Feeling the pressure.

ARD Annals of the Rheumatic Diseases Leaders Feeling the pressure The normal synovium occupies a tissue space confined by the joint capsule. This outer limit is sometimes clear, as in the volar plate of digital hinge joints, and sometimes imperceptible, as in the normal suprapatellar region where areolar synovial tissue appears to blend into the interstitial spaces of the thigh. The limit becomes obvious in chronic synovitis, however, when the capsule clearly confines the distribution of radiolabelled albumin and serves as a relatively non-compliant rind in the presence of pressur-ised effusions.` 3 Withip the capsule lies a synovial fluid filled articular cavity invested by the synovial intima which is in turn surrounded by areolar, fibrous, or fatty subintimal tissues. None of these contiguous regions (cavity, intima, and subintima) is separated by a basement membrane, and all of them are served by the same interstitial fluid. Thus they should share essentially the same concentration of physiological solutes and they should experience the same pressure fluctuations as the joint is used.4 We know very little about the dimensions of those fluctuations in normal joints. Observations from a number of centres in a number of species agree that resting pressures in the normal knee are slightly subatmospheric-usually at around-4 mm Hg.3 5 This baseline may differ in other joints; the average canine wrist, for example, had a resting pressure of +2-9 mm Hg,5 but that finding was the rare exception to the subatmospheric rule. In this issue of the Annals, Gaffney and colleagues greatly expand our meagre database in normal human subjects by reporting 'slightly subatmospheric' resting pressures in eight meta-carpophalangeal (MCP) joints, six wrists and one ankle.6 All of these data are interesting, but the resting joint should be only a brief chapter in the physiological story. What happens to intra-articular pressure when the joint is put to work? Perhaps surprisingly, Gaffney's group found no change with periods of isometric exercise.6 In contrast, Jayson and Dixon found that intra-articular pressure in the knee decreased by more than 100 mm Hg during isometric quadriceps contractions.7 There is a logical explanation for these discrepant findings. The synovial fluid within each normal joint is a highly viscous, bubble free film interposed between and adherent to the mobile surfaces of cartilage and synovium. When the joint moves, that film is subjected to shear, with resultant tensile stress in the fluid itself. A pressure manometer within the …