Sunday, January 27, 2013

Subscapularis and total shoulder

Importance of the subscapularis muscle after total shoulder arthroplasty.

These authors remind us that the primary stabilizing mechanism for normal shoulders and for anatomic shoulder arthroplasties is concavity compression - that is the pressing of the humeral ball into the glenoid socket by the muscles surrounding the joint.

In performing a shoulder arthroplasty, one of the important joint compressors, the subscapularis, is incised and repaired, often with resulting weakness or even loss of integrity (for additional posts on this topic search this blog for 'subscapularis'). 

The authors used an EMG-driven musculoskeletal model of the glenohumeral joint to compare joints with normal and deficient subscapularis muscles. They observed that to maintain joint equilibrium,  a deficient subscapularis induced a decrease of the force of the infraspinatus muscle, further decreasing joint compression. As a result, the upward pull of the deltoid induced an upward migration of the humeral head, an eccentric contact pattern and higher stress within the cement.  They suggest that the loss of stability induces the “rocking horse” effect that can lead to a progressive loosening of the glenoid component. 

This brings up the possibility that the delayed upward translation of the humeral head after shoulder arthroplasty previously noted here, may be in part due to subscapularis failure placing additional load on the supraspinatus, contributing to its failure.

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