Intraosseous pressure during loading and with vascular occlusion in an animal model

Objectives We studied subchondral intraosseous pressure (IOP) in an animal model during loading, and with vascular occlusion. We explored bone compartmentalization by saline injection. Materials and Methods Needles were placed in the femoral condyle and proximal tibia of five anaesthetized rabbits and connected to pressure recorders. The limb was loaded with and without proximal vascular occlusion. An additional subject had simultaneous triple recordings at the femoral head, femoral condyle and proximal tibia. In a further subject, saline injections at three sites were carried out in turn. Results Loading alone caused a rise in subchondral IOP from 11.7 mmHg (SD 7.1) to 17.9 mmHg (SD 8.1; p < 0.0002). During arterial occlusion, IOP fell to 5.3 mmHg (SD 4.1), then with loading there was a small rise to 7.6 mmHg (SD 4.5; p < 0.002). During venous occlusion, 10P rose to 20.2 mmHg (so 5.8), and with loading there was a further rise to 26.3 mmHg (SD 6.3; p < 0.003). The effects were present at three different sites along the limb simultaneously. Saline injections showed pressure transmitted throughout the length of the femur but not across the knee joint. Conclusion This is the first study to report changes in IOP in vivo during loading and with combinations of vascular occlusion and loading. Intraosseous pressure is not a constant. It is reduced during proximal arterial occlusion and increased with proximal venous occlusion. Whatever the perfusion state, in vivo load is transferred partly by hydraulic pressure. We propose that joints act as hydraulic pressure barriers. An understanding of subchondral physiology may be important in understanding osteoarthritis and other bone diseases.