In vivo and in vitro anterior cruciate ligament (ACL) studies have shown that greater than 50% of the total resisting force of anterior tibial translation on the femur is supplied by the ACL. 1,2 Although some research supports the mechanical ability of a functional knee brace to limit anterior tibial translation, 3–6 others studies 7,8 have shown that functional knee braces are incapable of eliminating pathologic anterior tibial translation. Wojtys et al. 6 investigated the effectiveness of six different braces on restricting anterior tibial translation and the brace’s simultaneous effect on select neuromuscular parameters. They found that the brace that reduced anterior tibial displacement the greatest also caused the greatest delay in hamstring reaction time. Conversely, the authors noted that the brace that allowed the greatest anterior tibial translation did not affect the thigh musculature reaction times.
Increased hamstring electromyographic (EMG) activity, increased quadriceps-to-hamstrings EMG levels, and the elimination or reduction of knee extensor torque during the stance phase of gait decreases anterior drawer and increases knee joint stability in the absence of the native ACL. 9–11 Although several researchers 3–5 have investigated the effects of braces in conjunction with joint compression or quadriceps activity, few have studied the effects of bracing in conjunction with hamstring muscular activity.
The purpose of this study was to determine: 1) the effectiveness of an off-the-shelf functional knee brace at preventing anterior tibial translation in subjects with ACL-deficient knees and greater than 6 mm of side-to-side difference, and 2) the effect of a 50% maximal voluntary hamstring contraction on knee joint laxity.
A total of 14 individuals, at a minimum 240 ± 42 days postinjury, who had been diagnosed (arthroscopically) as ACL- deficient (ACL-d) were screened. The criteria for the study were ACL-d unilaterally and a 6-mm or greater side-to-side difference, as measured by the Knee Signature System (KSS; Orthopedic Systems, Inc., Union City, CA). Nine ACL-d subjects (8 male, 1 female, mean age, 32.9 ± 5.2 years; mean mass, 75.0 ± 8.3 kg; mean height 177.4 ± 15.1 cm) met the criteria to participate in the study. Before testing, subjects gave their written informed consent.
An off-the-shelf, shell and hinge design (Edge Brace, Innovation Sports Inc., Irvine, CA) functional knee brace was used. The brace was modified by grinding out an area (≈4 cm) on the tibial cuff to allow the tibial tubercle pad of the KSS testing apparatus to rest on the leg. The femoral cuff was drilled to attach the testing apparatus. These modifications were considered not to affect the mechanical integrity of the brace according to the manufacturer. Displacement of the tibia relative to the femur was measured with the KSS system, a device that has shown adequate validity and reproducibility in previous studies. 12–18 A force transducer (Entran Devices Inc., Fairfield, NJ) measured the force (in Newtons) of the hamstring contraction during isometric knee flexion. The force transducer was placed on a foot extension under the participant’s heel and in line with the direction of pull of the hamstring musculature.
Participants performed a series of five 3-second maximal voluntary contractions (MVC) to measure knee flexion strength (force in N) under isometric conditions for both limbs. Then the participants practiced MVC three times with the knee in 30° of flexion. Visual feedback from the force signal was observed on a computer screen. The peak values of the five maximal isometric contractions were divided in half and averaged to obtain a 50% MVC target value. Subjects were then allowed to practice the 50% isometric contractions until they demonstrated competence in maintaining a consistent 50% MVC target force as observed on the computer screen.
During the static series, a standard Lachman examination with and without a 50% MVC of the hamstrings was conducted in braced and no-brace conditions. The opposite noninjured limbs were tested in the nonbraced condition.
A dynamic knee extension test was performed and divided into two conditions. First, passive extension, whereby the tester placed a hand under the subject’s heel and manually moved the subject’s leg to extension (0°) and back to 90° of flexion. Second, active knee extension, in which the subject actively moved the leg through the same range of motion (90° to 0° and back to 90°). Tibial translation relative to the femur was measured and graphed as a function of knee angle for both the active and passive conditions. From these curves, two displacement points, one from the passive phase and one from the active phase were digitized at 15°, 20°, and 30° of knee flexion. The difference between the passive and active absolute values at each angle yielded the relative amount of translation at these angles. These values were averaged over three trials to yield an average score for statistical comparisons. Because the passive value was used as a reference, all translation values account for the knee joint laxity inherent to each subject’s knee in an unloaded situation.
Displacement data from the Lachman tests were plotted as a continuous force versus displacement curve. Anterior knee joint laxity was measured at anterior loads of 89 N and 180 N of force. 2,19–22 A repeated measures ANOVA was used to detect differences between conditions for all tests with p ≤ .05 considered significant. A Bonferroni post hoc analysis test was used to detect specific differences between the conditions.
ANTERIOR TIBIAL DISPLACEMENT—STATIC LACHMAN TEST
Means and standard deviations of anterior tibial displacement for the Lachman tests (under 89 N and 180 N anterior loads) in the relaxed, braced, and 50% MVC of the hamstrings conditions are presented in Table 1. The mean side-to-side difference in anterior laxity between the ACL-d limbs and the uninjured limbs was 10.5 ± 5.5 mm at 89 N and 13.5 ± 7.5 mm at 180 N, respectively. Average 50% MVC force values during the hamstring contraction were 107.2 N for the ACL-d limb and 117.6 N for the opposite (uninjured) limb.
The ACL-d limb exhibited significantly larger displacement when compared with the uninjured limb at 89 N (p < .0001) and 180 N loads (p < .0001) (Figure 1). The 89 N Lachman tests conducted on the injured limb with a 50% MVC of the hamstrings resulted in a mean 86.0 ± 14.5% reduction of anterior displacement compared with the Lachman tests with no hamstring contraction (p < .0001). With a 180 N load, the 50% MVC of the hamstrings anterior tibial displacement of the ACL-d limb was significantly reduced by a mean 83.0 ± 15.5% (p < .0001). There were no significant differences in anterior tibial displacement between the ACL-d and uninjured limbs when a 50% MVC of the hamstrings was exerted in the ACL-d limb against an 89 N or 180 N anterior drawer (p > .457).
The application of a functional knee brace significantly reduced anterior tibial displacement by 43.7% (p < .0001) with a 89 N anterior drawer and 26.4% (p < .0003) with a 180 N anterior drawer. At both the 89 N and 180 N loading conditions, the 50% hamstring contraction was more effective at reducing anterior tibial displacement then the brace (Figure 2).
Mean and standard deviations of the relative differences of anterior tibial displacement between the active and passive knee extension test under braced and no braced conditions and at 15°, 20°, and 30° of flexion are provided in Table 2 and depicted in Figures 3, 4, and 5.
The KSS measures were able to differentiate between the uninjured and ACL-d knees as significant differences were detected between these groups at all angles tested (p < .004). During the dynamic knee extension, the knee brace decreased anterior tibial translation by 1.8%, 5.6%, and 13.4% at knee flexion angles of 15°, 20°, and 30°, respectively. However, these decreases were not statistically significant (all p > .309).
The present study was undertaken to evaluate the effects of hamstring force and functional knee bracing on knee joint laxity, specifically anterior tibial displacement. The results of this study indicate that the greatest decrease in anterior knee joint displacement was caused by the application of a 50% voluntary hamstring contraction and secondarily by a functional knee brace. This finding reiterates the need for rehabilitation to focus on restoration of hamstring neuromuscular function after ACL reconstruction.
Several investigators have studied the effects of joint compressive loads, 23 quadriceps forces, 4,24 or co-contraction of quadriceps and hamstring forces. 7 To our knowledge, the present study is the first in vivo study to delineate the effect of a quantifiable hamstring contraction and a functional knee brace at reducing anterior tibial translation.
Several studies such as the electromyographic (EMG) study by Branch et al., 25 the cadaveric study by Hsieh and Walker, 23 and the questions posed in the study by Wojtys et al. 6 propelled us in our present study.
In the EMG study, Branch et al. 25 measured the activity in hamstrings of ACL-d and healthy individuals in braced and unbraced conditions. They reported hamstring activity (EMG values) above 45% MVC. They noted several effects of the brace on EMG activity, such as a consistent decrease (15% to 25%) in integrated medial hamstring activity and quadriceps activity during both the swing and stance phases. From these results, the authors 25 suggested that that the ACL-d individual, while wearing a brace, requires less co-contraction for knee joint stabilization and that the brace, by either mechanical restraint or by changing performance kinematics, may reduce the demand for increased muscular output. Our study tends to support the latter. We found that hamstring contraction alone reduced anterior tibial translation more than the brace alone. Although in our study there appears to be a relationship supporting their findings, we do not know whether the 50% MVC force contraction used in this study can be directly comparable to the electromyographic MVC observed in the hamstring musculature during cutting as reported by Branch et al. 25 However, the hamstring force used in our study was sufficient in limiting anterior shear over and above the influence of a functional knee brace and can be considered the upper limit of hamstring force needed to restrict anterior shear in future studies.
Wojtys et al. 6 demonstrated that a functional knee brace reduces anterior tibial translation. However, they noted that the brace that reduced anterior translation the most, also caused the greatest delay in hamstring voluntary reaction time. Wojtys et al. 6 stated that this hamstring inhibition might be the trade-off braced individuals pay for increased tibial restraint. Undoubtedly, a delay in neuromuscular response times in any of the thigh musculature is considered an adverse effect. It is not yet clear whether this effect is due to cutaneous afferent inhibition, restricted blood flow, or some other physiologic phenomenon.
Hsieh and Walker 23 demonstrated the effect of joint compression on anterior-posterior knee laxity after sequential cutting of the anterior cruciate, posterior cruciate, medial, and lateral meniscus. With no load, anteroposterior laxity was greater than with 1/2 to 2 body weights of load, implicating a supportive role of bony joint geometry in limiting anteroposterior laxity, and to some degree rotary laxity, when the intrinsic knee joint structures (collateral ligaments, cruciate ligaments, and the menisci) are absent. In contrast, Flemming et al. 3 have recently demonstrated that weight bearing may actually strain the ACL or graft when compared with non-weight bearing. However, in the same study, these authors demonstrated that a functional knee brace can effectively reduce this strain. Similarly, our results demonstrate that a functional knee brace, in the absence of appropriate muscular stabilization, can significantly reduce anterior displacement. This may prove critical when low loads or unanticipated external forces are acting on the ACL-d limb.
We acknowledge that the examination of knee stability as outlined in the present study may not reflect true physiological loading or knee instability that may occur in gait or in competitive situations where the effects of a knee brace or a hamstring contraction may be called into play. However, we feel this methodology was able to differentiate between the importance of knee bracing and a hamstring contraction on limiting anterior drawer when all other confounding factors (factors that are not easily accounted for in “real life” situations) are controlled.
Ultimately, knee joint stability is achieved by a dynamic interaction of ligaments (cruciates and collaterals), meniscus, externally applied forces, bony geometry, and active muscle forces. Evidence from the present study indicates that a 50% hamstring contraction provides better control of anterior tibial displacement than a functional knee brace under the same loading conditions. However, the functional knee brace was able to reduce anterior tibial displacement to values similar to the uninjured limb. This observation suggests that a functional knee brace may provide sufficient restraint, particularly in situations where a hamstring contraction is insufficient to control the displacement.
The authors would like to acknowledge Michael J. Decker, MS, for technical assistance during the data collection and analysis of this work. The authors would also like to thank Charles Dillman, PhD, and Richard J. Hawkins, MD, and Lottie B. Applewhite for their review and comments of the final draft of this manuscript.
Funding of this research was funded in part by Innovation Sport, Irvine, CA, and from a grant from the National Football League Charities (grant to MRT). Institutional Review Board: Vail Valley Medical Center, Committee for Human Subject Testing, 181 W. Meadow Drive, Vail, CO 81657.
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Keywords:© 2001 Lippincott Williams & Wilkins, Inc.
Knee stability; hamstring muscles; knee braces; biomechanics