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 I = yHom + zVol + n = 1 N a n Other n ##EQU00001## where y and z are numerical coefficients, Hom is the measured homogeneity, Vol is the measured cartilage volume, and where Other.sub.n represents N further biomarkers each having a respective numerical coefficient a.sub.n, N being zero or an integer.