Abstract and Introduction
Weight-bearing CT (WBCT) scans of the foot and ankle have improved the understanding of deformities that are not easily identified on radiographs and are increasingly being used by orthopaedic surgeons for diagnostic and preoperative planning purposes. In contrast to standard CT scans, WBCT scans better demonstrate the true orientation of the bones and joints during loading. They have been especially useful in investigating the alignment of complex pathologies such as adult-acquired flatfoot deformity in which patients have been found to have a more valgus subtalar joint alignment than in a normal cohort and high rates of subfibular impingement. Studies using WBCT scans have also provided new insight into more common lower extremity conditions such as hallux valgus, ankle fractures, and lateral ankle instability. WBCT scans have allowed researchers to investigate pronation of the first metatarsal in patients with hallux valgus compared with normal feet, and patients with lateral ankle instability have been found to have more heel varus than healthy control subjects. Understanding the application of WBCT scans to clinical practice is becoming more important as surgeons strive for improved outcomes in the treatment of complicated foot and ankle disorders.
Weight-bearing imaging to assess pathology has become the standard of care in foot and ankle orthopaedics.[1,2] Weight bearing better represents and reveals underlying pathologies such as malalignment, impingement, joint space narrowing, and instability that may not be fully appreciated when the foot is offloaded. Until recently, radiographs were the only readily accessible imaging modality that could be done with the patient fully bearing weight.[1,3] The advent of cone-beam CT technology rather than conventional multidetector CT configurations allowed the detector to move around the patient without the patient moving through the scanner and made the development of the weight-bearing CT (WBCT) scanner possible.
Standard weight-bearing radiographs are limited by projection, foot orientation, and difficulty in assessing the three-dimensional relationship of structures because of the bones being superimposed. In addition, measurements on radiographs require calibration. CT scans overcome these deficits by producing three-dimensional images that do not require calibration. Measurements of joint alignment and angles on cone-beam WBCT scans have been shown to be more accurate than on conventional radiographs and nonweight-bearing CT scans, and this may have clinical implications for surgeons planning deformity correction before surgery. As an example, Hirschmann et al reported notable differences when comparing measurements of impingement (eg, fibulocalcaneal distance) and joint space width (eg, lateral talocalcaneal joint space width) between weight-bearing and nonweight-bearing CT scans in 22 patients (Figure 1, A and B).
CT scan images showing differences between weight-bearing (A and C) and nonweight-bearing (B and D) CT scans. Subtalar impingement and narrowing of the subtalar joint space is more clearly seen on the weight-bearing CT scan (A) compared with the nonweight-bearing CT scan (B), which is highlighted by the arrows. In addition, collapse of the medial longitudinal arch especially at the naviculocuneiform joint is more readily apparent on the weight-bearing CT scan (C) than on the nonweight-bearing CT scan (D).
WBCT scans have several other advantages over conventional imaging modalities. Radiation exposure in cone-beam WBCT scans has been estimated to be approximately 10% to 66% less than conventional multidetector CT scanners.[1,3,5] Less ionizing radiation exposure is a notable advantage of cone-beam WBCT scans over conventional CT scans and may be advantageous for those patients who require frequent imaging or have complex deformities. One recent study also demonstrated the cost effectiveness and decreased image acquisition time of WBCT scanners over conventional radiographic and CT imaging modalities. For centers that do not have access to WBCT scans at this time, a traditional CT scanner with an axial loading device that is able to produce greater than 70% of the patient's body weight has been shown to accurately represent full weight-bearing and can be used as a substitute for newer three-dimensional standing WBCT scanners. However, such devices still do not achieve full weight-bearing and do not truthfully represent the foot in a standing ("stance") position.
These advantages of WBCT scans, however, must be weighed against their higher cost and increased ionizing radiation exposure to patients over standard weight-bearing radiographs. Weight-bearing radiographs remain sufficient to adequately diagnose and treat most foot and ankle pathologies, and many of the studies included in this review do not directly compare WBCT scans with weight-bearing radiographs. Therefore, the superiority of WBCT scans over traditional weight-bearing radiographs in many foot and ankle diagnoses has not been established.
The purpose of this review article on WBCT scans in foot and ankle orthopaedics is to summarize the current literature, describe how WBCT scans have been used for research purposes to help better understand foot and ankle pathologies, and demonstrate how this technology may be applied to clinical practice.
J Am Acad Orthop Surg. 2020;28(14):e595-e603. © 2020 American Academy of Orthopaedic Surgeons