Study Design

This dual-center IRB-approved prospective study complied with the Health Insurance Portability and Accountability Act (HIPAA) and the Declaration of Helsinki. Informed consent was signed by all study participants.

Subjects

In the two involved tertiary hospital clinics, consecutive patients with clinical diagnosis of symptomatic flexible AAFD from October 2014 till June 2016 were recruited. We excluded patients who were younger than 18 years old, were not able to communicate efficiently with clinical study personnel, or were not able to stand still independently for at least 40 seconds. Individuals with the inability to bear weight and a rigid deformity or those who had standard contraindications for standard CT scans including pregnancy or those with serious medical or psychiatric issues were also not enrolled in this prospective study.

Coronal plane clinical hindfoot alignment of all study participants was measured in the physiologic WB position by the most experienced of the senior authors (LCS). Patients were instructed to stand in a comfortable and natural stance position, and the medial border of each foot was positioned over two parallel lines that were drawn into the floor, controlling for rotational misalignment. The measurement obtained here, named clinical hindfoot alignment angle (CHAA) (Fig. 16.1a), was similar to the standing tibiocalcaneal angle (STCA) and was based on clinical expertise of the senior author and evaluation of anatomical surface landmarks [8, 23].

Following clinical examination, all patients underwent WB CBCT examinations.

CBCT Imaging Technique

All CT studies were conducted using a CBCT extremity scanner (Generation II, Carestream Health Inc., Rochester, NY). All imaging studies were performed under physiological upright WB position with the patients standing with their feet almost at shoulder width, distributing body weight evenly between their both legs. We employed the same scan protocol that was described in prior studies [22, 24, 25].

WB CBCT Measurements

To develop computer-based measurements, the raw 3D data were used to generate and create axial, coronal, and sagittal image slices and were digitally transferred into our dedicated software (Vue PACS, Carestream Health, Inc., Rochester, NY). Image annotations were deleted, and a unique random number was allocated to each imaging study.

Following a mentored training protocol entailing five AAFD cases, three fellowship trained foot and ankle surgeons applied different hindfoot alignment measurements independently using the dedicated software. All observers were blinded, and the order of images was randomized. One month following the first assessment, a second set of measurements for intraobserver reliability was performed by one of the observers. The measurements were performed on the clinical reconstructed 3D images. Rotational position control was assured with the use of images where the medial aspect of the heel and the most medial aspect of forefoot and the medial eminence of the first metatarsal were in line with each other. The images used were also tangential to the floor.

The first measurement performed aimed to mimic the clinical hindfoot alignment evaluation and was named WBCT clinical hindfoot alignment angle (WBCT CHAA). It was obtained on 3D images where the windowing was set to maintain the surface anatomical landmarks, including the skin (Fig. 16.1b).

The second measurement was performed in an image with the same positioning, but the windowing was changed, removing the skin and subcutaneous, but keeping some of the overlying soft tissue structures including the Achilles tendon. That image also allowed a better evaluation of the calcaneal tuberosity, used as a bony anatomical landmark. The angle measured was formed by the longitudinal axis of the Achilles tendon and the longitudinal axis of the calcaneal tuberosity and was named Achilles tendon/calcaneal tuberosity angle (ATCTA) (Fig. 16.1c). To adequately assess the alignment of the calcaneal tuberosity, we used similar technique drawing ellipses as described by Johnson et al. [13].

The last four measurements were performed in images with the same positioning, however with different windowing that removed all the soft tissue structures, maintaining only the bony anatomy. The third measurement obtained was the tibial axis/calcaneal tuberosity angle (TACTA), which was formed by the intersection of axes of calcaneal tuberosity and the tibia (Fig. 16.1d). The fourth measurement was the tibial axis/subtalar joint angle (TASJA) formed by the intersection of tibial axis and the line connecting midpoint of the posterior facet of the subtalar joint and most inferior point of the calcaneal tuberosity (Fig. 16.1e). The fifth measurement was the hindfoot alignment angle (HAA), as described by Williamson et al. [26] (Fig. 16.1f), and the sixth measurement was the hindfoot moment arm (HMA) (Fig. 16.1g), as described by Saltzman et al. [12].

Positive values were considered valgus alignment.

Statistical Analysis

Data analysis was performed with JMP Pro version 12.2.0 (SAS Institute, Marlow-Buckinghamshire, UK). We used Shapiro-Wilk W test to evaluate the normal distribution of each set of measurements. The intraobserver and interobserver reliability was assessed by intraclass correlation coefficient (ICCs). Correlations between 0.81 and 1 were regarded almost perfect, 0.61–0.8 were considered as considered as substantial, 0.41–0.6 were considered as moderate, 0.21–0.4 were regarded as fair, 0.1–0.2 were considered as slight agreement, and less than 0.1 were regarded as poor agreement [22, 27]. Measurements obtained from clinical examination and WB CBCT images were compared by one-way ANOVA and paired Student’s t-tests or the Wilcoxon rank-sum tests and nonparametric comparison for each pair by the Wilcoxon method. We used a linear regression model to evaluate the relation between values of hindfoot moment arm and measurements obtained from clinical examinations as well as WB CBCT images. P-values of less than .05 were considered significant.