Association between scapular asymmetry and shoulder pain in juvenile baseball players

Abstract

Objectives: Scapular dyskinesis, defined as an alteration in scapular position and motion, has been suggested as a risk factor for upper extremity injuries. Assessing scapular asymmetry is a simple and easy way to identify scapular dyskinesis that is typically associated with throwing-related injuries. Despite the large number of investigations identifying scapular asymmetry in patients with shoulder pathology, little research has examined the incidence of scapular asymmetry and its association with shoulder injury in young, skeletally immature baseball players. The purpose of this study was to determine the incidence of scapular asymmetry in juvenile baseball players and investigate its association with shoulder injuries. Methods: Two hundred and twenty-three juvenile baseball players aged 9-12 years were included in this study. Each participant submitted a self-completed questionnaire and underwent a physical examination and an imaging assessment of scapular asymmetry. Questionnaire items included the subject’s sex, age, position, years of baseball experience, practice frequency and duration, and shoulder pain during throwing over the past 1 year. Tenderness of the proximal humeral epiphysis and pain during maximum external rotation were also assessed on the physical examination. Digital photos of each participant’s upper body were taken from behind, first with their arms held straight out to the sides and then with their hands in the air. Scapular asymmetry was evaluated using a computerized image analysis system (Image J version 1.33u; National Institute of Mental Health, Bethesda, MD, USA). Inter- and intra-observer reliability for scapular asymmetry was 0.65 and 0.77, respectively. All analyses were conducted using JMP 10 (SAS Institute, Cary, NC, USA) and p values <0.05 were considered statistically significant. Results: Fifty-three of the 223 subjects had experienced shoulder pain while throwing in the past 1 year. Of these 53 symptomatic subjects, 28 (52.8%) had experienced shoulder pain twice or more in that year, whereas 42 (79.2%) had some kind of throwing disability. The incidence of shoulder pain significantly increased with age and was significantly higher in pitchers than in players of other positions. There was no significant association between shoulder pain and years of baseball experience or amount of practice. Scapular asymmetry was observed 171 (76.7%) subjects. The scapula of the throwing side tended to tilt anteriorly, deviate inferiorly and medially, and rotate downwardly in both the arms at side and hands up positions compared with the scapula on the non-throwing side. Age- and position-adjusted logistic regression analysis revealed that scapular superior deviation and anterior tilting significantly increased the risk of shoulder pain as evidenced by odds ratios (95% confidence interval) of 2.41(1.17-4.98) and 2.36 (1.20-4.67), respectively. Conclusion: Our study demonstrated that approximately three-quarters of juvenile baseball players under the age of 12 years already had scapular asymmetry and that 24% experienced shoulder pain. Scapular superior deviation and anterior tilt, which might be induced by tightness of the pectoralis minor muscle, leads to increased rotational torque at the proximal humeral epiphysis by restricting the scapular posterior tilt during late cocking to the acceleration phase.