii muscle and divides into 2 portions. The short (thicker) tendon inserts onto both the radial tuberosity and the medial collateral ligament of the elbow. The long tendon (lacertus fibrosus) blends with the fascia of the forearm and the tendon of the extensor carpi radialis (Sisson 1975). Palmieri et al. (1986) proposed that the lacertus fibrosus may co-ordinate the action of both the biceps brachii and the extensor carpi radialis muscles and, therefore, synchronise shoulder, elbow and carpal joint movements.