Intra-osseous pressure of the equine third metatarsal bone

Intra-osseous pressure was measured in the third metatarsal bone of 8 pony foals at the age of 1, 3, 6 and 26 weeks of age, and on one occasion in the third metatarsal bone of 5 ponies aged 1-5 years; the animals were under general anesthesia and in lateral recumbency. Saphenous venous pressure and lateral superficial plantar metatarsal arterial pressure were also monitored throughout the procedure. There were no statistically significant changes in the epiphyseal, diaphyseal, venous or arterial pressures with increasing age of the foals. There was no statistically significant difference between the mean epiphyseal and mean diaphyseal pressures in the foals. Individually however, the diaphyseal pressure exceeded the epiphyseal pressure in 32 of the 36 measurements made in the foals. Saphenous vein occlusion and intravenous adrenalin administration showed that both venous and arterial pressures influence intra-osseous pressure. Intra-osseous injection of heparinized saline on one side of the growth plate produced an immediate massive increase in intra-osseous pressure on that side of the growth plate, but in almost all cases a concomitant rise in intraosseous pressure on the other side of the physis did not occur. Simulated effusion of the metatarsophalangeal joint was associated with an increase in intraosseous pressure in the adjacent metatarsal epiphysis, and flexion of the joint with an increase in intraosseous pressure in both the epiphysis and the metaphysis.