Comments: This intriguing and well-designed study shows that retrofitting a knee brace, by adding a spring that engages as the knee is extended, engenders 5 degrees greater knee flexion at the point of landing of single-leg jumps. The spring-loaded brace, provided by well-known manufacturer DonJoy, is the predecessor to the Fource Point, a brace which is now widely available and extensively marketed. DonJoy has chosen to place this study on its website, and includes it prominently in its promotion efforts for the Fource Point brace. The comments set forth herein reflect not only the study itself, but also the promotional efforts associated with the Fource Point brace. The comments pertaining specifically to said promotional efforts can be identified by their references to marketing claims, and should be considered separate from the Yu study itself.

Landing a jump activates the quadriceps group (which contract eccentrically in order to absorb the kinetic energy of landing). The quads pull on the patellar tendon; this pulling generates forwards shearing on the knee. However, due to the biomechanics of the knee, this forwards shearing force decreases as the knee is more flexed. Because said shearing stresses the ACL (just like the Lachmann and anterior-drawer tests do), it was hoped that landing a jump with a more-flexed knee would translate into less stress on the ACL. This was seen as a potential advantage especially for female athletes, who tend to land jumps with inadequate knee flexion. But the authors found that the use of the extension-constraining knee brace, regrettably, did not reduce the ground-reaction forces, nor did it reduce the resultant forces and moments at the knee joint. (Moments are twisting forces, therefore torques. Because there is no noninvasive way to measure stress in the ACL directly, we look at the ground-reaction force resulting from landing a jump. The greater the ground-reaction force, the greater the torque at the knee, and the greater the tensile stresses expected on the ACL. However, there are other factors to consider, as will be noted in the subsequent paragraph.) The authors note that simply landing jumps with knees flexed an extra 5 degrees due to the modified brace is not a panacea, and there is still much research to be done in this regard. (More details can be found in this article's discussion section, which should be read with extra care.) There is also the question of exactly how beneficial a mere 5 degrees of extra flexion at landing is. Although the 5 degrees of extra flexion at landing is statistically significant, this does not automatically mean it is biomechanically notable. The fact that no reduction in ground-reaction forces occurred could mean that the 5 degrees of extra flexion was simply too small to have a measureable and significant effect on ground-reaction force. Or, perhaps more likely, the aspects discussed in the following paragraph need to be taken into account.

There is one very important issue which the authors do not delve into, and it is this: some of the subjects (the male ones) had significantly smaller ranges of knee flexion motion with the brace than without the brace (see Figures 6 and 7). Yes, the spring-braced knees had 5 degrees more flexion at the point of landing, but thereafter, the male knees moved (i.e. flexed) through a smaller range of motion! (The authors simply write this off as being due to the fact that the knees were more flexed at point of landing...but this ignores the fact that for the males, the range of motion was 10 degrees less, and because the knees were 5 degrees more flexed to begin with, the actual loss of range of motion was 5 degrees.) The range throughout which the knee moves, from point of landing to the point of maximum flexion, is important because it determines the "shock-absorption action" obtained. The quadriceps muscles contract eccentrically (i.e. they are activated, but they are forced to lengthen due to the kinetic energy of the jump which needs to be absorbed in order to prevent the person from smashing into the ground and incurring massive injuries), and they do this during the time between the point of landing and the point of maximum flexion. One very plausible explanation for the reduced range of motion after landing is simply that the presence of the strapping behind the knee (a necessity with all braces) impedes deep flexion somewhat. As Figure 7 shows, for males, the loss in post-landing range of motion is about 10 degrees, while for females, it is about 5 degrees. But because the knees were 5 degrees more flexed to begin with, the males actually lost 5 degrees of flexion while the females lost none. The fact the amount of maximum knee flexion (and also the range of knee flexion) either decreased (for males) or remained the same (for females) explains why the ground reaction force was not reduced by the use of the spring-loaded brace. Because males naturally flex their knees more deeply, the result is that males are more likely to have their maximal flexion impeded by the presence of brace strapping behind the knee. And, Figure 6 shows that, indeed, the males lost 5 degrees from their maximum flexion when wearing the brace, while for females the maximum flexion remained the same. So, it is not surprising that the use of the spring-hinge bracing did not engender any statistically significant changes in ground-reaction forces. (As the authors note, more study is needed. There could be multiple factors at work here. The strap-impingement issue presents an interesting possible avenue for future research: maybe someone could repeat the study with the braces glued onto the legs, so as to avoid the use of strapping entirely. Then again, there really is no substitute for the strapping, since without the strapping the braces would not be able to protect against injurious hyperextension. Bracing offers reliable protection against sideways forcing and injurious hyperextension, and the presence of the strapping is an integral part of this.)

The fact that the ground-reaction forces were unchanged by the spring-hinge bracing means that this bracing did not reduce stresses on the ACL. Granted, the additional five degrees of flexion obtained (as a result of the spring-loaded brace hinge) would reasonably be expected to be beneficial in terms of reducing the likelihood of ACL injury, especially in females, because females are known to land jumps with insufficient knee flexion (and this insufficient knee flexion is suspected to be one of the causes for the elevated female-ACL-injury risk). But this small amount (just 5 degrees!) of increased flexion-upon-jump-landing is not enough to enable anyone to safely claim that this spring-loaded hinge is going to protect the female athlete against ACL injury! In other words, DonJoy's marketing people should not advance this study as "clinical proof" that spring-loading a brace hinge protects the ACL. As Yu et al. note, more research is needed first.

Although this study did look at dual-leg jumping, the task involved occurred in the controlled environment of a research laboratory. One would wonder what the effect of wearing an extension-constraining brace (on one knee only) would be on dual-leg jumping in the inherently unpredictable context of sports, particularly with regards to symmetry of landing and therefore the concern of maintaining balance and avoiding injury. One would need to consider the stiffness of the extension-constraining spring in the brace, as well as other factors. The concern of influencing symmetry of landing would be more acute if the extension-constraining spring were stiffer and set to engage earlier. For the purposes of this study, the extension-constraining spring force was rather gentle; the adjustable spring was able to exert a maximum torque of only 3 Newton·metres when the brace was 10 degrees short of full straightness. The subjects in this study reported that the spring-loaded brace did not harm their performance. However, to truly ascertain the impact of the bracing on performance, detailed studied into muscle-contraction patterns would be needed. Note, too, that all the people in this study were people with perfectly normal knees (hence no ligament damage, no proprioception losses due to ACL-injury histories, no muscle atrophy, no muscle-compensation effects resulting from knee injuries, no gait alterations due to knee injuries, etc.).

Note that the effects obtained by the use of the modified DonJoy 4Titude brace could in fact be obtained by any brace equipped with a spring that engages as the knee reaches near-full extension. A somewhat similar effect (comparable in terms of providing a gradual increase in resistance to extension as the knee is progressively extended) is generated by the XCL-strapping (crossed-strapping) option of the Innovation Sports functional knee braces (CTi2, Edge/Morph, C180), and also by older (and no-longer-made) versions of Bledsoe ACL braces. In any case, it would be relatively easy to add springs to any existing dual-upright-type functional knee brace in order to obtain the same effect as was obtained with the spring-equipped DonJoy 4titude, and therefore this study does not constitute an endorsement of DonJoy.

Note that the idea of reducing anterior-drawer forces on the ACL through encouraging more-flexed knees upon jump landing by the use of a specially-equipped knee brace is very different from the idea of using the brace directly to exert anterior-drawer-counteraction forcing on the knee. The latter entails tightening the strapping so that the brace exerts a rearwards force on the tibial tuberosity simultaneously with a forwards force just above the rear of the knee. (DonJoy markets this as its "Four Points of Leverage", despite that fact that nearly any dual-upright frame-type brace can accomplish this same effect.) Note that such exerted-directly-by-bracing anterior-drawer-counteraction forcing is most effective at full extension (and even then its effectiveness is limited by practical reasons, for example the presence of soft tissues and the hamstring tendons behind the distal end of the femur), and becomes progressively less possible as the knee is increasingly flexed. One important distinction between having spring-loaded brace hinges (as in this study) and having a crossed-strapping system is that the latter can very effectively use the quadriceps extension forces to exert anterior-drawer-counteraction forcing on the knee, whereas the former cannot. If the goal is to exert anterior-drawer-counteraction forcing on the knee (and also to provide a soft stop to extension), then a crossed-stapping system would make more sense than placing springs in the hinges. A crossed-strapping system would be especially desirable for an ACL-deficient knee that manifests a lot of laxity.

Please remember that a crossed-strapping system is capable of exerting anterior-drawer-counteraction forcing on the knee, while placing springs in the brace hinges cannot do this. In fact, placing an extension-constraining hinge in the hinge, as DonJoy has done, actually generates forwards-directed forces on the rear of both the tibia and the femur, via the straps immediately above and below the knee. (Any marketing claims that the Fource Point hinge enhances the Four-Points-of-Leverage action are erroneous.)

While it is true that the use of a spring-loaded hinge reduces the time which the knee spends at full extension (DonJoy refers to this as the "at-risk position"), keep in mind that ACL injuries can occur at a variety of flexion angles. When the knee is near full extension, injurious hyperextension is a serious concern, but this is something which any brace (equipped with the appropriate extension stops) will control. Please remember that many twisting-type ACL injuries occur with the knee partly flexed (i.e. quite far from near-full extension), for example during a plant-and-pivot maneuver, or during an off-balance landing (i.e. resulting in a fall, as the person crumples to the ground) from a jump. The reality is that near-full-extension is not the only position where the knee is at risk of ACL injury. In fact, having the knee flexed to 90 degrees means the femur can act most efficiently as a wrench on the knee; this phenomenon is most apparent in the context of alpine skiing. Because multiple factors play a role in ACL injuries, flexion angle is not the only factor which determines whether or not the ACL will be torn. The spring-loaded hinge is ideally suited for knees which extend too far as a result of injury (usually violent hyperextension or a high-kinetic-energy multi-ligament injury, also known as a total knee dislocation). The fact that the spring loading (in the Fource Point hinge) can be deactivated means that the user can try it out and experience it for himself/herself.

In terms of the bigger picture of general knee-injury-prevention strategies, it should be borne in mind that the habit of landing jumps with increased knee flexion can be developed through specific training exercises (or via training programs such as Cincinnati Sportsmetrics or Girls-can-jump). Such training is clearly still the single most valuable means towards reducing the ACL-injury-risk in female athletes, particularly since it also addresses other injury propensities of female athletes (e.g. weak hamstrings, co-ordination issues, feet too close together and knees angled inwards upon landing). And, as an added benefit, such training improves athletic performance as well as endurance (and these benefits themselves mean reduced injury risk).