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]

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. 

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

Related Post:

Owens BD, Campbell SE, & Cameron KL (2013). Risk Factors for Posterior Shoulder Instability in Young Athletes. The American Journal of Sports Medicine PMID: 23982394