Biomechanical basis of shoulder osseous deformity and contracture in a rat model of brachial plexus birth palsy

Background: The purpose of this study was to investigate the relative contributions of two proposed mechanisms, strength imbalance and impaired longitudinal muscle growth, to osseous and postural deformity in a rat model of brachial plexus birth palsy (BPBP). Methods: Thirty-two Sprague-Dawley rat pups were divided into four groups on the basis of surgical interventions to induce a strength imbalance, impaired growth, both a strength imbalance and impaired growth (a combined mechanism), and a sham condition in the left forelimb. Maximum passive external shoulder rotation angle (ERmax) was measured bilaterally at four and eight weeks postoperatively. After the rats were killed at eight weeks, the glenohumeral geometry (on microcomputed tomography) and shoulder muscle architecture properties were measured bilaterally. Results: Bilateral muscle mass and optimal length differences were greatest in the impaired growth and combined mechanism groups, which also exhibited >15° lower ERmax (p < 0.05; four weeks postoperatively), 14° to 18° more glenoid declination (p < 0.10), and 0.76 to 0.94 mm more inferior humeral head translation (p < 0.10) on the affected side. Across all four groups, optimal muscle length was significantly correlated with at least one osseous deformity measure for six of fourteen muscle compartments crossing the shoulder on the affected side (p < 0.05). In the strength imbalance group, the glenoid was 5° more inclined and the humeral head was translated 7.5% more posteriorly on the affected side (p < 0.05). Conclusions: Impaired longitudinal muscle growth and shoulder deformity were most pronounced in the impaired growth and combined mechanism groups, which underwent neurectomy. Strength imbalance was associated with osseous deformity to a lesser extent. Clinical Relevance: Treatments to alleviate shoulder deformity should address mechanical effects of both strength imbalance and impaired longitudinal muscle growth, with an emphasis on developing new treatments to promote growth in muscles affected by BPBP.