Validity and responsiveness of the SMFA-D questionnaire in patients with primary osteoarthritis of the knee and total knee artbroplasty.

Introduction: Scoring systems used so far in total knee arthroplasty are limited by their non-fulfillment of basic test criteria. The aim of the study was to demonstrate the validity and responsiveness of a German version of the new short musculoskeletal function assessment questionnaire (SMFA-D) in patients with primary osteoarthritis and total knee arthroplasty. Methods: 66 patients with a tricompartmental cemented PFC-Sigma total knee arthroplasty completed the SMFA-D and the WOMAC questionnaire preoperatively and at 12 to 16 weeks follow-up. Preoperatively, physicians' rating of function of the leg, patients' self-selected walking speed, pain and arthritis severity score were registered for demonstration of criterion validity. Construct validity was evaluated with the WOMAC. Discriminant validity was assessed by comparing patients with or without previous surgery at the knee, use of pain medication and use of walking aids. The function and bother indexes of the SMFA-D were correlated with these parameters. Standardized response means were calculated. Results: The function index correlated with physicians' rating (r = 0.51), walking speed (r = 0.61), pain (r = 0.36) and the arthritis severity index (r = 0.36). The bother index correlated with pain (r = 0.37) and the arthritis severity index (r = 0.25). The function and bother index correlated with the WOMAC (r = 0.77) and (r = 0.81), respectively. Patient groups with or without walking aids (p = 0.02) and with or without pain medication (p = 0.001) differed in the function index. The bother index of patients with or without pain medication (p < 0.001) and with or without walking aids (p < 0.006) differed. Function and bother index improved from 46 (SD 17) to 34 (SD 19, p < 0.001) and 43 (SD 18) to 33 (SD 22 p < 0.001), respectively. The standardized response mean was 0.86 for the function index and 0.53 for the bother index. Conclusions: The SMFA-D questionnaire is valid and responsive in patients with primary osteoarthritis of the knee and total knee arthroplasty. It measures function and bother from the perspective of these patients.