Hand injuries are common in baseball, and can not only result in impaired function and performance, but often create a significant amount of lost playing time in any given season. Given the location of the hands at the end of the kinetic chain, they are important in directing and stabilizing the power transferred by the lower half, core, and upper extremities into proper and accurate throws and swings. The hands and the numerous bones, ligament, tendons, and muscles therein are always at increased risk of insult due to their constant exposure to and importance in even the most trivial baseball tasks; thankfully, hand injuries heal in a more predictable fashion than other injury types, whether treated with surgery or non-invasive approaches.
For Toronto Blue Jays outfielder Melky Cabrera, it was a hand injury that ended his season prematurely — in particular, an avulsion fracture of his right pinky finger with involvement of an extensor tendon, suffered on a play that saw Boston Red Sox catcher Christian Vazquez pick Cabrera off first base. Lunging back to the bag with his right hand, Cabrera appeared to sustain blunt trauma to the pinky finger, with an eccentric mechanism causing the fracture. It also saw a small chunk of bone attached to the extensor tendon being pulled away from the main part of the bone. The extent of the avulsion led Cabrera to have surgery performed to repair the fracture, which was recently completed successfully.
While we don't have the exact tendon identified, given that it is Cabrera's pinky finger and it is a tendon of an extensor muscle, we have two options as to which muscle's tendon was inevitably injured: the extensor digitorum (ED) or the extensor digiti minimi (EDM). Having only two tendons to be concerned with can give a false sense of anatomical simplicity, but as we will see, when discussing the hands, this isn't necessarily the truth.
Each hand has twelve extensor muscles that comprise an extensor system that moves the wrist, thumb, and all fingers in the dorsal plane. Extensor tendons can also assist in radial and ulnar deviation of the wrist and contribute to the supination and pronation of the wrist and thumb. These muscles are extrinsic in nature, in that they do not originate from inside the hand, with our pinky finger muscles and tendons of interest originating from the lateral epicondyle of the humerus. As the tendons course toward the hand, they arrange themselves into a deep and superficial group, with the pinky tendons a part of it. Upon crossing the wrist, they course under a thickened band of tissue called the extensor retinaculum, which prevents displacement of the tendons via six compartments (or tunnels) of the retinaculum. As these extensors travel over the hand, bands of tissue called juncturae tendineae create an interconnecting web between tendons; this helps space the tendons, redistribute forces, coordination extension, and further stabilize the joints of the finger. This delicate anatomy is made even more complex through the interplay of these tendons and the incorporation of the intrinsic muscle system, and numerous interconnecting ligaments between the tendons and the volar plates of the phalanges, which are ligaments attaching two bones together. The splitting of extrinsic and interosseus muscle tendons into bands and slips also provide a large amount of anatomical variability and complexity to the hand. In terms of final attachment of the two pinky tendons of interest, they both insert on the middle, and distal phalanges, with the EDM also inserting onto the distal phalanx.
The aforementioned is not exhaustive, but provides a glimpse of the convoluted and tortuous details of anatomy of the hand and is enough to move forward and discuss the potential particulars of Cabrera's injury. Given the mechanisms involved, what he more than likely suffered was a mallet finger, which is a forced flexion of the distal interphalangeal (DIP) joint while the terminal extensor tendon — likely the ED — is actively contracting. For Cabrera's situation, his hand was fully extended in an effort to touch the base before a tag was applied and banged his fingertips into the side of the base, creating the unexpected flexion of the DIP joint. This can cause an acute 'drop finger' deformity, where a loss of function and congruency of the DIP joint causes the distal phalanx to be hyperflexed.
Mallet injuries can be further divided into soft and bony injuries; considering the avulsion sustained also required surgery, Cabrera's probably suffered a bony mallet fracture and one that included a large avulsed fragment. A large avulsed fragment typically is one that sees 40% or more of the articular surface of the tendon removed due to the insult. Displacement of the fracture can also add complexity to the situation, as it requires manipulation of the joint and an open reduction to completely repair the fracture. Surgical reconstruction of the tendon can also introduce postoperative complications as well as an increased potential for loss of joint congruency and degenerative changes of the pinky and the joint.
As far as baseball activities, Cabrera should be fine to resume them soon and be back for Spring Training; of course, the severity of his injury and the surgical procedure undertaken will complicate matters. Surgical pinning of the joint can be temporary or permanent, with permanent fixation of the DIP joint in a neutral position a small but real possibility, thereby preventing Cabrera from flexing the DIP joint. While this won't have any effect on his throwing, the return of his hitting wares would be the skill at most risk for sustaining any detrimental effects from the injury. The pinky finger is a significant contributor to grip strength and the ability to fully wrap his pinky around the bat (or lack thereof) could pose problems not only with finding a general level of comfort holding the bat, but also potentially with power from the right side, as the affected hand—his top hand when hitting righty—is responsible for providing much of the power of the swing and for maintaining a direct path to the ball. Batting lefty might still show Cabrera to have issues with the injury and could manifest itself in slightly slower bat speed and also a tougher time in getting the bat around on inside pitches. [Note: I have edited the last two sentences after a tweet - Cabrera is a switch hitter, not solely a righty. I have added a comment below with further clarification]
A return to play in time for Spring Training next season are all but guaranteed for Cabrera. However, the extent of the injury sustained and the complexity of the surgical procedure will ultimately determine how well he bounces back and how much, if at all, the unfortunate break will affect his hitting and power.
Dines, J. S. (2012). Sports medicine of baseball. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.
Henry, G.I., & Molnar, J.A.. Extensor Tendon Lacerations. Retrieved September 14, 2014, from http://emedicine.medscape.com/article/1286225