With two outs and runners on first and second in the fifth inning at Citi Field on Thursday night, Mets All-Star outfielder Michael Conforto swung hard through a 95-mph fastball from D-backs left-hander Robbie Ray. Conforto collapsed, grabbing his left shoulder and stayed down for the better part of a minute before being helped off the field by Mets manager Terry Collins and trainer Ray Ramirez. The slow-motion replay of the injury showed Conforto's shoulder moving in an obviously unnatural way beneath the pinstripes of his jersey.
"It turns your stomach," Collins said after the game. "A player was having a tremendous year and really making a name for himself. To go down like that, and that kind of injury, is tough to watch."
The Mets later announced that in addition to suffering a dislocation of his left shoulder, an MRI also revealed a tear in Conforto's posterior shoulder capsule, which is consistent with a dislocation.
What is a posterior shoulder capsule?
The shoulder is a ball-and-socket joint comprised of three bones: the upper arm bone (humerus), the shoulder blade (scapula) and the collarbone (clavicle). The head of the humerus fits into a socket of the scapula. Connective tissue known as the shoulder capsule surrounds the joint and is filled with fluid that lubricates the joint.
Dr. Christopher Ahmad, the president of the Major League Baseball Team Physician Association, used a creative and easy-to-visualize analogy to describe the joint and a capsule injury: "Imagine the shoulder ball-and-socket joint, which is much more shallow than the hip ball-and-socket joint, like a basketball on a dinner plate. If you touch the ball, it rolls off the plate, and that is a dislocation. So you want to keep the basketball in the plate. If you took some cake fondant and lined the rim of the plate, that would keep the ball in a little more easily. That fondant is equivalent to the labrum. The capsule is tissue, like fabric, that attaches from the fondant to the basketball itself. If you push the ball off the plate, you can have a tear through the fondant, which is the labrum, or the fabric, which is the capsule, or both."
Anterior shoulder dislocations, in which the humerus is forced out the front of the shoulder joint, are far more common than posterior ones, in which the humerus is forced out the back; posterior dislocations account for just 10 percent of all dislocations. Conforto's situation is even more rare because the injury affected his rear shoulder. Hitters more typically dislocate their lead shoulders, most often when reaching for outside pitches, when the shoulder is aggressively abducted. Dr. Ahmad said while he had certainly seen injuries to the rear shoulder of hitters, he had personally never seen a rear shoulder dislocation from swinging the bat.
"Conforto activated muscles during the swing, muscles that can force the shoulder out the back," Ahmad said. "The pectoralis and the latissimus are firing in a way that when his arm came across his body, the muscle forces caused it to come out the back."

What are Conforto's treatment options?
The fact that Conforto swung and missed exacerbated the problem. Like pitchers, hitters build energy from the ground that travels up the body through the kinetic chain and is then transferred into the bat and ball. During a swing-and-miss, there is no transfer of energy to the ball, and the shoulder must absorb all of the accumulated energy. In Conforto's case, that energy overcame the joint capsule.
Now, Conforto and the Mets must decide whether they will try to let the shoulder heal on its own or have it surgically repaired. With the non-operative treatment, the shoulder is rested to allow the tissues to heal, then the shoulder is aggressively strengthened to have the muscles dynamically stabilize the shoulder, then a hitting and back-to-play program is implemented. If it doesn't work, the player can have surgery.
How long will Conforto be out for?
When season and career timing, the severity of the injury and shoulder pathology are all taken into consideration, earlier surgery can be an option. While Conforto has never been on the disabled list for a shoulder injury, Collins did say in his postgame news conference that while he was unsure of specifics, he believed Conforto had had some shoulder issues in the past. Prior issues could increase the level of damage seen on the MRI and make surgery a more reasonable option.
"During surgery, we fix everything that's damaged," Ahmad said. "If the capsule is torn, we fix the capsule. If the labrum is torn, we fix the labrum. If the shoulder is extremely loose in general, we will tighten everything up with the stitching."
Following surgery, the player does not swing a bat for three months and typically returns to play in four to six months. So if Conforto had surgery immediately, a return in time for Spring Training would be realistic. Recovery could also be slightly easier and/or faster, because Conforto throws right-handed.
On Friday, the Mets placed the 24-year-old on the 10-day disabled list to give them time to evaluate his rehab options.