Dry taps and what to do about them: a pictorial essay on failed arthrocentesis of the knee.
PURPOSE: To determine and illustrate the causes of unproductive arthrocentesis of the knee. PATIENTS AND METHODS: Consecutive patients were studied who had inflammatory (rheumatoid or psoriatic) arthritis affecting the knees and experienced unproductive arthrocentesis during a randomized, controlled trial. Magnetic resonance imaging (MRI) was used, supplemented first by intravenous gadolinium contrast and subsequently by manual mixing of the diffused contrast to outline the furthest possible penetration of contrast within the joint cavity. RESULTS: In 4 out of 5 patients studied, failed arthrocentesis was due to combinations of inspirated joint fluid too viscous to be withdrawn or to mix with contrast, adipose tissue, and lipoma arborescens (thickened synovium with fat replacement). One MRI exam was normal. More free synovial fluid was imaged on the lateral side. CONCLUSIONS: Failure to aspirate synovial fluid from the knee is explicable to anatomic terms; in particular, fluid viscosity and lipoma arborescens play a role in chronic effusions. Although surface anatomic landmarks for knee arthrocentesis may be more visible medially, the lateral approach is more likely to yield fluid for synovial analysis in difficult cases. Internal medicine trainees should be taught the lateral approach.