Laxity, Instability, and Functional Outcome After ACL Injury: Copers vs. Noncope
Laxity, Instability, and Functional Outcome After ACL Injury: Copers vs. Noncopers
Abstract & commentary
Synopsis: The purpose of the study was to evaluate variables such as laxity, instability, and functional measures to determine whether they can serve as predictors of coping ability in the ACL-deficient patient population.
Source: Eastlack ME, et al. Laxity, instability, and functional outcome after ACL injury: Copers versus noncopers. Med Sci Sports Exerc 1999;31:210-215.
Patients who are anterior cruciate ligament (ACL) deficient and desire to return to a high level of activity usually opt for surgical reconstruction. However, a small number of patients continue to function at a high level of athletic activity with cruciate-deficient knees. The purpose of this study was to evaluate variables such as laxity, instability, and functional measures in order to determine whether these variables can serve as predictors of coping ability in the ACL-deficient patient populations. Specifically, the purposes were 1) "to assess the relationship among muscle performance, laxity, and function in a group of copers and non-copers; and 2) systematically characterize the coper and non-coper populations to identify tests that can differentiate between the populations." Copers were defined as those who are "able to return to function at a high level . . . after injury without complaint of instability." Noncopers were defined as those unable to return to pre-injury levels of activity because of demonstrated instability.
Forty-five subjects participated in the study; however, only 36 were categorized prospectively as copers (3) or noncopers (33). An additional nine copers were "recruited from a cohort of known copers." The noncopers were further subdivided into individuals who had acute (1-6 months; n = 18) or chronic (beyond 6 months; n = 15) injuries. The following variables were collected for each subject: 1) measurement of laxity using a KT2000; 2) functional outcome measures including the Lysholm scale, the KOS activities of daily living (KOS-ADL), the sports portion (KOS-SPORT) of the knee outcome survey, the self-report portions of the IKDC form, patient-reported global rating score of knee function; 3) active and passive range of motion; 4) strength testing using a voluntary isometric contraction of the quadriceps femoris; and 5) functional performance measures including the single-leg hop, cross-over hop, triple hop, and timed hop. Some of the data were converted into percentages, and some categorical data were "converted to numbers" for statistical comparison. A variety of statistical analyses were used to compare variables. Analysis revealed that "there were no statistically significant or clinically meaningful differences" in laxity measures. In addition, the copers scored "significantly higher than the subacute and chronic non-coper groups" on the Lysholm KOS, KOS-ADL, and KOS-SPORT measures. The copers also had a significantly higher quadriceps index and performed significantly better on all four hop tests. There were no differences, however, between groups in IKDC scores. In summary, the variables that "distinguished copers from noncopers were: 1) global rating scores; 2) KOS-SPORT scores; 3) quadriceps index (strength); and 4) cross-over hop test.
Comment by Clayton F. Holmes, EdD, PT, ATC
Eastlack and colleagues conclude that laxity as measured by the KT2000 does not "predict degree of dysfunction," that self-reported clinical outcome scales are better indicators of degree of dysfunction than are laxity measures, and that the two groups can be discriminated based on a set of the four tests listed above. These findings are important because they indicate that self-reported measures are accurate estimates of function; however, the findings should not be extrapolated too broadly because of questions regarding the study’s internal validity. First, internal validity is compromised by the sampling procedure. It is understood that copers are such a small group of the active athletic population and that they will be hard to find; nevertheless, because they chose a portion of that sample group, Eastlack et al may have biased the study results. Second, they mixed questions of differences and relationships. Initially, they asked questions about relationships and performed correlations on certain variables; however, Eastlack et al also chose to analyze data from some of these same variables with a MANOVA, indicating that they were looking for differences. Finally, a simple stepwise multiple regression was used to predict whether subjects would fall into one of two groups. This means that the criterion variable was dichotomous in nature. A simple stepwise regression is not appropriate for this prediction. If they were truly predicting dichotomous variables (copers and noncopers), they should have used a logistic regression instead of stepwise regression.
Regardless of these flaws in research design and methods, this study attempts to answer important questions regarding predictors of behavior. While to me the results do not satisfactorily answer these questions, they do demonstrate that functional measures actually measure daily, reported function. In so demonstrating, the study provides further evidence that functional outcome measures with demonstrated concurrent validity are among the most important clinical measures that healthcare professionals can perform.
Which of the following measures of patient function cannot distinguish copers from noncopers after ACL injury?
a. Laxity as measured by KT-2000
b. Cross-over hop test
c. Lysholm scale
d. Quadriceps strength
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