Biceps brachii long head overactivity associated with elbow flexion contracture in brachial plexus birth palsy

The incidence of brachial plexus birth palsy is 0.38 to 4.6 per 1000 live births1-5. Although the majority of affected infants recover spontaneously, as many as 34% experience long-term sequelae, including skeletal deformities and soft-tissue contractures6. Elbow flexion contracture is a well-recognized complication of brachial plexus birth palsy that can impair upper extremity function. In a previous study of 319 children with brachial plexus birth palsy, 48% of the patients had an elbow flexion contracture of ≥10°, with more than one-third of these patients having a contracture of ≥30°7. Despite this high prevalence of elbow flexion contracture in children with brachial plexus birth palsy, the etiology remains unclear. In 1994, Ballinger and Hoffer suggested that elbow contracture may result from elbow flexor function returning first and dominating extensor function8. Sibinski et al. later proposed that contracture could result from a muscle imbalance between stronger elbow flexors and weaker elbow extensors or a misdirection of regenerating neurons, causing cross-innervation and co-contraction of synergistic and antagonistic muscles9. Most recently, Nikolaou et al. studied brachial plexus birth palsy in a mouse model and proposed that elbow flexion contracture is due to impaired growth of denervated muscles10. Decreased volume and thickness of shoulder musculature also have been reported in magnetic resonance imaging (MRI) studies of children with brachial plexus birth palsy11. We propose a novel hypothesis based on our understanding of the adverse effects of brachial plexus birth palsy on the shoulder. Muscle weakness about the shoulder is a well-recognized finding in patients with brachial plexus birth palsy. Previous studies have demonstrated evidence of atrophy and fatty degeneration of the supraspinatus, infraspinatus, subscapularis, and deltoid muscles on MRI12,13. Partial paralysis of the muscles about the shoulder results in glenohumeral deformity, including glenoid hypoplasia, flattening of the humeral head, and posterior glenohumeral subluxation14-17. Previous studies have shown that the long head of the biceps brachii muscle plays a role as a shoulder flexor and humeral head stabilizer and may compensate for glenohumeral instability in athletes18-21. It also has been hypothesized that overactivity of unaffected or mildly affected muscle groups may lead to a flexion contracture in patients with poliomyelitis22,23. We propose that the long head of the biceps brachii muscle, which crosses the glenohumeral joint anteriorly, helps to stabilize the shoulder in children with brachial plexus birth palsy-associated weakness of shoulder rotators and abductors. Because contraction of the long head of the biceps brachii muscle also flexes the elbow, overuse of this muscle as a shoulder flexor may contribute to associated elbow flexion contractures. The purpose of the present study was to compare the mean activity duration of the long and short heads of the biceps brachii muscle in the affected limb with that in the unaffected limb of children with brachial plexus birth palsy-associated elbow flexion contracture as they performed standardized upper extremity tasks. Additionally, we tested the alternate hypothesis that elbow flexion contracture is due to an elbow flexor-extensor muscle imbalance by measuring isometric muscle strength with surface dynamometry. Finally, we studied the impact of elbow flexion contracture on upper extremity function with use of three-dimensional motion analysis and three validated function measures.