Study Design

We prospectively recruited consecutive patients in our tertiary hospital clinic from September 2014 through June 2016. We used the following inclusion criteria: clinical diagnosis of symptomatic flexible AAFD; age 18 years or older; ability to communicate effectively with clinical study personnel; ability to stand or sit still, unassisted, for at least 40 seconds; and availability of comparative imaging study (CT scan, magnetic resonance imaging scan, or radiograph) performed for a clinical purpose within the past 3 months. We excluded pregnant patients and those with major medical or psychiatric illness that could prevent completion of the procedure. Screened, enrolled, and included patients are presented in a CONSORT diagram (Fig. 15.1).

Subjects

Twenty patients (14 right feet, 6 left feet) were included in the study. The cohort consisted of 12 men and 8 women, with a mean age of 52 years (range, 20–88) and a mean body mass index value of 30 (range, 19–46).

CBCT Imaging Technique

All CT studies were performed on a CBCT extremity scanner (generation II, Carestream Health Inc., Rochester, NY) [27]. Participants underwent two consecutive scans of the symptomatic foot: one NWB scan (sitting position with knee extended, ankle joint in neutral position, and foot placed plantigrade over a foam surface in the CBCT gantry) and one WB scan (physiological upright WB position). For the WB position, the scan was performed with the participant standing with feet approximately at shoulder width and distributing body weight evenly between both legs. The nominal scan protocol was based on previous technical assessment [26, 27]. We applied 90 kVp and 72 mAs (6 mA and 20 msec for each frame, 600 frame acquisition) for all scans to optimize contrast-to-noise ratio per unit of dose within the limits of our CT system power. Conversion factors for size-specific dose estimates were 1.4 for the foot and ankle (8 cm diameter); therefore, the size-specific dose estimate for CBCT ankle imaging was estimated to be 12 mGy.

The weighted CT dose index for CBCT ankle acquisition has been estimated to be 15 mGy using a Farmer chamber in a stack of three 16 cm CT dose index phantoms [27]. The CBCT image data were reconstructed using a “bone” algorithm using iterative reconstruction with scatter correction to images with 0.5 × 0.5 × 0.5 mm³ isotropic voxels.

Measurements

The raw 3D data were converted to sagittal, coronal, and axial image slices that were transferred digitally into dedicated software (Vue PACS; Carestream Health, Inc.; Rochester, NY) for computer-based measurements. Image annotations were removed, and each study was assigned a unique and random number. After completion of a mentored training protocol with five AAFD cases in which the observers learned how to use the software and how to perform the measurements, two fellowship-trained foot and ankle surgeons and one fellowship-trained radiologist performed the measurements in an independent, random, and blinded fashion. One of the observers (surgeon) performed a second set of measurements for intraobserver reliability assessment 1 month after the first assessment was completed (to reduce memory bias). WB and NWB images were compared with respect to averaged values of measurements analogous to AAFD radiographic parameters [17, 21] on the axial, sagittal, and coronal views.

Axial Plane Measurements

The axial plane was defined as parallel to the horizontal platform, with the horizontal edge of the images aligned to the axis of the first metatarsal. Two axial parameters were assessed (Fig. 15.2). The first was the talus-first metatarsal angle, which is formed by the intersection of two lines representing the axis of the first metatarsal and the axis of the talus [17]. Values were considered positive when the angle had a medial vertex, indicating relative increased forefoot abduction. The second was the talonavicular coverage angle, as described by Sangeorzan et al. [28].

Coronal Plane Measurements

The coronal plane was defined as perpendicular to the horizontal platform, with the horizontal edge of the images aligned to a line perpendicular to the ankle bimalleolar axis. Nine coronal parameters were assessed (Figs. 15.3 and 15.4). The first was the forefoot arch angle, as described by Ferri et al. [22]. Positive values represented a relative higher positioning of the medial cuneiform to the fifth metatarsal. The second was the navicular to skin distance, also as described by Ferri et al. [22]. The third was the navicular to floor distance, measured from the most inferior aspect of the navicular to the floor line. The fourth was the medial cuneiform to skin distance, measured from the most inferior aspect of the medial cuneiform to the plantar skin. The fifth was the medial cuneiform to floor distance, measured from the most inferior aspect of the medial cuneiform to the floor line. The sixth was the calcaneofibular distance, which was the shortest distance between the distal fibula and the lateral/superior surface of the calcaneus [20, 21, 23]. The seventh was the subtalar horizontal angle, which was the angle between the posterior facet of the talus and the floor (horizontal line) measured at 25% (posterior aspect), 50% (midpoint), and 75% (anterior aspect) of the posterior subtalar joint length [19, 25, 29, 30]. Positive values represented valgus alignment of the joint.

Sagittal Plane Measurements

The sagittal plane was defined as perpendicular to the horizontal platform with the horizontal edge of the images aligned to the axis of the second metatarsal. Eight sagittal parameters were evaluated (Fig. 15.5). The first was the talus-first metatarsal angle, which was formed by the intersection of the axis of the first metatarsal and the axis of the talus [17]. Values were considered positive when the angle had a plantar vertex. The second was the navicular to skin distance, measured from the most inferior aspect of the navicular to the plantar skin surface. The third was the navicular to floor distance, measured from the most inferior aspect of the navicular to the floor line. The fourth was the cuboid to skin distance, measured from the most inferior aspect of the cuboid to the plantar skin surface. The fifth was the cuboid to floor distance, measured from the most inferior aspect of the cuboid to the floor line [21]. The sixth was the medial cuneiform to skin distance, measured from the most inferior aspect of the medial cuneiform to the plantar skin surface. The seventh was the medial cuneiform to floor distance, measured from the most inferior aspect of the medial cuneiform to the floor line [31–33]. The eighth was the calcaneal inclination angle, which was the intersection of the floor line and a line connecting the most inferior point of the calcaneal tuberosity and the undersurface edge of the anterior process of the calcaneus [34].

Statistical Analysis

Data from each type of measurement were evaluated for normality by the Shapiro-Wilk test. The intraobserver reliability (one observer) was calculated using the Pearson’s or Spearman’s correlation test and 95% confidence intervals. Interobserver reliability was assessed using intraclass correlation coefficients (ICCs) and 95% confidence intervals for each measurement in each image type, considering the amount by which bias and interaction factors can reduce the ICC. Correlations of 0.81–1.00 were considered almost perfect, 0.61–0.80 were considered substantial, 0.41–0.60 were considered moderate, 0.21–0.40 were considered fair, and <0.20 were considered slight agreement [35, 36]. Measurements performed on WB and NWB images were compared using paired Student’s t-tests or Wilcoxon rank-sum tests. P < 0.05 was considered significant.

Source of Funding

This work was based on an industrial grant from Carestream, Inc., which provide monetary incentive to subjects who undergo CBCT examinations. The decision to recruit the proper subjects who meet the criteria was based on clinical presentation and decided by the orthopedic surgeon.