Variations in articular calcified cartilage by site and exercise in the 18-month-old equine distal metacarpal condyle

Summary Objectives To interrelate articular calcified cartilage thickness, mineralisation density, tidemark count and tidemark linear accretion rate by site in the equine third metacarpal distal condyle. To determine the effects of exercise during early life on articular calcified cartilage. Method Six of 12 pasture-raised Thoroughbred horses were exercised from 10 days old. Calcein labels were given 19 and 8 days prior to euthanasia at 18 months old. Osteochondral specimens were cut from the distal third metacarpal condyle and imaged using confocal scanning light microscopy (CSLM) and quantitative backscattered electron scanning electron microscopy (qBSE). Articular calcified cartilage thickness and total thickness mineralisation density were measured on montaged qBSE image sets, and inter-label mineralisation density, tidemark count and linear accretion rate measured on registered CSLM–qBSE image pairs. Results Calcified cartilage thickness, mineralisation density, tidemark count and linear accretion rate varied significantly between sites. Regions with thinner calcified cartilage had greater linear accretion rates, hence rapid chondroclastic resorption. Mineralisation density was positively correlated with linear accretion rate. Fewer multiple tidemarks were counted in regions with greater linear accretion rates. Lag time between the tidemark and cement line was estimated (180 days; in the range of 0–648 days). Exercise had little effect on measured parameters. Conclusion The major determinant of articular calcified cartilage thickness is the rate of chondroclastic resorption, not tidemark linear accretion rate. Our evidence supports coupled, mechanosensitive regulation of chondroclastic resorption and linear accretion rate in articular calcified cartilage. Exercising pasture-reared foals causes little additional adaptation in distal third metacarpal articular calcified cartilage.