Ask a Sports Medicine Doctor column: Preventing an ACL injury
January 12, 2015
Question: I work as a ski instructor, and I was wondering what I can do to limit the chances of tearing my ACL?
Answer: Most ACL injuries occur in athletes under the age of 40. Female athletes are on average three times more likely to tear their ACL than male athletes in most sports, although in alpine skiing the female-to-male injury ratio for ACL tears is fairly equal. The majority of ACL injuries do not involve a collision, but rather most ACL tears occur as a result of a noncontact injury. In my office, most athletes who tear their ACL report a twisting type fall or a having a flat landing off of a jump. In these mechanisms, the anatomic structures supporting the knee fail to dissipate the forces and stabilize the joint and the ACL tears.
When one lands, whether riding in the terrain park or catching some air while skiing, ideally the ground reaction forces are absorbed in an accordion like fashion. In this way, the musculature supporting the foot and ankle, knee, hip and spine all absorb and dissipate these forces so that the ACL sees less of these forces. If an athlete has weak core strength and more easily lands in the backseat, then they are less able to land with their knee over their toes and dissipate these forces. Instead, they land "stiff-legged" and can tear their ACL. If one lands with the knee over the toe, those forces can be much better absorbed and the ACL protected.
If an athlete is excessively knock-kneed, then they are predisposed to ACL tears. With this alignment, the lower leg bone (tibia) can internally rotate excessively, which places increased stress on the ACL. Furthermore, studies have shown that female athletes tend to land with more of a knock kneed alignment, and they also demonstrate this alignment when decelerating or pivoting.
Several studies have shown that specific neuromuscular training or reprogramming the way athletes jump, land and decelerate can significantly decrease the incidence of ACL tears. Landing with the knee over the toe is critical to a safe landing and preventing an ACL tear. Furthermore, athletes can minimize landing with a knock kneed alignment by working especially on hip and gluteal muscle strength. One study collected and analyzed video of World Cup skiers who tore their ACL's. Most of the ACL tears seen were due to the "slip-catch mechanism." In this, the athlete was leaning backward and/or leaning into the hill and losing pressure with the downhill ski while turning around a gate. While in this off balance position, the racer extends their downhill ski in an effort to get pressure back on this ski. However, when the inside edge of their downhill ski suddenly catches the snow, their nearly-straight leg weighting the downhill ski is forced into a knock kneed alignment. The lower leg bone (tibia) is also forcefully internally rotated and there is increased compression across the knee joint and then the ACL tears. Avoiding this "back seat" position while skiing and maximizing core, hip, and gluteal strength might help prevent this sort of ACL tear mechanism.
Dr. Rick Cunningham is a knee and shoulder sports medicine specialist with Vail-Summit Orthopaedics. Visit http://www.vsortho.com.
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