The biomechanics of subscapularis repair - all sutures are not equal!

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.

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.

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Rotator cuff repair: all sutures are not equal

Differential suture loading in an experimental rotator cuff repair.

The primary mechanism of failure of a rotator cuff repair attempt is suture tension overloading the ability of the tendon edge to resist pull out. Normally, of course, the rotator cuff tendon is attached to the tuberosity via millions of tiny but strong fibrils. Load applied to the tendon is shared among these many fibrils - like a parachute with many points of attachment to the canape

In a rotator cuff repair, even with the most modern methods, the points of attachment of the cuff to bone are few.

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Noting that repairs of large rotator cuff tears often fail to heal because the suture pulls through the tendon, this study explored the loads applied by the suture to the tendon.

After a 4-suture supraspinatus repair with transosseous sutures, the arm was placed in 12 different positions. The tension in each suture was monitored using individual load cells.

This study demonstrated that loads applied to the repaired tendon were not shared equally among the sutures. When the arm was externally rotated relative to the plane of the scapula, the tension in the anterior suture was over 10 times that in the posterior suture (P < .001). When the arm was internally rotated, the tension in the posterior suture was over 10 times that in the anterior suture (P < .0005).

The authors concluded that suture tension could be concentrated in one of the repair sutures, increasing the risk of suture tension overload, especially in the presence of suboptimal quality of tendon tissue at the margin of the tear.

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Suture Number Determines Strength of Rotator Cuff Repair - JBJS

Suture Number Determines Strength of Rotator Cuff Repair - JBJS

This study uses a sheep cadaver infraspinatus repair model to test the hypothesis that more sutures make a stronger rotator cuff repair.

We recognize that the primary mechanism of failure of cuff repair is the sutures pulling through the tissue of the tendon. While other mechanisms (suture stretch, suture breakage, knot failure, and bone failure) can occur, they do not seem to be nearly as much of a problem as pull-through.  Now common sense would suggest that the load necessary to cause pull-through of each suture passed through a tendon is primarily dependent on the quality of the tendon. Thus if the quality of the tendon is uniform (as was probably the case in this model) and if the nature of pull allowed for each suture to be loaded equally (as was probably the case in this model), twice the number of sutures should be able to withstand twice the load. This expected linear relationship is demonstrated by our graph of the authors' data shown below.

Even though we know that the quality of tendon is not uniform in a clinical cuff tear, it does stand to reason that more sutures will do a better job of distributing the load applied to them so that there is less load per suture. The load per suture can also be reduced by not repairing the cuff under tension and by immobilizing the arm in some abduction after surgery.

Thus, even though this is an oversimplified repair situation, it does make the point that more is more.

There is another issue to be raised, however. The infraspinatus, in animal or man, has a relatively uniform direction of pull. By contrast, the most commonly torn human tendon, the supraspinatus, lies on the top of the shoulder so that its directions of pull vary with shoulder position. These changes in shoulder position result in differential loading of the sutures as demonstrated by Howe et al, the abstract of which is reproduced here:

Differential suture loading in an experimental rotator cuff repair.

BACKGROUND:
Repairs of large rotator cuff tears often fail to heal. A possible factor in these failures is excessive tension in the repair sutures, causing them to pull through the tendon.

HYPOTHESIS:
Arm positions encountered during early rehabilitation after cuff repair can dramatically increase the relative tension in the different sutures of the cuff repair.

STUDY DESIGN:
In a human cadaver model, a 4-suture supraspinatus repair was carried out with transosseous sutures. After the repair, the arm was placed in 12 different positions. The tension in each suture was monitored using individual load cells.
RESULTS:

When the arm was externally rotated relative to the plane of the scapula, the tension in the anterior suture was over 10 times that in the posterior suture (P < .001). When the arm was internally rotated, the tension in the posterior suture was over 10 times that in the anterior suture (P < .0005). When the arm was in neutral rotation, there was no significant difference in the suture tension.

CONCLUSIONS:
This study is the first report of direct suture tension measurement after a model rotator cuff repair. In this model, 30 degrees of either internal or external rotation of the arm in relation to the plane of the scapula created substantial imbalances in the tension between the most anterior and most posterior sutures of a supraspinatus repair, regardless of the position of abduction.

CLINICAL RELEVANCE:
Avoiding external rotation stretching during the healing of supraspinatus repairs may prevent tension overload in the critical anterior suture.

From the study of Howe et al we can conclude that arm positioning can cause high individual suture tensions, risking individual suture pull-through, even with large numbers of sutures.

The final issue with the recent JBJS article is that one of the authors'  failure endpoints was formation of a gap at the repair site of 20 mm - i.e. two centimeters!! We suggest that failure of cuff healing is likely with much smaller gaps separating the tendon from the bone.

This is why we prefer an 'inlay' technique (shown below) to an 'onlay' technique in cuff repair.

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