Vail Daily letter: ACL injuries
February 26, 2013
In contradiction to the recent letter in the Vail Daily about skiing and ACL injuries, long tails of alpine skis are not (by themselves) the main cause of this leading injury.
In the combination of peer-reviewed medical papers by leading ski injury researchers, Professor Robert J. Johnson M.D. (University of Vermont), Professor Jacques DeGuise (University of Montreal), we find the following: Purely rear-weighted loading through the tails of skis, called “boot-induced anterior drawer,” has a prevalence of 10 percent to 15 percent of all skiing ACL injuries.
All of us longstanding skiers know that back in the 1970s, boots and long-tailed skis allowed us to exert massive rear-weighting if we inadvertently formed a “jet turn,” but few of us ever sustained a skiing ACL injury from a jet turn. Longitudinal studies show that, indeed, despite diagnostic deficiencies during the 1970s, there were few skiing ACL injuries in the 1970s. Loading (alone) has been around for a long time without causing skiing ACL injuries.
However, the introduction of a large “abduction force” (a lateral force applied to the medial edge of the ski near the projected axis of the tibia) acts over the length of the tibia to generate “valgus torque” about the knee – in combination with rear-weighted loading – causing “phantom foot” (or “slip catch”) loading that has a prevalence of 70 percent to 80 percent of all skiing ACL injuries.
The combination of valgus torque plus boot-induced anterior drawer loading produces maximal strain across the ACL.
Why are we seeing more abduction loading (that causes valgus torque) plus rear-weighted loading during the past 20 years?
Because of shaped skis (I love shaped skis ).
Before shaped skis, when a skier caught an inside edge during large rearward loading, one end of the ski would slide out. Torque about the tibia was produced. Binding toe pieces avert tibia torque and they have performed well since the mid-’70s. That’s why that are few tibia fractures (prevalence of 2 percent to 3 percent of all skiing injuries) since the mid-’70s.
However, with shaped skis, when a skier catches an inside edge during large rearward loading, one end of the ski does not slide out. Both ends bite, causing the lateral components of the two forces formed at the tip and tail to become superpositioned into a single abduction force located under the projected axis of the tibia.
The ski does not rotate about the long axis of the tibia. It translates laterally, or in an equal and opposite context, the ski remains trapped while the skier’s center of gravity moves laterally. A large abduction force is produced that generates valgus torque about the knee. The combination of valgus torque plus rear-weighted loading equals … pop.
How can combined valgus and rear-weighted loading (that’s a consequence of a trapped shaped ski) be averted? Lateral heel release. I expressly developed non-prereleasing lateral heel release bindings to address this proven situation.
This way, we can have it both ways: We can mitigate (never eliminate) the leading cause of ACL injuries and enjoy our (long-tailed) beautiful shaped skis.