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Document Title: Hame-AJSM-Jul02.shtml
Article Title: Injury to the Anterior Cruciate Ligament During Alpine Skiing -- A Biomechanical Analysis of Tibial Torque and Knee Flexion Angle
Authors: Sharon L. Hame, MD, Daniel A. Oakes, MD and Keith L. Markolf, PhD
Publication: American Journal of Sports Medicine, Baltimore, Maryland
Date: July 2002
Volume 30, Number 4, pages 537-540
Keywords: Alpine-skiing knee-injury occurrence, injury prevention, torque, lever arm, moment arm, injury biomechanics, cadaver study.
(Reference-denoting numbers appear in the same font and point size as the document text. As with all Knee Library documents, this article is provided in full-text form, complete with all figures and tables.)
Comments: This article concludes that hyperflexion (i.e. flexion of knees so far that one's heels go past one's buttocks [which is exactly what occurred to forum founder Bob Willmot]) and hyperextension (falling forwards with one's skis remaining straight forward) are the major dangers to alpine skiers. One concern in this cadaver-based study (besides the fact that it cannot directly take into account the effect of muscle activation) is that the torque applied to the knees was a mere 10 Newton-metres, which is much lower than what can be applied by skis in real-life situations (and which is roughly 25% of what would cause ACL rupture). The authors conclude that, for alpine skiers, twisting is less of a concern than hyperflexion and hyperextension. But because the forces used in this study were so low, some further experimentation with loadings more representative of what can be expected in real life would be desirable. Also desirable would be further research into the effects of various multi-part series of forces (for example bipartite combinations such as hyperextension with twisting, hyperflexion with twisting, and sideways forcing with twisting, or tripartite combinations such as hyperextension, sideways forcing, and twisting). However, it is clear that hyperflexion and hyperextension represent genuine dangers to the alpine skier's knee, and the authors are to be commended for their thought-provoking research.
For people with knee-ligament-injury histories in particular, note that any well-fitted and well-designed dual-hinged functional knee brace protects against hyperextension via blocking devices in the hinges; additionally, many functional braces can be equipped with flexion-limiting devices in their hinges. This leaves twisting as a major concern. Note that while alpine-ski bindings can be set to release appropriately, the problem is that the ski bindings cannot detect the torque at the knee, because this torque itself depends on the angle at which the knee happens to be flexed at that point in time. This study shows that the tension generated in the ACL at 90 degrees of flexion, by the low-level (10 Nm) forces used, is less than the tension generated at full extension and full flexion. Unfortunately, the authors do not delve into the concerns that result from higher torques; had they used torques more representative of real-life situations, they probably would have found that ACL injuries easily occur when the skier's knee is flexed to 90 degrees. Consider that many plant-and-twist injuries occur in cutting-type sports such as soccer and basketball, with the knee flexed to about 90 degrees. (It would be wonderful if data were available on the flexion angles at which real-life skiing knee injuries occurred, but this would require all skiers to equip themselves with data-recording knee-flexion-angle sensors -- albeit then such sensors could be connected, perhaps via a wireless connection, to some yet-to-be-invented "smart" ski bindings, which then could theoretically use this information to calculate when to release most appropriately. However, keep in mind that hyperextension and hyperflexion are something which it is inherently nearly impossible to design ski bindings to protect against.)
It is an undeniable fact of simple mechanics that the femur is most effective as a wrench when it is oriented at 90 degrees to the tibia; thus if the tibia is the rotation axis, then (because the skis are also oriented 90 degrees to the tibia), the result is that the tibia can be conceptualized as a rod with two wrenches applying opposing torques at both ends. The weak point of such a system is the knee, and because the ankle is stronger in twisting, the knee incurs the twisting injury. (Protection against such injuries could be achieved by wearing a functional knee brace equipped with a twist-resisting connection to the skier's boot. As long as the knee is partly flexed, this ski-boot-attachment arrangement would ensure that the "weak point" of the system occurs at the binding [exactly as it should], and thus the binding releases reliably and in a timely fashion. Such a brace, if also equipped with the aforementioned protective devices against hyperextension and hyperflexion, would therefore be very helpful in protecting the skier's knee against a wide variety of potentially knee-ruinous situations. Although the purchase cost of two such braces would be considerable, the advantage of protecting against hyperflexion and hyperextension [identified as worrisome by this study], together with protecting against twisting [which, due to the aforementioned lever arms of the skis and femur, will always remain a major threat to the skier's knee] and sideways forcing [not touched upon by this study, but nonetheless a factor in some skiing injuries], are abundantly clear.)
The authors note the risk of injury during situations in which the leg musculature might not be activated in time, for example during unexpected or very rapidly occurring situations; again, bracing would seem helpful in this regard. (Skiing accidents are notorious for the speed at which they occur. Skiers move at high speed, which translates into diminished reaction time. And, kinetic energy increases with speed squared, which means that a skier moving at five times the speed of a walking person will, in the event of an accident, incur injuries 25 times as severe. Regrettably, the authors did not discuss these issues.) The authors of this study also noted the importance of good strength in the leg musculature, although they did not expound on this nor on other injury-prevention aspects. The authors also did not delve into the biomechanical limitations of muscles, in particular the fact that the leg musculature is biomechanically disadvantaged with regards to controlling twisting. (Consider that, with the tibia serving as rotation axis, the lever arms of the femur [at 90 degrees flexion] and the skis are very long...while the lever arm of the leg musculature is no more than a few centimetres. It is reasonable to expect that the torques likely to be encountered during real-life alpine-skiing knee-twisting incidents will generate tensile forces in the ACL that are far greater than the 2160-Newton ultimate strength of a normal ACL. Similar limitations of muscle capabilities exist also with regards to efforts to protect the knee against hyperextension and other modes of forcing; these limitations are an inevitable consequence of the enormously long lever arms provided by the leg bones. Although strong leg musculature is essential, depending on muscle strength alone for skiing-injury prevention is asking for trouble.)
Skiers should remember that in addition to muscle strength, endurance is of prime importance -- especially when the temptation to squeeze in "just one more run" at the end of a long day of skiing appears. Another helpful ski-injury prevention measure involves learning to recognize knee-injury precursors and how to deal with them (see, for example, Ettlinger-AJSM-Sep95.shtml, here in the Knee Library). Skiers should also understand the inherent dangers in overestimating their abilities (consider, for example, the enormous kinetic energy involved in barrelling down a triple-black-diamond near-vertical run), and should be reminded that the use of any type of protective gear (including knee bracing as well as the now-ubiquitous helmet, even though both of these are indeed beneficial) is not a licence to take massive risks.
ABSTRACT
Background: The anterior cruciate ligament has been shown to be particularly susceptible to injury during alpine skiing. Tibial torque is an important injury mechanism, especially when applied to a fully extended or fully flexed knee.
Purpose: We wanted to record the forces generated in the anterior cruciate ligament with application of tibial torque to cadaveric knees in different positions.
Study Design: Controlled laboratory study.
Methods: Thirty-seven fresh-frozen cadaveric knees were instrumented with a tibial load cell that measured resultant force in the anterior cruciate ligament while internal and external tibial torques were applied to the tibia at full extension, 90° of flexion, full flexion, and forced hyperflexion.
Results: At each knee flexion position, mean force generated by 10 N·m of internal tibial torque was significantly higher than the mean generated by 10 N·m of external tibial torque. Mean forces generated by tibial torque at 90° of flexion were relatively low. During flexion-extension without tibial torque applied mean forces were highest (193 N) when the knee was hyperflexed.
Conclusions: Application of internal tibial torque to a fully extended or fully flexed knee represents the most dangerous loading condition for injury from twisting falls during skiing.
Clinical Relevance: Understanding of the mechanisms of falls can be used to design better equipment and to better prevent or treat injury.
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