Anterior cruciate ligament (ACL) injuries are an ever-increasing injury, especially in the game of football, and, man, are they the worst.
Minnesota Vikings standout rookie running back Dalvin Cook is one of the latest NFL players to suffer an ACL tear this past Sunday, abruptly ending what appeared to be a promising and fruitful season. While the science behind ACL reconstruction and rehabilitation has progressed steadily towards becoming an “exact science” and the ligament’s presence in the average sports fan’s lexicon has increased, the discussion, in the media and amongst fans, is typically lacking anything beyond “it’s torn and the athlete will be out until next year.”
The goal of this article is to highlight what the ACL is, causes of ACL injuries and to provide some clarification about all things ACL-related. For those interested, I previously wrote about former Minnesota Timberwolf and current Chicago Bull Zach LaVine’s ACL tear here).
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The ACL is a strong ligament located within the knee joint and its three main functions are: 1. to resist excessive knee extension (straightening); 2. to resist excessive internal rotation of the femur (imagine rolling your right femur to the left without your lower leg moving) with respect to the tibia; and 3. to keep the tibia in close proximity to the femur by restricting anterior translation (imagine the tibia moving straight forward without the femur moving). If one or more of these motions occur to an excessive amount (which is variable from person to person) the ACL is subject to rupture.
The ACL is a strong ligament located within the knee joint and its three main functions are: 1. to resist excessive knee extension (straightening); 2. to resist excessive internal rotation of the femur (imagine rolling your right femur to the left without your lower leg moving) with respect to the tibia; and 3. to keep the tibia in close proximity to the femur by restricting anterior translation (imagine the tibia moving straight forward without the femur moving). If one or more of these motions occur to an excessive amount (which is variable from person to person) the ACL is subject to rupture. The ACL is the yin to the posterior cruciate ligament’s (PCL) yang, as the PCL essentially protects the knee from excessive movements in the opposite directions (knee flexion [bending], external rotation of the femur, and posterior translation).
Knee extension, internal rotation of the femur, and/or anterior translation of the tibia commonly occur when a player is attempting a cut or an abrupt stop and often result in non-contact injuries, such as Cook’s. This results in the player’s knee often being thrust into valgus (the knee “bending” inwards toward the midline of the body) while their foot is plantarflexed (toes pointed down while the heel is off the ground), which is the most common position for non-contact ACL ruptures (contact ACL injuries in football usually occur when a player’s knee is hit from the side or front, which throws the knee violently into valgus or extension, respectively).
As can be seen in the video above, when Cook attempts to perform a cut, his knee is thrust into valgus and his foot is plantarflexed. Although his knee was flexed (which, in theory, should’ve put the ACL on slack), the sudden valgus force was enough to rupture the ACL.
ACL tears can only be definitely diagnosed via magnetic resonance imaging (MRI), although a physical exam can provide a good idea whether or not it’s torn (for those interested: the two physical exam techniques used to diagnose a torn ACL are the anterior drawer test and the Lachman test). Only an MRI can determine a partial tear versus a complete tear, but, as it pertains to the ACL, it doesn’t really matter; once the ACL is compromised, even partially, its ability to stabilize the knee sufficiently for sport is gone.
One of the common myths surrounding ACL injuries is that they often occur in isolation. While the reports that are often distributed by coaches or team PR departments only disclose the ACL injury, it’s far more likely that the player also suffered from other musculoskeletal injuries. It’s been estimated that bone bruises occur in as many as 80% of all ACL tears and chondral (cartilage) and meniscus tears are common as well. However, and this is likely why they’re rarely disclosed in addition to patient confidentiality, they rarely affect the overall recovery timeline.
When it comes time for surgery, it is determined if the player will have part of his/her hamstring or patellar tendon harvested for use as the new ACL (neither has an effect on overall rehab time, though the patellar tendon graft typically has slightly better outcomes). The tendon is drilled into the bone, which provides the blood supply for the tendon to transform into a ligament over the next 6-12 months (this process is known as re-ligamentization and would be impossible without blood flow; the natural ACL actually has a minuscule blood flow, which is why they can’t heal on their own).
The greatest fear when dealing with ACL injuries, at least in my opinion, is not necessarily whether or not the player will be able to return to their prior level, but rather the risk for re-tear or tearing of the ACL in the opposite knee. Recent studies have shown that players who have torn an ACL have a 5.8 percent chance of re-tear and an 11.8 percent chance of tearing their opposite ACL; they also are six-times more likely to suffer a second injury when compared to a player who hasn’t torn an ACL. Proper rehabilitation and allowing for complete healing can mitigate these risks to an extent, but Cook will always be at a greater risk for ACL injury from here on out.
The current best evidence timeline for return to play is 9-to-12 months, however, there has been a recent push by doctors and researchers at the Mayo Clinic to push the timeline back further to 24 months. Their argument revolves around a mountain of evidence that has been collected indicating that the new ACL, surrounding structures, and person, in general, are not optimally healed physically and sometimes mentally for at least two years. Not only does the ACL have to reform, but the nerves need to regrow, the muscles need to re-strengthen, the mind needs to re-acclimate to making quick movements. These things take time, however, while a two-year recovery time may be ideal, it just isn’t very practical, in my opinion. Waiting two years to get back, especially in professional sports, has major financial and career-altering implications; there has to be a risk-reward trade-off when it comes to long-term rehabilitation and return to play with professional athletes.
As for Cook, I’d expect him to make a full recovery physically and should be able to return to his prior abilities. There will always be an increased risk for another ACL tear, but as long as he is able to overcome the mental hurdle of returning to play and playing with the same mentality as before, he should be able to play at a high level once again. The road to recovery will be long, but I’d expect to see him back in purple and gold by this time next year.
Academic sources used for this article:
Nagelli CV and Hewett TE. Should return to sport be delayed until 2 years after anterior cruciate ligament reconstruction? Biological and functional considerations. Sports Med. 2016; DOI 10.1007/s40279-016-0584-z
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