These authors point out that coracoacromial ligament (CAL) excision and acromioplasty increase superior and anterosuperior glenohumeral translation. They used a cadaver model to estimate how much of an increase in rotator cuff force is required to re-establish intact glenohumeral biomechanics after acromioplasty.
Nine cadaveric shoulders were subjected to loading in the superior and anterosuperior directions in the intact state after CAL excision, acromioplasty, and recording of the translations. The rotator cuff force was then increased to normalize glenohumeral biomechanics.
At 150 and 200 N of superior and anterosuperior loading, an increase in the rotator cuff force of 25% was required to eliminate the increased translation resulting from CAL excision.
At 150 and 200 N of superior and anterosuperior loading, an increase in the rotator cuff force of 25% and 30%, respectively, was required to eliminate the increased translation resulting from acromioplasty and CAL excision.
The authors concluded that after subacromial decompression, the rotator cuff has to increase its force production to maintain baseline glenohumeral mechanics. Under many circumstances, in vivo force requirements may be even greater after surgical attenuation of the coracoacromial arch.
Comment: As Codman pointed out in 1934 "The coracoacromial ligament has an important duty and should not be thoughtlessly divided at any operation."
He recognized then, as we should today, that the coracoacromial arch provides an important stabilizing function resisting the superiorly directed force applied by the deltoid or when pushing up from a chair, bed, floor or bar. He pointed to the normal articulation between the superior aspect of the cuff and the undersurface of the coracoacromial arch.
The center of curvature of the arch is the same as the center of curvature of the humeral head.
The stabilizing effect of the arch remains the same if there is ossification of part of the coracoacromial ligament.
We have previously demonstrated that the acromion is loaded when superiorly directed force is applied through the humeral head (whether or not the cuff is intact).
Sacrifice of the coracoacromial arch in the cuff deficient shoulder is a common cause of anterosuperior escape and pseudoparalysis.
Where does that leave the concept of 'impingement'? See this link.
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