In recent years, the Wolverines have seen more than their share of student-athletes sidelined by serious knee injuries, most notably torn anterior cruciate ligaments (ACL), including senior cornerback Blake Countess in 2012, former linebacker Jake Ryan in 2013, junior tight end Jake Butt in 2014, and redshirt junior tailback Drake Johnson in the first game of the 2013 season and again in this year's Ohio State game.

The writing is entirely hers, unless noted by italics.

The ACL is a thick band of elastic tissue that connects the upper and lower leg, providing vital stability to the knee. Dr. Bruce Miller, head orthopedic team physician for Michigan football, says that ACL tears are increasingly common among NCAA athletes.

While the actual numbers are unknown at this present time, Miller and other experts have observed changes in injury patterns impacting elite athletes (both college and professional), high school athletes, and recreational athletes.

ACL injuries occur when the knee is severely twisted, most commonly while landing, turning, or when the leg is hit during actual play.

"We all learn how to jump and run as children, but injuries can occur during otherwise routine activities - slowing down after a run, changing directions on a soccer field, landing after a rebound," explained Miller, who added that many ACL injuries occur in noncontact situations.

Butt's injury in Feb. 2014, in fact, occurred during team conditioning.

The highest risk sports include football, basketball, soccer, and gymnastics.

Although football players account for the largest numbers of ACL tears, female gymnasts have the highest rate of injury based on the number of injuries per exposure.

Several factors contribute to the heightened risk of ACL injuries. Among the potential reasons, Miller believes increased training demands play a key role.

"The volume of practice is large and growing. Because of the way athletes train today, many more exposures occur in the practice setting than in actual games," he said.

Miller continued, "It may be a player's third training session of the day, and the last repetition of the last exercise, so the body is fatigued."

Something routine that was easy earlier in the day might result in injury.

Miller also notes that today's collegiate athletes are "bigger, stronger, and faster" than ever. As a result, the magnitude of force that affects the knee is also greater, including non-contact forces.

The role of playing surface is another consideration. Artificial playing surfaces are increasingly common today and some evidence suggests that this may be a risk factor for ACL injuries, although the association remains controversial.

Finally, genetics and anatomy can contribute to injury risk. This includes factors intrinsic to the individual athlete such as increased joint laxity or having a smaller ACL.

With ACL tears, the diagnosis can usually be made immediately after injury. In fact, Miller says this is often the ideal time to examine an injured knee, before the pain and swelling have set in fully. Additional studies, including an MRI scan, are usually done to confirm the diagnosis and also to assess the extent of other damage to the knee.

Without an intact ACL, the knee is unstable. Full stability can only be regained through surgical repair. The timing of such repair can vary.

"We wait until the injured knee looks and feels like the other [healthy] knee," said Miller.

For the average person, this usually takes about six weeks. However, Miller notes that elite athletes are not average. He explains that a Michigan football player who experiences an ACL tear may "work all day, every day" getting the knee into condition for surgery.

"In theory, there are unlimited resources that can be put toward the pre-surgical rehabilitation process, so the timing [of repair] may be sooner," he said.

Pre-surgical rehabilitation focuses on improving the knee's range of motion and strengthening the surrounding muscles. Miller adds that the sports calendar is sometimes taken into account, but only as a secondary consideration.

"We might look ahead and think about when a player might be ready for the Big Ten opener," he said.

Although each athlete's timeline differs, Miller says it is not uncommon for an ACL repair to occur sooner than the typical six weeks, if (and only if) the knee is otherwise ready.

Many years ago, surgeons would simply sew the disrupted ends of the ACL together. This resulted in a weakened ACL that was prone to repeat injury. Surgical techniques have evolved and today ACL repair involves a graft, either taken from an adjacent area (usually the patellar tendon or hamstring tendon) or an allogenic graft (human tissue from a cadaver).

The preferred route for a young, healthy athlete is overwhelmingly to use his/her own tendon. Miller added that cadaver grafts are associated with more complications, including a higher technical failure rate, as well as the rare, but potentially devastating occurrence, of infection due to contamination of the grafts.

Post-surgical recovery is a lengthy process that entails an intensive program of physical therapy. According to Miller, the usual time frame for a player's return is about six months.

"Before returning, athletes must pass through a series of milestones, including strength parameters and agility tests. Six months may or may not be enough," he said.

He also highlights the difference between full recovery and the ability to play safely.

"The graft continues to mature for about three years, but by that time, a collegiate career would be over," Miller noted, adding that the vast majority of NCAA athletes that experience an ACL tear are able to return to their pre-injury activity level.

Repeat injuries (on either the same or opposite side) - such as the one suffered by Johnson this past fall to the same left knee - can pose additional challenges.

Besides the continued exposures, Miller notes that some players might be predisposed to repeat injury for the same reasons the initial injury occurred. A second injury on the same side can also present technical difficulties that complicate surgical repair.

For instance, there may be more damage to the cartilage and other surrounding tissues, so the required repairs may be more complex.

According to Miller, the overall success rate for a first time ACL repair is about 95 percent - an estimate based on the collective experience of hundreds of surgeons and thousands of patients.

Although the exact numbers are unknown, the odds of success after a second surgery are lower and depend to a large extent on how much other damage is present in the surrounding structures. In extreme situations, multiple operations are needed (known as a "staged" repair), in which case return to full function is much less likely.

In this setting, Miller explains that the risks related to re-injury are a major consideration since additional surgical repair may not be possible.

Despite these sobering prospects, the majority of athletes experience an uneventful, albeit lengthy, recovery after an ACL tear. Long term, some data suggest that ACL injuries are associated with an increased risk of knee arthritis but this too is an area of controversy.

"Is it the ACL injury or everything else the knee is being put through?" asked Miller, rhetorically.