Case Report Bilateral hypoplasia of the minor tubercle of the humerus with medial luxation of the biceps tendon in two Quarter Horses C. M. Lund*, C. A. Ragle, H. C. Rice and T. E. Wilkinson Jr Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, Pullman, USA. *Corresponding author email: lundcaleb@gmail.com Keywords: horse; hypoplasia; humerus; tubercle; biceps brachii; shoulder; lameness Summary Two Quarter Horses were examined at the Washington State University Veterinary Teaching Hospital with forelimb lameness. Case 1 was a 4-year-old female with a 4 month history of intermittent forelimb lameness that had partially responded to oral anti-inflammatories. The horse was in full training and actively competing in cutting. Case 2 was a one-year-old filly bred for cutting that presented with a right forelimb lameness of 3 weeks’ duration, which was not responsive to oral anti-inflammatories. The horse was not in training. On lameness examination, Case 1 was grade 1/5 (American Association of Equine Practitioners scale) lame in the left forelimb in a straight line on a hard surface, extension and flexion of the shoulder was resented and exacerbated the lameness. Case 2 was grade 3/5 lame in the right forelimb in a straight line on a hard surface, flexion and extension of the shoulder was resented and exacerbated the lameness. Both horses had a characteristic dished appearance to the dorsal aspect of the shoulder, with prominence of the proximal aspect of the humerus. Ultrasound and proximodistal oblique (skyline) radiographic views of the scapulohumeral joints demonstrated bilateral hypoplasia of the minor tubercle of the humerus with bilateral medial luxation of the proximal biceps tendon in both cases. To the authors’ knowledge this is the first report of 2 cases of bilaterally affected horses, as well as the first report of the condition in the Quarter Horse breed. Introduction Lameness in the forelimb of the horse is less frequently localised to the shoulder region as compared to other areas of the forelimb (Adams and Blevins 1989). Commonly described causes of lameness in the shoulder region include osteochondrosis and osteoarthritis of the scapulohumeral joint, joint luxation, fractures, bicipital bursitis and tendonitis, infraspinatus bursitis, and neurogenic muscle atrophy (Hubert and Stashak 2011). The scapulohumeral joint is unique among forelimb joints as it does not have collateral ligaments; the joint function and stability is maintained by the surrounding muscles and tendons (Adams and Blevins 1989). The joint is supported caudally by the triceps muscle group, laterally by supraspinatus and infraspinatus muscles, and medially by the subscapularis muscle. The biceps apparatus (biceps brachii muscle and tendon), supports the cranial aspect of the joint, and has been reported to play an important role in the forelimb passive stay apparatus and in protraction of the forelimb (Gyuru and Zajer 1982; McDiarmid 1999). The biceps brachii originates on the supraglenoid tubercle of the scapula. The bilobed tendon runs over the prominent proximodorsal aspect of the humerus and glides between 3 prominent tubercles (major, intermediate and minor). As the biceps tendon glides over the humerus it is enveloped by a synovial sheath called the intertubercular (bicipital) bursa. The proximal humerus tubercles in the region of the bursa are covered by fibrocartilage. The superficial pectoral fascia forms a ligament-like band that extends between the minor and major tubercles and binds the musculotendinous unit to the humerus. The distal portion of the biceps is tendinous and inserts on the radial tuberosity on the dorsomedial aspect of the radius (McDiarmid 1999). The muscle has a unique make-up in that the tendon spans from origin to insertion, giving it some function in the passive stay apparatus preventing shoulder flexion when the elbow is extended (Nevens et al. 2005). Previously reported causes of lameness attributed to the biceps apparatus include bicipital bursitis and tendonitis, as well as osteochondral disease of the humeral tubercles. (McDiarmid 1999; Little et al. 2009). Ultrasonographic examination is the imaging technique of choice for evaluating the bicipital apparatus (McDiarmid 1999). Congenital dysplasia of the bicipital apparatus is a less commonly reported cause of lameness in the horse. Three reports exist in the literature in a total of 6 horses (McDiarmid 1997; Heinen et al. 2002; Coudry et al. 2005). Medial luxation of the tendon of the biceps brachii muscle was first reported as a cause of lameness in a Shire foal (McDiarmid 1997). Intertubercular groove dysplasia was reported as a cause for dislocation of the biceps tendon in a Welsh pony (Heinen et al. 2002). Coudry et al. (2005) described hypoplasia of the minor tubercle of the humerus as the cause of medial biceps tendon luxation in 4 mature horses of various breeds and disciplines. All 3 reports describe dysplasia of the proximal humeral tubercles of various degrees accompanied by medial dislocation of the biceps musculotendinous unit. This condition has been hypothesised to be congenital due to absence of the minor tubercle with no evidence of fracture or trauma (Coudry et al. 2005). Hypoplasia of the minor tubercle of the humerus has also been reported in other species, including dog and man (Boemo and Eaton-Wells 1995; Friedman et al. 2008; Szabó et al. 2008). To the authors’ knowledge, hypoplasia of the minor tubercle of the humerus with medial biceps tendon luxation has not been reported in the Quarter Horse. This is also the first report containing 2 cases of bilaterally affected animals. This report discusses the clinical presentation and treatment outcomes of bilateral hypoplasia of the minor tubercle of the humerus with bilateral medial luxation of the biceps brachii tendon in 2 Quarter Horses bred for cutting. EQUINE VETERINARY EDUCATION 467 Equine vet. Educ. (2014) 26 (9) 467-472 doi: 10.1111/eve.12176 © 2014 EVJ Ltd