Documentation:FIB book/Lower Extremity/Ski Bindings and Lower Limb Fractures

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Overview

This page discusses the mechanisms of lower-limb fractures (primarily tibia, but also fibula) in the context of skiing, and the importance of releasable ski bindings to mitigate the risk of fracture. The relevance of this is shown by the embedded video below, in which a French downhill skier fractures both of his legs during the 2017 World Cup. You can skip to 1:40 to see it happening.

Introduction

Percentage of tibia fractures of all alpine skiing injuries from 1961 to 1997.[1]

Almost 60 years ago, in 1961, lower leg fractures, known as "boot top" fractures, were very common in the environment of alpine skiing, making up 12.9% of all alpine skiing injuries[1]. A boot top fracture consists of both a fracture where the tibia breaks close to the ski boot top and the fibula breaks higher, close to the knee. These fractures are common because of the rigid connection between the boot and the ski during skiing accidents.[2] Due to advances in ski bindings during the ‘70s and ‘80s, most notably "releasable bindings", which allow the bindings to release the boot when subjected to high bending or torsional forces, the frequency of boot top fractures has drastically decreased. This decrease is illustrated in skiing injury statistic, as these efforts to increase the safety of skiing equipment reduced the amount of tibia fractures from 12.9% of all alpine skiing injuries in 1961 to 2.1% in 1997.[1]

The introduction and improvement of releasable ski bindings was necessary, as these bindings decrease the likelihood of skiers suffering from a boot top fracture, which is a severe injury with a long healing process. These tibia/fibula fractures often also result in long-term consequences, such as ankle osteoarthritis, foot and ankle deformities and discomfort due to metal implants.[3]

Numerous studies have been done on the injuries related to alpine skiing, including lower leg fractures. Several academic papers touch on the studies that are conducted to identify the correlation between ski binding release and lower leg fractures. Several statistics indicate that the decline in the injury rate can be attributed to improved binding designs.[4]

Biomechanics of Bindings

A classic releasable ski bindings used to this day is called the "heel/toe binding" for which it needs to satisfy two main conditions: retention, meaning that the binding must hold the boot firmly in place while skiing, and protection, meaning that the binding must release when a pre-set fracture threshold of bending and/or torsion is experienced by the lower limb (with a factor of safety implemented). Hull & Allen in 1981 used existing quantitative data to determine an average for this fracture threshold more known as the average quasi-static tibia strength. This was found to be 225 Nm in bending and 75 Nm in torsion.[5]

Furthermore, Crawford & Mote expanded on the releasable "heel/toe binding" with the idea of conventional binding. This allows the heel to release first when the pre-set fracture threshold in bending is met and/or the toe to release first by sliding out when the pre-set fracture threshold in torsion is met.[6]

Early Development

In the earlier development stages of releasable ski bindings, a few prototypes were designed to reduce lower leg fractures as a result of alpine skiing. Biomechanics data is used in additional design and development of these bindings, often resulting in an exponential improvement in performance and potential injury reduction.

In 1981, Hull & Allen’s prototype consisted of a dynamometer, an analog computer controller and an electromechanical release mechanism in order to collect biomechanics data to reduce lower leg fractures. The electromechanical release mechanism consisted of a closed-circuit hydraulic system which allowed the binding to withstand non-injurious high magnitude and short-duration loads without releasing but instead allowing the binding to release right before the quasi-static loads reach injurious levels. This allowed Hull & Allen to determine maximum binding loading before release in order to better implement their design prototype.[5]

8 years later in 1989, Wunderly & Hull built on this prototype to design a new prototype consisting of 4 subsystem: a retention mechanism, a "double pivot" (explained below), load sensing, and a release mechanism. The retention mechanism is simply used to latch the boot in to the binding, causing the top plate to latch down and the jaws to close. This keeps in the boot firmly held until the actual release value of combined loading is reached.[7]

An important concept addressed in Wunderly & Hull's work that was lacking in Hull & Allen's prototype was the "double pivot", which allows rotations about the Y and Z axes. This restricts the binding to sensing the loads in bending and torsion together. Additionally, it places the boot 2.54 cm above the surface of the ski, which increases elevation and the skier’s ability to control the ski. In other words, the double pivot not only allows the sensing of the desired loads but also provides more friction control. The load sensing allows a programmable cam to sense the bending and torsional moments in order to exactly trigger the release mechanism when the motion reaches a predetermined limit.[7]

Instrumented Binding Experiments

Several snow skiing experiments have also been conducted in efforts to better predict binding retention requirements with respect to tibial failure. In one such experiment by Quinn & Mote[8], six healthy males skied a course with an instrumented left ski (including dynamometers at the toe and heel of the binding) to predict boot top forces and moments. Linear regression analysis determined that the vertical binding forces were not satisfactory predictors of anterior-posterior boot top moment.[8] This contradicted the conventional practice of binding designers, who used such forces to determine when the bindings should release.

Crawford & Mote[6] built on this work by adding electromyographic electrodes to Quinn & Mote's instrumentation techniques, since Quinn & Mote had not considered musculature effects in their model. This study used more subjects (36) than Quinn & Mote, and subjects skied courses multiple times. Results found that the user's weight can help predict boot top moment thresholds, but not torque thresholds. This was an important discovery, as commercial binding release settings could be adjusted according to weight at the time.

Limitations

Although there were many strengths to these studies, as with all biomechanics experiments, there were several limitations to this work.

Modelling Limitations

Most of the above studies list a major limitation as the assumption of negligible change in inertia.[5][8] This is not consistent with real-life scenarios, as inertia changes constantly with the skier's change in posture and centre of gravity while skiing a course. Additionally, with the exception of Crawford & Mote, most models do not account for the effects of musculature, which has been cited as a potentially major influencer in determining tibia threshold values.[9] Finally, with the exception of Hull & Allen's prototype, most ski bindings and therefore models only consider bending and torsion independently, whereas most tibial fractures occur by the combination of both mechanisms.

Live Skiing Experiments Limitations

As with many clinical human volunteers studies, Quinn & Mote and Crawford & Mote were limited to fairly small sample sizes (6 and 36, respectively). With different subjects, and in Crawford & Mote's case, different ski courses, repeatability was difficult to maintain, making data analysis more complicated. Most importantly, the prototypes discussed above were not tested in live skiing experiments, meaning that prototype verification was never completed.[5][7]

Overall Limitations

Although ski binding work has led to a drastic decrease in lower limb fracture over the past thirty years, this advancement in technology has not helped to mitigate knee injuries. Many of the above studies list the lack of consideration of ligamentous knee injuries as a major limitation.[5][6][7][8] As a result, most of the work done surrounding ski injuries today is focused towards the knee. Therefore, the research studying tibial fracture in ski injuries is quite dated, and unfortunately minimal.

Persisting Issue and Future Works

As mentioned above, although the improvement in ski binding design and biomechanics parameters has decreased the frequency of lower limb fracture, no bindings have been able to help mitigate knee injuries, particularly ligamentous injuries. Much research and development has been done with the goal to further prevent ski injury with sensor-integrated ski binding developments from 1990s.

Lowering Ski Binding Settings to Incorporate Sex-specific Differences

In a retrospective study of 498 recreational skiers who sustained an ACL injury, it was reported that the bindings failed to release in 78% of cases, with a much higher incidence in female than male skiers. [10] Releasable binding technique cannot fully protect the skier from knee related injuries. However, lowering the binding setting thresholds to make the ski easier to self-release may decrease the chance of a knee injury, as it provides more flexibility to skiers to self-release their bindings.

Knee-injured male recreational skiers reported a failure of binding release at the moment of accident in 55–67% of cases compared to 74-88% of cases reported by female skiers, despite the fact that sexes do not seem to differ with regard to neither the date of the last binding adjustment nor not correctly adjusted bindings. Falls at slow speed and falling backward resulted in a higher incidence of failed binding release than did falls at higher perceived speeds. Overall, failure of the binding to release was substantially associated with female sex, slow perceived speed, and complete rupture of the ACL.[10]

The current International Standards Organization ISO 11088 standard for binding setting values does not consider sex-specific differences, and the failure of binding release associated with a knee injury is significantly higher among females compared to males. The ISO 11088 standard for binding setting values allows a lowering by 15%, which could prevent injuries of the lower extremities. Research showed that four times more females were able to self-release their ski at least once with both legs with a 15% lowered binding settings compared to their default settings.[11]

New Measurement Techniques with Dynamometers

Meyer et al. developed a new embedded force platform with a dynamometer to perform force and torque measurements.[12] The compact platform can be embedded onto the skier's own equipment as a removable interface between the boot and the binding. The platform is composed of two stages; an upper platform using two strain-gauge dynamometer to measure Fy and Mz component, and the lower platform with 4 sensors that can measure Fz, Mx and My. The platform based on strain-gauge sensors has a benefit over piezo-electric sensors developed by Stricker et al.[13] as signals from strain gauges do not drift and there is no need to calibrate the platform after each trial. Furthermore, the 3D modelling and finite element analysis are used to evaluate the load distribution to obtain an optimal ski binding design. Future development would allow the measurement of kinematic measurements to quantify loads acting on the knee to put a focus on reducing ligamentous knee injuries.

Electromechanical Ski Binding

Neptune et al. developed electromechanical ski bindings, in which the design criteria for the new binding system includes a release sensitivity to binding loads that correlate strongly with the twisting and anterior/posterior binding moments developed at the injury sites of the leg. Resistive strain gauges were implemented to measure the load and to accurately measure the twisting moment about the tibia shaft as well as the anterior/posterior bending moment at the boot top. If the system recognized an excessive load above the configured release level, the circuitry actuated a trigger release mechanism in the heel piece to release the boot from the ski.[14]

Conclusion

The development of releasable ski bindings has greatly contributed to the reduction of the occurrence of boot top fractures in the alpine skiing environment. Although boot top fractures have drastically decreased in the past few decades, further biomechanics work could decrease incidence rates further. Future works on the electromechanical ski bindings to automate the releasing system, as well as the new measurement development using dynamometer embedded platform should have a greater focus on ligamentous knee injuries, as the amount of knee-related injuries has not seen a decrease and continues to be a prevalent issue for alpine skiers.

References

  1. 1.0 1.1 1.2 A. Natri, B. D. Beynnon, C. F. Ettlinger, R. J. Johnson, and J. E. Shealy, “Alpine Ski Bindings and Injuries,” Sports Medicine, vol. 28, no. 1, pp. 35–48, 1999.
  2. R. Johnson and M. Pope, “Tibial shaft fractures in skiing,” The American Journal of Sports Medicine, vol. 5, no. 2, pp. 49–62, 1977.
  3. S. Milner and C. Moran, “(v) The long term complications of tibial shaft fractures,” Current Orthopaedics, vol. 17, no. 3, pp. 200–205, 2003.
  4. R. J. Johnson, M. H. Pope, G. Weisman, B. F. White, and C. Ettlinger, “Knee injury in skiing: A multifaceted approach,” The American Journal of Sports Medicine, vol. 7, no. 6, pp. 321–327, 1979.
  5. 5.0 5.1 5.2 5.3 5.4 M. L. Hull and K. W. Allen, “Design of an Actively Controlled Snow Ski Release Binding,” Journal of Biomechanical Engineering, vol. 103, pp. 138–145, Aug. 1981.
  6. 6.0 6.1 6.2 R. P. Crawford and C. D. Mote, “Ski Binding Minimum Retention Requirements,” Skiing Trauma and Safety, vol. 11, pp. 93-108, 1997.
  7. 7.0 7.1 7.2 7.3 G. S. Wunderly and M. L. Hull, “A Biomechanical Approach to Alpine Ski Binding Design,” International Journal of Sport Biomechanics, vol. 5, no. 3, pp. 308–323, 1989.
  8. 8.0 8.1 8.2 8.3 T. P. Quinn and C. D. Mote, “Prediction of the Loading Along the Leg During Snow Skiing,” J. Biomechanics , vol. 25, no. 6, pp. 609–625, 1992.
  9. C. Y. Kuo, J. K. Louie, and C. D. Mote, “Control of Torsion and Bending of the Lower Extremity During Skiing,” Skiing Trauma and Safety: Fifth International Symposium, pp. 91–109, 1985.
  10. 10.0 10.1 Ruedl G, Helle K, Tecklenburg K, Schranz A, Fink C, Burtscher M, "Factors associated with self-reported failure of binding release among ACL injured male and female recreational skiers: A catalyst to change ISO binding standards?", Br J Sports Med, vol. 50(1), p. 37-40, 2016.
  11. Posch M, Burtscher M, Schranz A, Tecklenburg K, Helle K, Ruedl G, "Impact of lowering ski binding settings on the outcome of the self-release test of ski bindings among female recreational skiers", Dove Medical Press, vol.8, p. 267-272, 2017
  12. F. Meyer, A. Prenleloup and A. Schorderet, "Development of New Embedded Dynamometer for the Measurement of Forces and Torques at the Ski-Binding Interface", Sensors, vol.19, 2019
  13. G.Stricker, P.Scheiber, E. Lindenhofer, and E. Müller, "Determination of forces in alpine skiing and snowboarding: Validation of a mobile data acquisition system", Eur. J. Sport Sci. vol. 10, pp. 31-41, 2010
  14. R. Neptune, M.L. Hull, "A New Electromechanical Ski Binding With Release Sensitivity to Torsion and Bending Moments Transmitted by the Leg", International Journal of Sport Biomechanics, p331-349, 1992.