Type II collagen is a major component of articular cartilage and its breakdown 
is a key feature of osteoarthritis. Products of cartilage collagen metabolism 
can be detected in the blood, synovial fluid and urine. Several biomarker assays 
have been developed which can be used to measure the synthesis and degradation 
of collagen, and therefore provide information regarding cartilage turnover. 
This is the first part of a two-part review and describes the need for accurate, 
reliable information regarding collagen turnover, the processes by which the 
biomarker epitopes are generated, their application to the study of both healthy 
and diseased cartilage and the results of currently published studies, with 
particular reference to the veterinary species. The second part of the review 
considers the non-collagenous biomarkers of cartilage matrix turnover.