ABSTRACT
OBJECTIVE: The Oxford Knee Score (OKS) and OKS Activity and Participation Questionnaire (OKS-APQ) are patient reported outcome measures used to assess people undergoing knee replacement surgery. They have not explicitly been tested for unidimensionality (whether they measure one underlying trait such as 'knee health'). This study applied item response theory (IRT) to improve the validity of the instruments to optimise for ongoing use. STUDY DESIGN AND SETTING: Participants undergoing primary total knee replacement (TKR) provided pre-operative and post-operative responses for OKS and OKS-APQ. Confirmatory factor analyses (CFA) were performed on the OKS and OKS-APQ separately, and then on both when pooled into one. An IRT model was fitted to the data. RESULTS: 2972 individual response patterns were analysed. CFA demonstrated that when combining OKS and OKS-APQ as one instrument, they measure one latent health trait. A user-friendly, free-to-use, web app has been developed to allow clinicians to upload raw data and instantly receive IRT scores. CONCLUSIONS: The OKS and OKS-APQ can be combined to use effectively as a single instrument (producing a single score). For the separate OKS and OKS-APQ the original items and response options can continue to be posed to patients, and this study has confirmed the suitability of IRT-weighted scoring. Applying IRT to existing responses, converts traditional sum scores into continuous measurements with greater granularity including individual measurement error.
ABSTRACT
AIMS: There is increasing evidence for fixation as opposed to revision for Vancouver B fractures around polished taper slip stems, however it is remains unclear whether fixation is associated with stem loosening in the longer term. This study aims to assess survival of plate-fixation of Vancouver-B-fractures around a polished-taper- slip cemented stem and identify factors associated with failure. METHODS: Retrospective cohort study assessed 129 consecutive unilateral Vancouver-B-fractures around cemented Exeter stems at a minimum of 5 years following open-reduction-internal-fixation (ORIF) with unilateral non-locked unilateral plating+/-cerclage cables. The primary outcome measure was reoperation for any reason. Kaplan Meier survival analysis was performed. RESULTS: Fractures (B1 n = 31 (24%); B2 n = 91 (71%); and B3 n = 7 (5%)) occurred at median of 6 years (IQR 1.2-9.2) after primary surgery. Mean patient age was 78.2 (SD10.56, range 46-96) and 54 (42.9%) were female. Mean follow up was 8.7 years (SD 2.48, 5.7 to 14.4). Symptomatic femoral stem loosening requiring revision occurred in two B2 fractures of metaphyseal split and short spiral patterns. The most common mode of failure was non-union (n = 7, 5%). Both fixation failure (n = 6/31 Vs n = 3/91 vs n = 0/7, p = 0.008) and reoperation (n = 8/31 vs n = 6/91 vs n = 0/7, p = 0.008) were significantly higher following fixation of B1 fractures compared to B2 and B3 fractures. Overall 5year survival free from reoperation was 88.8% (82.0-94.7 95%CI). B1 fracture types were associated with an increase risk of failure (endpoints fixation failure p = 0.010; and reoperation p = 0.016). Transverse fractures (B1) were associated with a relative risk of reoperation of 4.22 (1.63-10.9 95% CI, p = 0.008). CONCLUSION: Fixation of Vancouver-B fractures around cemented Exeter stems, when the bone-cement interface is intact and the fracture is anatomically reducible, had an excellent 5-year survival. Only 2 (1.6%) cases of late femoral stem loosening occurred, however, B1 type transverse fractures were associated with a higher rate of reoperation.