Impact of context specificity on response to the shortened WOMAC function scale in patients undergoing total knee arthroplasty

Objective: To determine, in patients undergoing total knee arthroplasty (TKA), whether increasing context specificity of selected items of the shortened version of the Western Ontario and McMaster Universities Osteoarthritis Index function (WOMAC-F) scale (ShortMAC-F) (i) enhanced the convergent validity of the ShortMAC-F with performance-based mobility measures and (ii) impacted on mean scale score, structural validity, reliability, and interpretability.

Design: Secondary analysis of randomized clinical trial data.

Setting and participants: Patients undergoing TKA (n=114).

Interventions: Not applicable.

Main outcome measures: The ShortMAC-F was modified by specifying the "ascending stairs" and "rising from sitting" items to enquire about difficulty in performing the tasks without reliance on compensatory strategies, while the modified "level walking" item enquired about difficulty in walking 400 meters. Before and 12 weeks after TKA, patients completed the WOMAC-F, modified ShortMAC-F, knee pain scale, sit-to-stand test, fast gait speed test, and stair-climb test. Interpretability was evaluated by calculating anchor-based substantial clinical benefit (SCB) estimates.

Results: The modified ShortMAC-F correlated significantly more strongly than ShortMAC-F or WOMAC-F with pooled performance measures (differences in correlation values, 0.12-0.14). Increasing item context specificity of the ShortMAC-F did not influence its psychometric properties of unidimensionality (comparative fit and Tucker-Lewis indices >0.95; root mean square error of approximation, 0.05-0.08), reliability (Cronbach alpha, 0.75-0.83), correlation with pain intensity (correlation values, 0.48-0.52), and SCB estimates (16 percentage points); however, it resulted in lower mean score (4.5-4.8 points lower).

Conclusions: The modified ShortMAC-F showed sufficient measurement properties for clinical application, and it seemed more adept than WOMAC-F at correlating with performance-based measures in TKA.