Intra‐articular pressure levels were determined for joint positions throughout the normal physiological range of movement of dogs' knee joints. Change in joint position resulted in change in intra‐articular pressure. It was demonstrated that intra‐articular pressure is highest with the joint in the fully flexed position. Minimum pressure was recorded at a position between 80 degrees and 120 degrees. Minimum pressures were usually subatmospheric. The rate of change of joint position affected intra‐articular pressure. The relationship of intra‐articular pressure and joint position before and after full flexion demonstrated a hysteresis effect; the pressures were lower than for the same joint position before flexion. Maintenance of the joint in the fully flexed position for increasing periods of time between repeated movement cycles resulted in a similar reduction, of constant magnitude, in pressure between joint positions before and after each period of flexion. However, there was also a progressive decrease in pressure for all joint angles over the total number of movement cycles. There is a contribution to intra‐articular pressure of joint capsular compliance and fluid movement into and out of the joint (both of which are time‐dependent). The recording of intra‐articular pressure in conscious, upright dogs revealed similar pressure levels to those measured in anaesthetized supine dogs. The major determinants of intra‐articular pressure in normal dog knee joints include joint size, synovial fluid volume, position of joint, peri‐articular tissue and joint anatomy, membrane permeability, capsular compliance, and movement of fluid into and out of the joint.

SUMMARY1. Single-and multi-unit recordings were obtained from the medial articular nerve (m.a.n.) of knee joints in the anaesthetized dog. The single-unit recordings were confined to low threshold (group I and II) articular mechanoreceptors.2. Multi-unit recordings revealed that the m.a.n. discharge was maximal in extension, submaximal in flexion, and minimal at intermediate angles, i.e. a U-shaped profile.3. Subatmospheric intra-articular pressures do not appear to influence the m.a.n. discharge.4. Intra-articular infusion of even small quantities of fluid, although not affecting the U-shaped profile, reversed the m.a.n. discharge pattern with maximum neural activity occurring in flexion and being submaximal in extension.5. Recordings from single units indicated that the enhanced discharge after fluid infusion was a result of increased discharge frequency and ' recruitment ' of individual afferents.

SUMMARY1. This study has investigated changes in intra-articular hydrostatic pressure in the knee joints of normal dogs in response to continuous and stepwise infusions of fluids. The relationship between pressure and volume in the joint was examined over the pressure range of -8 to +50 mmHg, and also at much higher pressures often associated with joint disease or injury. The effects of joint angle and dog weight on the pressure-volume relationship and on elastance of the dogs' knees were also examined.2. With liquid paraffin B.P. the pressure was found to increase more with each unit volume infused at subatmospheric pressures than at pressures around atmospheric, and increased more again at higher pressures. The pressure-volume curve with saline infusions was affected by egress of fluid from the joint at supra-atmospheric pressure. Above + 5 mmHg the rise in pressure per unit volume infused was less than that for paraffin at the same volume.3. Elastance and compliance of the normal joint capsule were calculated from the pressure-volume data. Elastance was high at subatmospheric pressures, decreased rapidly as atmospheric pressure was approached and rose as a linear function of pressure above 12 mmHg.4. The biphasic shape ofthe elastance-pressure curve is discussed, and explanations for the shape are suggested.5. After intra-articular pressure in the knee was raised by infusion of paraffin oil the joint was moved through the range of positions from 125 deg extension to 50 deg flexion. Intra-articular pressure did not change across the range 125-110 deg. However, increasing the angle of flexion from 110 to 50 deg resulted in a rise in pressure which became steeper for each volume increment.6. Increasing intra-articular fluid volume caused a decrease in the total range of movement of the joint.7. The pressure-volume curves measured at extended angles of 110, 125 and 140 deg, where the starting pressures were subatmospheric, were the same. At flexed joint positions of 80 and 50 deg, where the starting pressures were supra-atmospheric, the pressure-volume curves became steeper with greater flexion. * Authors names are in alphabetical order.14 pHY 357S. NADE AND P. J. NEWBOLD 8. Elastance of the joint tissues increased with flexion. The elastance at each joint angle depended also on the volume or pressure.9. Significant differences were found to exist between pressure-volume curves for three groups of animals of different weight. Curves were steeper in small animals.10. Acrylic casts of the knee joint space were constructed at 90 and 40 deg of flexion to demonstrate channels of communication between anterior and posterior regions. The anterior compartment provided two-thirds of the total surface area.

SUMMARY1. The normal activity of the medial articular nerve of the knee joint of nine dogs was measured in response to continuous passive movement. Whole nerve recordings were made simultaneously with joint position and angular velocity.2. When the knee was moved the medial articular nerve discharged at all joint angles with maximum activity at flexion in some animals, and at extension in others. For an individual animal the discharge pattern was reproducible over several cycles of movement.3. The discharge was direction sensitive, responding with a lesser activity whenever the joint was moved away from the extremes of movement range.4. With higher velocities of movement, the discharge activity of the nerve increased at the ends of joint movement.5. The results indicate that the discharge activity of the whole medial articular nerve is influenced not only by joint position but also by the velocity and acceleration of movement. This may reflect changes in the tension of capsular and extra-capsular tissues.