Evaluation of Forgotten Joint Score in total hip arthroplasty with Oxford Hip Score as reference standard.

Background and purpose - Total hip arthroplasty (THA) is performed mainly because of pain. To evaluate the result after surgery, different questionnaires measuring the patient-reported outcome regarding quality of life are used. Forgotten Joint Score (FJS), designed to chart postoperative symptoms, was developed to find subtle differences between patients who report that their operated hip is "very good" or "excellent." We evaluated whether FJS provides additional information compared with the Oxford Hip Score (OHS) and ceiling and floor effects with use of these instruments. We also studied level of internal consistency for OHS and FJS, and the reproducibility of the FJS. Patients and methods - 111 patients who underwent unilateral primary THA in 2015 were included. The participants answered 2 questionnaires: Forgotten Joint Score and Oxford Hip Score. Floor and ceiling effects were recorded for each of the instruments and agreement between them. The FJS was studied with respect to reproducibility and level of internal consistency. Results - OHS ceiling effect (31%) was higher compared with FJS (21%), whereas the OHS seemed to provide a more nuanced picture of patients with an inferior clinical result. Floor effect for FJS was 3% and 0% for OHS. The degree of explanation was 68% between the 2 questionnaires (linear regression, r2 = 0.68). FJS items had a high internal consistency (Cronbach's a = 0.93) and reproducibility (Pearson correlation = 0.87, ICC = 0.93); 92 patients answered on 2 distributions of the FJS questionnaires, 19 patients had identical answers. Interpretation - OHS had a larger ceiling effect than FJS, which could indicate that FJS is a more fine-tuned instrument to separate patients with good to excellent outcome after THA. The high internal consistency of FJS indicates that the items of the instrument consistently cover the construct of joint awareness.