Introduction

Anterior cruciate ligament (ACL) tears are a common knee injury, with a reported estimate of 200,000 ACL injuries annually in the United States. Annual ACL reconstruction rates have been reported to be increasing in the United States, with recent estimates ranging from 60,000 to 175,000. Functional bracing often plays an important role in postoperative management of ACL reconstructions and in nonoperative management of ACL-deficient knees.

Early in the 20th century, functional ACL bracing sought to correct the forces imparted on the knee in an attempt to restore stability to ACL-deficient patients. Bracing evolved and soon became a common aspect of postoperative ACL reconstruction management. A survey conducted among American Orthopaedic Society for Sports Medicine members published in 2003 found that only 13% of physicians never braced patients following ACL reconstruction and only 3% of physicians never braced ACL-deficient patients. Accounting for the reported prevalence of ACL injuries and ACL reconstructions performed annually, an estimated 100,000 functional braces are prescribed each year in the United States.

The goal of functional ACL bracing is to reproduce the knee-stabilizing forces of the native ACL. Regardless of whether a patient requires a brace to stabilize an ACL-deficient knee or to unload an ACL graft following reconstruction, the goal is to reduce the risk of further injury such as meniscal tears or early onset osteoarthritis. Generally, functional ACL braces are used in four situations: nonoperative management of a chronic ACL-deficient knee, nonoperative management of a young patient with an ACL tear who will undergo surgery upon reaching skeletal maturity, preoperative management of an ACL tear prior to surgery, and postoperative management following ACL reconstruction.

While functional bracing is a common component of ACL management, biomechanical and clinical results remain inconclusive. Additionally, a variety of functional ACL braces currently exist ( Fig. 112.1 ), and they differ in cost, design, and recommended use. The purpose of this chapter is to summarize the current state of ACL functional bracing literature and offer direction for future functional ACL bracing research.

Two available functional anterior cruciate ligament braces on a left knee, presented in alphabetical order. A, DonJoy A22 Custom Knee Brace, B, Ӧssur CTi Brace.

Biomechanical Findings

Functional ACL braces should strive to re-create the kinematics of the native ACL. Numerous studies have investigated the strain, elongation, and in situ forces of the ACL throughout normal range of motion and during various rehabilitation exercises and activities of daily living. Beynnon et al. have extensively investigated the in vivo strain of the native ACL under a variety of parameters. They reported that maximum strain of the ACL occurred at full extension, with decreasing strain as the knee was flexed to 90 degrees. Additionally, they reported that hamstring-dominated exercises strained the ACL minimally, while quadriceps-dominated exercises placed more strain on the ACL. Unsurprisingly, studies investigating the elongation of the ACL have reported similar findings. Both the anteromedial and posterolateral ACL bundles have been reported to elongate during extension and shorten as the knee was flexed toward 90 degrees. In vitro studies have also reported similar findings, demonstrating that the in situ forces in the ACL posterolateral bundle increased with increasing knee flexion beyond 90 degrees of flexion; however, the in situ forces in the anteromedial bundle remained relatively constant. Therefore ACL kinematic studies have demonstrated the dynamic stabilizing role of the ACL, from extension through increasing degrees of flexion.

The primary role of the ACL in anterior tibial translation is well known. Biomechanical studies investigating the utility of functional ACL braces have focused on the ability to limit anterior tibial translation and subsequently minimize the strain of an ACL graft or an ACL-deficient knee. A study by Beynnon et al. investigated the ability of three different functional ACL braces—The DonJoy Legend (dj Orthopedics, Inc., Vista, California), SofTec Genu (Bauerfeind USA, Inc., Kennesaw, Georgia), and the Townsend Rebel (Townsend Design, Bakersfield, California)—to limit anterior tibial translation in chronic ACL-deficient patients during weight bearing and non–weight bearing. They reported that functional ACL bracing significantly limited anterior tibial translation in both non–weight bearing (2.1% of normal) and weight bearing (23.4% of normal), but none of the functional ACL braces reduced anterior tibial translation to within normal limits during the transition from non–weight bearing to weight bearing. Their findings suggest that one limitation of functional ACL braces may be the ability to provide translational stability during common athletic activities where the transition between non–weight bearing and weight bearing is common, such as in landing while jumping or during pivoting movements.

Interestingly, a study by Cook et al. investigated the effect of functional ACL braces in chronic ACL-deficient athletes during cutting and running movements. They reported that bracing improved running and cutting abilities; however, bracing did not prevent abnormal anterior tibial translation. Additionally, a study by Giotis et al. demonstrated using motion capture technology that functional bracing reduced tibial internal rotation in patients with a recent history of ACL reconstruction; however, it was not restored to normal levels. Overall, biomechanical studies have demonstrated that functional ACL braces are able to somewhat limit anterior translation and internal rotation, and provide stability during activity. However, the braces are unable to restore knee kinematics to the uninjured state.

Biomechanical Findings

Functional ACL braces should strive to re-create the kinematics of the native ACL. Numerous studies have investigated the strain, elongation, and in situ forces of the ACL throughout normal range of motion and during various rehabilitation exercises and activities of daily living. Beynnon et al. have extensively investigated the in vivo strain of the native ACL under a variety of parameters. They reported that maximum strain of the ACL occurred at full extension, with decreasing strain as the knee was flexed to 90 degrees. Additionally, they reported that hamstring-dominated exercises strained the ACL minimally, while quadriceps-dominated exercises placed more strain on the ACL. Unsurprisingly, studies investigating the elongation of the ACL have reported similar findings. Both the anteromedial and posterolateral ACL bundles have been reported to elongate during extension and shorten as the knee was flexed toward 90 degrees. In vitro studies have also reported similar findings, demonstrating that the in situ forces in the ACL posterolateral bundle increased with increasing knee flexion beyond 90 degrees of flexion; however, the in situ forces in the anteromedial bundle remained relatively constant. Therefore ACL kinematic studies have demonstrated the dynamic stabilizing role of the ACL, from extension through increasing degrees of flexion.

The primary role of the ACL in anterior tibial translation is well known. Biomechanical studies investigating the utility of functional ACL braces have focused on the ability to limit anterior tibial translation and subsequently minimize the strain of an ACL graft or an ACL-deficient knee. A study by Beynnon et al. investigated the ability of three different functional ACL braces—The DonJoy Legend (dj Orthopedics, Inc., Vista, California), SofTec Genu (Bauerfeind USA, Inc., Kennesaw, Georgia), and the Townsend Rebel (Townsend Design, Bakersfield, California)—to limit anterior tibial translation in chronic ACL-deficient patients during weight bearing and non–weight bearing. They reported that functional ACL bracing significantly limited anterior tibial translation in both non–weight bearing (2.1% of normal) and weight bearing (23.4% of normal), but none of the functional ACL braces reduced anterior tibial translation to within normal limits during the transition from non–weight bearing to weight bearing. Their findings suggest that one limitation of functional ACL braces may be the ability to provide translational stability during common athletic activities where the transition between non–weight bearing and weight bearing is common, such as in landing while jumping or during pivoting movements.

Interestingly, a study by Cook et al. investigated the effect of functional ACL braces in chronic ACL-deficient athletes during cutting and running movements. They reported that bracing improved running and cutting abilities; however, bracing did not prevent abnormal anterior tibial translation. Additionally, a study by Giotis et al. demonstrated using motion capture technology that functional bracing reduced tibial internal rotation in patients with a recent history of ACL reconstruction; however, it was not restored to normal levels. Overall, biomechanical studies have demonstrated that functional ACL braces are able to somewhat limit anterior translation and internal rotation, and provide stability during activity. However, the braces are unable to restore knee kinematics to the uninjured state.