How serious is Michael Wacha's scapular stress injury?

The mechanisms and factors that led to Cardinals starting pitcher Michael Wacha's stress injury—described as either a stress reaction or stress fracture—of the scapula are complex. Rarely seen in the overhead throwing athlete, the scapular stress injury is one that can often go undiagnosed because of its ambiguous symtptoms and its rarity in the sport. Both a blessing and a curse in that its primary treatment is simply rest and it rarely requiring surgical intervention, a successful return from a scapular stress injury lies in a safe resumption of activity, doing so without sustaining continued insults to the scapula that often can lend themselves to a fracture of a more severe nature.

The scapula plays a number of crucial roles in generating shoulder and arm actions in throwing. It exhibits three planes of motion and translations in two directions that allow for the scapula to perform the following actions: elevation, depression, abduction, adduction, lateral rotation, medial rotation, anterior tipping, and posterior tipping. It provides bony anatomy for two of the three articulations of the bones of the shoulder (the clavicle, the humerus, and the scapula)—the glenohumeral and the scapulothoracic—while also providing insertion/origin points for seventeen muscles, including all four muscles that comprise the rotator cuff.

The mechanism of scapular stress injury in baseball players appears to be secondary to both repetitive muscle action on the scapula and muscle fatigue. With increased and prolonged muscular action on bone, rotator cuff muscle fatigue can also result in stress injury to the scapula. As these muscles fatigue, their ability to dissipate force and stress away from bone diminishes, placing the bone at increased risk for stress injury. With this, a fatigued muscle may also result in unfettered pull from an antagonistic muscle group, causing increased deformative stresses on the bone.

Also, given the weightlifting routines inherent to all players' routines, the muscular hypertrophy derived from exercise and weight training proceeds faster than the bone can remodel and repair itself from minor insults. This can lead to stress injury because it creates an abnormal imbalance of forces placed on the bone, with tiny cracks in the bone the result of these imbalances and stresses.

With respect to pitching mechanics, the delicate balance between correct mechanics and control of the scapula are highlighted during the pitching motion. This puts the scapula in prolonged positions of increased upward scapular rotation and retraction during pitching, which is important during the late cocking phase of pitching, as it aids proper movement of the glenohumeral joint. The late cocking phase is one of heightened rotator cuff activity, with the trapezius and serratus anterior muscles—both of which have attachments on the scapula—firing to stabilize and retract the scapula, allowing the initiation of acceleration of the humerus and hand towards the batter.

The deceleration phase of the pitching cycle is also one that finds the scapula susceptible to injury, as forceful contraction of the muscle groups of the shoulder, in particular, those of the posterior shoulder with attachments to the scapula, provides the necessary dissipation of energy to smoothly slow the arm down, post-throw. It is at this point that the rotator cuff contracts most powerfully, as it tries to stop the humerus from dislocating out of the glenohumeral joint.

The faster the humerus is pulled away from the scapula and the greater the rate of internal rotation of the humerus, the more active the muscles of the posterior shoulder must be to maintain joint stability. It is here that potential strength imbalances and fatigue arising from the muscles that attach to the scapula (especially the teres minor) can predicate a pitcher to scapular stress reactions and fractures. Previous studies have shown no particular pattern with respect to the location of the stress injury in baseball players, with one player sustaining a fracture of the lateral border of the scapula, with other reports finding fractures occurring at the junction between the neck and body of the bone.

In athletes in general, the incidence of stress fractures is reported to be between 1.4 and 4.4 percent, with most fractures occurring in the ribs and feet. Scapular stress injury in baseball is even more rare; aside from Wacha, the only other documented cases being suffered are by Edwin Correa, Kurt Ainsworth, and Brandon McCarthy. Of these three historical scapular stress injuries, McCarthy is one that is most frequently referenced, not only due to the rarity of the injury, but his ability to persevere through multiple flare-ups of his original scapular fracture, which was suffered in late 2007, and continue his career, something which neither Correa or Ainsworth could boast.

The pairing of McCarthy and Wacha doesn't end with this diagnosis, as both are righthanded pitchers who not only are very tall and slender—McCarthy is listed at 6'7", 200 pounds, Wacha at 6'6", 210 pounds—but also display (or at least displayed at the time of initial insult) mechanics that put the scapula at increased levels of elevation, retraction, and lateral rotation, which were consistent with a pronounced tilt of the spine in order to allow for an unhindered arm path at a high three-quarters, 'over the top' delivery. Wacha, in particular, uses this arm angle to his benefit, mostly for added downward plane on his changeup. McCarthy has evolved mechanically to a lower three-quarter arm slot, but did exhibit the higher slot with spine tilt similar to Wacha in 2007, when he first suffered a scapular stress injury. A comparison of the horizontal (left) and vertical release points over the course of the their careers (at least that has been captured by PITCHf/x) shows these similarities.

McCarthy:

Wacha:

As seen in the graphs, both pitchers averaged vertical release points roughly higher than their heights, with McCarthy exhibiting a lot of vertical height to his release point in 2007. A concomitant 'small' horizontal release point—right around a half foot away from their bodies—broadly confirms this very exaggerated, over the top delivery by both. As a brief comparison, Justin Masterson, a similarly tall (6'6") pitcher with a pronounced sidearm delivery, sits around five and a half feet of vertical height and a little over two feet of horizontal extension away from his body in his delivery.

Could the prolonged and extensive amounts of elevation, retraction, and lateral rotation and the coinciding rotator cuff muscle imbalances and weakness, along with Wacha's body type, predispose him to bouts of scapular stress injury?

While there could be numerous explanations for McCarthy's drop in arm slot after his first stress injury, the possibly problematic mechanics and anatomy that McCarthy displayed at the onset of his scapular issues could very well have been remedied with the slight drop in arm slot; while he has had recurrences of the stress fracture, over time, they have become less limiting, with McCarthy not spending significant time out of his given team's rotation due to stress fractures since 2011.

With Wacha undergoing another MRI today to determine whether the next course of action will be further rest and treatment or the initiation of a throwing program in preparation for his return to the Cardinals this season, there is much riding on the next 24-48 hours for Wacha's immediate future. Longer term, if McCarthy is any indication, Wacha's future remains less concrete, as the possibility of a flare-up or a full-blown fracture sustained still looms large. Even with a worst case scenario, a recurrence of the injury still lends itself to be one that will allow Wacha to be productive, albeit in spurts. Whether Wacha nips the scapular issues in the bud for good remains to be seen; the impact of his pitching mechanics and his adherence to them, despite their potential for incurring shoulder issues, remain in question.

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Data courtesy of Brooks Baseball.

Stuart Wallace is an associate managing editor and writer at Beyond The Box Score. You can follow him on Twitter at @TClippardsSpecs.

Other References:

Dines, J. S. (2012). Sports medicine of baseball. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.

Herickhoff PK, Keyurapan E, Fayad LM, et al.. Scapular stress fracture in a professional baseball pitcher. Am J Sports Med 2007; 35: 1193–1196.

Levine BD, Resnick DL. Stress Fracture of the Scapula in a Professional Baseball Pitcher: Case Report and Review of the Literature. J Comput Assist Tomogr. 2013; 37(2):317-319.