Assessment of differences between the modified Cincinnati and International Knee Documentation Committee patient outcome scores: a prospective study.

BACKGROUND Functional outcome questionnaires have become a common part of patient follow-up in the orthopaedic community. The modified Cincinnati Knee Rating System and the International Knee Documentation Committee (IKDC) subjective scale were designed to provide clinicians with information regarding a patient's functional and clinical status after knee surgery. HYPOTHESIS The functional outcome data reported on the modified Cincinnati Knee Rating System and the IKDC subjective questionnaire are equivalent. STUDY DESIGN Cohort study (diagnosis); Level of evidence, 2. METHODS Between 2000 and 2007, all patients with complex knee disorders seen in our orthopaedic clinic were prospectively followed and given simultaneously the modified Cincinnati Knee Rating System and the IKDC subjective questionnaires to report their functional outcomes. The total scores of each instrument were compared at each time of evaluation. As a measure of responsiveness, the standardized response means were calculated. A second within-patient analysis was also performed to determine if the results would be different when looked at by the patient. RESULTS There were a total of 130 patients with a total of 444 modified Cincinnati Knee Rating System questionnaires and 462 IKDC subjective complete questionnaires included in the study. Overall total scores on both the modified Cincinnati Knee Rating System and the IKDC subjective questionnaires were equivalent in a population analysis. Overall total scores showed improvement in function over time from preoperative measurement through the 2-year follow-up. The individual patient analysis demonstrated that, for a specific patient, it was possible that up to 34% of the population would report a total score of more than 10 points difference, depending on the scale selected. CONCLUSION The modified Cincinnati Knee Rating System and the IKDC subjective rating questionnaires reported by patient population provide similar results. The practitioner who is not performing a population-based study but following individual patients for recovery will find that the individual questionnaires may offer different functional limitations. Individual differences were lost in the population means, with the approximate balance of positive and negative score differences masking the individual patient differences.