Joint fluid pressure in chronic knee effusions.

With sterile precautions and after infiltration with local anaesthetic, a special type of needle was introduced into the lateral aspect of the knee near the upper border of the patella. This needle (S.W.G. 18, Down Bros., with a short bevel and perforations made in the sides 0 5 cm. from the point to prevent blockage) had a tap and a side-arm. A manometer filled with normal saline was attached to the side-arm and the tap was opened. The manometer level fell rapidly and the pressure equalized with the atmospheric pressure with a minimal loss of joint fluid (Fig. 1, opposite). The tap was closed and the initial readings in millimetres of saline and joint fluid were taken with the limb at rest. The pressure was then opposed by air pressure generated by a sphygmomanometer bulb communicating by a Y-tube with a sphygmomanometer. Communication between the two systems was interrupted by a sterile cotton wool plug. The manometer recorded in mm. Hg the pressure required to maintain the saline level at the zero point. Thus a null-point method of recording pressures was used. A strain-gauge method had been used initially but was discarded because it did not give sufficient sensitivity over the wide range of pressures that were recorded. Pressures were measured in the supine position, at rest, with a venous cuff applied to the thigh at 70 mm. Hg and with maximal quadriceps contraction. The patient then stood by the couch and readings were taken with the leg hanging and with the leg bearing weight; in some cases readings were taken with the leg bearing weight and maximally braced. The procedure took 45 to 60 minutes, and afterwards joint fluid was taken for culture and 40 mg. depot methyl prednisolone was introduced. No growth of organisms was recorded in the 22 joints examined.