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May 3, 2012
ACL Tears
Past Precedent

by Kevin Pelton

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Since Derrick Rose suffered a torn ACL last Saturday, there has been plenty of discussion by players, coaches, media members and fans about whether Rose's injury, along with another torn ACL for Iman Shumpert of the New York Knicks, could be traced back to this season's compacted post-lockout schedule. The evidence, including my comprehensive research and a slightly different study posted by Doug VanDerwerken on TeamRankings.com, suggests that injuries are flat or even down this season.

I will say this for the cynics: Derrick Rose's injury was in one sense far out of the ordinary. It was the first ACL tear suffered by a current All-Star--that is, Michael Redd doesn't count as an All-Star in 2009--since Danny Manning in 1995. To find a player of truly comparable ability suffering a torn ACL, we'd have to go all the way back to Bernard King in 1985. So Rose's injury, along with Shumpert's and one suffered by rookie standout Ricky Rubio during the regular season, makes it worthwhile to investigate the nature of ACL injuries in the NBA. To study the issue, I updated a spreadsheet I began several years ago with the help of detailed research by the good folks at the APBR.

Going back to the 1999-00 campaign and not including this year's results, I found 40 ACL tears suffered by NBA players in games, practices, or even during summer workouts while under contract. That averages out to 3.3 per year, lower than the figure ("about five") Stern quoted to ESPN Radio's Colin Cowherd. My data may be incomplete; either way, the four we've seen this year is nothing unusual.

Of those 40, 22 involved players with usable pre- and post-injury numbers to compare. That's a relatively small sample size, but such is the nature of rare injuries. On average, these players saw their per-minute winning percentage drop from .452 to .405, a 10.4 percent decline in their effectiveness. 15 of the 22 got worse.

As interesting is a comparison of some of their key statistics before and after the injury:

Period     MPG    Usg    TS%   Reb%   Ast%   Stl%   Blk%   FTA%   Win%
----------------------------------------------------------------------
Before    24.0   .206   .528   .113   .026   .013   .013   .119   .452
After     21.7   .194   .502   .114   .024   .013   .012   .114   .405

Despite the obvious impact on athleticism, players coming back from torn ACLs have generally maintained their performance in the "hustle" statistics--rebounding actually up slightly, steals and blocks similar. The change is most evident in their scoring numbers, as players coming back from torn ACLs tend to use fewer possessions and are much less efficient. They also play fewer minutes and tend to miss more time, playing in 75.7 percent of their teams' games after returning.

Unlike what I found with microfracture knee surgery, there does not appear to be much connection between how long players missed and how much their performance suffered. For the most part, the time missed is simply a factor of when in the season players suffer an ACL injury, as most return in training camp. However, it's going to be tough for Rose and Shumpert to make it back by opening night. Of the four players who previously suffered torn ACLs in the month of April, Bonzi Wells (April 6) and Pat Garrity (April 15) were ready at the start of the season. Vitaly Potapenko (April 17) returned in December and Leon Powe (April 19) wasn't back until after the All-Star break. Only one other player (Steven Hunter, about four and a half months) made it back faster than Garrity.

There does seem to be a relationship between a player's age and how much their performance drops off, as shown in the following chart:

Because of the sample size, the relationship between the two is not statistically significant, but it suggests that a player of Rose's age (23.6) at the time of the injury could be expected to decline by 6.3 percent the following season. Shumpert, not yet 22, is in better position. In his case, the projected decline is just 2.7 percent. (The outlook is even stronger for Ricky Rubio, five months younger than Shumpert and projected to decline by less than two percent.) Of course, in any case, rehabbing a torn ACL means taking away opportunities for Rose and Shumpert to add to their game and develop over the summer.

Looking a year forward (and removing David West, still just one year out from his torn ACL), there is no evidence to support the conventional wisdom that players are better the second season back from an ACL tear. 11 of the 21 players saw their winning percentage decline between their first and second years back after the injury, and on average their winning percentage declined slightly. This is not to say that Rose, Rubio and Shumpert won't improve as young players, just that there's nothing magical about being two years out from an ACL tear.

The good news is that ACL repairs have become fairly routine in the modern NBA. Still, even when the three young guards who have suffered torn ACLs recently are back on the floor, the effects of their injuries could linger.

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Kevin Pelton is an author of Basketball Prospectus. You can contact Kevin by clicking here or click here to see Kevin's other articles.

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