The subscapularis tendon was incised from the lesser tuberosity in 11 fresh-frozen human cadavers. A four-strand suture repair was performed to each of the two sites: (1) the lesser tuberosity (LT) and (2) the humeral neck (HN).
After each repair, the humerus was placed in four different positions (0, 30, 60, 90 degrees) of abduction with the arm in neutral rotation with respect to the plane of the scapula. In each position, a total load of 40 N was applied to the repaired tendon in the direction of action of the subscapularis.
The proportion of the total tension in each suture was determined for each repair site (LT & HN) and for each position of abduction.
With the arm in low angles of abduction, the superior suture (blue in the graphs below) experienced the greatest proportion of the overall tension in the subscapularis (HN=65% vs. LT=37%, p<0.01) in comparison to the mid-superior (red), mid-inferior (yellow) or inferior (purple) sutures.
As the arm was brought into greater degrees of abduction this difference lessened (HN=35% vs. LT=4% at 30 degrees, p<0.01; HN=4% vs. LT=2% at 60 degrees, p<0.05). In 90 degrees of abduction the tension of the superior suture at both sites was not different (HN 4% vs. LT=6%, p=0.71).
Comment: This is an important study in that in helps us realize that all sutures of a subscapularis repair do not share equally in the load and that the amount of abduction changes the load distribution among the sutures.
Because the superior suture is the most important (see this link) and because it carries the most load when the arm is adducted, we avoid the stretch shown below after shoulder arthroplasty.
We have a new set of shoulder youtubes about the shoulder, check them out at this link.
Be sure to visit "Ream and Run - the state of the art" regarding this radically conservative approach to shoulder arthritis at this link and this link
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