A computer based calculation of a prognostic index I of osteoarthritis based on biochemical and imaging based biomarkers a mathematical combination of said values, wherein a first imaging based biomarker is a measure of the quantity of a cartilage in a joint compartment, a second imaging based biomarker relating to the quality of said cartilage in said joint compartment, and wherein a value of said first biomarker indicative of a larger quantity of cartilage affects the index to make it predictive of more disease progression, and a value of said second biochemical marker indicative of a greater departure from the quality of disease free cartilage affects the index to make it predictive of more risk of disease progression, exemplified by; ]]> ;where y and z are numerical coefficients, Hom is the measured homogeneity, Vol is the measured cartilage volume, and where Othern represents N further biomarkers each having a respective numerical coefficient an, N being zero or an integer.
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