It’s a bluebird day out on the mountain. Clear skies, and the holidays have brought a lot of skiers to the mountain. Not a big deal, but snowfall hasn’t been all that great this year either, leading to some icy conditions on the limited open steep vertical terrain. You got out to the mountain early so that you could avoid the jerry’s as much as possible, but not much terrain is open yet. After your first few runs of the day you begin descending a fairly steep incline run. You feel the edges carve into the slope when out of nowhere a snowboarder straight lines down the run straight into your turn path. The sudden redirection causes you to shift balance, you lose the inside edge of your downhill ski, which then re-catches quickly. You were making a right hand turn, so your non-dominant leg was facing downhill. The sudden catch twists your knee inwards. You hear and feel a pop in your knee and crumple while still hurtling down the icy slope. By the time you come to a stop the snowboarder is nowhere to be seen. Another skier comes to your aid. Still in a slight daze your try to stand but feel excruciating pain in your twisted knee. Ski patrol has to carry you down the mountain. Once inside the ER tent your medical provider lets you know that the ACL and potentially other ligaments of the knee are ruptured and will likely need surgery if you are ever going to ski again. 

This is not an uncommon scenario. The knee is the most common location of injury in skiers, and the ACL is injured in approximately 50% of all serious knee injuries that require hospitalization (Posch et al., 2020). The ACL or anterior cruciate ligament is a necessary stabilizer of the knee. Its job is to keep the lower leg bone, the tibia, from sliding forwards while we are walking or running. Without this ligament we can have serious stability issues that can lead to damage of other structures in the knee such as the meniscus, other collateral ligaments, and many of the tendons of the knee. The main way that the ACL gets injured is when the knee is twisted and buckles inwards (Excessive valgus load with internal rotation for you kinesiology nerds out there). There are three major ways in which this occurs while skiing: the slip catch is the most common and described in the excerpt above, back weighted landing, and dynamic snow plowing (Bere et al., 2011). 

In the slip catch mechanism, The outer ski inside edge loses contact with the snow which forces it to drift away from the center of mass. When the ski edge abruptly catches the knee is forced inwards and rotates leading to excessive force on the ACL and rupture. 

In the back-weighted landing, the skier loses balance mid air during a jump. Then they land with their weight too far backwards which causes strain on the ACL as biomechanical forces push the tibia forwards. 

In the dynamic snow plow mechanism, the skier loses contact with the inner skis, which makes it drift outwards. Their balance is thrown off as their legs are split apart, sending their weight backwards and onto a single bent leg. The inside edge of the weight bearing ski then catches, causing the knee to buckle inwards. 

In all of these cases, the ski typically fails to release from the bindings. Our feet were not meant to be three feet long and immobilized for a reason. Whenever we have limited range of motion in one joint of the body, that range has to be taken up in another. In this case we have immobilized the ankles, so the motion that was once taken up in the ankle now is distributed in the hip and knee. As a hinge joint, the knee is not meant to take the rotational and pivoting forces that are commonly absorbed by the highly mobile ankle joint. This is why releasing the ski is so important, and also why we will spend the next paragraph talking about DINS.

The DINS or Deutsches Institut für Normung International Standard settings are the calibration of how much force it takes to eject the boots from the ski. These settings are calibrated based on your height, weight, age, and terrain difficulty (Owens et al., 2018). Beginners should have their DIN settings placed relatively low so that when they fall the boots more easily disengage from the skis, which will save the knee from excessive twisting load. The issue with more elite skiers is that the forces they normally place on their skis would be enough to eject the skis if on the same DINS as a beginner skier. Thus they have to increase the DINS on their skis so that they don’t come off during their normal turns, but this also predisposes them to not having a ski release during the mechanisms of injury mentioned before. There are many free websites that are able to calculate the DIN settings for your skis based on your anatomical metrics. It is important to make sure that your DINS are set correctly at the beginning of each season before you go out while renting skis.

The next question you may be asking yourself is, besides ensuring that my DINS are set correctly, what kinds of things can I do off the mountain that will decrease my chances of sustaining an ACL injury. Unfortunately there isn’t a ton of research looking at the effectiveness of ACL prevention programmes in skiers. There is some evidence to show that using a knee brace will offer some protection against reinjury in skiers that have already ruptured their ACL, but it is not recommended to brace as a prophylactic measure. While there has not been a lot of research on ACL rupture prevention programmes in skiing, there has been a lot of research in a much larger sport that has an epidemic of ACL injuries; soccer (European futbol). ACL injuries are the plague of many soccer players. This led the medical branch of the FIFA organization to investigate the matter and research effective prevention programs. This investigation led to the creation of the FIFA 11+ warm up; a warm up designed to be implemented in youth soccer teams around the world. Research on teams that implemented the 11+ protocol found a 44% decrease in knee injuries and potentially a 50% decrease in “severe injuries.”  (FIFA 11+ workbook)

The strengthening exercises that have been shown to decrease these injuries by such a high rate are a surprisingly simple series of dynamic strengthening and jumping exercises designed to strengthen the muscles of the hips and thighs so that they are stronger and more able to resist the forces acting to internally rotate and buckle the knee. There is also significant emphasis placed on knee position during these dynamic exercises. Basically it is important to make sure that the knee is tracking forwards instead of to the inside while doing squats and jumps. An easy way to think of it is to look at your patella (the bone on the front of the knee) and pretend it is a flashlight, when you jump or land is the flashlight still pointing straight or is it pointing to the side? It is very important to keep the knee pointing straight ahead as you do dynamic activities and jumps (unless you have certain anatomical variants that necessitate different anatomical positioning). 

The ACL injury prevention protocol is as follows 

Part 1 (2 sets of 10 repetitions of each exercise)

• Squat with hip circle outwards 

• Squat with hip circle inwards 

• Reverse Nordic curls 

• Nordic hamstring curls 

Part 2 

• Plank (2 sets of 30s holds)

• DNS side bridge (hold 2 sets of 30 seconds each side)

• Single leg squats (2 sets of 10 repetitions)

Part 3

• Jump squat 

• Lateral hops

• Single leg hops 

• Split squat jumps (tele skiers)

• (2 rounds of 30 seconds of each exercise separated by 30 seconds of rest) 

References:

Owens, Brett D., Christopher Nacca, Andrew P. Harris, and Ross J. Feller. 2018. “Comprehensive Review of Skiing and Snowboarding Injuries.” The Journal of the American Academy of Orthopaedic Surgeons 26 (1): e1–10.

Bere, Tone, Tonje Wåle Flørenes, Tron Krosshaug, Hideyuki Koga, Lars Nordsletten, Christopher Irving, Erich Muller, Robert Cortas Reid, Veit Senner, and Roald Bahr. 2011. “Mechanisms of Anterior Cruciate Ligament Injury in World Cup Alpine Skiing: A Systematic Video Analysis of 20 Cases.” The American Journal of Sports Medicine 39 (7): 1421–29.

Posch, Markus, Alois Schranz, Manfred Lener, Katja Tecklenburg, Martin Burtscher, and Gerhard Ruedl. 2020. “In Recreational Alpine Skiing, the ACL Is Predominantly Injured in All Knee Injuries Needing Hospitalisation.” Knee Surgery, Sports Traumatology, Arthroscopy: Official Journal of the ESSKA, August. https://doi.org/10.1007/s00167-020-06221-z.

“11plus_workbook_e.pdf.” n.d. https://www.fifamedicalnetwork.com/wp-content/uploads/cdn/11plus_workbook_e.pdf.