1. The absorption of fluid from the joint cavity was studied by measurement of the flow of Ringer solution or paraffin oil, from an infusion reservoir, into rabbit knee joints at constant pressures between 0 and 25 cm water.

2. Although no oil was absorbed across the synovium, oil flowed continuously into the joint cavity at constant intra-articular pressure. It was concluded that the joint capsule expanded with time (delayed compliance, viscous creep). Viscous creep of the capsule could explain, in part, the poor correlation observed clinically between effusion volume and pressure.

3. The rate of absorption of Ringer solution by the synovium was calculated by subtraction of the volumetric rate of creep from the volume inflow of Ringer solution. The absorption rate increased as a linear function of intra-articular pressure up to 9 cm water but as a much steeper (six-times) function of pressure above 9 cm water (the ‘breaking point’ phenomenon). Since pressure increases upon joint flexion, flexion may minimize synovial fluid volume by promoting fluid absorption.

4. The absorption rate was unaffected by ligation of the lymphatic drainage of the joint, but was reduced to between 79 and 85% by intermittent interruption of blood flow to the joint. Fluid accumulated in connective tissue outside the synovium.

5. It is concluded that these artificial effusions are absorbed partly into the synovial micro-circulation, in accordance with Starling's hypothesis of fluid exchange, and partly into compliant connective tissue outside the joint capsule. The increased sensitivity of flow to pressure above 9 cm water is explained by a progressive reduction in synovial resistance to flow, and implies facilitation of absorption of joint effusions of pressures over 9 cm water.

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