Knee effusion: normal distribution of fluid.

Although visualization of articular fluid on MR images of the knee is common, no specific MR criteria that enable assessment of the quantity of the effusion have been established. We performed MR of three cadaveric knee specimens after the instillation of increasingly large volumes of fluid and studied the distribution of the fluid. When 4 ml of fluid was injected, the anteroposterior diameter of the suprapatellar recess was 4 mm on midline sagittal MR images and 10.0-12.5 mm on lateral sagittal MR images, corresponding to the usual routine radiographic criteria for a knee effusion.