If you ski, you’ve probably heard about Anterior Cruciate Ligament injuries. This injury is very common in both recreational skiers as well as professional skiers. The most common mechanism for recreational skiers has been reported to be “THE PHANTOM FOOT.”[1] In this situation, the skier is out of balance backward with the hips below the knees. The uphill arm is back, and the upper body generally faces the downhill ski. The injury occurs when the inside edge of the downhill ski tail engages the snow surface, forcing the knee into internal rotation in a deeply flexed position. The ski acts as a lever to twist or bend the knee, hence the term ”phantom foot.”
Interesting what happens on the world cup has not been studied… until….
In a recent study, Bere et al. (2011)[2] examined 20 cases of ACL injuries reported through the International Ski federation Injury Surveillance System for 3 consecutive World Cup Season (2006 – 2009). Seven international experts in the field of skiing biomechanics and sports medicine related to alpine skiing performed visual analyses of each case to describe the injury mechanisms in detail (skiing situation, skier behavior, biomechanical characteristics).
MECHANISM OF ACL INJURIES in WORLD CUP SKIERS
The three main mechanisms of ACL injuries (13 male, 7 female) that were identified based on video motion analysis were:
- Slip-Catch (n = 10)
- Landing Back Weighted (n=4)
- Dynamic Snowplow (n = 3 also common in recreational skiers)
Ski Discipline
Most injuries occurred in the downhill discipline (n = 10), followed by giant slalom (n=7), slalom (n=2) and super-G (n=1). These results have contributed to Alpine Canada’s decision to restrict downhill racing until athletes are 18 years of age.
Skiing Situation
The researchers reported, “In more than half of the cases (n =12), the skier was turning at the time of injury. In all of these cases, the skier was out of balance backward and/or inward, mainly during the steering phase out of the fall line. The second most common skiing situation (n = 4) at the time of injury was landing back-weighted after jumping. In 2 other cases, the skier was traversing and was out of balance backward and/or to the side. In the remaining 2 cases, 1 skier hooked the gate in a balanced position, and another skier was tumbling after having lost control. In 17 of the 20 cases, there was no binding release on the injured side. In the remaining 3 cases, the binding released well after the identified time of injury”
Slip-Catch
Is the most common mechanism of ACL injuries in World Cup Skiing where the skier was turning and out of balance backward and/or inward. The skier lost pressure on the outer ski, which then drifted away from the body’s center of mass. The skier extended the leg, attempting to reestablish grip with the outer ski. The outer ski then abruptly caught the snow surface, forcing the nearly straight knee into flexion, internal rotation, and valgus at the time of injury.
Image Source: Click Here
Landing Back Weighted
During the flight phase of the jump, the skier lost balance backward and, as a result, landed on the ski tails with a large clap angle2 and nearly extended knees. As the tail of the ski was loaded, the skis rotated forward, and the skier attempted to recover his balance. The suggested loading mechanism was a combination of tibio-femoral compression37 and anterior drawer of the tibia related to the femur.
Image Source: Click Here
Dynamic Snowplow
The skier was out of balance backward with more weight on 1 ski than the other. The un-weighted ski then drifted away from the body’s center of mass, forcing the skier into a split position. The loaded ski then rolled from the outside edge to the inside edge, which subsequently engaged the snow surface and forced the knee into internal rotation and/or valgus. The positioning of the skis at the time of injury was similar to a snowplow, hence the term ”dynamic snowplow.”
Image Source: Click Here
Important points to consider
- Dynamic valgus inward movement the knee) of the knee with internal rotation are similar mechanism for ACL injuries across many sports (basketball, volleyball, soccer)
- While the most common mechanism of ACL injuries in professional skiers differs from recreational athletes (Slip-catch & Dynamic Valgus vs. Phantom Foot) the underlying dynamic valgus and internal rotation of the knee are consistent mechanisms of ACL rupture in both groups.
- In all cases of ACL ruptures investigated by Bere et al. the skiers were all out of balance backward and/or inward, mainly during the steering phase out of the fall line. This highlights the importance of skill, ski conditions, visibility, course set-up and the interaction between each factor.
- Neuromuscular training of the knee has been shown to significantly reduce (1.5 – 4 times compared to controls) the incidence of ACL ruptures in many sports. Studies of neuromuscular training of alpine ski racers for ACL prevention to my knowledge have not been conducted, although virtually all dryland programs encompass ACL prevention strategies.
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According to the authors designing bindings to release in response to a load that will cause ACL rupture vs. loads that commonly occur with ski-racing is next to impossible.
1. Natri, A., et al., Alpine ski bindings and injuries. Current findings. Sports Med, 1999. 28(1): p. 35-48.
2. Bere, T., et al., Mechanisms of anterior cruciate ligament injury in World Cup alpine skiing: a systematic video analysis of 20 cases. Am J Sports Med, 2011. 39(7): p. 1421-9.
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