The term is usually reserved for a focal or isolated crater caused by an injury, where the joint surface and a bit of underlying bone has been knocked off. The person lands with force onto the bent knee, or there is a direct blow to the knee, knocking off a piece of articular cartilage.The chondral defect is the crater. The loose body is the bit that breaks off.
Cartilage defect location and stiffness predispose the tibiofemoral joint to aberrant loading conditions during stance phase of gait. Zevenbergen L, Smith CR, Van Rossom S, Thelen DG, Famaey N, Vander Sloten J and Jonkers I. PLoS One. 2018; 13(10): e0205842.