Quantifying self-report measures' overestimation of mobility scores postarthroplasty.

BACKGROUND

Self-reports of function may systematically overestimate the ability of patients to move around postarthroplasty.

OBJECTIVE

The purpose of this study was to estimate the magnitude of systematic differences in Lower Extremity Functional Scale (LEFS) and Western Ontario and McMaster Universities Osteoarthritis Index physical function subscale (WOMAC-PF) scores before and after primary total knee arthroplasty (TKA) or total hip arthroplasty (THA) by referencing the values to Six-Minute Walk Test (6MWT) distances and Timed "Up & Go" Test (TUG) times.

DESIGN

This study was a secondary analysis of data from a prospective cohort study.

METHODS

The LEFS, WOMAC, 6MWT, and TUG were administered to 85 patients prearthroplasty and once at 9 to 13 weeks postarthroplasty. Regression analysis was applied using a robust error term for clustered data. With the self-report measures as dependent variables and performance measures, occasion (prearthroplasty or postarthroplasty), and performance measure-by-occasion as independent variables, 3 propositions were examined: (1) the relationship between self-report and performance measures is identical prearthroplasty and postarthroplasty (ie, regression lines are coincident); (2) the relationship differs between occasions, but is consistent (ie, regression lines are parallel); (3) the relationship is not consistent (ie, the regression lines are not parallel).

RESULTS

For all analyses, the results supported the second proposition (ie, the relationship differed between occasions, but was consistent). The systematic differences varied by location of arthroplasty, but were similar for both performance tests. For the LEFS, the difference was approximately 11 points for patients who received TKA and 13 points for patients who received THA. For the WOMAC-PF, the difference was approximately 12 points for patients who received TKA and 19 points for patients who received THA. These differences exceed the minimal clinically important change for an individual patient.

LIMITATIONS

The findings are specific to 9 to 13 weeks postarthroplasty.

CONCLUSION

Dependence on scores of self-report measures alone, without knowledge of the magnitude of the identified systematic differences, will result in overestimating the ability of patients to move around postarthroplasty.