Magnetic resonance imaging of lipoma arborescens explaining dry tap

Failed arthrocentesis due to chronic synovitis with lipoma arborescens in a patient with psoriatic arthritis. T1 weighted (left) and T2 weighted (right) axial images through the mid-pole of the patella show massive frond-like, fatty proliferation of the synovium both on the medial and lateral sides of the joint (short arrows). Although a large joint effusion is present (E), placement of a needle, particularly from the medial side (long arrow), would probably enter the hypertrophied synovium and would fail to yield fluid. Aspiration of 10 mL of low-viscosity, inflammatory fluid was achieved by the lateral approach.