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General Knee-Injury Epidemiology and Prevention
See also Factors Influencing Knee-Injury Risk and Functional Knee Bracing.
For a brief overview of knee anatomy, physiology, and biomechanics, please click here.
For profound insight into the fundamental material differences (e.g. lower strength, easier breakability, less elasticity, more frangibility) between the female and male ACLs, please see the superb December 2006 article Sex-based differences in the tensile properties of the human anterior cruciate ligament, by Naveen Chandrashekar et al., in the Female-Athlete Knee-Injury Incidence and Prevention Subsection. Understanding and Preventing Noncontact Anterior Cruciate Ligament Injuries -- A Review of the Hunt Valley II Meeting, January 2005, Letha Y. Griffin et al.; American Journal of Sports Medicine, Baltimore; September 2006, Volume 34, pages 1512-1532. Comments: This article, a compilation of insights from dozens of knee experts from around the world, provides a comprehensive overview of noncontact (i.e. planting-and-twisting, also known as cutting-type or pivoting) ACL injuries. The authors note that degenerative changes continue to plague ACL-reconstructed knees, thus showing that top-notch reconstruction and rehabilition is not enough. They note that functional knee bracing does not prevent planting-and-twisting-type knee injuries. This is an obvious finding, given the presence of the easily sheared soft tissues surrounding the leg bones. (Although this article focuses on noncontact ACL injuries, the authors could have noted that bracing is ideal for protecting against sideways forcing and injurious hyperextension, two modes of forcing which also cause a lot of knee injuries. It should be kept in mind that sideways forcing and injurious hyperextension can occur in noncontact settings as well as in contact settings. In other words, noncontact ACL injuries are not always of the twisting type.) The authors do note that using a functional brace with an extension constraint (ideally either a spring in each hinge or a system of crossed straps behind the knee) results in more flexed knees during the landing of jumps -- a desirable situation for female athletes in particular. The authors discuss various training measures aimed at preventing ACL injuries, and also delve into the reasons why females are 2-8 times as likely to incur an ACL tear. They note that there is still a lot of research to be done.
For insight into the male-versus-female knee-injury rates in lacrosse, soccer, and basketball players, see the June 2006 article Comparing the Incidence of Anterior Cruciate Ligament Injury in Collegiate Lacrosse, Soccer, and Basketball Players -- Implications for Anterior Cruciate Ligament Mechanism and Prevention, by Mihata et al., in the Female-Athlete Knee-Injury Incidence and Prevention Subsection. For penetrating insight into the 2-to-8-fold elevated ACL-injury incidence in females and prevention avenues in this realm, please see the March 2006 articles Jump-Land Characteristics and Muscle Strength Development in Young Athletes -- A Gender Comparison of 1140 Athletes 9 to 17 Years of Age, by Sue Barber-Westin et al., and Effect of Gender and Maturity on Quadriceps-to-Hamstring Strength Ratio and Anterior Cruciate Ligament Laxity, by Christopher Ahmad et al., in the Female-Athlete Knee-Injury Incidence and Prevention Subsection. Lower extremity biomechanics during the landing of a stop-jump task, Bing Yu et al.; Clinical Biomechanics; March 2006, Volume 21/3, pages 297-305. Comments: This study found that more motion at the hip and knee translates into lower potentially-ACL-injurious forces. This makes sense intuitively as well: anyone who lands a jump with knees locked at full extension or hyperextension is guaranteed to incur massive, permanent, severe injuries to the knee ligaments and articular cartilage. The take-home message here is to always land jumps with ample knee flexion.
Peak Torque and Rotational Stiffness Developed at the Shoe-Surface Interface: The Effect of Shoe Type and Playing Surface, Glen A. Livesay et al.; American Journal of Sports Medicine, Baltimore; March 2006, Volume 34, pages 415-422. Comments: This intriguing study looks at the peak torque, hence maximum torque attained prior to slippage, developed with different combinations of shoe type (large-cleated standard grass shoes, as common for outdoor soccer for decades, versus the newer small-cleated turf shoes) and playing surface (natural grass versus traditional Astroturf, versus the newer artificial surfaces [FieldTurf and AstroPlay, which utilize sand and/or crumb-rubber infill]). Ten different shoe-and-surface combinations were tested. The highest peak torques arose with the grass shoe–FieldTurf tray and turf shoe–Astroturf combinations, whereas the lowest peak torques consistently appeared with the grass surface; however, certain combinations of shoe and infill-type synthetic surfaces brought peak-torque values very similar to those of the natural-grass surface. It would seem reasonable to conclude that a thick, healthy natural grass (which unfortunately is difficult to maintain on a heavily-used field, especially in arid and hot climates) would be most conducive to low twisting-type knee-injury rates. Natural grass also appears to be the most forgiving in terms of shoe type: grass shoe or turf shoe can be used without engendering worrisome increases in peak torque. Meanwhile, the infill-type synthetic surfaces are a good substitute for natural grass, but only if used with the appropriate footwear (i.e. small-cleated turf shoes). Using traditional large-cleated grass shoes on artificial surfaces (including the newest infill types) appears to be risky. The authors note that although the testing loading was a mere 511 N (much less than the weight of the average football player), there is a linear relationship between peak torque and compressive loading, and so the results could be scaled up. The authors also note that because human reaction time must be taken into account when analyzing injury situations, the rate at which the torque rises to its peak value also merits consideration. Finally, other factors play a role too; these include attributes of the athletes themselves, for example training, muscle strength, knee anatomy, knee alignment, and vestibular/proprioceptive/neuromuscular firmware aspects. Given that twisting-type knee injuries occur on all playing surfaces, it is also clear that the athlete's pivoting technique plays a major role in these injuries. For this reason, regardless of the playing surface used, the athlete would be well-advised to practice knee-friendly pivoting techniques, in particular pivoting only on the front portion of the foot (instead of planting the entire shoe sole prior to pivoting).
For insight into the use of prophylactic bracing for injury prevention in the context of competitive contact/collision sports (and also for comments pertaining to the use of functional braces in prophylactic capacities), see the April 2005 article Team Physician's Corner - The Use of Knee Braces, Part 1: Prophylactic Knee Braces in Contact Sports, by Najibi et al., in the Functional Knee Bracing Subsection. For insight into the various interrelated factors (including, but not limited to, gender) which influence ACL-injury propensity, see The Familial Predisposition Toward Tearing the Anterior Cruciate Ligament -- A Case Control Study, by Kevin Flynn et al., in the Factors Influencing Knee-Injury Risk Subsection. Upper and Lower Extremity Muscle Fatigue After a Baseball Pitching Performance, Michael J. Mullaney et al.; American Journal of Sports Medicine, Baltimore; January 2005, Volume 33, pages 108-113. Comments: This study looked at repeated baseball pitching (which involves the entire body, not just the pitching arm) and the fatigue resulting therefrom. Although much of the study focuses on shoulder musculature, the authors note that fatigue in the leg muscles is correlated with reduced knee flexion, among other kinematic indicators. Since reduced knee flexion is correlated with increased risk of knee injury (especially in sports rich in running, jumping, and pivoting), and because fatigue-related concerns arises from any vigorous and potentially fatiguing activity (not just ball-throwing), it follows that endurance-training should be pursued as part of any rehabilitation or sports-training regimen.
For insight into the proprioceptive and performance benefits of special knee-injury-prevention training programs, see Effectiveness of a Neuromuscular and Proprioceptive Training Program in Preventing Anterior Cruciate Ligament Injuries in Female Athletes: 2-Year Follow-up, by Bert Mandelbaum et al., in the Female-Athlete Knee-Injury Incidence and Prevention Subsection. Effect of Fatigue on Knee Kinetics and Kinematics in Stop-Jump Tasks, Jonathan D. Chappell et al.; American Journal of Sports Medicine, Baltimore; July 2005, Volume 33, pages 1022-1029. Comments: This intriguing article clearly demonstrates the neuromuscular consequences of fatigue. Motor-control performance progressively deteriorates (as evidenced by reduced jump height, reduced knee flexion upon jump landing, and other changes) as the person becomes increasingly fatigued. Because muscle activation, timing, and capability all decline with fatigue, and because the knee depends heavily on the surrounding musculature for protection against ligament injuries, it is to be expected that a reduction in the quality of muscular performance translates directly into increased demand on the knee ligaments. A higher likelihood of knee-ligament injuries must therefore be expected. It is incumbent upon all athletes to include some form of endurance training as part of their regimens.
Incidence, Causes, and Severity of High School Football Injuries on FieldTurf Versus Natural Grass: A 5-Year Prospective Study , Michael C. Meyers and Bill S. Barnhill; American Journal of Sports Medicine, Baltimore; October 2004, Volume 32, pages 1626-1638. Comments: This study looks at injuries (with a particular focus on the knee and also head) in team sports played on a synthetic playing surface known as FieldTurf. The heavily advertised FieldTurf consists of infill layers of ground synthetic rubber underlain by sand, and held in place by friction resulting from HDPE fibres that have been combed erect by a special machine. The authors report a higher incidences of 0-day time loss injuries, noncontact injuries, surface/epidermal injuries, muscle-related trauma, and thermal-related injuries during games on FieldTurf. But there were more 1- to 2-day time-loss injuries, 22+ days time-loss injuries, head injuries, and ligament injuries on natural-grass games. So, while playing on FieldTurf does seem to translate into reduced ACL injuries, more MCL injuries appear. As with anything that is marketed as protective equipment, one must be cognizant of the revenge effect: that is, if the new equipment (in this case, not something that is worn but rather the playing surface itself) makes players feel safer, then they might take greater risks. The authors also note that the more consistent artificial composition enhances the speed of the game. But they don't directly note that kinetic energy increases with the square of speed...and that increased kinetic energy (as manifested during acceleration/deceleration and direction-changing manoeuvres) is what makes high-speed injuries exponentially worse than low-speed ones. (Additionally, higher speed translates into diminished reaction time.) The authors note that the higher ACL injury incidence on natural turf is likely attributable to the fact that in many places, natural grass is often allowed to dry out and become dormant, or is overseeded with more durable turf strains that sacrifice cushioning capability for toughness...and so the numbers of injuries on natural turf would probably be substantially lower if the grass were always vigorous and the soil were always moist. (Note that at least one of the authors has a conflict of interest, likely in the form of ownership in the FieldTurf Corporation. Note that there are competing products which use very similar infill-type cushioning arrangements, for example Sprinturf, Sportexe Momentum/Omnigrass, and Sportfield NT/Realgrass. It might be interesting to compare the properties of these different products, but given that they are all extremely similar in both design and construction, it is probably safe to say that the results would all be nearly identical. As a comparison standpoint, keep in mind that the old-fashioned product known as AstroTurf is basically an outdoor carpet with no infilling.)
For a good review and thorough discussion of the evidence in favour of ACL reconstruction, in particular as it applies to preventing reinjury of the ACL-injury-history knee, please read The Effect of Anterior Cruciate Ligament Reconstruction on the Risk of Knee Reinjury, by Dunn et al., in the Long-term Consequences of ACL Injuries Subsection. A Prospective Cohort Study of Injury Incidence and Risk Factors in North Carolina High School Competitive Cheerleaders, Mark R. Schulz, et al.; American Journal of Sports Medicine, Baltimore; Mar/Apr 2004, Volume 32/3, pages 396-405. Comments: The author notes that a number of studies have identified cheerleading injuries as being "unusually severe". The high-elevation stunts and pyramid means that falls involve considerable height, and this translates into increased kinetic energy (again, kinetic energy increases with the square of speed).
Increased Injury Risk Among First-Day Skiers, Snowboarders, and Skiboarders, Mike Langran and Sivasubramaniam Selvaraj; American Journal of Sports Medicine, Baltimore; January 2004, Volume 32, pages 96-103. Comments: This article is an eye-opener for anyone who is considering taking up a new snow sport. Simply being new and inexperienced raises the injury risk enormously. And, although taking lessons from a professional is advisable, keep in mind that newly learned skills do not confer true experience, and therefore the newcomer to snowsports is reminded to not be overly confident. It was noted that using gear borrowed from friends or family was associated with almost an eightfold increased risk of injury...and so the financial savings of such borrowing are likely to prove illusory. Also to be kept in mind is that although the wearing of protective equipment and padding is indeed wise, beware of the "revenge effect". No protective gear is a license to take greater risk. (If donning a helmet automatically makes you feel capable of pursuing a near-vertical triple-black-diamond run -- as is often depicted in media advertisements -- on your first day, then please take a moment to ponder the implications of the following fundamental law of physics: kinetic energy, and hence injury risk, increases with speed squared; additionally, higher speed means reduced reaction time.) Note, too, that because alpine-ski bindings are incapable of detecting the torque at the knee (since said torque is a function of the knee flexion angle), even the best-adjusted ski bindings cannot reliably protect the knee against twisting-type injuries. (For more insight into skiing injuries in particular, see the July 2002 article by Hame et al.)
For insight into the use of functional knee bracing in protecting against repeated giving-way in ACL-deficient knees in the context of alpine skiing, see the April 2003 article Effect of Functional Bracing on Subsequent Knee Injury in ACL-Deficient Professional Skiers, by Kocher et al., in the Functional Knee Bracing Subsection. Injury to the Anterior Cruciate Ligament During Alpine Skiing -- A Biomechanical Analysis of Tibial Torque and Knee Flexion Angle, Sharon L. Hame et al.; American Journal of Sports Medicine, Baltimore; July 2002, Volume 32/4, pages 537-540. Comments: This article concludes that hyperflexion (i.e. flexion of knees so far that one's heels go past one's buttocks [which is exactly what occurred to forum founder Bob Willmot]) and hyperextension (falling forwards with one's skis remaining straight forward) are the major dangers to alpine skiers. One concern in this cadaver-based study (besides the fact that it cannot directly take into account the effect of muscle activation) is that the torque applied to the knees was a mere 10 Newton-metres, which is much lower than what can be applied by skis in real-life situations (and which is roughly 25% of what would cause ACL rupture). The authors conclude that, for alpine skiers, twisting is less of a concern than hyperflexion and hyperextension. But because the forces used in this study were so low, some further experimentation with loadings more representative of what can be expected in real life would be desirable. Also desirable would be further research into the effects of various multi-part series of forces (for example bipartite combinations such as hyperextension with twisting, hyperflexion with twisting, and sideways forcing with twisting, or tripartite combinations such as hyperextension, sideways forcing, and twisting). However, it is clear that hyperflexion and hyperextension represent genuine dangers to the alpine skier's knee, and the authors are to be commended for their thought-provoking research. (Further comments provided in the article.)
For insight into the biomechanical differences between females and males during rapid acceleration-deceleration activities, including greater anterior-drawer shearing and inwards torquing at the knee, see the following article by Kerrigan et al.: A Comparison of Knee Kinetics between Male and Female Recreational Athletes in Stop-Jump Tasks, in the Female-Athlete Knee-Injury Incidence and Prevention Subsection. For excellent insight into the biomechanically horrific consequences of even occasional use of high-heeled footwear, and also for insight into the benefits of refraining from using such footwear, see the following two articles by Kerrigan et al.: Knee osteoarthritis and high-heeled shoes and Women's shoes and knee osteoarthritis, both housed in the The Degenerate Knee: Arthritis Subsection. Another article of particular interest to women is Knee Joint Torques: A Comparison Between Women and Men During Barefoot Walking.
Effects of Protective Knee Bracing on Speed and Agility, David L. Greene et al.; American Journal of Sports Medicine, Baltimore; July 2000, Volume 28/4, pages 453-459. The title of this study indicates that it sets out to examine the "effects of protective knee bracing on speed and agility", and it does indeed provide a lot of very useful insight. However, the major concern is that this study blurs the distinction between off-the-shelf functional (DonJoy Legend, Breg Tradition) knee braces and low-end protective-only devices (e.g. McDavid Knee Guard). This is not an apples-to-apples comparison. This same blurring of distinction extends to the authors' review of previous studies. (Further comments provided in the article.)
See also the March/April 2000 article Electromyographic and kinematic analysis of cutting maneuvers: Implications for anterior cruciate ligament injury, by Scott Colby et al., in the Knee Biomechanics, Functional Anatomy of ACL subsection. Skiing Injuries, Robert E Hunter; The American Journal of Sports Medicine, Baltimore; May/Jun 1999, Vol 27/3, p. 354. Comments: This article provides a very good overview of alpine-skiing injury mechanics. Hunter focuses on injuries in general, including not only knee injuries but also wrist, shoulder, and head injuries. Good information on general injury-prevention measures and their potential consequences as well as possible revenge effects. A program to help reduce the risk of serious knee injuries among alpine skiers, Robert Johnson, Jasper Shealy, and Carl Ettlinger. Comments: This article, published by Ettlinger's Vermont Safety Research organization, offers a wealth of practical hints and knee-injury-prevention techniques for the alpine skier. (It is hereby recognized that this injury-prevention programme comprises an analogue video tape which must be purchased. Readers might wish to consider approaching their local librarian and submitting a request to purchase said tape.) Note that many of the points covered by Johnson, Shealy, and Ettlinger are discussed in Hunter's May 1999 AJSM article, which is here in the Knee Library. See also the September 1995 ASJM Article by Ettlinger et al., also available here in the Knee Library. Skiing Injuries in Children, Adolescents, and Adults, M.C. Deibert et al.; The Journal of Bone and Joint Surgery, American Edition; Jan 1998, Vol 80, p. 25-32. Comments: This thought-provoking article discusses the injury epidemiology of alpine skiing, and covers knee injuries in the context of all the injuries associated with this sport. As Figure 2 shows, knee injuries rank at or near the top of injuries for skiers of all ages. Hand injuries are a concern too, as are head injuries. The authors point out two worrisome trends: an increase in head and spinal injuries (despite insufficient data to nail down statistical significance) and an increase in serious knee injuries (i.e. ACL tears). Although some trends are encouraging (e.g. fewer injuries overall, fewer broken bones, more skier hours between injuries), the increase in severe injuries (defined as injuries which can bring lifelong consequences, for example ACL tears and head injuries) is cause for concern. Although the change in lower-extremity injuries (i.e. drop in tibial fracture, but growth in ACL tears) is attributed primarily to the advent of stiff, high-backed ski boots, another aspect to keep in mind is the fundamental physical fact that kinetic energy increases with speed squared. Modern skis have ultra-slippery fluorocarbon-polymer bases and high-tech waxes, and ski resorts have brought out ever-more-challenging runs (e.g. more precipitous slopes). This, combined with the trend towards "extreme" sports, has resulted in alpine skiing involving higher speeds than ever before. It is not surprising, then, that Olympic skiers can reach speeds of about 100 kilometres per hour (62 miles per hour). A skier moving at this speed harbours 25 times (i.e. 5×5=25) as much kinetic energy as a cross-country skier moving at 20 kilometres per hour (which is still brisk). This Olympic skier, in an crash with a solid object, would incur injuries 25 times as severe as the cross-country skiier, would require 25 times as much distance to come to a full stop without colliding, and would have a correspondingly diminished reaction time (i.e. travels five times as far during the time in which it takes for the brain to register an occurrence and effectuate a response accordingly). A novice alpine skier moving at 60 kilometres per hour still carries nine times the kinetic energy of the aforementioned cross-country skier. In short, alpine (downhill) skiing is a high-risk sport because of the high speeds it typically involves.
A Method to Help Reduce the Risk of Serious Knee Sprains Incurred in Alpine Skiing, Carl F. Ettlinger et al.; The American Journal of Sports Medicine, Baltimore; Sep/Oct 1995, Vol 23/5, p. 531-537. Comments: This article is a classic with regards to alpine-skiing knee-injury prevention. Readers might wish to inquire at their local public libraries for further material from Vermont Safety Research (either the training video or the ACL Awareness Training Handbook). Bindings -- Conditional Release, Seth Masia; Ski Magazine; Nov 1991. Comments: This general-level article describes the limits of what alpine-ski bindings can protect against. Note that the major reason ski bindings cannot protect against twisting is because they cannot detect the torque at the knee, since said torque is a function of the angle at which the knee happens to be flexed at a given point in time. Masia also touches on the issues of taking unsafe risks, and also mentions speed. (Remember that speed is a major concern, since kinetic energy, hence injury severity, increases with the square of speed. Higher speeds also translate into reduced reaction time.) Additional comments are provided in brackets in the main text.
Can Knee Injuries Be Prevented? Hopefully, they can be, if skiers and the ski industry at large get behind the effort., Carl Ettlinger and Robert Johnson, MD; Skiing Magazine; March 1991. Comments: An intriguing, insightful, and highly readable general-level article. The research done by Ettlinger and Johnson is a timeless classic. Be certain to read the peer-reviewed medical-journal article Ettlinger-AJSM-Sep95.shtml. Note that alpine-skiing ACL injuries can occur via various means, including anterior-drawer forcing (boot-induced ACL tear) and twisting (phantom-foot ACL injury). The long skis act as lever arms on the knee, thus raising serious concerns about injuries in general. Bindings cannot protect against anterior-drawer forcing, but they are designed with twisting in mind. However, the problem with twisting is that the binding cannot detect the actual torque at the knee, because this torque is itself a function of the flexion angle of the knee. (This is the prime reason why, in the many years since this article was written, no major improvements in ski bindings have occurred.) Knee injuries can also occur via hyperextension, hyperflexion, and sideways forcing. Bindings provide little or no protection against against these modes of injury. With regards to ACL injuries, modern bindings (including those available 15 years after this article was written) are not much better than those available in the 1980s. Various multi-pivoting designs, including some which appeared promising, seemed to have generated ample advertising rhetoric from equipment manufacturers but have brought very little in terms of tangible benefits for the alpine skier. (Further comments accompany the article.) 1991 and Beyond: A look at what new technology may be in store for skiers in the next few years—and a look beyond, Carl Ettlinger, with illustrations by William Hamilton; Skiing Magazine; Spring 1987. Comments: This intriguing general-level article discusses knee injuries in the context of ski-binding-and-boot system design. Ettlinger also delves in some novel (and now historical) conceptualizations in the realm of alpine-skiing bindings, including the Spademan, Nava, and Cubco designs. The root cause of knee injuries is that the binding cannot accurately read the twisting forces (torques) at the knee, since these forces vary with the angle at which the knee happens to be flexed at each point in time. There is one way to enable the binding to read the torque at the knee, but this requires wearing a functional knee brace and connecting the lower shell of such a brace to the ski boot in a twist-arresting fashion. Designing a standalone binding (i.e. without requiring a knee-brace-and-ski-boot attachment) so that the skier remains in control, and so that the binding does not release inappropriately and yet releases reliably when knee injury is imminent, remains a major challenge. (Further comments on this topic are provided in the other Ettlinger articles in this section.) Why All The Knee Injuries?, Carl Ettlinger; Skiing Magazine; Spring 1986. Comments:
This intriguing and highly insightful general-level article, one of several by Carl Ettlinger, describes how the modern ski boot and binding have brought an epidemic of knee injuries which continues to this day. Ettlinger hints at the consequences of intensification of sport: "Improvements in footwear and playing surfaces only seem to increase the frequency and severity of the injury." Keep in mind that improvements in safety equipment (for example, the advent of alpine-skiing helmets) are often accompanied by a revenge effect, as users feel safer and therefore take greater risks. Alpine skiing is a high-kinetic-energy sport, since it involves high speeds. Kinetic energy, hence injury severity, increases with speed squared. Competitive downhill racers can reach speeds of about 100 km/hr. At that speed, the racer has a 625 times as much kinetic energy as a person jogging at 4 km/hr (since 100=4×25, and 25 squared is 625).
Remember, too, that kinetic energy also dictates stopping distance. Note, too, that high speed means decreased reaction time. All these factors, combined with the high boot which encases the lower leg and therefore makes the already-very-vulnerable knee even more vulnerable, conspire to make alpine skiing a notably knee-risky sport. Because no ski binding can detect the twisting forces at the knee (since said torque is itself a function of the flexion angle of the knee), the only reliable hardware-based way to protect a knee against twisting-type skiing injuries (including the "phantom-foot ACL tear") would be via a knee brace connected to the boot in a twist-controlling fashion; such an arrangement would ensure timely and appropriate release of the ski binding. However, such a system would not protect against anterior-drawer-type ACL injuries (also known as the boot-induced ACL tear). Ettlinger suggests a ski boot that has a rear section capable of flexing backwards might be helpful in enabling an skier's buttocks to contact the ground during a fall without having the ski carve out first and thereby twist the knee. Such an arrangement would not directly protect against twisting, but could help the skier slow down and regain control without having his/her weight suspended in mid-air behind the ski (which is what enables the ski to carve undesirably). Additionally, Ettlinger notes the importance of learning to fall (i.e. bail out) correctly, and to recognize and safely deal with ACL-injury precursors. Ettlinger also notes that it is wise to avoid complete extension and complete flexion of the knee. Here is where strong musculature and good training (including neuromuscular and endurance) is very valuable. People with knee-injuries histories who use functional bracing should keep in mind that many braces can be set to prevent both hyperextension- and hyperflexion-type knee injuries. (For more insight into the concern of hyperflexion-type knee injuries in alpine skiing, see the article Hame-AJSM-Jul02.shtml.) Click here to return to the Main Entrance Page of the Knee Library.
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