Morphometric study of the equine navicular bone: variations with breeds and types of horse and influence of exercise

Navicular bones from the 4 limbs of 95 horses, classified in 9 categories, were studied. The anatomical bases were established for the morphometry of the navicular bone and its variations according to the category of horse, a fter corrections were made for front or rear limb, sex, weight, size and ag e. In ponies, navicular bone measurements were smallest for light ponies an d regularly increased with body size, but in horses, navicular bone dimensi ons were smallest for the athletic halfbred, intermediate for draft horse, thoroughbreds and sedentary halfbreds and largest for heavy halfbreds. The athletic halfbred thus showed reduced bone dimensions when compared with ot her horse types. Navicular bones from 61 horses were studied histomorphomet rically. Light horses and ponies possessed larger amounts of cancellous bon e and less cortical bone. Draft horses and heavy ponies showed marked thick ening of cortical bone with minimum intracortical porosity, and a decrease in marrow spaces associated with more trabecular bone. Two distinct zones w ere observed for the flexor surface cortex: an external zone composed mainl y of poorly remodelled lamellar bone, disposed in a distoproximal oblique d irection, and an internal zone composed mainly of secondary bone, with a la teromedial direction for haversian canals. Flexor cortex external zone tend ed to be smaller for heavy ponies than for the light ponies. It was the opp osite for horses, with the largest amount of external zone registered for d raft horses. In athletic horses, we observed an increase in the amount of c ortical bone at the expense of cancellous bone which could be the result of reduced resorption and increased formation at the corticoendosteal junctio n. Cancellous bone was reduced for the athletic horses but the number of tr abeculae and their specific surfaces were larger. Increased bone formation and reduced resorption could also account for these differences.