Nine months after undergoing ACL reconstruction, Minnesota Vikings running back Adrian Peterson rushed for 2,097 yards — the second-highest total in a season ever — and won the 2012 NFL MVP award. Derrick Rose, the 2011 NBA MVP, underwent ACL surgery in May 2012 and has yet to play another game in a Chicago Bulls uniform.

Which of these scenarios is closer to normal? Better yet, what can we learn from each one?

Approximately 200,000 people suffer ACL (anterior cruciate ligament) injuries in the United States each year. Athletes and most active people choose to undergo ACL reconstruction. The surgeon uses tissue from the knee to make a new ligament in order to stabilize the knee and help the athlete return to sports and exercise.

Historically we have assumed that these surgeries and the athletes who undergo them fare well. A 2010 survey asked NFL team doctors what percentage of NFL players return to play. Ninety percent of the physicians answered “90 to 100 percent.” Unfortunately, studies published in the last few years paint a more ominous picture.

A study of NFL players who underwent ACL reconstruction showed that only 63 percent returned to play in a regular-season game within two seasons after surgery. A study of high school and college football players showed similar results. Sixty-three percent of high school football players and 69 percent of college players made it back two years after surgery. Only 43 percent of them had returned to pre-injury levels.

Finally, a 2011 study of competitive athletes in a variety of sports showed that only 33 percent of athletes had returned to their sports at their pre-injury levels 12 months after surgery.

Re-injury after ACL surgery also appears to be more common than we formerly believed. A 2013 article estimates that about one in every four athletes who returns to play suffers a second knee injury. A study by Donald Shelbourne, who developed the accelerated ACL rehab program used today, showed that 17 percent of athletes 18 years old and younger sustained a second ACL tear within five years.

How then can we predict when it is appropriate to allow an athlete to return after ACL surgery? And what factors can indicate whether an athlete will return to the same level of performance?

In the Adrian Peterson and Derrick Rose examples, using only nine months or 17 months and counting as baselines to judge when other players should return to play misses the point. Time from surgery is not the determining factor. It probably isn’t even all that important.

The use of time as a criterion for return to play evolved from studies of the incorporation of the ACL graft into the knee. In the early weeks after ACL surgery, the graft matures from a cellular and structural level. In this early stage, if the knee was subjected to forces seen in sports, the graft would likely fail. In that sense, time from surgery is probably a better minimum standard — a player shouldn’t return sooner than three months out, for instance — than as a framework for expected return to the same level of ability.

What does seem to be important is the rigorous process where the medical team works to restore functional ability of the knee and neuromuscular control of the lower extremity. Weeks of training for coordinated movements like single- and double-leg hops, landing on uneven surfaces on the injured leg, balance and coordination work, and plyometric exercise are critical.

Strength, motion, and neuromuscular control of the hip and knee in multiple planes, and for both legs, can all predict the risk of re-injury. All of them can be improved with training, however. This process can take as little as four months, but it can require 12 to 24 months. It is vital not just for preventing injury of the ACL graft or the opposite knee’s ACL. The work is crucial to getting that athlete back to playing at his former level. These are the criteria we are starting to use.

Adrian Peterson’s return from ACL surgery was truly amazing. His MVP-caliber performance only nine months after surgery was far more impressive than just getting back on the field. Maybe Peterson serves as a model other athletes can emulate after their surgeries. Derrick Rose might serve as a better example of the typical recovery and return to play for many athletes.

Dr. David Geier is an orthopaedic surgeon and sports medicine specialist in Charleston. For more information about ACL injuries and other sports medicine topics, go to his blog at drdavidgeier.com.