Guide to lower extremity radiologic measurements: part 2 knee.

Although most imaging assessments are made qualitatively, quantitative measurements in orthopedic imaging are becoming more important in detecting subtle findings and assessing degrees of abnormality, which can help direct surgical management. In the knee, patellofemoral maltracking is a common cause of anterior pain, particularly in younger patients. Failure to recognize this pathophysiology may result in accelerated chondral loss and osteoarthritis (OA), and the imaging findings may be subtle. As a result, clinicians and radiologists have developed numerous measurements to detect and quantify patellofemoral alignment. Additional entities, such as femorotibial subluxation or angular abnormalities, may only be detected by using standardized measurements and can help detect ligamentous insufficiency or developmental malalignment, which can lead to instability and OA in the medial and lateral femorotibial compartments. Finally, the proper positioning and hardware selection for knee arthroplasty are critical to preventing early hardware failure and postsurgical pain. This review, which focuses on the knee, is the second in a three-part series discussing the appropriate imaging modalities on which to obtain specific lower extremity measurements as well as proper measurement techniques, grouped by pathology. Furthermore, the normal value or range of values according to current literature is reported, along with the significance of abnormal measurements.