Effect of posterior shoulder tightness on internal impingement in a cadaveric model of throwing

Mihata T, Gates J, McGarry MH, Neo M, Lee TQ. Knee Surg Sports Traumatol Arthrosc. 2013 Jan 16. [Epub ahead of print] 

Take Home Message: Posteroinferior glenohumeral capsule tightness may cause internal impingement of the shoulder when positioned in maximum external rotation. 

Internal impingement, a common problem among athletes performing overhead throws, is best described as the supraspinatus and infraspinatus tendon getting pinched between the posterior-superior glenoid and the greater tuberosity of the humerus. It has been suggested that glenohumeral internal rotation deficits (GIRD) can alter the arthrokinematics of the glenohumeral joint to cause internal impingement. However, this has yet to be experimentally demonstrated. Therefore, the purpose of this cadaveric study was to determine if inducing GIRD by tightening the posterior-inferior shoulder capsule altered glenohumeral contact pressure, surface contact area of the rotator cuff, and humeral head position. For this study the authors arthroscopically examined 7 fresh-frozen cadaveric shoulders (average age 51 years).to verify that there was no internal derangement and that all rotator cuff tendons were intact. The scapula was secured at 30° of upward rotation while the humerus was positioned at 60° of abduction (equals 90° combined with scapular motion) and in maximal external rotation in order to simulate the late-cocking phase of throwing. The authors then induced anterior shoulder capsule laxity resulting in a 20% increase in shoulder external rotation. Next, the authors tightened the posterior-inferior capsule to decrease internal rotation by ~24 degrees (GIRD). Joint contact pressure and contact area were measured with a thin force sensor within the joint. Readings were taken at maximum external rotation under a standard load. Humeral head shift during maximum external rotation was measured relative to the glenoid with three-dimensional analysis. The authors found that after inducing GIRD there was a change in external and internal rotation range of motion, as well as the total arc of motion when compared to the intact specimen. Furthermore, after inducing GIRD there was posterior humeral head translation, increased contact pressure on the rotator cuff, and decreased surface contact area on the rotator cuff. 

While internal impingement has been observed and discussed for well over 10 years, we now have quantifiable data demonstrating that GIRD (specifically posterior-inferior capsule tightness) may be a contributing factor to internal impingement among overhead athletes. The data also reaffirms the Burkhart et al. theory, that suggests a tight posterior capsule will cause the humeral head to shift in a posterior-superior direction thereby placing the posterior-superior structures (supraspinatus, infraspinatus, and superior labrum) at risk. Although it has been suggested that internal impingement may be a normal physiologic occurrence, the repetitive stresses of overhead throwing coupled with anatomical adaptations (posterior capsule tightness) appear to play a role in whether or not internal impingement becomes structurally damaging. One item to note is that the surface area of the rotator cuff being impinged actually decreased. This was not hypothesized; however, it is conceivable that a horizontal abduction component is missing from this study or that decreased surface area involvement results in higher stress. Many overhead throwers, especially baseball pitchers, forcefully retract their scapulae while simultaneously horizontally abducting their upper arms in an effort to gain more throwing velocity. Futures studies may want to consider expanding upon this study while incorporating horizontal abduction. What do you notice when an overhead thrower presents with internal impingement? Do your overhead athletes with internal impingement always have significant GIRD?

Written by: Mark Rice
Reviewed by: Stephen Thomas

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Mihata T, Gates J, McGarry MH, Neo M, & Lee TQ (2013). Effect of posterior shoulder tightness on internal impingement in a cadaveric model of throwing. Knee Surgery, Aports Traumatology, Arthroscopy PMID: 23322267