Joint capsular stiffness in knee arthritis. Relationship to intraarticular volume, hydrostatic pressures, and extensor muscle function.

Increased intraarticular hydrostatic pressure (Pia) may inhibit juxtaarticular muscle function, obstruct blood supply to joint structures and promote anoxic joint destruction in chronic arthritis. Joint capsular stiffness together with synovial fluid volume determines Pia at rest. Seventeen knee joints with effusive arthritis and different degrees of radiological cartilage involvement in 13 patients with chronic arthritis were examined. Since capsular elastance was difficult to standardize, we introduce a measure of joint capsular stiffness where the intraarticular volume yielding a pressure of 50 mm Hg (V50) is used. After normalization of injected volumes according to the V50, pressure volume curves became similar. Intraarticular hydrostatic pressure and maximal voluntary isometric extensor torque were measured simultaneously, while altering the intraarticular fluid volume in 9 knee joints. In 5 of these, quantified electromyography (EMG) of the vastus medialis and lateralis portion of the quadriceps muscle was also monitored. Progressive inhibition of extensor torque and EMG was found as the intraarticular pressure volume was increased in both intact and destroyed joints. No difference in inhibition was found for the 2 portions of quadriceps muscle tested. Increased intraarticular hydrostatic pressure Pia levels between 200 and 1150 mm Hg were observed during maximal voluntary activation of extensor muscles. The reproducibility was good for all variables studied. In a few instances evidence of intraarticular compartmentalization was found at low volumes. We conclude that the V50 is a convenient expression of capsular stiffness. Furthermore, increasing Pia caused by joint effusion inhibits knee extensor muscle function and impairs synovial blood flow. Awareness of these relations will facilitate more rational therapeutic approaches in chronic arthritis.