New York Mets pitcher Matt Harvey and starter Jeremy Hefner may undergo Tommy John surgery, but how does the unusual procedure work?

Harvey has been diagnosed with a "partially torn ulnar collateral ligament in his pitching elbow," and is unlikely to play again this season, ESPN reported. He may decide to use Tommy John surgery, but may opt to work on strengthening the area instead.

Hefner has a partially torn medial collateral ligament, also in his pitching arm. After a second opinion, Hefner will have Tommy John Surgery.

"I kind of knew it was coming, and I had some time to process it, too, before they told me," Hefner said on the day of his second opinion, ESPN reported. "I'll see what the second doctor says and go from there."

Hefner admitted to holding off on telling his team about his elbow discomfort and "pitching through the pain," this may have worsened the injury.

Harvey has not yet decided whether or not to get the surgery, but "pitching through partial tears" is often unsuccessful.

"Doctors will always tell you that if you could avoid surgery you should," general manager Sandy Alderson, said. "However, we have had situations in the past -- not just here, but across baseball -- where the conservative approach doesn't work, and what you end up with is a loss of time and therefore a delay in recovery. That all has to be taken into account. We're all aware of what [a] delay might cause, and so we will be monitoring the situation. And I'm sure Matt will be making a decision in conjunction with us that takes that into account."

So what is Tommy John surgery?

The procedure was first performed on Los Angeles Dodgers pitcher Tommy John in 1974, WebMD reported.

"A surgeon replaces the injured UCL with a tendon taken from somewhere else in the patient's body," according to WebMD.

Any type of repetitive stress on the elbow can cause UCL damage, but candidates for Tommy John surgery are primarily athletes who participate in "throwing sports," such as baseball, tennis, and gymnastics.The procedure has a recovery period of about 12 weeks.

Symptoms that come along with a UCL injury include: pain on the inside of the elbow, "looseness or instability in the elbow," inability to throw objects (such as a baseball), and irritation of the "funny bone."

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