Rationale for a Parsimonious Measure of Subjective Knee Function Among Individuals With Anterior Cruciate Ligament Reconstruction: A Rasch Analysis

Context The International Knee Documentation Committee Subjective Knee Evaluation Form (IKDC) is the most frequently used patient-reported measure of subjective knee function among individuals with anterior cruciate ligament reconstruction (ACLR). Yet, due to the limitations of traditional validation approaches, whether the IKDC measures knee function as intended is unclear. Rasch analysis offers a robust validation approach, which may enhance the clinical interpretation of the IKDC. Objective To assess the psychometric properties, ability to classify health status, and relationships between the IKDC and objective measures of strength and functional performance relative to a newly proposed reduced-item instrument. Design Cross-sectional study. Setting Laboratory. Patients or Other Participants A total of 77 individuals with primary unilateral ACLR (age = 21.9 ± 7.8 years, time postsurgery = 6.2 ± 1.0 months) and 76 age-matched control individuals (age = 22.0 ± 4.2 years). Main Outcome Measure(s) Rasch analysis was used to assess the psychometric properties of the IKDC. Receiver operator characteristic curves and logistic regression were calculated to assess the accuracy of classifying participants with ACLR versus control participants. Pearson product moment and Spearman rank order correlation analyses were conducted to evaluate relationships among subjective knee function, quadriceps torque, and single-limb hop performance. Results Rasch analysis aided the development of a reduced 8-item instrument (IKDC-8), which yielded improved psychometric properties in the rating scale performance (IKDC-8 = 0, IKDC = 3 nonmonotonic “misbehaving” items), percentage of variance accounted for by 1 dimension (IKDC-8 = 71.5%, IKDC = 56.7%), and precision in item separation (IKDC-8 = 9.79, IKDC = 5.02). The IKDC was an outstanding diagnostic tool, and the IKDC-8 was excellent, correctly classifying 87.2% and 82.7% of cases, respectively. Using the Hanley-McNeil formula, we found no difference in the areas under the respective receiver operator characteristic curves. Equivalent associations between subjective and objective knee function were observed regardless of the instrument used. Conclusions We demonstrated evidence of enhanced reliability and validity for a parsimonious measure of subjective knee function. The proposed instrument reduces the number of items, increases the score interpretability as measuring a single construct, and improves the rating scale functioning while not diminishing its ability to classify participants with ACLR versus control participants or changing existing relationships with objective measures of recovery. We suggest the IKDC-8 may enhance clinical use by reducing administration time, improving the interpretation of the subjective knee function score, and clarifying functional ability.