Brandon McCarthy: Scap Load Failure

Yesterday, for the second time in his 3-season stint with the Texas Rangers, Brandon McCarthy was diagnosed with a stress fracture of the scapula. With consideration to the number of muscles that move and stress the scapula when throwing a baseball, it's amazing how rarely scapular stress fractures are diagnosed in pitchers.

In 1987, Texas Rangers pitcher Edwin Correa was diagnosed with a stress fracture in his scapula. Correa never again pitched in the Major Leagues.

In 2003, Kurt Ainsworth, then pitching for the San Francisco Giants, was also diagnosed with a stress fracture in his scapula. Ainsworth recovered but pitched in only 7 more games in the Majors, all in 2004.

The most detailed information that I can find on this type of injury is what I know from McCarthy's previous stress fracture and what I learned from reading "Scapular Stress Fracture in a Professional Baseball Player," a study published in the American Journal of Sports Medicine in February 2007.

The study takes a look at the injuries suffered by an unidentified right-handed Major League Baseball starting pitcher. This pitcher's mechanics were apparently a mess. Prior to his scapular stress fracture, the pitcher had been through Tommy John (ulnar collateral ligament replacement) surgery and a "transient episode of subacromial bursitis" in his shoulder. In the three years that followed his recovery from the stress fracture, this pitcher had surgery on both his elbow and shoulder, though neither was directly related to his scapula injury.

Pitcher X's stress fracture was located on the lower outside edge of the scapula bone, called the lateral border. The doctors who authored the study concluded based on the fracture's location that the likely cause of Pitcher X's stress fracture was "repetitive stress in the area of the teres minor attachment."

Repeated stress on muscles and bones causes them to grow stronger and more resilient over time. When the growth can't keep up with the stress, stress fractures occur in bones and tendonitis occurs in muscles. In the case of Pitcher X, his teres minor grew stronger at a faster rate than his scapula. Eventually, the stress fracture developed.

Muscles with origins on the scapula. Subscapularis and biceps brachii not shown. Click to enlarge.

The teres minor attaches the lateral border of the scapula to the outside edge of the back of the humerus (see the diagram). It is one of the muscles of the rotator cuff, and its primary function is external rotation. The teres minor is stretched by internal rotation when the back of the humerus turns away from the scapula. It is also stretched as the humerus moves away from the scapula.

In the delivery, the rotator cuff contracts most powerfully during the follow-through as it tries to stop the arm from twisting and flying out of socket. The faster the humerus is moving away from the scapula and the greater the rate of internal rotation, the more powerful the contraction must be to maintain joint stability.

Brandon McCarthy's follow-through is a little unique in this regard. I missed it when I reviewed his mechanics last month because I wasn't looking for it. Of course, maybe I'm just seeing what I want to see. Here's the clip I used for my original analysis:

Notice that, after release, McCarthy's arm continues to move away from his scapula and towards his head. Even at 210 frames per second, it only lasts for a few frames, so look closely. You might even see his shoulder blade "hump up" a little around this time.

This high-intensity eccentric contraction stresses the teres minor muscle more than the other rotator cuff muscles because its scapular attachment is the furthest from the joint.

A reasonable outlook

Rangers general manager Jon Daniels has reportedly said that McCarthy will not pick up a baseball for several weeks. That might be a bit of an exaggeration, depending of course on the severity of the stress fracture. McCarthy himself said the pain has been there for a while and that he feels like he did in 2007.

McCarthy suffered and recovered from an injury similar to this one back in August/September 2007. I can not say how similar because I do not know the exact location and severity of either his 2007 or his 2009 stress fracture. McCarthy recovered from the 2007 injury fairly quickly and missed only a month of Major League action.

Rest is the only way to recover from a stress fracture. While the bone heals and gets stronger, the offending muscle atrophies and weakens - killing two birds with one stone. Often times, the pain will subside long before the bone fully heals, sometimes months after diagnosis. It might be that McCarthy's original stress fracture never healed.

I believe the Rangers are more or less "on the lookout" for injuries like this with McCarthy, so I feel that this injury is probably an early-stage stress fracture. They will probably keep him from throwing until scans no longer show signs of the fracture. Hopefully, this is no longer than 3 or 4 weeks.

Last month, I said that if McCarthy could stay healthy and have success with his mechanics, there was no reason to change them. I now believe there's sufficient reason to start that process. McCarthy and the Rangers need to give serious consideration to making major changes in his delivery.

[I suspect that Pitcher X is Darren Dreifort, though the article was written by doctors in Baltimore.]