Factors affecting intraosseous pressure measurement

Background Although a raised intraosseous pressure (IOP) has been found in osteoarthritis and osteonecrosis, the normal physiology of subchondral circulation is poorly understood. We developed an animal model and explored the physiology of normal subchondral perfusion and IOP. Methods In 21 anaesthetised rabbits 44 intraosseous needles were placed in the subchondral bone of the femoral head (n=6), femoral condyle (n=7) or proximal tibia (n=31). Needles were connected to pressure transducers and a chart recorder. In 14 subjects the proximal femoral artery and vein were clamped alternately. In 5 subjects arterial pressure was measured simultaneously in the opposite femoral artery. Results The average IOP at all 44 sites was 24.5 mmHg with variability within (SD 6.8) and between subjects (SD 11.5). IOP was not significantly influenced by gender, weight, site or size of needle. Needle clearance by flushing caused a prolonged drop in IOP whereas after clearance by aspiration, recovery was rapid. IOP recordings exhibited wave patterns synchronous with the arterial pulse, with respiration and with drug circulation time. There was a correlation between IOP and blood pressure (13 sites in 5 subjects, Pearson correlation 0.829, p<0.0005). There was a correlation between IOP and the associated pulse pressure (PP) in 44 sites among 21 subjects (Pearson correlation 0.788, p<0.001). In 14 subjects (31 sites) arterial occlusion caused a significant reduction in IOP and loss of PP (p<0.0001). Venous occlusion significantly raised IOP with preservation of the PP (p<0.012). Conclusion Our study shows that subchondral cancellous bone behaves as a perfused tissue and that IOP is mainly a reflection of arterial supply. A single measure of IOP is variable and reflects only perfusion at the needle tip rather than being a measure of venous back pressure. Alternate proximal vessel clamping offers a new means of exploring the physiology of subchondral perfusion. We describe a model that will allow further study of IOP such as during loading.