The relationship between intra-articular and juxta-articular intraosseous pressures in the metatarsophalangeal region of the pony.

Eight metatarsophalangeal (fetlock) joints were studied in anaesthetized young ponies. Pressure measurements were made in the joint and at various sites in the marrow cavity of the third metatarsal bone with simultaneous measurement of systemic arterial and venous blood pressures. Fetlock joint flexion was always associated with a statistically significant initial decrease in intra-articular pressure (p < 0.05) followed by a marked increase in pressure (p < 0.05). Subsequent extension was accompanied by a similar pressure pattern, although there appeared to be a hysteretic relationship. Fetlock joint distension induced by incremental injection of saline caused a stepwise increase in the intra-articular pressure. Both joint flexion and effusion were associated with an increased intraosseous pressure within the third metatarsal bone. This effect was most pronounced at the distal end of the bone, probably through partial obstruction of the vessels draining the epiphysis and metaphysis.