Sunday, October 16, 2016

Understanding and caring for the subscapularis

Subscapularis tendon loading during activities of daily living

These authors uses the Newcastle shoulder to model the subscapularis with 3 lines of action encircling the humeral head. The load was measured in the entire subscapularis, and the percentage of this load in each of the 3 tendinous bands was calculated.

The activities modeled are shown below.

The maximum force produced by the entire subscapularis muscle for the various activities ranged from 3 to 43 N. Load sharing between the 3 subscapularis bands showed that the superior band bore the largest percentage of the total load of the muscle (95% ± 2%).  The graph below shows the loading of the different bands for the ten activities.

The load in the subscapularis, particularly in the superior band, increased significantly when a supraspinatus tear was simulated (P < .0001).

Comment: Nature planned for the fact that the superior band would be most heavily loaded by making a strong upper rolled border for the tendon shown as "SC" in this figure from the classic work by Clark and Harryman .

While this is the strongest part of the tendon, it is also sees the greatest load so that tears of the subscapularis tendon usually start there.

This is all of great relevance to the repair of the subscapularis after shoulder arthroplasty.

Even though multiple sutures are placed, the greatest load is on the upper ones, especially when the adducted arm is externally rotated 

which is why it is important to 

until the subscapularis tendon repair is completely healed.

See this related link.