Tensile properties of the knee-joint capsule at an elevated intraarticular pressure

In order to evaluate the capsular reaction to high intraarticular pressure (IAP) in the knee during arthroscopy, we examined 13 patients admitted for elective surgery with joint infusion to 30, 70, 120 and 170 mmHg IAP (4, 9.3, 16 and 22.7 kPa). The infusion was repeated once, at all IAP levels except 30 mmHg, after 2 min and was recorded for another 2 min. A pressure-time curve was recorded as a combined effect of viscoelastic properties of the capsule and extraarticular fluid absorption. 3 more knee joints were infused directly to 170 mmHg and a pressure-time curve was recorded for 15 min, after which time the joint was reinfused to 170 mmHg in order to estimate the change in joint volume due to absorption or relaxation. In 10 knees, the curves were uniform. At infusion, the pressure curve was almost linear above 70 mmHg. There were no signs of plastic deformation of the joint capsule at pressures below 120 mmHg, while at 170 mmHg there were signs of capsular deformation. At each pressure level, the curve revealed a rapid fall in initial pressure that gradually decreased because of capsular relaxation or fluid absorption. Repeated infusion delayed the fall in pressure, due to increased capsular stiffness. In 3 knees infused directly to 170 mmHg IAP, the slope for the first 2 min of the pressure-time curve did not differ from that found at maximal IAP in knees examined with stepwise increasing pressures. Discontinuity of the capsule, even of puncture size, influenced the pressure/volume correlation considerably. We conclude that at IAP levels of 170 mmHg, there are signs of plastic deformation of the joint capsule. In order to avoid capsular damage, knee arthroscopy should be done at intraarticular pressure levels below 120 mmHg.