REPEATABILITY OF CT OSTEOARTHRITIS KNEE SCORE (COAKS) MULTICOMPONENT MEASURES

INTRODUCTION

The CT Osteoarthritis Knee Score (COAKS) is a semiquantitative system for grading structural disease features in knee OA from weight bearing CT (WBCT). Previous work has demonstrated excellent inter- and intra-observer reliability of COAKS with the aid of a feature scoring atlas, but test-retest repeatability has not yet been evaluated. There is growing interest in multicomponent measures in knee OA imaging research because they may provide granularity in structural feature evaluation, in particular with respect to study baseline stratification and monitoring progression. The multi-feature and multi-compartment nature of COAKS means that it could provide novel insights into OA morphotypes and structural disease progression if found to be robust.

OBJECTIVE

To evaluate test-retest agreement of COAKS multicomponent scores based on WBCT imaging.

METHODS

14 individuals recruited and consented at the University of Kansas Medical Center had baseline and follow-up WBCT imaging suitable for analysis. Participants were (mean ± SD) 61.3 ± 8.4 years old, with BMI 30.7 ± 4.3 kg/m² and had a male:female ratio of 8:6. All scanning was performed on a single XFI WBCT scanner (Planmed Oy, Helsinki, Finland) with the mean ± SD interval between baseline and follow-up attendances 14.9 ± 8.1 days. A Synaflexer^TM device was used to standardize knee positioning during scanning. Imaging acquisition parameters were 96 kV tube voltage, 51.4 mA tube current, 3.5 s exposure time. A standard bone algorithm was applied for reconstruction with 0.3 mm isotropic voxels and a 21 cm vertical scan range. All scans were anonymised prior to analysis both according to the individual and imaging attendance. All knees were reviewed for COAKS by an experienced musculoskeletal radiologist (T.D.T.). Scores were recorded in a cloud-based file on Google Sheets (alongside the feature atlas in Google Docs) and read by custom MATLAB scripts to generate baseline versus follow-up difference plots and intraclass correlation coefficients for absolute agreement from a single observer, Shrout-Fleiss ICC(3,1). Scores for individual COAKS features (JSW, osteophytes, subchondral cysts, subchondral sclerosis) were combined across compartments. Compartment scores (medial tibiofemoral, lateral tibiofemoral, patellofemoral, proximal tibiofibular) were combined across features. Multicomponent scores were also summated for the whole tibiofemoral compartment (medial-lateral combined) and from across the whole knee joint.

RESULTS

ICC values were excellent (>0.81) for all multicomponent scores apart from subchondral sclerosis combined across all compartments (0.69, 0.43-0.84) and all features combined at the proximal tibiofibular joint (0.65, 0.38-0.82). Best agreement was seen for osteophytes combined across all compartments (0.93, 0.85-0.96) (Figure 1), all features combined at the medial tibiofemoral compartment (0.95, 0.90-0.98) and the lateral tibiofemoral compartment (0.97, 0.94-0.99). Full ICC results are given in Table 1. ICCs for all features combined across the whole tibiofemoral compartment (0.93, 0.86-0.97) (Figure 2) and across the whole knee joint (0.90, 0.79-0.95) were also near-perfect with data values.

CONCLUSION

These data support excellent agreement for COAKS multicomponent scores by compartment, by feature, and as a whole. These results suggest that a multicomponent approach could offer sensitivity in distinguishing morphotypes and monitoring structural progression as personalised medicine approaches become more realistic in developing treatment strategies for OA. Having established the excellent repeatability of a multicomponent COAKS approach, it will now be essential to validate.