ACL and Related Knee Injuries.....Shaped Skis Vs Straight Skis

If you're looking for a ski technology rant look more at how many more people dart in to the woods and die after catching a shaped ski edge than occurred when everyone was using straight skis.  Hook a shaped edge and your momentary out of control direction radically changes whereas hooking a straight ski edge keeps you going straight but locked.

 

Still, I'll accept the additional risks in return for the ease of skiing on newer skis.

 

Have a close look at rick howell's posts on this forum, specifically the range of distances from the heel he has isolated as being of interest.    

USSA and FIS are using increased ACL injuries as a rationale for dialing sidecuts back to higher radius year after year.  It's now looking to be 35 next year, up from 21 a few years back for GS.

Not sure how accurate these are but they are certainly on point-

 

http://www.ski-injury.com/specific-sports/alpine

The rate of serious knee sprains (in particular anterior cruciate ligament - ACL - sprains) initially jumped by some 240% (from the 1970's to the early 1990's), then remained static for about ten seasons with an ACL injury incidence rate in MDBI of about 2200. Since about the year 2000, the ACL injury rate has begun to decline. The next paragraph explains...

 

and

http://www.ski-injury.com/specific-injuries/knee

The introduction of carving (“super sidecut”) skis, which possess improved turning characteristics compared to traditional skis, was initially linked to an increase in knee injury rates. This was indeed seen in some of the alpine racing teams when they first started using carving skis in the early 1990's. This risk of injury with carving skis now seems to have diminished and in fact for the last 5 years or more the injury stats show that carving skis in fact protect against injury - the hypothesis is that established skiers who changed from traditional 'skinny' skis have now got used to carving skis and their improved carving characteristics.

 

and here is an Ortho`s take http://www.drstaub.com/ski.html

 The ligaments along the inside of the knee are 
injured when a skier catches the inner edge of a ski in the snow and the foot rotates outward as the 
body continues forward.  Stress is then placed on the inner side of the knee, which is forced into a 
“knock-knee” position as the skier falls.  The ligament will be stretched (sprained) if the force is 
minimal and if the bindings release.  Otherwise, the ligament can be completely torn...

 

The new shorter, shaped skis place more stress on the knees than the former, longer skis.

Especially stress on the medial collateral ligament  (MCL).

Prevention of these injuries mainly centers around adequate bindings.  It has been reported that a 
high percentage of lower extremity injuries result from a failure of bindings to release.  Quality 
bindings properly mounted and maintained will prevent many knee injuries.  Various knee braces can 
also be worn and, to some extent, can provide protection.  A brace with metal sidings and hinges can 
comfortably be worn under ski pants and allow an acceptable degree of knee mobility

Shaped skis tend to be shorter. Shorter skis put less torque on the knees - shorter lever arm physics. (Also they can be lighter.) Shaped skis also improve the skills of a skier - hopefully a skilled skier will fall less. Perhaps high level skiers may be able to push the envelope further and sustain more severe injuries, the paranoid race powers seem to think so.
For me, they seem to help as I have not hurt my knees since switching to shaped skis - despite an appetite for bumps and serious graying.
Eric

Shaped skis are beautiful performers — and here to stay:  however, the facts, both biomechanically and epidemologically are as follows:

 

Basic physics of Isaac Newton show that when T = f x d  and when properly functioning bindings supply a 'torque-limit', a long ski will produce the same torque about the tibia as a short ski (again, only when a limit is supplied by a binding), until — in the presence of the human anatomy — were the lateral force applied to the ski becomes incrementally closer and closer to the projected axis of the tibia — then the lateral force 'transforms' to produce more of a lateral shear-force ( in orthopedic terms, this means more of an 'abduction force' applied at the distal end of the tibia )  that is generating more valgus torque (about the femur) than torque about the tibia, when knee flexion is approximately 80 to 110-degrees (excluded angle).  Large valgus torque takes the ACL (and the MCL) directly past their elastic limit (the limit for a 50th-percentile male is ~400 Newtons of abduction-force applied to the ski ~15cm aft of the projected axis of the tibia = ~ 25 daNm of valgus torque).  Under the same loading conditions — where the lateral force that's applied to the medial edge of the ski operates under the same 'limit-conditions' of a properly functioning ordinary binding — and when the abduction force becomes applied incrementally closer and closer to the tibia, torque about the tibia approaches zero (the limit of a properly functioning 'ordinary' binding becomes irrelevant).  This is all basic physics-101.   These facts are proven both mathematically and experimentally by actual physical measurements.   So far, I am the only one on the planet measuring (experimentally) torque about the tibia and valgus torque as a function of the position of the applied force on the ski:  once others start to measure these interactions, everyone's light bulbs will go on. 

 

The experimental measurements produce clear results that dramatically prove how the forces supplied by longer skis that can 'slide-out' at either the tip (or, alternatively, at the tail) produce a single force that acts over the length of the ski to generate torque about the tibia, which torque about the tibia is limited by a properly functioning 'ordinary' binding (the resultant load on the tibia is crystal clear);  AND we also see, that experimental measurements produce results that dramatically illuminate how the two forces supplied at BOTH THE TIP AND THE TAIL by shorter-shaped skis (neither the tip or the tail slide-out when edged) produce a SINGLE RESULTANT CENTROID located under ( or near ) the projected axis of the tibia, where this singular loading-condition ( in orthopaedic termonology this is an 'abduction force' ) generates torque about the tibia that is well BELOW the limit supplied by a properly functioning 'ordinary' binding ( tibia torque approaches zero ) BUT WHICH ABDUCTION FORCE BECOMES TRANSFORMATIVE TO ACT OVER THE LENGTH OF THE TIBIA to produce massive valgus torque about the femur when knee flexion is large ( as during Phantom Foot and Slip-Catch events ).   This above loading scenario matches the most prevalent load-events, on-snow (Phantom Foot and Slip-Catch events) which are believed by leading epidemiologists to be associated with ~ 70% to 80% of all (prevalence) of skiing ACL injuries.

 

(( When the quantity of the applied load is BELOW the limit supplied by a binding, no release occurs:   When T = f x d and d =0, T = 0. ))

 

(( Straight backward loading ( as in "BIAD" events  — "Boot Induced Anterior Drawer loading events ) — where the anterior drawer component of the load-event is greater than the valgus torque produced by Phantom Foot or Slip-Catch events — appear to be present in approximately 15% of all skiing ACL injuries. ))

 

In addition to the plausible biomechanical / experimental measurements produced in the lab ( above ) regarding the dominant injury-mechanism (Phantom Foot or Slip-Catch events), epidemiological data consistently show that the incidence of ACL injuries (yes, that's expressed as mean-days-between-injuries) were very, very low when proper evidence-based data was first collected by the leading epidemiologists starting in the mid-seventies, then steadily and significantly rose during the eighties, plateauing during he nineties, and has only very, very slightly tapered-off since the mid-nineties.  The epidemiological data clearly also show that ACL injuries are BY FAR the most prevalent ( expressed as "% of all" ) type of skiing injury, today.

 

Binding toe pieces with upward release play no ( ZERO ) role in mitigating Phantom Foot or Slip-Catch events ( again, which PF and S-C events have a prevalence of ~70% to 80% of all ACL skiing injuries ):  bindings with upward toe release do directly effect BIAD events ( which BIAD events have a prevalence of ~10 to 15% of all ).  

 

On the other hand, bindings that have a release mechanism positioned along the kinematic pathway between the snow surface where load-events produce SINGLE RESULTANT CENTROIDS that enter the ski under (or near) the projected axis of the tibia — and the hip — (between the snow surface located under the projected axis of the tibia and the hip) — serve to limit the abduction forces that become transformed from producing torque-about-the-tibia to becoming valgus torque (torque-about-the-femur) — which bindings with special release mechanisms that are placed between the snow surface under the tibia and the hip directly effect Phantom Foot and Slip-Catch events (again, which events appear to have a prevalence of ~ 70% to 80% of all skiing ACL injuries).

 

The intersection of plausible biomechanical modeling (experimental measurements both in-computo and in the physical lab) coupled together with proper epidemiological data provide the proof.   There is much talk — but very few ( if, any ) measurements by those who can make a positive impact toward solving this problem.  Anyone ( 'well ... almost anyone ) who would like to observe these measurements are welcome to visit my lab here in Stowe, VT.   This work was also presented at the following ISSS (International Society for Skiing Safety) conferences:   ISSS-Switzerland, 2003:   ISSS-Japan, 2005;  and ISSS-Scotland, 2007.   At the ISSS-Germany, 2009 conference, the work was not presented but was published in the abstract before the conference.   The abstracts of the work from '05, '07 and '09 are available, publicly, and can be sourced through ISSS (this work was not published by the 'Journal of ASTM International' because I don't write papers, I make bindings) and can also be sourced through the peer-reviewed  Journal of Knee Surgery, Arthroscopy and Sports Traumatology.  The work from '03 can be purchased from the peer-reviewed journal of  Medicine & Science in Sports & Exercise  [St. Onge, N., Chevalier, Y., Hagemeister, N., Van de Putte, M., De Guise, J., “Effect of Ski Binding Parameters on Knee Biomechanics: A Three-Dimensional Computational Study,” Vol. 36, No. 7, pp 1218-1225, May, 2004].  More of this work will be presented at ISSS-Argentina, 2013.

 

There is only one non-pre-releasing binding that provides a mechanism that resides within the kinematic pathway between the snow surface and the hip which reads and reacts to the single abduction force that becomes transformed from producing torque-about-the-tibia (where a single abduction-force enters the ski NOT under or near the projected axis of the tibia — as can typically occur with a long straight ski) to becoming a producer of valgus torque (where the abduction force enters the ski as a SINGLE RESULTANT CENTROID under or near the projected axis of the tibia — as can typically occur with a shorter, shaped ski).  Anyone can engineer a binding to release in any mode:  not everyone can engineer a binding to release in a special mode that does not cause pre-release.  Here, my 'lab' is a result of my experience racing DH for many years at a high level on incrementally-improved experimental bindings that do not pre-release (independently of the settings);  from proprietary pendulum (dynamic impact) test methods since 1976;  and from the on-snow lab here on the mountain in Stowe, Vermont.   There is only one binding with this special mode of release that does not pre-release.   (( Mitigating pre-release is a higher priority than release (in a binding that is intended to release) because mitigating injuries that can be caused by pre-release ( head and spinal cord injuries ) is FAR more important than mitigating broken bones and ruptured or sprained ligaments. ))

 

Shorter shaped skis cause this transformation ( from producing tibia torque to valgus torque ) — and bindings that are utilized on shorter shaped skis must therefore apply the concepts of evolutionary biology (specifically, 'adaptive radiation') to deal with the transformitive-changes that are caused by our beautifully-functioning shorter shaped skis.   Bindings that can do this allow us to have it both ways:  easily carved-turns + lesser ACL injuries.

 

Rick Howell, Stowe, Vermont

Edited by Richard Howell - 5/16/12 at 4:37am

* ...of course, one would be remiss to not add that all alpine ski-bindings that are "intended to release" MUST also meet (1) minimum international alpine ski-binding standards ISO 9462, 9456 and 11087 (and their sub-standards);  AND  (2) must also meet  de facto  'standard industry practice'  for anti-pre-release and durability.   The term 'must' applies to  de facto  American case law;  to statutory German law;  and to enforcement actions in Switzerland by the BfÜ (Bureau for the Prevention of Injuries, based in Bern, Switzerland) who remove alpine ski-bindings from retail stores that do not gain approval from TÜV according to the ISO standards ( because Switzerland has learned that skiing injuries notably impact the Swiss-GDP ).   There is presently only one independent testing lab in the world that tests alpine ski-bindings according to the ISO standards (International Standards Organization — ISO is based in Geneva, Switzerland)  — and that independent lab is TÜV (based in Munich, Germany).   The ISO ski-binding standards are developed by 'consensus' from delegates representing DIN (Germany),  ASTM (USA),  AfNOR (France)  and  Ö-Norms (Austria).     'Standard industry practice' is  de facto  among binding manufacturers — and is the high-engineering-bar of field testing that forms a  de facto  economic barrier-to-entry, resulting in only a handful of alpine ski-binding manufacturers that can cash-flow operations past this key hurtle of anti-prerelease and durability, pre-revenue.

 

   :)  :)

 

Rick Howell, Stowe, Vermont
 

Does anyone want to 'vote' against Isaac Newton ?
 

... re the ref to 'the DIN standard' — there are thousands of DIN (German Industrial Standards) standards — but by your ref, you probably mean DIN 7881, which was adapted (with some changes) to become ISO 8061 (Selection of Release Torque Values).  This standard can be purchased by anyone through ANSI (American National Standards Institute) and therefore, this standard is 'still available for you'. 

 

However, pls note that the standards I reference (above), ISO 9462, 9465 and 11087 relate to ski-binding design for binding manufacturers, not to the selection of release torque values for ski-retailers and for skiers.  However, all of these standards, including not only ISO 8061 but also ISO 9462, 9465 and 11087 can be purchased by anyone through ANSI, on-line.
 

(( Of course, there is no binding manufacturer standard ( yet ) that applies to the measurement of valgus torque — because the keepers of the flame still only observe bindings with their proverbial magnifying glasses (measuring tibia torque) rather than with telescopes (measuring valgus torque):  until others begin measuring valgus torque, the effects of 'shorter' shaped skis ('shorter' than older traditional long straight skis) will cause ACL injuries.  The classic required reading book for freshmen engineers at MIT,  Structure of Scientific Revolution,  by Thomas Kuhn, discusses this problem about those who hold the old-paradigm and who refuse to re-think in new ways.  The solutions to this problem ( the "Not Invented Here Syndrome") are not for the feint hearted ( I will not spoil the story for you ) ... but meanwhile, the people who are the 'old-guard' / keepers of the flame (who, ironically, are the same people today that were the 'radicals' that helped all of us skiers by solving the tibia-fracture problem back in the late 1960's and early 1770's) are the root of why we (as an industry) cannot break out of the skiing-ACL problem to run with the new flag (bindings that respond to valgus torque) to solve the knee-friendly problem.   (( Also of course, anyone can purchase a pair of these new bindings that respond to valgus torque at retail, then utilize a simple fish-scale to measure the new mode of release force as a function of setting indicator number to derive the selected release force (not valgus torque) values ... though that measurement (alone) will not, directly, illuminate the effects of the valgus torque relative to strain across the ACL:  measurement of femur torque directly correlates to valgus torque, especially when different length 'tibias' and different knee-flexion angles are introduced. ))  ))

 

:)   :)

Edited by Richard Howell - 5/16/12 at 6:27am

 

It is also could be that the average carving and racing ski sidecut depth has decreased some along with the average turning radius increasing some over the past 5 years.  We're not seeing as many people on skis under 17r.  GS racers are now up above 29.  Powder and back country skis are also way up there.  I would assert that the decrease in injuries over the past 5-8 years correlates with the increase in fat powder ski and higher radius GS and carving skis quite well.

The Vatican..  hahahaha

This is what happened to me but it was the outside edge of the inside foot (turning left to right) 

The above ref to ACL injuries is incorrect:  the facts show that, longitudinally over time, the rate of change of the incidence of ACL injuries is decreasing slightly, but that the actual incidence remains BY FAR the worst of all skiing injuries and that skiing ACL-injuries also remain the highest in terms of prevalence.  'Safety' ( I don't like that term ) is also measured both as a function of frequency and severity:  by this index also, skiing ACL-injuries remain EVEN MORE BY FAR the leading injury-problem in skiing, today.  BY FAR.  There is only one study that attempts to find that shaped skis do not correlate to skiing ACL-injuries — and that study was funded by Austrian ski manufacturers, and it does not properly (statistically) compare injured skiers to control-skiers (the sample sizes are too small and the p-values are not good).  Virtually all other studies, especially including the repeatedly-proven epidemiological studies by Johnson, Shealy and Ettlinger ( which are randomized, double-blind, evidence-based studies with statistically-valid sample sizes that compare injured populations and their equipment factors to  a control-population and their equipment factors and where diagnosis is conducted by on-site MD's (and where both the injured and the control pop's are 'closed' — meaning that very few skiers 'escape undetected by this study' to a 3rd-party clinic, hospital, etc.) , show, conclusively, that skiing-ACL injuries are BY FAR the most frequent in terms of both incidence and prevalence, TODAY, BY FAR.
 

The above discussion of individual (singular) events have been repeatedly proven to be unreliable in several extensively-researched studies spanning over 40-years where injured skiers who were videotaped were asked to describe their injuries (before they saw their injury-mechanism), then upon review of the videotapes, the actual injury-mechanism was typically VERY DIFFERENT from the injury-mechanism perceived by the injured skier. 

 

We must rely upon statistically-valid sample sizes of injured skiers compared to control-populations of skiers within properly conducted evidence-based research to properly point to trends in injury-mechanisms ... then utilize that information to construct plausible biomechanical models either 'in-computo' or with 'physical lab measurements' to simulate the injury-mechanisms that are discovered through the properly conducted evidence-based research.

 

That is all I'm going to offer at this time ( no more posts for a while ) ... because I must deal with other critical issues that are on my immediate horizon, which issues, if properly resolved, will help all of us move-forward with knee-friendly skiing.

Einstein successfully campaigned against Newton. Irrelevant here.

My two serious injuries would never happen with modern equipment. Boottop fracture from horribly designed antique Nordica red racer boots (I still carry a prejudice against Nordica products). Twisted out of one binding on a helicopter, rode out on one ski until the safety strap gacked my knee when the trailing wedged in a bump. Modern technology has completely removed these risks. Thanks!

I'm not sure I understand the issue. Does a knee injury usually happen at the first slip and twist? Or does the injury happen during the tomahawk? High level racers who can't make a turn because the ski's turn radius is too big would appear to be at a much higher risk of injury.

With a skiing population that is getting older and fatter, any reduction in injury rate is a huge endorsement of the safety of modern equipment. Do wall sits and quad tighteners to really protect your knees.

Eric

What happens is that they transfer a lot more g-forces to the knees because the CAN turn tighter at higher speeds.  When they're skidding, pivoting, and chattering the energy gets dispersed.  When the skis rail and hold in the tighter arc at very high speeds a lot more force is exerted on the knees.. and the knee goes POP!

So the knee fails causing the fall? Seriously you want me to believe that?

I did abandon a project to make padded underwear for old people because the orthopod figured enough hip fracture falls happened because the hip fracture preceded the fall. The liability was too high because of that. But our targets were not conditioned athletes.

Avoid the fall, avoid the injury on skis.

Eric

It is more that the ability to go faster and ski courses at higher rates of speed results in more injuries.  Slip CATCH POP is more the cause than hold hold popped knee.  They're dialing back to older sidecuts to slow down the skiers and reduce both the g forces and the likelyhood of crashes.  Make the skis too good and people get hurt on them pushing themselves beyond their physical limits

 

OK, better skis let you go faster and the injury risk goes up with speed. But the proposed rules confuse me. NASCAR puts restrictor plates in the engine not limits on the steering wheel. Perhaps outlawing flouro wax is more appropriate than requiring skis that don't turn

I borrowed super G skis for a masters GS race. I am not a great racer so I was challenged by the course anyway. I did OK time wise but was very uncomfortable and felt more at risk for blowing out of the course. I would struggle with the new rules. But I don't race much so the serious racers will adapt and enjoy.

Eric

Typo correction:   The elastic limit of the ACL is approached at 600 N ( not 400 N ) when an abduction force is applied to the medial edge of the ski ~15 cm aft of the projected axis of the tibia:  it is at this point ( ~ 15 cm ) where the resultant of mostly valgus torque is vector-added to a small amount of tibia torque to produce maximal resultant valgus-tibial torque at 24 daNm ( not 25 daNm ).   Thank you for your attention to this correction.  :)   :)

 

Rick Howell, Stowe, Vermont

OK, but for me, without the rush of speed and edge I won't race as much and when I do I will be quite timid. Injury issue solved but declining participants issue exacerbated. And the pros will push the limits no matter what.

Eric

wow

I'm glad to see that math and physic has a place here at Epicski.  Its to much for me.

 

My respones to Oldschoolskier  statement in the first post  "One of the things that I have noticed is that since stepping from straight skis to shaped skis this year (2012), there does seem to be additional loading placed on the knees."  I haven't had any increase loading on my knees and I have been the shape ski since Elan SCX's came on to the market. We do need to look at shape ski and binding but let's not forget the skier style ie the park and ride folks. I think that if you just drop inside the turn and the ski  hooks up on you will find that the knee will be taking a lot of abuse IMHO.

 

Hank 

Flawed pole.

 

Both styles of skis are equally likely to cause injury to knees, one is not more likely than the other.  Just my opinion.

 

BTW my old race skis are just as torsionally stiff as modern skis.

gravity kills.....