This is an excerpt from Understanding and Preventing Noncontact ACL Injuries.
By Bruce D. Beynnon, PhD, and James R. Slauterbeck, MD
This chapter reviews what is currently known about the effectiveness of knee bracing in preventing injury to the ACL and reinjury of ACL grafts. We present the relevant literature on the mechanisms whereby braces influence ACL biomechanics. In addition, we review clinical studies focusing on the efficacy of prophylactic and functional knee braces in preventing ACL injuries, specifically those that provide the highest level of evidence: randomized, controlled trials. Our review revealed that very little is known about the effectiveness of prophylactic braces in preventing ACL injuries. As well, very little is known about the effectiveness of functional knee braces in preventing injury of healing ligaments and ACL grafts.
The Biomechanics of Knee Bracing
Knee braces have been used by the sports medicine community to treat instability of the knee due to an ACL disruption, to protect an ACL graft, and to prevent knee ligament injuries during sport. Both functional and prophylactic braces are designed with the objective of allowing normal joint kinematics while limiting unwanted displacements and rotations between the tibia and femur that might detrimentally strain a healing ligament or graft or produce intra-articular injury.
Subjective studies have shown that braces help most subjects with knee ligament injuries feel better and can even improve athletic performance in those with torn knee ligaments; however, many athletes report that knee bracing hinders their performance and that braces migrate and slide out of position on the leg during activity (Greene et al. 2000).
The Effect of Knee Braces on ACL Biomechanics
Indirect and direct techniques have been used to measure brace performance on knee and ligament biomechanics. Indirect measurement techniques (i.e., kinematic studies of the position of the tibia in relation to the femur) have shown that functional braces increase knee stiffness and reduce anterior-posterior displacement of the tibia relative to the femur, but only for low intersegmental shear loads. Direct measurement techniques have been performed by our group through instrumentation of the ACL of cadavers (Arms et al. 1987). This study showed that application of a functional brace resulted in an increase in ACL strain values with passive flexion-extension of the knee joint (i.e., the braces had a detrimental prestraining effect on the ACL), and suggested that bracing may be harmful for an injured ACL or a healing ACL graft.
This study motivated us to perform measurements of ACL strain in human subjects to determine if braces were harmful to the ACL, and that investigation revealed that bracing did not prestrain or harm the ACL (Beynnon et al. 1992). This finding was in direct contrast to those of our earlier cadaveric study and raised the question whether it is valid to use cadavers to study the effect of braces on ACL biomechanics. Further, we found that both custom and off-the-shelf brace designs significantly reduced ACL strain values for anterior-directed loads applied to the tibia (relative to the femur) up to the maximum anterior load of 140 N. Similarly, bracing significantly reduced ACL strain values in response to internal and external torques applied about the long axis of the tibia up to the maximum torque of 6 Nm. This is important because torque applied about the long axis of the tibia produces a substantial load on the ACL at the extremes of knee flexion and extension (Markolf et al. 1995) and can injure the ACL (Ryder et al. 1997). For both anterior loading and internal-external torques of the tibia, we determined that the protective strain-shielding effect of a functional brace on the ACL was dependent on the magnitude of applied load and torque. The protective effect of the brace on the ACL decreased as the magnitude of applied anterior load and internal-external torques increased.
From this investigation, we concluded that knee brace performance was determined by at least four different characteristics. The first is the brace design, composed of parameters such as the hinges, the uprights, and the strap versus shell attachment technique. The second is the brace-limb interface. This was thought to be of importance because the magnitude and sequence of contraction of the leg musculature alters the compliance of the soft tissue at the brace-leg interface, and therefore the capability of the brace to mechanically control the skeletal system, protect the ligaments, and prevent intra-articular injury. Third, we determined that it was important to monitor the forces produced on the leg by the brace strap tensions. Fourth, we found that the magnitude of contraction of the leg musculature, as well as the compressive joint load produced by body weight, affected ACL strain values and therefore were important to include.
We then progressed to an investigation in which we controlled the brace design and brace-limb interface variables by studying a single brace (the DonJoy Four Point Brace) while systematically examining the effect of the extrinsic forces produced by different brace strap tensions on ACL strain biomechanics. This was done in the presence of the intrinsic forces produced by leg musculature and body weight (Beynnon et al. 1997b). Bracing significantly reduced ACL strain values for anterior-directed shear loading of the tibia (to the limit of 140 N) with the subject in nonweight-bearing and weight-bearing postures. Similarly, functional bracing significantly reduced ACL strain values with internal-external torque applied to the tibia up to the limit of 6 Nm with the subject nonweight bearing. The posterior-directed load applied by the brace strap to the proximal tibia was adjusted between a low (22 N) and a high (45 N) setting (loading that we hypothesized would protect the ACL), but this did not modulate the effect of the brace on ACL strain values for the nonweight-bearing and weight-bearing conditions. Our most recent investigation of the same brace design confirmed that it can protect the ACL in response to anterior-posterior-directed shear loading with the knee nonweight bearing and weight bearing, as well as in response to internal torque applied to the nonweight-bearing knee (Fleming et al. 2000).
The Effect of Knee Braces on the Biomechanics of the ACL-Deficient Knee
Functional knee braces can reduce the abnormal anterior-posterior laxity of the ACL-deficient knee to within the limits of the normal knee during nonweight-bearing and weight-bearing activities (Beynnon et al. 2003). However, in the ACL-deficient knee, braces do not reduce the abnormal increased anterior displacement of the tibia relative to the femur that occurs as the knee transitions from nonweight-bearing to weight-bearing conditions (Beynnon et al. 2003, 2002).
The Effectiveness of Braces in Preventing ACL, ACL Graft, and ACL-Deficient Knee Injuries
In an effort to determine if bracing is effective in preventing ACL injury, ACL graft injury, or injury to the ACL-deficient knee, we considered the highest level of scientific evidence available in the literature. Anterior cruciate ligament injury or injury to the ACL-deficient knee is a relatively rare event, and therefore researchers focusing on the effectiveness of bracing in reducing these injuries must carefully study a large sample of athletes over a long time interval. Consequently, it is not surprising that only a few studies based on prospective, randomized, controlled study designs have been reported. A majority of the research has focused on the sport of American football, in which the injury mechanisms involve both direct and indirect contact.
The Effectiveness of Prophylactic Knee Braces in Preventing ACL Injuries
The use of prophylactic knee braces to prevent knee injuries has long been a contentious area of research. The best study to date of prophylactic knee braces and ACL injury remains a prospective, randomized study of 1396 cadets playing intramural tackle football at the U.S. Military Academy (Sitler et al. 1990). This study showed that prophylactic knee brace use did not significantly decrease the severity of ACL and MCL injuries. There was, however, a trend toward a reduced rate of less severe ACL and MCL injuries in athletes who used braces (Sitler et al. 1990). On the basis of the data published in that paper, we compute that the rate of ACL injury in the nonbraced cadets was 3.0 times higher (95%CI: 1.0-9.2) than in braced cadets. It should be noted that the number of ACL injuries was small. Only 16 ACL injuries occurred: four in the braced and 12 in the nonbraced condition. Other large epidemiologic studies conducted to date have focused on the effectiveness of prophylactic braces in reducing MCL injuries in the sport of football; however, these investigations have not dealt with the effect of bracing on the reduction of ACL injuries (Albright et al. 1994a, 1994b; Hewson, Mendini, and Wang 1986; Teitz et al. 1987). The efficacy of prophylactic knee braces in preventing ACL disruptions remains an unanswered question (Najibi and Albright 2005).
The Effectiveness of Functional Knee Braces in Preventing Injury to an ACL Graft
A prospective, randomized, multicenter study examined 100 cadets at the three largest U.S. military academies who underwent ACL reconstruction and were randomized to either functional brace use or no brace use (McDevitt et al. 2004). At one year postsurgery, the use of a functional brace appeared to have no effect on the incidence of graft injury; however, there were only five reinjuries (two in the braced and three in the nonbraced group).
The Effectiveness of Functional Knee Braces in Preventing Injury to an ACL-Deficient Knee
A prospective study of 180 ACL-deficient skiers, identified from screening of 9410 professional skiers, showed a higher risk of knee injury in skiers who did not wear a functional knee brace compared to those who wore functional knee braces (risk ratio of 6.4, based on a total of 12 injuries) (Kocher et al. 2003).
This is an excerpt from Understanding and Preventing Noncontact ACL Injuries, edited by the American Orthopaedic Society for Sports Medicine, Timothy E. Hewett, Sandra J. Shultz, and Letha Y. Griffin.