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Doctor’s Note: Mechanisms of ACL injuries

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By Dr. Jeff Daniels EBS Medical Columnist

Why are there so many knee injuries during the winter? From my perspective, the joy of ski season is diminished by every anterior cruciate ligament injury that comes through the ski patrol’s doors and into our clinic. In late February, during a day with new snowfall and a noontime warm-up, we saw 10 skiers with torn ACLs.

While plenty of skiers hurt other ligaments in the knee, particularly the medial collateral ligament, the ACL is particularly vulnerable when the knee sustains a twisting injury. It doesn’t require a disastrous tumble down the Big Couloir – a slow fall coming down from the magic carpet may be all it takes to turn a bluebird day into several months of limping, surgery and rehab.

Located in the interior of the knee joint, the ACL is an important stabilizing ligament. The flat ends of the femur and the tibia meet to form the knee joint. Many ligaments, tendons and other types of soft tissue hold these two bones together and allow for the knee joint to function normally. One of the most important components providing stability is a pair of ligaments that crisscross one another, hence the term “cruciate.” The ACL inserts into the anterior part of the tibia, while the posterior cruciate ligament emanates from the back, or posterior, of the tibia.

The PCL is a robust piece of fibrous tissue and it’s rarely injured in a ski or snowboard accident. The ACL is much thinner and becomes vulnerable to injury when it twists around the PCL. Many different types of falls can produce ACL injuries, but with so many people throwing themselves down mountains on two thin slats of laminated steel, wood, and fiberglass, I wouldn’t be surprised if skiing causes the greatest number of ACL tears among all sports.

It’s hard to study this phenomenon scientifically, but certain patterns of falls are more likely to cause ACL injuries. Watching videos of people falling and then following up on their injuries, the Vermont Safety Council – comprised of ski professionals and physicians – was able to identify a consistent fall pattern.

VSC named this the “phantom foot” injury, where the tail of the downhill ski, along with the stiff back of the downhill ski boot, serve as a lever to combine twisting and bending of that ski.

The phantom foot injury pattern occurs when a skier who is losing his or her balance tries to recover that balance, particularly when falling backwards. The fight made by the muscles of the leg and the rest of the body to resist the fall is what sets up the ACL for injury.

Skiers commonly injure ACLs when they are thrown off balance and fall backward causing the uphill arm to swing further back, and the hips to bend and sink below the knees. All the weight is on the downhill ski, and when hitting the slope, the upper part of the body is facing the downhill ski. The downhill knee is then injured.

When this sequence of events occurs in the order described above, the ACL is likely to tear, often accompanied by an audible pop. Pain is variable, but the knee immediately becomes unstable and will swell within minutes.

Learning how to fall, throwing yourself forward instead of settling backward, and not fighting a fall are all ways of reducing the risk of an ACL injury.

Dr. Jeff Daniels has been practicing medicine in Big Sky since 1994, when he and his family moved here from New York City. A unique program he implements has attracted more than 700 medical students and young doctors to train with the Medical Clinic of Big Sky.

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