Pressure-volume relationships in equine midcarpal joint.

This research studied how different variables, including joint angle, fluid infusion, history-dependence, and time-dependence, affect the pressure-volume relationship of healthy horse midcarpal joints.

Research Design and Methodology

• The horse subjects were anesthetized and made to lay on their backs in order to measure intra-articular pressure (IAP) and record data from fluid-infused midcarpal joints, i.e., the equivalent of the human wrist.
• 24 horses with a total of 48 midcarpal joints were studied.
• They used two types of fluid for infusion into the dorsal joint capsule, saline and synovial.
• The researchers observed the resulting pressure-volume (PV) curves, which they found to be sigmoidal, i.e., curve-shaped like the letter ‘S’.

Findings and Observations

• The PV curves were best described by the equation IAP = A x e(B x volume) – C, where e is the base of natural logarithms, A and C are constants, and B refers to the relative change in pressure per unit of volume.
• No compartmentation was observed in the joints, implying there were no specific, isolated sub-structures responsible for the mechanical behavior.
• Elastance, a measure of the ability of the tissue to return to its original shape after distension, was greater with low pressures compared to high ones, at 90 degrees than at bigger angles, and with saline infusion compared to synovial fluid.
• Hysteresis, the lagging of a physical effect behind its cause, was found to be more significant at 90 degrees than at 135 degrees and when dealing with the natural synovial fluid than with saline.
• The researchers noticed that with sequential distention at high intra-articular pressures, the elastance gradually increased.
• The drop in IAP over time showed a positive logarithmic relationship.

Conclusion and Implications

• The study illustrates the important role that synovial fluid plays in the pressure-volume relationship within joints.
• The findings also underscore the importance of considering the joint angle, the history of distention cycles, and the reduction of IAP over time in future research into the pressure-volume relationships in joints.