Comments: This intriguing study involved implanting tiny Hall-effect transducers directly, via spikes, into the ACLs of the subjects. (These were people with normal ACLs who were scheduled to undergo either meniscectomy [extremely worrisome in its own right, even if only partial meniscus removal is done] or diagnostic/explorative arthroscopy. Loadings applied in the study consisted of anterior-posterior [i.e. in the sagittal plane] shearing, inwards tibial twisting, outwards tibial twisting, quadriceps contraction with knee held at 30 degrees, and active flexion-and-extension movements. These types of loadings cover some of the most common ACL-injury situations, except for those involving hyperextension.) The result of using an implanted Hall-effect transducer is a measurement of ligament tensile stresses in the most direct way possible. A strain-shielding effect was noted with some of the braces, but only at low loadings. One major concern with this study is that of tightness of the brace strapping and standardization therefor. Since different knee braces employ different designs and construction techniques, the strap-tightening methodology should consider this. (Note, too, that the straps on a given brace should not all be of the same tightness. Seasoned knee-brace users know that the strap immediately below the knee should be tightest, to ensure good anchorage of the brace on the leg; also, the topmost strap should not be too tight or else the entire brace will be forced down the leg.) In any case, simply using a spring scale to measure and equalize strap tension across the different braces being tested leaves much to be desired...and this particular study did not even do this. However, the topic of strap tension is touched on (inadequately) in a successor study, available here in the Knee Library as Beynnon-AJSM-May97. One potential solution to the strap-tightness problem might entail using a tissue-compression sensor underneath each strap, thus enabling each strap to be tensioned so that a certain target value of tissue compression is generated (e.g. with muscles relaxed or perhaps maximally activated).

Beynnon astutely points out that previous cadaver studies are hampered by their inability to account for muscular contractions. (Another problem with putting braces on cadavers is that soft tissues behave differently in a corpse than when alive, and so it is easy to greatly overtighten the strapping and thereby inadvertently generate spurious results in such a study. Note that when a brace is applied to the leg of a real live person, the soft tissues automatically compensate for minor misalignments of the brace hinges with respect to the exact movements of the leg bones; soft-tissue shearing also explains why the brace does not have to mimic the complex triplanar motion of the knee, albeit a hinge which accurately mimics the sagittal-plane roll-and-glide of the knee is advantageous because it reduces piston-type sliding of the brace's upper shell with respect to the thigh.)

On a logistical note, keep in mind that some of the brace manufacturers of the products in this 1992 study no longer exist. Note, too, that the braces which are still available today have been revised or updated since the time of the study. (The Townsend brace studied is today known as the Townsend Original; note that Townsend's most popular brace is now the Premier, a model which is vastly different from the Original. The C.Ti. has been revised, and is now the CTi2; however, the CTi2 retains the same full-tibial-shell frame shape as its predecessor. The posterior-tibial-cuff design of the DonJoy 4-Point is the same as all the present-day DonJoy frame-type braces. Lenox Hill, today owned by USMC, still makes its trademark derotation brace, now named the Custom Classic, albeit it now has newer models too.)

Beynnon et al. conclude by noting that soft-tissue compliance, in particular shearing of the soft tissues surrounding the leg bones, is likely to be the limiting factor in terms of the protection a brace can provide. This seat-of-the-pants conclusion is intuitively very logical (as well as rather obvious), and it explains why a used-in-isolation brace is useful for protecting against injurious hyperextension and sideways forcing, but not against twisting-type injuries.