Guide to lower extremity radiologic measurements: part 3 ankle and foot

The ankle and foot rely on a complex, coordinated interaction of numerous small bones and supporting soft tissues to produce efficient locomotion and weight bearing. Subtle osseous malalignments or insufficiency of tendons and ligaments can result in pain, instability and arthropathy. Although gross malalignment can be seen qualitatively, detecting subtle malalignment often requires standardized measurement on imaging studies with prescribed techniques. Moreover, stress imaging techniques, such as weightbearing or angular stress, are more often used in ankle/foot imaging compared with other lower extremity joints, which results in an added layer of complexity when performing measurements. In the ankle/hindfoot, quantitative assessment is most commonly used to detect and/or characterize ankle instability, calcaneal fractures and posterior tibialis tendon dysfunction. Furthermore, quantitative assessments of ankle arthroplasties may help diagnose device failure. In the midfoot, assessment of the tarsometatarsal joints can help detect subtle Lisfranc instability, while in the forefoot, quantitative characterization of hallux valgus is often used to guide surgical management. This review article is the last of a three-part series discussing measurements of the lower extremities and focuses on common measurements used in the foot and ankle, grouped by pathology, with attention to the imaging study of choice for each measurement, as well as the appropriate technique of measurement.