Monday, January 30, 2017

The subscapularis - the importance of the superior band.

Subscapularis tendon loading during activities of daily living

These authors used a computer model to estimate the relative amount of load transmitted through the superior portion of the subscapularis during activities of daily living as compared with the load that is transmitted through the middle and inferior portions in a normal shoulder and in a shoulder with a supraspinatus tear.

The model suggested that the maximum force produced by the entire subscapularis muscle for the various activities ranged from 3N (reaching to opposite axilla, eating with hand to mouth) to 43 N (reaching back of head, lifting a block to head height).

The superior band bore the largest percentage of the total load of the muscle (95% ± 2%).

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

Comment: This study reinforces the idea that the superior aspect of the subscapularis is its most important aspect. Additional evidence can be found in the disproportionately large 'footprint' for the insertion of the upper subscapularis into the lesser tuberosity of the humerus (see below) and by the beefy 'upper rolled border' of the tendon seen at surgery.

When the subscapularis is incised in the surgical approach to the shoulder and then repaired at the end of the case, it is critical that the upper subscapularis be securely repaired back to bone.

A few years back, one of our residents, Vinko Zlomislic (now on faculty at UC San Diego), did a laboratory study demonstrating that the site of repair and arm position affect suture tension in subscapularis repair.

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). 

The authors concluded that the position of abduction had a profound effect on the tension in the different sutures of the subscapularis repair. Tension was concentrated at the superior suture, particularly in low angles of abduction. This may increase the risk of subscapularis failure at this critical aspect of the repair site.

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.