Metabolic activity in the calcified zone of cartilage: observations on tetracycline labelled articular cartilage in human osteoarthritic hips.

The tidemark is a metabolically active zone in the calcified layer of the cartilage in which it is possible to show that there is calcifying activity even over a short period of time using a tetracycline labelling technique. The tidemark slowly advances in the direction of the non-calcified cartilage and analysis of double-tetracycline labelled cartilage shows that this is not an appositional phenomenon like that occurring in bone, but that, where present, several tidemarks can be labelled at the same time. Each tidemark may therefore be metabolically active and it is not just the tidemark adjacent to hyaline cartilage which incorporates calcium.