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Synopsis: Functional or cruciate knee braces protect the ACL during anterior-posterior shear loads directed at the knee.
Source: Fleming BC, et al. The influence of functional knee bracing on the anterior cruciate ligament strain biomechanics in weightbearing and nonweightbearing knees. Am J Sports Med 2000;28(6):815-824.
The purpose of this study was to test the strain in the ACL in the loaded and unloaded knee under different compressive loading conditions. This study addresses an important clinical concern: Does an off-the-shelf knee brace increase ACL strain under low loads?
Patients were included in this study if they had normal ACLs on routine arthroscopy for other pathology. The knee arthroscopy was performed under local anesthesia so that the patient could participate in the testing. Reproducible data were obtained from 11 of the 15 subjects. The Don Joy Legend functional knee brace was appropriately fit and placed onto the knee with the straps under uniform tension. Strain was measured using a differential variable reluctance transducer (DVRT), which was placed through the anterolaterel portal into the ACL during the knee arthroscopy. The loading fixture was designed to apply anter-posterior (AP) forces, internal-external torques, and varus-valgus moments to the tibia under conditions with and without compressive load to the knee. A load cell was used to measure the forces placed on the knee.
The results for the AP load tests showed that the strain in the ACL was reduced in the braced knee for anterior sheer loads up to 130 N. For internal torque, the ACL strain was less than in the non- braced knee in the loaded condition. The brace did not reduce strain in the ACL under conditions of external torque or varus-valgus moments.
This is another study by the Vermont group, which has capitalized on a clever intra-operative experimental model and has made several significant contributions to the orthopaedic literature. This article specifically tests the strain in the ACL under different loading conditions with and without a brace. The study concludes that the off-the-shelf Don Joy cruciate brace effectively reduces the strain on the ACL at low anterior-posterior loads.
Although the loads applied in this experiment were relatively low, they were similar to those loads applied to the ACL during a Lachman exam. However, one cannot extrapolate this information to higher applied loads, nor can one assume that if the strain increases in the ACL that the force in the ACL increases. However, this study does provide valuable information.
A design problem is inherent in this model. The strain measurements are reflective of the small area on the ACL where the strain gauge is in contact. Multiple DVRTs would increase the value of this information but the space in the notch is limited. Therefore, only one can be placed. The anatomy of the ACL is complex, and multiple bundles are present, which complicate the interpretation of the results. Little changes measured in the strain may be larger changes at other ACL locations. In my opinion, this is a design problem that cannot be overcome in this model, but the value of the information is paramount. Orthopaedic surgeons typically try to reconstruct the anteromedial bundle of the ACL, which is the bundle where the DVRT is placed. Fleming and colleagues make great effort to consistently place the DVRT in the same place in the ACL, which adds to the reproducibility of the results.
A 4-point functional brace like the Don Joy Legend reduces strain in the ACL. The opposite of this is also very important. The brace does not increase the strain in the ACL under the conditions in this experiment. This is very valuable information, especially for those who believe wearing braces may cause increased strain in the ACL. In my opinion, it is reasonable to assume that the ACL strain (change in length) must increase in order for significant injury to occur within the ligament. This study determined that ACL strain was not increased; therefore, ACL injury is not likely to occur with a properly fit functional knee brace at low loads. This study does not address improperly fitted braces.