COMPUTED TOMOGRAPHY OSTEOARTHRITIS KNEE SCORE (COAKS) REVEALS PATTERNS IN FEATURE AND COMPARTMENTAL CONTRIBUTIONS TO WHOLE JOINT OSTEOARTHRITIS LOAD

INTRODUCTION

The Computed Tomography Osteoarthritis Knee Score (COAKS) has been shown to be reliable in semi-quantitatively assessing structural knee OA from weight-bearing CT (WBCT), as well as being able to display feature scores across different compartments as phenotype heat maps. COAKS has the advantage of being able to assess structural features of OA at the knee joint in 3-D, which may reveal insights into disease patterns beyond the scope of radiography.

OBJECTIVE

To evaluate the distribution of structural features and multi-compartmental involvement as a proportion of whole knee joint OA burden using COAKS.

METHODS

96 bilateral knee WBCT examinations were sourced from existing research projects at the Department of Rehabilitation Medicine, University of Kansas Medical Center, USA. An experienced musculoskeletal radiologist (TT) chose one knee from each participant to provide a study group with a subjectively wide range of disease. Stratification by existing radiographic scoring systems was purposefully avoided. Demographic information was also not included in this structural analysis. Each knee was scored by a single trained observer (JJ) using the COAKS atlas guide and 4-point system (0-3) for each OA feature (J = joint space width; O = osteophytes; C = subchondral cysts; S = subchondral sclerosis) in each joint compartment (MTF = medial tibiofemoral; LTF = lateral tibiofemoral; PF = patellofemoral; PTF = proximal tibiofibular), summing up to a total joint score out of possible maximum of 48. Descriptive analysis was performed by looking at the relative contribution of individual structural feature scores and individual compartment scores as a proportion of the whole joint score.

RESULTS

The histogram of score distributions is presented in Figure 1. Median whole joint score was 18.5 points with an interquartile range of 9 to 27.25 and overall range of 3 to 45. The lowest quartile scores were considered “initial” structural disease, uppermost quartile scores “widespread” disease, and those within the interquartile range “intermediate” disease. Using these boundary definitions, a pattern of MTF predominance was seen in “initial” whole joint disease, logically levelling out across all compartments when “widespread”, but still with heterogeneity in compartmental contributions at all stages (Figure 2). Alongside MTF involvement, “initial” disease tended to include predominantly LTF involvement to a greater extent than PF involvement, with the opposite predominance of PF over LTF involvement in “intermediate” disease: this suggests some exclusivity between LTF and PF involvement before widespread disease is established. Reasonably uniform contributions were seen from the PTF compartment across all disease stages, suggesting that this compartment is linked to whole overall joint OA burden. From a structural feature perspective, knees with “initial” disease tended to be dominated by joint space narrowing and osteophytes, with cysts appearing in “intermediate” disease, all well-recognized radiographic phenomena (Figure 3). Sclerosis contributed to total score across all stages, suggesting the feature is being readily identified using the COAKS definition. “Widespread” disease was also logically characterized by similar relative contributions from all features, but with greater uniformity implying a loss of sensitivity in discriminating between the prevalence of these features beyond the “intermediate” stage.

CONCLUSION

Semi-quantitative analysis of whole knee joint OA burden using COAKS reveals that there is heterogeneity of compartmental involvement across a wide range of disease severities, an important consideration if only relying on radiographic or MRI features at the MTF compartment for disease stratification. The COAKS construct appears to be sensitive to compartmental involvement distinct from whole joint severity but may lack sensitivity to compare relative proportions of structural features in more advanced disease. These results support the value of COAKS evaluation in larger cohort studies as well as characterizing spatial and temporal relationships between knee OA features in preparing for clinical validation.