We undertook a prospective MRI study to measure the retroversion of the humeral head in 33 consecutive infants with a mean age of 1 year 10 months (3 months to 7 years 4 months) who had an obstetric brachial plexus lesion (OBPL). According to a standardised MRI protocol both shoulders and humeral condyles were examined and the shape of the glenoid and humeral retroversion determined.

The mean humeral retroversion of the affected shoulder was significantly increased compared with the normal contralateral side (−28.4 ± 12.5° v −21.5 ± 15.1°, p = 0.02). This increase was found only in the children over the age of 12 months. In this group humeral retroversion was −29.9 ± 12.9° compared with −19.6 ± 15.6° in the normal shoulder (p = 0.009), giving a mean difference of 10.3° (95% confidence interval 3.3 to 17.3). This finding is of importance when considering the operative treatment for subluxation of the shoulder in children with an OBPL.