After many years in development, the 2008/09 winter season will see the commercial release of a new alpine ski binding - called simply the Knee Binding - specifically designed to protect against the commonest cause of ACL injury in alpine skiers, the so called Phantom Foot (PF) scenario. The set of events that constitute a PF fall have been the subject of detailed video analysis and are thought to account for approximately 70% of all ACL injuries from alpine skiing. If the Knee BindingTM can help to reduce the numbers of such injuries, then it will truly be a major breakthrough. This product won  "Innovation of the Year" at the SIA ski trade show in Las Vegas, Nevada in February 2008.

Anterior Cruciate Ligament Injury

The anterior cruciate ligament (ACL) is a vital ligament for an alpine skier. It attaches between the posterio-lateral aspect of the femur (thigh) bone and the anterio-medial aspect of the tibia (shin) bone and holds the knee stable in an anterior-posterior direction. Without it (or an effective surgical replacement) the knee commonly feels "unstable" and it is difficult to ski at anything above an average intermediate skier level. For professional skiers (racers, instructors and patrollers), ACL injury can represent a real threat to their careers. ACL injuries comprise approximately 25% of all alpine ski injuries and are more common in females - this is thought  to be due to the influence of female hormones on ligament strength and also possibly due to differences in knee posture between men and women.

There are three recognised mechanisms of ACL injury amongst alpine skiers. These have largely been researched thanks to hours of painstaking video analysis by a team from Vermont, USA lead by Professor Bob Johnson, Professor Jake Shealy and Carl Etllinger from Vermont Ski Safety. This work has identified that approximately 70% of all ACL injuries result from a set of events they have termed the "Phantom Foot" scenario.  The Knee Binding is specifically designed to provide protection for the ACL in the phantom foot scenario by providing lateral release of the ski boot at the heel below the elastic limit of the ACL (i.e. before the force applied across it becomes too great that it ruptures). I will now try and explain how the phantom foot scenario happens in more detail below.

The Phantom Foot Scenario explained

The PF scenario is essentially a specific type of backwards fall on skis that applies force across the ACL to such a degree that the ligament is at risk of rupture. It derives it's (some would say strange!) name from the fact that the tail of the downhill ski acts as a “phantom foot” - providing a force from the back of the ski as if an imaginary foot were pushing the tail of the ski.  In combination with the stiff back of a ski boot, this force acts as a lever to apply a unique combination of twisting and bending force across the knee joint which can ultimately lead to ACL injury. It occurs when the knee is flexed (bent) to an angle of between 45 and 140 degrees.

It is important to know that PF ACL ruptures can and do happen at slow speed and on very gentle slopes. Don't think it only happens to ski racers - the video clip later on this page shows a PF ACL injury and you'll see that it happens at relatively slow speed.

The components of a PF fall

So, how do we know what makes up a PF fall scenario? The answer is video analysis of more than 14,000 skiing injuries!! This work has identified a typical PF profile which comprises six elements that come together in the event of (usually a backwards) fall and which characterise this mechanism of injury. These six elements are:

1) Uphill arm back
2) Skier off balance to the rear
3) Hips below the knees
4) Uphill ski un-weighted
5) Weight on the inside edge of downhill ski tail
6) Upper body facing downhill ski

These components usually occur in the order shown above, although this does not have to be the case. However, when all six elements of the phantom foot profile are present, injury to the ACL of the downhill leg is extremely likely. There are certain relatively common situations that predispose to the PF scenario developing and which it is important to be aware of if you wish to avoid this injury occurring. These situations are:-

a) Attempting to get up whilst still moving after a fall
b) Attempting to recover from an off-balance position to the rear
c) Attempting to sit down after losing control


Video of a Phantom Foot ACL injury scenario

The video clip below is from YouTube and shows a skier sustaining a PF ACL rupture of their left knee after a small jump goes wrong, resulting in them falling backwards. It all happens pretty fast, but watch the uphill ski become unweighted, the hips go below the knees, the skier's body faces down the slope and (not very pleasant this) if you turn the sound up you can actually hear their reaction (A loud "Oh") at the moment the ACL pops.


The new binding - Knee Binding

Knee Binding has been developed by Rick Howell and his team from Stowe, Vermont, USA. Rick was previously responsible for inventing toe clip pedals for cycling. Key to the success of the device was the development of a new test method to both simulate and quantify the loads applied during a phantom foot fall. This was then used to compare the responses to PF fall forces of both standard alpine ski bindings as well as the new KneeBinding device. It was found that traditional ski bindings could not detect (and therefore respond) to these forces whereas the KneeBinding could - and crucially at force levels well below those that would lead to ACL rupture.

The KneeBinding offers a unique system that allows the binding to release laterally (side to side) at the heel, and it is this specific feature which is believed to offer the ACL protection in a Phantom foot fall. Click on the thumbnail picture to the right to see the full picture of the binding.

I have included a video below posted by the binding's manufacturer for you to look at. on You Tube . The video features the binding's inventor and has two components. The first describes the background to the problem. The second shows you the test system used in the development of the binding and then shows you a couple of simulations when forces are applied to the ski with both a traditional ski binding in position and then with the new binding in place. In particular, note the dramatic difference in forces used between the two bindings when the PF scenario is simulated. The traditional binding only releases when a serious amount of force is applied (see how far back Rick stands from the ski!) whereas the lateral heel release allows the KneeBinding to respond at a much lower level of force.

KneeBinding Video



Video  - The concept behind the binding (7 mins 22 secs)

More information

For more information on the KneeBinding, visit the manufacturer's website at


St Onge N et al. Effect of ski binding parameters on knee biomechanics: A Three Dimensional Computational Study. Medicine and Science in Sports and Exercise. 36 (7): 1218-1225, May 2004.



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