AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting
of the American Society of Biomechanics
Clemson University, South Carolina
September 24-27, 1997
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| AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting |
Functional testing of the ACL reconstructed (ACLr) and ACL deficient (ACLd) knee is important for evaluating the stability of the knee joint. Unfortunately, a limited number of functional tests have been reported in the literature (Barber et al, 1990, Harrison et al, 1994, Noyes et al, 1991, Risberg and Ekeland, 1994, Wilk et al, 1994). The goal of this study was to develop a functional test that could be used to differentiate between the injured and uninjured knee in ACLd and ACLr populations, and to establish the reliability of such measurement parameters.
Previous studies evaluating knee function after ACL injury have examined several types of tests including a one-legged hop for distance, a one-legged hop for time (Barber et al, 1990, Noyes et al, 1991, Wilk et al, 1994), and a single limb stance test (Goldie et al, 1989). However, it has been suggested that these tasks may not be sensitive enough to detect differences between the injured and uninjured limb (Barber et al, 1990, Noyes et al, 1991, Risberg and Ekeland, 1994, Wilk et al, 1994), or that they are testing an individual's ability to generate force, rather than the stability of the knee (Wilk et al, 1994). Additionally, the reliability and validity of many of these tests has not been established.
Stability has been defined as the ability to transfer the vertical projection of the center of gravity to the supporting base, and keep the knee as still as possible (Goldie et al, 1989). Subluxation in an ACLd or ACLr knee is most likely to occur at foot strike during running and jumping activities (McNair and Marshall, 1994). Since many activities involve landing on one leg, it is important to determine if an individual with an ACLd or ACLr knee has any type of instability when landing from a step down or a hop. Therefore the purpose of this study was threefold: 1) to establish normal variation for the step and hop tests, 2) to determine the reliability of criterion force plate measures of functional stability, and 3) to determine if criterion measures of functional stability could differentiate between injured and uninjured knees.
Twenty-five healthy (Group 1), eleven ACLr (Group 2), and thirteen ACLd (Group 3) subjects were tested. The mean time since surgery was 158 (± 31) days for the ACLr subjects and 1213 (± 596) days for the ACLd subjects. Twelve healthy subjects participated in three testing sessions to determine reliability of the force plate measures. Force and moment data were collected for 3 seconds (200 Hz) while the subjects performed one-legged hop and step down tests onto a force plate (Bertec). The step test consisted of a single-limb step down from a height of 19 cm onto the force plate. The hop test consisted of a single limb hop with the distance relative to each subject's leg length (greater trochanter to lateral malleolus). The subjects practiced the movements before testing began, being instructed to look straight ahead, keep their hands on their waist, and to stabilize as quickly as possible at impact. Ten trials on each leg for each functional test were collected (Group 1 - dominant vs. non-dominant; Groups 2 and 3 - injured vs. uninjured). In addition, the ACLr group performed ten trials while wearing a brace.
The force and center of pressure signals (Fap,Fml,Fv,CoPap,CoPml) were analyzed to determine when they stabilized. A sequential estimation algorithm determined stabilization time when the sequential moving average fell within ñ ¬ of a standard deviation of the overall mean (Clarkson et al, 1980). The standard deviations of the force and CoP signals were calculated as well. Another measure of vertical force stability (Fv%) was calculated as the time for the vertical force component to reach and stay within 5% of the subject's body weight after landing (McKinley and Pedotti, 1992). Intraclass correlations were computed to determine reliable parameters. Means and 95% confidence intervals were calculated for stabilization times and standard deviations of the force and CoP parameters for the percent difference in dominant vs non-dominant legs of normal subjects. Repeated measures ANOVA was used to determine differences in functional stability between injured and uninjured limbs.
The test-retest reliability results are shown in Table 1. Parameters with a correlation coefficient of 0.8 or higher on at least one limb were used for further analysis. The means and 95% confidence intervals for stabilization times and standard deviations for the normal subjects are listed in Table 2. A positive number represents a percent difference in favor of the dominant limb. Figure 1 shows the mean differences in stabilization times between the injured and uninjured limbs during the step down test for the ACLr group. Stabilization times based on Fv took significantly longer for the injured limb with the brace (p<0.001) and without the brace (p<0.001) than for the uninjured limb. For the hop test, the standard deviation of Fv was significantly greater for the injured limb in the ACLd and less for the injured limb in the ACLr populations (p < 0.05).
Table 1. The intraclass reliability coefficients for the stabilization times and standard deviations calculated from the forces (F) and centers of pressure (CoP) parameters.
Table 2. Means and 95% confidence intervals for the normal subjects. Expressed as a percent difference between the dominant and non-dominant limb.
Figure 1. Stabilization times for the injured and uninjured limbs during the step down test for the ACL reconstructed group.
This study demonstrated that the stabilization times produced from Fap, Fml, Fv, and Fv%, as well as the standard deviation of the Fap, Fml, and Fv signals were the most reliable measures during the hop test. These results are similar to the findings of Goldie et al (1989) who concluded that force measures were more reliable than center of pressure measures for quantifying postural steadiness during single limb stance. For the step down test, the stabilization times produced from Fap, Fml, Fv, CoPap, CoPml, and Fv% as well as the standard deviation of the Fv, CoPap, and CoPml signals were the most reliable parameters. The vertical force parameter did differentiate between injured and uninjured limbs for the ACLr population during the step down test, and for the ACLd and ACLr populations during the hop test.
The changed performance during the step down on the injured limb might be the result of an altered pattern of motion in compensation for increased knee instability (Gauffin and Tropp, 1992). In this study, there was a significant decrease in peak vertical force at foot strike during the step down for the injured limb for the ACLr population (p < 0.01). Differences in stabilization times between the injured and uninjured limb may be explained by kinematic and/or joint stiffness regulations made by the injured limb during the impact phase when absorbing body weight (Gauffin and Tropp, 1992) and/or during the support phase when transferring body weight. Either of these strategies could have altered the stabilization time between limbs. Future studies involving kinematics would help clarify this issue.
In conclusion, force plate stabilization times were reliable and did differentiate between ACLd, ACLr and uninjured limbs.
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