The Effects of Tommy John Surgery on Batters

The new prevailing trend in major league baseball is a disturbing one. It is a trend of exponentially more frequent Tommy John surgeries. During the surgery, the ulnar collateral ligament is replaced by a different tendon from elsewhere in the body. As would be predicted, pitchers suffer from the injury much more than batters because they are constantly stretching their arm to full extension and pitching at high velocities. However, there are times when batters must have their UCL repaired. The unfortunate truth is that there is little data on what may happen to batters when they return. Most analysts report that the surgery has little to no affect on batters' performance. This isn't true.

My search for answers began when I heard the news that Matt Wieters, the Baltimore Orioles catcher, would need to undergo Tommy John surgery. Suddenly, I realized that nobody really knows how he will fare when he returns next season. Same thing applies to Minnesota Twins' top prospect Miguel Sano. Sano, the Twins' powerful third baseman of the future, had to have his UCL replaced before the season began to the disappointment of prospect and Twins' fans alike. The same kind of disappointment felt when Jose Fernandez needed to have Tommy John surgery. The injury is affecting more players at an exponential rate and there is little data (particularly in regards to batters) that suggests how it will affect them when they return.

I scoured the internet for the complete list of players who have undergone the procedure and came across a massive list of 737 confirmed players (major and minor leagues) and crossed out everyone that was not a position player. I was left with a meager list of 29 names from the major leagues (minor league players were excluded because of the distinct differences from each minor league level). After removing even more names of players who may have appeared briefly in the major leagues or had the surgery and never returned to playing, I was left with just 15 confirmed names. Stars of the times like Paul Molitor, one of the very first recipients of the surgery, and lesser known players like Kyle Blanks both stood out on the list.

The next step in the process of unraveling the mystery behind the surgery was to figure out how the surgery affects the batters. In other words, I wanted to test if different tools were affected and in what ways. Did batters hit for the same amount of power as they did before? To begin, I collected data to test for three different measures of arm strength.

Batting Average on Balls in Play (BABIP) determines the rate at which balls put into play are turned into hits. While this is not entirely based on arm strength, arm strength is a large factor in placement of the ball coming off the bat. A more powerful swing will lead to more balls in play being turned into hits. More on that here.

Slugging percentage (SLG) was the next piece of data I tested for. If a batter could hit the ball further, then they could have more extra-base hits.

Similarly, I tested for Home Run to Fly Ball percentage (HR/FB). This measures the rate at which fly balls go over the outfield walls and become home runs. Another barrier to success, as can be seen in the image below, was that there was no recorded advanced fielding data prior to 2002. So it is possible that the HR/FB data is less diluted by sample size than the other measures.


Honestly, the results were surprising. Like most analysts, I believed that they would be right in saying that the surgery has little to no affect on batter strength. I found this to be wrong though because, on average, most batters did experience a non-negligible decrease in BABIP, SLG, and HR/FB.


Of the 15 tested batters, 12 experienced a decrease in BABIP, culminating to an average decrease of 0.015. In the sabermetrics world, statistics dictate all research and this is no exception. A 0.015 decrease is another way to say, "1.5% less balls in play lead to hits". Whether this can be attributed to luck, fielding, or less power is another question. But with over 65,000 at bats worth of data, there should be a sizable amount of batter-driven results rather than deferring the results to worse luck or better fielding. In perspective, a 1.5% decrease in batting average causes a drop from .300 to .285.

Slugging percentage was the most impactful finding though because, of the 15 batters, 11 experienced a decrease in slugging percentage. A reminder that each surgery occurred at different points in the batters' careers, meaning that natural weakening with age should be filtered out. Overall, the data combined to form a 0.419 drop in slugging percentage or an average 0.028 decrease post-Tommy John surgery. 2.8% less hits were extra-base hits for the remainder of these batters' careers. A significant amount when considering that some of these batters had careers lasting fifteen years or more.

Home Run to Fly Ball rate had to be adjusted to take into account the emergence of fly ball data in 2002 (I removed the home runs hit before 2002 before calculating). Of the 9 batters tested, now 7 of them experienced a decrease in their HR/FB rates. This all comes out to be a 0.018 decrease, meaning 1.8% less fly balls zoomed out of the park and into the stands. The major league average usually stands at 10% but these batters saw their power drop from 10.1% to 8.3% after the surgery.

The only thing left to say is that analysts and fans alike need to recognize the fact that Tommy John surgery does have a negative effect on a batters' power. Mostly though, I'm disappointed Miguel Sano's power will never be what it could have been.
Thanks to Fangraphs for all batting and advanced fielding data and for the complete Tommy John surgery encyclopedia

This is a repost from where I originally posted the article - on my website: