Sunday, March 5, 2023

The subscapularis - anatomy, failure and reconstruction

An active man developed glenohumeral arthritis after a prior stabilization procedure on his right shoulder. He elected a ream and run arthroplasty.

His initial rehabilitation went very well, with excellent range of motion and return to all activities including cross country skiing and light weights at five months after surgery. Around that time a well-meaning relative suggested dumbbell curls with the arm adducted. However, these were associated with increased range of external rotation and pain at the subscapularis insertion on active internal rotation.

He had an incident in which he was pulling on his stockings up and felt a sudden sharp severe pain in the shoulder that radiated down to his fingers. Since that episode, he had persistent instability of the right shoulder and feelings that it slides out to the front. If he put his right hand in front of the right shoulder and pushed in, the sensation improved.

On examination he had external rotation to 90 degrees

and weakness of internal rotation.

A metal subtraction MRI was inconclusive about the status of the subscapularis tendon.

He declined a reverse total shoulder and instead elected to proceed with exploration of the shoulder and a plan for subscapularis reconstruction. At surgery the upper subscapularis tendon was torn from the repair site at the lesser tuberosity as if it had been unzipped from top down.

The glenoid had healed over with smooth fibrocartilage.

The subscapularis reconstuction was accompliashed using a braided hamstring allograft passed through drill holes in the lesser tuberosity

and then through the subscapularis medially.

The free ends of the graft were then secured to the humerus.

The shoulder was immobilized for two months, followed by a careful rehabilitation program avoiding stressing the subscapularis by passive external rotation or active internal rotation.

He recently reported: "Good morning. Everything is going very well. We arrived at Whistler yesterday for a ski week. I happily celebrated my one year anniversary following our second surgery. I continue to do my shoulder exercises on a regular basis using 10 pound hand weights for press ups while sitting for 30 to 40 reps, 12 pound weights for press ups while supine for 30 to 35 reps, 15 pound weight lift from floor while kneeling on chair for 30 reps, isometrics, band stretch. I continue to do the pulldown bar with 55 pounds for 30 reps. I use a bench type device where I sit and pull 55 pound weight for 30 reps. I do a few reps using the 70 pound weight. As far as skiing goes, I have no shoulder pain at all with either cross country skiing or downhill skiing. So as far as I’m concerned, I’m cured."

This case points to the importance of both careful repair of the subscapularis at shoulder arthroplasty

with particular attention to the suture at the superior part of the tendon

and considering reinforcing the repair with sutures in the rotator interval.

It is of interest that the load on the subscapularis is not evenly distributed along the superior-inferior extent of the tendon. The authors of Subscapularis tendon loading during activities of daily living 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 to the load that is transmitted through the middle and inferior portions.

The model indicated 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 chart below shows the distribution of load during 10 different activities.

It is evident that the superior aspect of the subscapularis is most important. This is consistent with 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 shown as "SC" in this figure from the classic work by Clark and Harryman Tendons, Ligaments, and Capsule of the Rotator Cuff

in this cadaver dissection (blue arrow)

and in this arthroscopic image (see SSc)

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

The disproportionate loading of the upper subscapularis was also demonstrated by one of our residents, Vinko Zlomislic (now on faculty at UC San Diego), who 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 in comparison to the mid-superior (red), mid-inferior (yellow) or inferior (purple) sutures. The proportion of the load carried by the superior suture became less as the shoulder was abducted.

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 it pointed out 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.

The authors of Tears of the Subscapularis Tendon: A Critical Analysis Review found the tendon footprint to be broad proximally (a tendinous insertion) and tapered distally (a muscular insertion) along with a number of anatomic insertions that are separate from the main tendinous attachment. These include a medial capsuloligamentous insertion, a inferior musculocapsular insertion, a “lateral hood”, and a superior “tendinous slip,” which is attached to the fovea capitis of the humerus and con- tributes to biceps stability. Once more they found that the majority of tears of the subscapularis tendon begin at the articular surface of the superior insertion of the tendon and extend inferiorly.

Comment: It is concluded that the upper aspect of the subscapularis insertion to the lesser tuberosity is disproportionally important. After arthroplasty it needs to be carefully repaired and protected from loading while it is healing. 

Stretching in flexion does not challenge the upper aspect of the subscapularis repair.

By contrast, stretching in external rotation with the elbow at the side disproportionally loads the critical attachment of the upper subscapularis.

as do falls

and unexpected jerks on the arm.

If the subscapularis should fail after shoulder arthroplasty, consideration can be given to reinforcing the repair with a braided hamstring graft.

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Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link).
The total shoulder arthroplasty (see this link).
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link).
Shoulder rehabilitation exercises (see this link).