The carefully curated MLB preview plan that Grantland’s baseball brain trust put together last month said that today’s feature would be about the depth of two NL East rotations: the Washington Nationals’ and the New York Mets’. The schedule stayed that way until Monday morning, when news broke that Mets starter Zack Wheeler’s sore right elbow, which he said last week wasn’t painful enough to have cost him a start during the regular season, would cost him all of his starts during the regular season. Wheeler has a fully torn ulnar collateral ligament that will require Tommy John surgery, ending his season before it could begin.

Wheeler’s absence opens a rotation spot for Dillon Gee — an uninspiring but acceptable replacement by fifth-starter standards — at least until he’s displaced by one or more members of the Mets’ trio of just-about-big-league-ready pitching prospects: Noah Syndergaard, Steven Matz, and Rafael Montero. Although the Mets’ depth isn’t as enviable as it was a week ago, that’s only because some of their surplus has already served its purpose, cushioning the rotation from what must have seemed to GM Sandy Alderson like an almost inevitable blow.

Just as Wheeler’s injury reshaped the right-hander’s anatomy, the Mets’ roster, and the NL wild-card race, it altered our topic for today. Instead of writing about the ways in which teams build depth, I’m writing about the way depth gets depleted. It’s a relevant topic at this time of year, because while March probably can’t make a bad baseball team better, it can kill a competitive one.

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Although the annual casualty counts haven’t followed a smooth upward progression, Tommy John surgery has become more common in recent seasons.

With increased incidence comes increased scrutiny. We have some sense of why so many UCLs snap, and we have a good grasp of return timetables. There’s surprisingly little research, however, into TJ seasonality. Do Tommy John–causing injuries strike most often at certain times of the year? And is there any point in the season when we can trust pitchers to make their scheduled starts?

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Click here for more MLB Preview! Lately, ligament replacements have seemed like a spring training tradition, a depressing counterpoint to the influx of players in the best shape of their lives. Wheeler joins teammate Josh Edgin, Rangers ace Yu Darvish, Royals reliever Tim Collins, Pirates swingman Brandon Cumpton, and free agent Joel Hanrahan on this month’s major league ligament-replacement list. Last March claimed even more pitchers: Luke Hochevar, Kris Medlen, Brandon Beachy, Patrick Corbin, Jarrod Parker, Bruce Rondon, A.J. Griffin, Brian Moran, Josh Johnson, Cory Gearrin, and David Hernandez (not to mention Bobby Parnell and Matt Moore in the early days of April). Even when a pitcher is pulled in “precautionary” fashion, we’re now conditioned to preemptively lament the loss of anyone who admits to feeling tightness or discomfort in the forearm/elbow area. There’s something extra sad about our season pass to a promising pitcher getting canceled just when we were about to start watching, but by this point in the spring, baseball withdrawal could be skewing our perceptions of how often the awful occurs. Is spring really Super-Scary Tommy John Time?

We can answer that question with the injury database assembled by Corey Dawkins, an athletic trainer who works as an injury prevention specialist at the Micheli Center for Sports Injury Prevention in Massachusetts. Dawkins’s database, much of which is hosted at Baseball Prospectus, contains more than 800 Tommy John surgery records going back to the surgery’s namesake, including more than 350 at the major league level. Crucially, his records include not only each TJ victim’s surgery date, but also the date on which he first missed time because of his injury. That extra level of detail gives us a better indication of when each pitcher’s problem actually appeared (although, as I’ll explain, it’s not a perfect proxy).

Over the last 10 seasons, June has been the busiest month for Tommy John surgeons (albeit only barely). But surgery dates tell us only so much, since players will often try the conservative, rest-and-rehab route before conceding that going under the knife is necessary. By the time those rehabbers reluctantly get into their hospital gowns, months may have passed since their last big league appearance. While we can’t always determine when they felt the first twinge, we can tell when the pain progressed to the point that they could no longer pitch.

Here’s the key image, which shows how responsible each month was for all 197 of the database’s TJ-inducing injuries from 2005 to 2014. A few players had surgery over the offseason without having missed any games during the previous year, so their surgery months stood in for their injury months.

The graph confirms that we’re not imagining things: March, at 27 percent, really is the big, bad month for finding out about elbow injuries that lead to Tommy John surgery. That figure tells us that if there were 100 Tommy John procedures performed on MLB pitchers in a year — a made-up, implausible figure, fortunately — then 27 of those pitchers on average would have first missed time because of the injury in March. (If a pitcher hurts his elbow on March 15 and tries rest and rehab for a few months before giving in and getting the surgery in June, he still counts as a March injury, assuming he didn’t come off the DL in the interim.) In recent seasons, this tendency toward preseason UCL tears has become more pronounced: From 2012 to 2014, 36 percent of MLB Tommy John victims succumbed to injury in March. The most devastating months in the 10-season sample were March 2014 and March 2012, with 12 victims apiece. At the major league level, at least, Tommy John surgeries aren’t actually up significantly during the regular season; the increase has been concentrated during the spring. After March, the world gradually becomes a safer place for pitchers, save for a slight uptick in August.

So there’s real reason for TJ terror at this time of year. Should we worry about other injuries as well? The image below plots the TJ trends against two samples of control surgeries: non-TJ surgeries for pitchers and all surgeries for position players. Again, the graph reveals what percentage of the injuries that led to those surgeries first manifested in each month. We’re dealing with proportions here, so don’t take this to be a comparison of overall pitcher and position-player injury rates.

The takeaway: March is a tough time for pitchers, period. (We miss you, Marcus Stroman.) Compared with other pitcher surgeries, TJs are far more front-loaded, but even non-TJ surgeries for pitchers reach their highest point as April approaches. Batters, meanwhile, don’t have it noticeably harder in the spring than they do during the summer. Both groups see significant bumps in the fall, as players break down from fatigue and those who’ve been playing through injuries finish their seasons and have no reason to avoid getting whatever is broken fixed.

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Lisa Blumenfeld/Getty Images

There are two persuasive explanations for why there are so many injuries in March. The first is physiological: After spending several months at sub-season intensity, a pitcher might go from “zero to full in too short a time,” says Dr. Glenn Fleisig, research director at the American Sports Medicine Institute. “What happens is, your muscles, tendons, and ligaments didn’t have enough time to adapt. Your muscles are not conditioned yet for pitching, and they may overload the ligaments and tendons because the muscles haven’t been conditioned to midseason form.”

However, the elevated rate of spring injuries isn’t only a byproduct of what Dr. Jeff Dugas, an orthopedic surgeon at the Andrews Sports Medicine & Orthopaedic Center, notes is a relatively stressful “ramp-up period” between two plateaus that’s presumably grown steeper in an era when mid-90s velocity is no longer exceptional. Although I excluded from the sample any player who missed time at the start of a season while recovering from a previous surgery, that doesn’t mean every player I included reported to spring training in perfect health.

“The other group of people that you get in March are the people who you tried to rest, or you tried to inject, or you tried to [fix with] physical therapy, or you tried to go the conservative route,” Dugas says. “And their first chance to really demonstrate whether or not that plan worked is when they get back on the mound and face live hitting in February and March.” Between the pitchers who fall prey to the ramp-up and the “people who can’t make it because the nonsurgical approach didn’t work,” Dugas says, “you’re almost dealing with a dual number of potential patients.”

Wheeler belongs to the second crew. He suffered from elbow soreness last season, and though he never missed a start, he skipped some bullpen sessions. His MRIs in November and January showed no structural damage, but MRIs sometimes produce false negatives, and it’s not difficult for small tears to go undetected, particularly if the doctor doesn’t recommend the dye-enhanced option (which involves an injection that’s a roadblock for needle-averse pitchers and that features a small risk of infection and allergic reaction for all recipients). Given how quickly Wheeler’s complete tear manifested this spring, it’s unlikely it was totally unrelated to his problems last year.

The closer a pitcher comes to the end of the season, the more likely he is to stay silent about an injury, or at least to pitch through one without taking time off. The pennant race and postseason give pitchers extra incentive to stay on the field, and the prospect of several months off makes it easier for them to obey the common late-season dictate to “rest in November.” Postponing surgery is usually the more prudent approach, but if the damage is severe enough, one offseason might not be enough to undo it.

“What happens is, they come back in spring training, they felt good while they were resting or doing light toss,” Fleisig says. “But when they try to ramp back up, it turns out that the damage did not repair sufficiently.”

Losing a starter in the spring is a less crippling setback if his team is aware of his history, as the Mets were with Wheeler’s. It can be a bigger setback if the player keeps his pain to himself.

“That happens all the time,” says Stan Conte, vice-president of medical services for the Los Angeles Dodgers. “We get calls a lot of times in January saying, ‘Jeez, this thing just isn’t better.’ And we say, ‘Uh, what thing?’ ‘Oh, the thing that bothered me in September that I didn’t tell you about. I just figured it would get better over the offseason.’ Now we’re in January … and they come to spring training and fail, and then we end up having a problem.”

The greater the risk of injury, the greater the incentive teams have to inoculate themselves against it. A club that could avoid the injuries decimating other rotations during the exhibition season would have a huge advantage once the real games begin, but it’s hard to know how to optimize spring usage patterns.

“You go back to what seems to make the most sense, and that progression in spring training in regard to bullpens and velocities and those kinds of things is still pretty much the same as it’s always been, unfortunately,” says Conte, whose team is taking it especially slow with its intentionally surgery-scarred pitching staff. “You only have X amount of time to get ready. It’s interesting to watch each team, even in the number of innings they’re using their starting pitchers from the beginning. Some will start with one or two innings, others will start at three or four innings and progress up. I wish I knew which one was right.”

Altering players’ preparation over the winter might lay stronger foundations for the spring. “It may be that these guys are not transitioning into high-intensity work as well,” Conte says. “In other words, they’re working on their lifting, their conditioning, but they’re not really doing high-intensity baseball and throwing or swinging really, really hard. That’s where the injuries occur, when they up the ante from 85 or 90 percent intensity to 100 percent intensity.”

Chris Geary, chief of sports medicine at the Tufts University School of Medicine, agrees that tinkering with players’ fitness plans has the potential to reduce the demands on the body. “The Jose Canseco workout, with or without the PEDs, isn’t necessarily going to be the best thing for a pitcher,” Geary says. “There’s more evolution, I think, in terms of the trainers still figuring out what’s the stuff these guys should be doing in the offseason in terms of core work versus arm strength versus leg strength.” But Geary isn’t optimistic that any workout approach is a panacea. “I don’t think you’re ever going to completely reach a point where you say, ‘All right, the number of injuries in March is the same as the number of injuries in June,’” Geary says.

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Without putting pitchers on Sodium Pentothal, there’s no way to know what percentage of the March spike is attributable to new injuries and what percentage is explained by an exacerbation of old injuries that we (and in some cases, the player’s team) never knew about. From a fan’s perspective, or a fantasy owner’s, though, it doesn’t much matter whether the injury is entirely new or a more aggressive form of an existing flaw. Either way, March, more than any other, is the month pitchers fall apart. And while April is the next riskiest, it’s also a distant second, so a team whose staff escapes spring training unscathed has reason to celebrate.

Spring training, formerly dismissed as insignificant in analytical circles, is enjoying a revival in 2015. Recently, The Economist’s Dan Rosenheck showed that with the proper adjustments applied, exhibition stats do have some predictive power. And now we have strong evidence that March is a make-or-break month for team health, the origin of more than a quarter of the cases of a dreaded type of season-ending surgery. How training staffs handle the transition from winter to spring is only one skirmish in a larger war against injuries, but in some seasons, those skirmishes have a decisive effect on the standings. Mess with March, and you run the risk of being DOA on Opening Day.

Thanks to Matthew Trueblood for inspiring this inquiry in a year-old email, and to Jessie Barbour, Dan Brooks, Corey Dawkins, and Rob McQuown for research assistance.