Risk Factors for Posterior Shoulder Instability in Young Athletes
Owens BD, Campbell SE, Cameron KL. Am J Sports Med. 2013 Aug 27. [Epub ahead of print]
Take Home Message: Athletes with increased glenoid retroversion may be at risk for posterior shoulder instability. Increased internal and external rotation strength was also associated with instability but it is unclear whether these differences were causative or compensatory to the differences in glenoid anatomy.
Posterior glenohumeral instability has become increasingly common in young athletes. However, little is known about the risk factors associated with these injuries. If we can identify modifiable risk factors for posterior shoulder instability, we may be able to develop injury prevention programs. Therefore, the purpose of this study was to determine the modifiable and nonmodifiable risk factors for posterior shoulder instability among young athletes at the United States Military Academy (intercollegiate and intramural sports). The authors performed a prospective cohort study with 714 young athletes who were followed over a 4-year period. Baseline measurements included subjective history of instability; clinical assessment of shoulder instability, range of motion, and strength; and bilateral noncontrast magnetic resonance imaging of the shoulder. On magnetic resonance images, the authors measured various anatomic features: glenoid version (e.g., retroversion), height, depth, rotator interval height/width/area/index. Participants were followed for 4 years and one orthopaedic surgeon evaluated and documented all possible acute posterior shoulder instability events. The authors reported that 46 shoulders sustained glenohumeral instability events, with only 7 (1%) in the posterior direction. Baseline factors associated with posterior instability were increased glenoid retroversion as well as increased external rotation and internal rotation strength.
This study identified increased glenoid retroversion as the most significant risk factor for posterior instability. This is consistent with previous studies that have also identified increased glenoid retroversion in patients with posterior instability. However, these studies could not determine whether the observed differences were a cause or effect of the instability event. The current study is unique in that it identifies increased retroversion before injury as a risk factor for subsequent shoulder instability. This study also examined modifiable risk factors for posterior instability, specifically rotator cuff strength. Contrary to their hypothesis, the authors found that increased internal/external strength was associated with subsequent instability. However, it is unclear whether the increased rotator cuff strength caused the instability or was instead a result of compensation for the increased retroversion observed in these athletes. The rotator cuff provides dynamic stability and compression of the humeral head on the glenoid. Differences in glenoid anatomy may alter the direction of the compressive forces acting on the glenoid and thereby alter the muscle forces required to maintain joint stability. These compensatory strategies may account for the differences in rotator cuff strength observed in this study. As a result, these strength measurement findings should be interpreted with caution. In conclusion, results from this study confirm that increased glenoid retroversion is the most significant risk factor for posterior shoulder instability. Future studies should determine the amount of retroversion in patients with posterior instability that is necessary for soft tissue repair and also the critical level of glenoid retroversion that would warrant the need for modification of glenoid anatomy through glenoid osteotomy with concomitant repair. Unfortunately, the authors did not identify modifiable risk factors that would allow for development of primary prevention strategies for shoulder instability in athletes and therefore future work is needed.
Questions for Discussion: Do you think athletes should be pre-screened with imaging to identify risk of posterior shoulder instability? Do you have any prevention protocols you would employ in high risk groups?
Written by: Katie Reuther
Reviewed by: Jeffrey Driban