We repeated some of our own previous experiments, as well as some of Torzilli's recent experiments (1 1) on which he bases his conclusions relating to a nonexchangeable "trapped water" in cartilage. We are unable to confirm Torzilli's findings. We observed partition coefficients for 3H.H0 very close to unity. That both the extrafibrillar and most of the intrafibrillar water is freely exchangeable and behaves as available water towards small solutes has been independently shown (3) for other collagenous tissues. All the different permutations of partition experiments have yielded results that are fully consistent with our original picture of the very major fraction of cartilage water being free. Key Wards: Cartilage-Trapped water-Exchangeable water-Free water-Nonexchangeable water.
How freely exchangeable and how available to solutes of different types is water in cartilaginous tissues is a question of importance with respect to the basic physiology of these tissues. From studies of mass transport in human articular cartilage it was found that the partition coefficients of small unchanged solutes in all zones of the tissue correlated very well with the total local water content. Hence, it was concluded that all, or at least most, of the water present-whether extra-or intra-fibrillarwas available to these solutes (4). The free exchangeability of the water was subsequently directly confirmed by Maroudas and Venn (6) who studied the partition and diffusion of tritiated water in cartilage. Our main findings were that partition coefficients for tritiated water are equal to the total water contents of the tissue and that the diffusion coefficients are equal to approximately 40% of their