On the Anatomy, Biomechanics, and Dumb Luck Behind Zach LaVine’s Torn ACL
It was announced Saturday afternoon that Zach LaVine tore his ACL during the third quarter of the Wolves’ recent loss to the Detroit Pistons meaning that his season is done. “Torn ACL” is a common injury across all sports and has worked its way into nearly every fan’s lexicon, but what exactly is the ACL? How does it tear? What does rehab look like and how long does it take to return to play? I thought it would be a good idea to answer these questions. (Full disclosure: I am currently in my second year of graduate school and am studying to become a physical therapist.)
What is the ACL?
Let’s start with the basics. ACL stands for anterior cruciate ligament and it is one of the four main ligaments of the knee alongside the posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL). While the MCL and LCL lie outside of the joint capsule of the knee (read: they aren’t actually inside the knee joint), the ACL and PCL are housed within the knee joint and connect to the tibia, or shin bone, and the femur, or thigh bone. The ACL and PCL are so named because they run past each other, which make them look like a cross (cruciate means cross).
The ACL runs from the bottom, back, and outside border of the femur to the top, front, and inside border of the tibia; essentially, it runs diagonally. Because of its orientation within the knee, the ACL’s main functions are to resist extension, or straightening, and rotation of the knee as well as preventing the tibia from excessively moving forward when the femur is fixed (the biomechanical term for this is anterior translation or anterior drawer). Basically, it is the ACL’s job to prevent hyperextension and excessive rotation of the knee and to keep the tibia in close proximity to the femur.
What are the mechanisms behind ACL injuries?
There are three main mechanisms that cause rupture of the ACL. The first two are the ones that are probably instantly thought of when you think of ACL injuries: getting hit with a strong force on the outside of the knee when the foot is planted (think of a running back getting hit by a linebacker from the side) and rotation of the knee when the foot is planted (think typical non-contact injury when the player goes down holding their knee). The third mechanism, and the one I originally thought was the mechanism behind LaVine’s injury, is hyperextension of the knee. However, after looking at the video, I believe LaVine most likely ruptures his ACL when he lands.
When I saw it happen live and when the news was announced that LaVine tore his ACL, I thought his injury occurred when he collided with Pistons big man Andre Drummond in the air; I thought his knee hyperextended. However, this still image of the moment when LaVine and Drummond collided clearly shows that LaVine’s knee is actually bent to roughly 90-degrees; this is important because the ACL is most slacked when the knee is bent and is most taught when the knee is in full extension (totally straight).
It’s a little tough to see on video due to it happening so quickly, so here’s the still of the moment I believe LaVine’s injury occurs.
Even though his knee is bent when he lands (so theoretically the ACL should be slacked), the sudden impact of LaVine landing thrusts his tibia forward while his femur remains relatively still. Remember, this movement (anterior translation) is one of the movements that the ACL resists. The force of the tibia moving away from the femur is too great to be controlled and overcome by the ACL and it gives way.
If you recall the three mechanisms that cause ACL rupture from before, this sudden anterior translation with a bent knee was not one of them, at least not directly. You see, in at least two of the three mechanisms (getting hit from the side and hyperextension), excessive anterior translation of the tibia relative to the femur is a component.
But what makes LaVine’s injury such a freak accident is that even though the tibia excessively anterior translates relative to the femur, you don’t really ever see it happening due to a player landing on their butt/back with their knee bent in such a fashion that the force of said landing drives their tibia forward relative to the femur instead of upward and into their femur (that’s a common mechanism for a meniscus injury, but that’s an article for a later day). There’s nothing that could have prevented this injury; it is pretty much the definition of bad luck.
The only other possible moment that LaVine could have torn his ACL is when he first takes off to jump. However, there was no contact to the outside of his, his leg didn’t rotate as he jumped, and his knee was still in a bent position when he takes off (so it didn’t hyperextend). When you also consider that LaVine didn’t react until right after his hard landing, all of the evidence strongly suggests that the impact was the culprit.
What does rehabilitation look like?
Most post-ACL surgery rehabilitation programs follow a similar progression and guideline. Rehab typically consists of two focuses: strengthening the muscles of the major muscle groups of the leg and progressing from basic cardiovascular exercise to more functional (in the case of LaVine more basketbally) activities. The strength progression begins at simply contracting the muscles via electrical stimulation and/or self-contraction to eventually lifting weights and the cardiovascular progression begins at biking and moves towards jogging and eventually cutting drills and basketball activities. Rehab usually begins as soon as possible after surgery and most of the time it begins the same day.
What is the timeline for return to play?
Generally, players return to game action 6-9 months after surgical reconstruction of their ACL, though, with many players, they don’t return to their “old self” until at least a year after surgery. The main reason why recovery takes so long is because of the poor blood flow to the ACL, meaning it takes a while for the graft, the new ACL, to receive the key nutrients and building blocks it needs to heal. Despite the new graft being fully healed (in the sense that all of the new ACL tissue has been laid down by the body) after approximately 32-weeks post-surgery, its strength is typically only 50-60% of that of a normal, healthy ACL one year later. In addition, the musculature of the thigh (the quadriceps, hamstrings, and groin) are typically fairly atrophied from being underutilized and need time to return to their previous strength.
However, where things get a little murkier is when there is other structural damage in addition to the ACL. Other structures that may be involved in ACL ruptures are the meniscus, the cartilage of the femur and tibia, and one of the other three main ligaments of the knee. Many times, this “extra damage” is caught on the MRI, though sometimes, rather unfortunately, it isn’t discovered until the surgery begins; any extra damage would throw off the timeline for return to play.
Luckily for Zach LaVine and the Minnesota Timberwolves, ACL reconstruction and rehabilitation has become a fairly exact science over the last couple of decades plus; that is to say, it is no longer the career-ender that it used to be. It’s difficult, and would be irresponsible of me, to speculate how this injury will affect LaVine and his career moving forward; many times players return to full strength, but sometimes they do not. I wouldn’t feel comfortable speculating until we know when the surgery is taking place, whether or not the surgery was successful, and to what extent, if at all, his other knee structures were damaged. And even then, it could be a year before we start seeing the real impact. Right now and for the foreseeable future, all we can do right now is wait and see.