Evaluation of physeal abnormalities of the knee with MRI.

Longitudinal skeletal growth occurs at the physis by a continuous process of chondrocyte division and hypertrophy. The physes of the distal femur and proximal tibia are the main contributors to skeletal growth, and therefore physeal abnormalities are most consequential in the knee. The normal growth structures are well depicted by conventional MR imaging, which demonstrates the integrity of the physeal cartilage, zone of provisional calcification, and primary spongiosa. Insults to the physeal cartilage including trauma, infection, chronic nonbacterial osteomyelitis, and repeated stress are also well depicted by sequences that show discontinuity of the cartilage and zone of provisional calcification. The dreaded complication of these physeal disturbances is the formation of a physeal bar, a bridge of tissue, generally bone, that joins the epiphyseal and metaphyseal bone and tethers growth. MRI shows the size of the abnormality of the physis as well as its location and depicts the abnormal growth by showing abnormal orientation of the growth recovery lines. Abnormal growth activity can be shown by diffusion tensor imaging tractography and quantified by metrics such as tract volume, apparent diffusion coefficient (ADC) and fractional anisotropy (FA).