The Patient-Specific Functional Scale: Its Reliability and Responsiveness in Patients Undergoing a Total Knee Arthroplasty

STUDY DESIGN: A prospective cohort study, with assessments before, at 3 months after, and at 12 months after total knee arthroplasty (TKA).

OBJECTIVES: To determine and compare the responsiveness and reliability of the Patient-Specific Functional Scale (PSFS) in patients undergoing a TKA.

BACKGROUND: Reliable and valid measurement instruments are important to measure functional status. The PSFS is frequently used in several patient populations, but its methodological characteristics in a population of patients with TKA in the perioperative phases of surgery are unknown.

METHODS: The PSFS, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), and global perceived effect (GPE) were administered at 3 time points in 150 patients undergoing a TKA. Correlations, effect sizes, and standardized response Means were calculated. The PSFS was administered twice to assess its reliability in terms of intraclass correlation coefficients and limits of agreement.

RESULTS: The correlation coefficients between the PSFS and WOMAC at 3 months and 1 year were 0.41 and 0.48, respectively; those between the PSFS and GPE were -0.37 and -0.55, respectively. The effect sizes of the PSFS at 3 months and 1 year were 1.71 and 2.89, respectively; those of the WOMAC were 1.45 and 1.64, respectively. The standardized response means of the PSFS at 3 months and 1 year were 0.96 and 1.48, respectively; those of the WOMAC were 1.28 and 1.37, respectively. The intraclass correlation coefficients ranged between 0.73 and 0.86. The systematic error was between 0.12 and 0.54. The limits of agreement ranged from +/- 2.17 to +/- 2.72.

CONCLUSION: The reliability of the PSFS is good. Its responsiveness is high, especially in the long term. However, the PSFS cannot be used interchangeably with the WOMAC in the immediate postoperative period.