knee injuries


by Kim Hewson, MD

In medicine, a syndrome is a group of signs and symptoms that are consistently observed that are characteristic of a single condition. As a retired orthopaedic surgeon and a ski instructor, I am often sought out for consultation on injuries suffered by friends, clients and fellow instructors. Over the past two seasons, a symptom of medial (inside) knee pain has emerged in some skiers with a common pattern of fat ski use on groomed terrain.

The common sign observed has been tenderness over and below the joint line of the knee associated with mild soft tissue swelling and tenderness. The skier symptoms are complaints of progressive soreness and difficulty initiating turns on the inside edge of the outside ski. After skiing, walking and nighttime discomfort are common. Some report temporary pain relief with ice and anti-inflammatory agents. The skier often will not return to skiing for several days and upon return uses a narrow waisted ski.

This syndrome with fat ski use occurs only when using fats on groomed or rough terrain, not in powder terrain. We define fat skis as skis >100mm underfoot.

Biomechanics of Inversion stress and varus thrust: a two-phase oscillating micro-trauma to the knee.

Phase I – Compression: The leveraged outside foot is forced into inversion stress. As a result, the outside ski flattens on the snow and the leg rotates slightly externally. The medial boot cuff adds subtle pressure [varus thrust] to the inside lower leg creating a varus stress or bowing compression force at the knee joint.

Phase II – Strain: In repeated attempts to recover the flattening ski, the skier corrects ski inversion by active foot eversion and internal leg rotation. As a result, medial hamstring muscle insertions just below knee are repeatedly put under strain resulting in tendonitis and bursitis.

Kim Hewson is an Orthopaedic Surgeon and former Director of Orthopaedic Sports Medicine and Team Physician at the University of Arizona. He is currently a veteran Telluride Ski School Alpine Instructor in his 15th season and Staff Trainer in the Biomechanics of Alpine Skiing.


When the FIS reduced side cut on GS skis, many were confused as to why GS was singled out over other disciplines. A new injury study (1)  sheds some light on this issue. And while the study falls short of actually identifying the injury mechanism, it provides enough clues to connect the dots. The study, Here are some key statements.

Competitive alpine skiing is considered to be a sport with a high injury risk. Injury rates per competition season and per 100 World Cup (WC) athletes were reported to be 36.7, with the knee being the most frequently affected body part.

“The injury rate was highest for giant slalom,

Associating the number of injuries per hour in WC skiing with skiers’ mechanical characteristics, injuries in super-G and downhill seem to be related to increased speed and jumps, while injuries in giant slalom may be related to high loads in turning.

 It has recently been found that many injuries occur while turning, without falling or being the result of a crash.

“Recently been found?” Seriously? Skiers have been experiencing knee injuries for years without falling and while apparently skiing in control. Without seeing any data, I can predict with confidence that, with rare exceptions, knee injuries are always associated with the outside ski of a turn. The other issue I can state with confidence is that if the moments of force (torques or twisting forces) acting on the outside ski are not tending to rotate the ski into the turn, they will be tending to rotate the ski out of the turn. In the mechanics of an outside ski on its inside edge, there can be no neutral. There are implications to moments of force that tend to rotate the outside ski out of the turn that are associated with speed and the length of any moment arm that exists between the reaction force at the inside edge and the center of the force applied at the sole of the foot of the skier. The force of gravity causes skier to accelerate in the fall line and decelerate as the skier crosses the fall line. (Force = Mass x Acceleration). The shorter the radius of the side cut of a ski, the longer the potential moment arm.

Skiers are turning for approximately 55% of the time in downhill, 80% in super-G and 93% in giant slalom. Moreover, it was shown that small turn radii might be related to an increased injury risk in giant slalom since they provoke the skiers to use their full backward and inward leaning capacities, and thus skiers have less buffer if an additional factor causes an out-of-balance situation.

Note the reference to additional factor.

Downhill had the largest mean turn radius, while giant slalom had the smallest mean turn radius.

Out-of-balance situations themselves are known to be a critical part of typical injury mechanisms, such as the ‘slipcatch’ and ‘dynamic snowplow’.”

What constitutes balance?

This is the central issue. Postural responses on a single limb involve two coordinated and interdependent balance strategies or synergies; 1) a plantarflexion strategy that resists the tendency of a disturbing force (typically gravity) that tends to topple the vertical column supporting CoM forward by causing the ankle to dorsiflex and, 2) an inversion strategy that resists the tendency of a disturbing force to topple the vertical column supporting CoM sideways (ergo, cause the foot to evert or turn away from the centre of the body). In terms of the latter strategy, if one is standing on the right foot, the disturbing force will tend to cause the foot to rotate about it’s inner or medial aspect into the ground. This is called eversion. In skiing, the external forces would tend cause the ski to rotate into the turn. The postural responses of the skier’s balance system act to control the degree of eversion. Eversion is mechanically coupled through a joint in ankle complex called the subtalar joint. When the angles at the knee are relatively small, the leg as a whole will rotate about its vertical axis on a 1:1 ratio with rotation of the foot about its long axis in eversion. Elite skiers don’t consciously cause these rotations to occur. The external forces cause them to occur. The balance system of the elite skier controls the rotations.

 The ‘balance problem’ is that there is only a very short window when the outside ski of the new turn is flat on the snow between edge changes in which to set up the biomechanics that engage the external forces that drive the moments of force into the turn. Among other things, this requires that the skier be able to rapidly dorsiflex their ankle so they can move CoM forward; ergo so the foot can pronate. It is relatively easy to prevent the foot from pronating, but extremely difficult to stop the foot from supinating especially under the influence of a high instantaneous peak force. The configuration that engages the external forces must be established before the outside ski acquires a significant edge angle and especially before the external forces start to increase.

The ‘slipcatch’ technique is the worst possible way to engage the inside edge of the outside ski. The outside ski is slipping sideways as the angle of inclination of the skier and associated edge angle are increasing until a point is reached where the inside edge catches or locks up. Should the slipping ski encounter a frozen ice formation the ski could suddenly decelerate. Should this happen, the offset or moment arm between the inside edge and the centre of the force applied at the sole of the foot will tend to rapidly rotate the foot about it’s long axis out of the turn. This will also rotate the tibia on its vertical axis against a well-stabilized femur.

Comparing the mean and minimal turn radii between discipline, it is evident that giant slalom has substantially smaller turn radii than super-G and downhill. Additional analysis of the data showed that the radial component is the main contributor to the increased FGRF in giant slalom. Thus, the combination of small turn radii and speed leads to larger mean and maximum FGRF in giant slalom compared with super-G and downhill. Furthermore, in giant slalom, skiers’ balance might be challenged simultaneously by small turn radii and high forces.

Injuries in giant slalom were linked to high loads in turning;

It gets worse.

First, the model for the computation of FGRF does not capture the high frequency force components and, therefore, might underestimate the work load (impulse), in particular for giant slalom.

In other words, the actual impulse forces in a GS turn could be much higher than the model predicted.

Furthermore, giant slalom includes a larger number of turns (52.0±3.5) compared with super-G (40.0±3.5) and downhill. Hence, skiers have to find balance in turning more frequently in a run and thus might be more often susceptible to balance-related mistakes in turn initiations.

 The implications are that the skier needs to be able to set up the processes responsible for dynamic equilibrium in the outside leg at the initiation of every turn.


sidecut radius + length of moment arm + instantaneous peak moment of unbalanced force out of the turn on the outside foot + high GRF

  1. Mechanics of Turning and Jumping and Skier Speed Are Associated With Injury Risk in Men’s World Cup Alpine Skiing – A Comparison Between the Competition Disciplines: Matthias Gilgien, Jörg Spörri, Josef Kröll, Philip Crivelli, Erich Müller: Br J Sports Med. 2014;48(9):742-747′, is available for free at Medscape (






In LESS REALLY IS MORE I talked about how I gone in a direction opposite from that of the industry after my perfect fit experience with Mur. I was now removing material from ski boots instead of adding material and expanding shells where necessary to make room for the structures of the foot. While this seemed to generally have a positive effect on skier balance and the ability to control skis, especially edging, removing material from the sides of the boot liner  exacerbated the fact that in the majority of cases I was encountering the shell wasn’t loading the instep of the foot. The reason for this turned out to be  that there was a void between the top of the tongue of the liner and the inner surface of the shell over the forefoot. This was allowing the foot to move upward into the void space or unload from contact with the sole plate (aka boot board) in response to changes or perturbations in ground reaction force. I coined the effect Separation Anxiety because of the alarm bells it was setting off in the skier’s balance system.

After I became aware of this effect, I started doing experiments to try and understand how it was affecting a skier’s balance and ability to control their skis. While riding ski lifts with foot rests (the old slow chairlifts) I would let one of my feet drop off the foot rest and try and feel what was happening with my foot and leg inside the boot when the foot unloaded from the boot board. At that time, I wasn’t thinking in terms of trying find a solution for knee injuries. I saw this as an issue that would be addressed by refinements in bindings which at that time were rapidly evolving. Through my experiments I had come to the realization that the unloading and reloading of the sole of the foot with the boot board, such as occurs when a skier is moving over irregular terrain, was setting off a chain-reaction of physiological events that were creating balance issues. Although I didn’t know exactly how, this unload/load cycle  seemed to be placing stress on the knee. But my focus was trying to find a way to reduce the effect on skier balance. In effect, I was trying to achieve a net improvement in skier balance by reducing negative balance artifacts.

The standard solution in those days was to attempt wedge the heel with heel or L-pads inserted in the liner. The objective was to keep the foot from lifting. I tried this approach. But I  found it didn’t work as advertised. The pads invariably caused problems with the Achilles tendon or they prevented the heel from seating in the back of the shell, or both. The latter had the effect of making the liner shorter and the boot hell to put on. I was looking for a better solution. But it wasn’t until 1980, while working on Podborski’s boots, that I came up with a device that eventually led to my being granted US Patent Number 4,534,122.


When I started skiing in 1970, the buzz was all about the new safety bindings. Debates raged in magazines and ski shops over which binding was the best as in the safest. After years of skiing being perceived as dangerous because of the incidence of broken legs, a new era had arrived with the introduction of a generation of sophisticated bindings. This created the perception that it was finally safe to go out play on the ski hills. But as the sound of snapping leg bones faded into the background it was replaced by an even grimmer sound; the popping of knee ligaments, in particular, torn ACLs. Before the introduction of the rigid plastic ski boot, few skiers had ever heard of an ACL. That was about to change.

It was about the time that I started working with National Ski Team members in 1977 that I began to hear of racers suffering knee injuries. Knee injuries seemed to start with a trickle. I can’t even recall hearing of a recreational skier suffering one. Like most skiers, I believed that the new bindings had addressed the injury issue. Even after knee injuries started to increase in frequency I thought it only a matter of time before refinements would be made to ski bindings and that this would be the end of them. As the popping of ligaments got more frequent, panic seemed to set in in the industry. Skiing had entered a period of vigorous growth. The last thing it needed was a good news, bad news story as in, “The good news is that the rigid plastic boot has made skiing easier. Now for the bad news…..”. As best I can recall, it was around 1980 that a team of spanish orthopaedic surgeons published a study linking the introduction of the rigid plastic boot to knee injuries noting that the incidence appeared to be rising in lock-step with sales of the boot. A classic problem-solving strategy is to go back to the time when a problem first emerged and look for anything that changed. In this case, the most significant change was in the boot. Meantime, those with expertise in biomechanics were pointing out that by stiffening the ankle the boot was sending the forces of skiing upward to the relatively weak knee.

In retrospect, it seemed like it should have been obvious that encasing the foot within what amounts to an orthopedic splint would act to transfer force up the leg. It’s ironic, if not erroneous, that the industry, even today, talks about the boot transferring energy to the ski as if this were the end game of skiing. The reality is that unless the ski industry has repealed Newton’s Third Law (which is doubtful), if a skier were to transfer energy to anything through the boot it would be through the stack of equipment between the sole of the boot to the source of Ground (or Snow) Reaction Force at the snow. This being the case, according Newton’s Third Law which states; “For every action there is an equal and opposite reaction”, the snow will transfer an equal amount of energy through the stack of equipment back up the skier’s leg to the knee. The issues are way more complex than a simple transfer of energy. But I will start with the simple and obvious then build from here.

The question is, “Given the established reputation of skiing as being a dangerous sport prior to the introduction of the rigid plastic ski boot and the fact that skis attached to the foot and leg act as force multipliers, did anyone consider the implications of trying to immobilize the foot and leg within a rigid plastic ski boot?”