The ACL and The Skier--Part 1
It's every skier's nightmare--the dreaded feeling of landing in the back seat, the sickening 'pop', the sudden pain, and the collapse of your knee that signal an ACL tear. We've all heard the horror stories, some of which are set on a busy resort, others, more frighteningly, occur deep in the back country, far away from rescue. As a skier, the initial reaction is often to shudder and think something along the lines of: "I hope that never ends up being me!". Of course there is a lot more that can be done to prevent an ACL injury than just hoping for the best. One way is simply to educate yourself on the matter, which I aim to help with via this blog. In two posts I will discuss 1) what the ACL is and how it is often injured, and 2) how to best protect your precious ligaments. Let's begin by looking into just how common ACL injuries are among skiers.
The Prevalence of ACL Injuries Among Skiers
There have been quite a few studies undertaken which look at ski injuries. I have selected a few of them to discuss here (though there are many others which have drawn similar conclusions). Keep in mind that the recency of these studies means that they largely look at skiers in the era of modern ski boots and bindings, which of course impact the commonality of certain injuries. The same studies would likely have found different results had they been conducted in the 60's.
In one study, the Authors analyzed data from high level French National Alpine ski teams over a period of 25 years (1980-2005). The study included 379 athletes with roughly even numbers of men and women. The results revealed at least one ACL injury in 28.2% of the female athletes and 27.2% of the male athletes. Furthermore, 19% of the injured athletes suffered a second ACL tear to the same knee and 30.5% experienced ACL injury in the opposite knee (Pujol et al., 2007). These numbers indicate the significance of ACL injury in elite level skiers, but what are the numbers like for the average resort skier?
Another group of researchers gathered data on skiing and snowboarding injuries at a resort in Vermont between 1988 and 2006. Their data showed that the ACL was the most commonly injured structure in skiers--17.2% of all injuries sustained on the hill involved the ACL (Kim et al., 2012).
Ekeland and Rovden (2010) investigated the various injuries on ski hills in Norway, and once again the knee was found to be the most commonly injured area among skiers. However, in this study (unlike in the first one discussed), females were found to have a knee injury rate nearly double that of their male counterparts (30% vs 16% respectively). They propose the reason for this may be technique or anatomy related, but confess that there is no definitive data to explain this discrepancy yet. Burtscher et al. had previously reported this difference between male and female injury rates in their 2009 paper. Out of 344 injured skiers surveyed on an Austrian ski hill in 2005, 58.7% were female and of these, 41.3% had sustained knee injuries. The authors analyzed these injuries individually and have suggested the following potential risk factors for knee injuries in female skiers: older age, less regular physical activity, skiing earlier in the morning or in colder weather and having less frequently adjusted bindings. These findings, however, do not indicate cause and effect. Rather they suggest potential areas for future, more in depth research. That being said, conditioning your muscles, staying warm and keeping your skis tuned are certainly all good ideas.
Let's talk briefly now about the structure and function of the ACL.
The Mighty ACL
The ACL (anterior cruciate ligament) is a tough bundle of ligamentous tissue that connects the front/centre of the tibia (shin bone) to the back of the femur (thigh bone). It runs diagonally so that it starts slightly towards the inside of the knee and finishes slightly towards the outside of the knee (Domnick et al., 2016).
One of the ACL's jobs is to prevent the tibia from sliding forward with respect to the femur. An example of this occurs when you land a jump and find yourself leaning too far back. The firm support of the ski boots holds your tibia stable, but your body weight pulls your femur backwards. Your ACL is tensioned and stretched to the max and will usually hold the knee together effectively, but if the force is too great it will fail. The ACL is also important in preventing the tibia from twisting inwards under the femur. This can happen if you catch an edge while turning so that the downhill leg is forced into a snowplow position while your momentum continues to carry you forward. Again, this position puts the ligament under a loaded stretch that, if too intense, will cause it to rupture (Domnick et al., 2016).
With these facts in mind, we can move on to the common mechanisms of injury encountered on the ski hill.
Mechanisms of Injury
Ettlinger, Johnson & Shealy (1995) conducted a 22 year study of ski injuries at a resort in Vermont and analyzed videos and reports of more than 1400 ACL injuries to come up with 2 main mechanisms of ACL injuries:
The Phantom Foot: refers to the fact that the tail of the ski points opposite to the human foot and can act as a lever to torsion the knee unnaturally. This type of injury typically occurs when the downhill ski becomes excessively loaded in an unbalanced position, transferring weight onto the inside edge of the tail. This is usually coupled with: the uphill arm going back, the hips dropping below knees (weight back), the uphill ski coming off the snow and the body twisting to face the downhill knee. This was the most common of the two mechanisms.
The Boot Induced Mechanism: typically refers to when the skier takes a jump in an off-balance position (typically back-weighted) and lands in much the same way. The tails contact the snow first and as the rest of the ski makes its way towards the snow, the structures in the knee are gradually stretched away from the shin, which is held fast by the ski boot. By the time the heel of the boot lands, the ACL has absorbed as much force as it can take. Therefore, it is likely to rupture at this point in the landing as the stiff boot shears the tibia forward relative to the femur.
Both these mechanisms are probably familiar to most skiers, though it is only the unlucky few who actually suffer a complete ligament tear. In the second part of this post, I will build on what I have written here and discuss the measures one can take to prevent an ACL injury ruining your ski season and what happens if an injury does occur...Look for Part Two in the next week or so!
References:
Burtscher, M., Sommersacher, R., Ruedl, G. & Nachbauer, W. (2009). Potential Risk Factors for Knee Injuries in Alpine Skiers. J ASTM Int, 6(1): 1-4.
Domnick, C., Raschke, M.J. & Herbort, M. (2016). Biomechanics of the anterior cruciate ligament: Physiology, rupture and reconstruction techniques. World J Orthop, 7(2): 82-93.
Ekeland, A. & Rovden, A. (2010). Skiing and Boarding Injuries on Norwegian Slopes during Two Winter Seasons. J ASTM Int, 7(4):139-149.
Ettlinger CF, Johnson RJ, Shealy JE. (1995). A method to help reduce the risk of serious knee sprains incurred in alpine skiing. Am J Sports Med. 23(5):531-537.
Kim, S., Endres, N.K., Johnson, R.J., Ettlinger, C.F. & Shealy, J.E. (2012). Snowboarding injuries: trends over time and comparisons with alpine skiing injuries. Am J Sports Med, 40(4): 770-6.
Pujol, N., Blanchi, M.P.R. & Chambant, P. (2007). The Incidence of Anterior Cruciate Ligament Injuries Among Competitive Alpine Skiers. Am J Sports Med, 35: 1070 – 4.