['Children with residual brachial plexus birth palsy often develop internal rotation contractures with subsequent glenohumeral dysplasia seen on axial imaging. Coronal deformity (characterized by humeral head subluxation), and angular deformity (characterized by glenoid retroversion) have been defined. We hypothesize that the location of the biceps tendon characterizes rotational deformity.', 'A retrospective study was performed on 91 children (average age 3.2+/-2.2 y) who lacked external rotation beyond neutral and had magnetic resonance imaging (MRI) scans of bilateral shoulders performed at our institution between 2000 and 2007. The charts were reviewed for the measurement of the external rotation of the involved shoulder with the arm adducted and the scapula stabilized. The glenoscapular angle (glenoid version), the percentage of the humeral head anterior to the middle of the glenoid fossa (PHHA), and the angle of rotation of the biceps tendon (biceps angle) were measured on MRI scans of both shoulders. Statistical analysis was performed to compare these MRI measurements for the involved and uninvolved sides, and to identify the correlations between them and the external rotation.', 'The average biceps angle was 47.9+/-15.2 degrees on the uninvolved side and 26.2+/-15.0 degrees on the involved side. The average differences between the 2 shoulders in the biceps angle (21.7+/-20.5 degrees), the version (18.9+/-15.0 degrees), and the PHHA (19.8+/-13.6%) were all significant (P<0.001). Only the biceps angle correlated significantly with external rotation (P<0.001). This correlation remained significant even when the version and the PHHA were held constant (P=0.004).'] Do the biceps tendon as a measure of rotational deformity in residual brachial plexus birth palsy?