Early changes in the distal intertarsal joint of Dutch Warmblood foals and the influence of exercise on bone density in the third tarsal bone

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Abstract
It was hypothesised that imposition of different exercise levels at a young age would lead to differences in bone density in the third tarsal bone and to difference in the prevalence of pathological lesions that might contribute to the development of bone spavin later in life. Furthermore, based on earlier literature, it was hypothesised that such lesions could be classified as a manifestation of osteochondrosis. Changes in bone density in the third tarsal bone and early pathological changes in the articular cartilage of the distal intertarsal joint were studied in the offspring of sires with radiographic evidence of osteochondrosis in either stifle or hock. Twenty-four foals were studied at age 5 months after having been subjected to different exercise programmes (box-rest, box-rest with sprint training, pasture exercise) from age one week. Nineteen other foals that originally belonged to the same exercise groups were studied at age 11 months, after they had been weaned, housed together and subjected to an identical low level exercise regimen for an additional 6 months. Bone density was quantified using a microscopic technique. Histomorphological analysis was performed semiquantitatively and using high detail radiography techniques. At age 5 months, mean +/- s.d. bone density in the compact bone of the third tarsal bone was significantly lower in the box-rested foals (37 +/- 4%) than in both the trained and pastured foals (48 +/- 7% and 52 +/- 11%, respectively). After 6 months of identical exercise the previously box-rested foals showed an increase in bone density (53 +/- 12%) which became similar to the value found in the formerly pastured foals (52 +/- 8%). Major pathological lesions (chondrocyte necrosis, fragmentation and chondrone formation) of the articular cartilage of the third and central tarsal bones were already present at age 5 months, but were significantly more numerous at 11 months. There was no relation between the number of cartilage lesions and the osteochondrosis status of the foals. Only 2 lesions in 11-month-old foals had histological characteristics compatible with osteochondrosis, all other lesions were degenerative in nature. It is concluded that bone density of the compact bone of the subchondral bone plate in the third tarsal bone reacts strongly to variations in exercise at a very young age. Low bone density, caused by lack of exercise, can be compensated for when exercise is later increased. Pathological changes in the distal intertarsal joint are common at 5 months and increase to 11 months. These lesions are degenerative in nature and seem not to be related to osteochondrosis. Although the clinical relevance of these abnormalities is uncertain, they may be relevant for the development of osteoarthritic processes in this region later in life.

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