Study Design

In this prospective, dual-institution study, we recruited consecutive patients in our tertiary hospital clinics from September 2014 through June 2016. We included patients aged 18 years or older with a diagnosis of symptomatic, flexible AAFD. We excluded patients who were unable to stand independently for at least 40 seconds, those incapable of communicating effectively with clinical study personnel, and those with contraindications for standard CT scans.

Subjects

Nineteen patients (13 right feet, 6 left feet) were included in our study. The study group consisted of 11 men and 8 women, with a mean body mass index of 31 (range, 19–46) and a mean age of 52 (range, 20–88) years.

WBCT Imaging Technique

Imaging studies were conducted on a cone beam WBCT extremity scanner (generation II, Carestream Health Inc., Rochester, NY). Participants were scanned in a physiological upright weight bearing position, standing with their feet at shoulder width and distributing their body weight equally between both lower extremities. We applied a protocol similar to that used in previous technical assessments [16–18]. The contrast-to-noise ratio per unit of dose within the boundaries of the CT was enhanced by 90 kVp and 72 mA (6 mA and 20 msec for each frame, 600 frame acquisition). The size-specific dose estimate for WBCT ankle imaging was calculated to be approximately12 mGy.

A Farmer chamber in a stack of three 16 cm CT dose index phantoms was used to calculate the weighted CT dose index and was found to be approximately 15 mGy [12]. Images of 0.5 mm³ isotropic voxels were reconstructed using a bone algorithm.

Measurements

The raw 3D data were used to generate axial, sagittal, and coronal image slices that were transferred digitally into Vue PACS software (Carestream Health, Inc., Rochester, NY) for computer-based measurements. Image annotations were eliminated, and a unique, random number was assigned to each study. The investigators consisted of a board-certified foot and ankle surgeon, an orthopedic surgery resident, and a medical student. Each investigator completed a training protocol with five AAFD patients who were not included in the study. After the protocol, each observer performed the measurements twice, independently and blindly, using a dedicated software. The training protocol included a standardized assessment of the full dataset of images; however, the final choice of which image to use to perform each measurement was made freely and independently by each observer. The second set of measurements was performed 30 days after the first assessment. Investigators were blinded to the patient’s identification and other measurements, and the order of the patient images was randomized.

Axial Plane Measurements

The axial plane was defined as parallel to the horizontal plane, represented by the platform where the patient was standing, with the horizontal boundary of the images aligned to the axis of the first metatarsal bone. Two axial measurements were defined: the talonavicular coverage angle (Fig. 17.1a) [17, 19] and the talus-first metatarsal angle (Fig. 17.1b) [20].

Coronal Plane Measurements

The coronal plane was defined as perpendicular to the horizontal plane, with the horizontal margins of the images perpendicularly aligned to the bimalleolar ankle axis. Nine coronal measurements were defined. The first three measurements involved the subtalar horizontal angle, which comprises the angle formed by the intersection of the horizontal line of the floor and the tangent line to the posterior facet of the talus. The angle was measured at three levels: 75%, anterior aspect (Fig. 17.1c); 50%, midpoint (Fig. 17.1d); and 25%, posterior aspect (Fig. 17.1e) of the posterior subtalar joint length. Positive values signified valgus alignment of the subtalar joint. The fourth measurement was the calcaneal-fibular distance, which was obtained by measuring the shortest distance between the superior or lateral surface of the calcaneus and the distal part of the fibula (Fig. 17.1f). The fifth measurement was the forefoot arch angle (Fig. 17.1g) [21]. A positive value showed a relative lower positioning of the fifth metatarsal to the medial cuneiform. The sixth measurement was the navicular to skin distance [21]. The seventh measurement was the navicular to floor distance (Fig. 17.1h). The eighth measurement was the medial cuneiform to skin distance. The ninth measurement was the medial cuneiform to floor distance (Fig. 17.1i).

Sagittal Plane Measurements

The sagittal plane was defined as perpendicular to the axial and coronal planes. The second metatarsal axis was used to determine the horizontal border of the images. Ten sagittal measurements were assessed. The first was calcaneal inclination angle (Fig. 17.1j) [22]. The second and third measurements were the navicular to floor and navicular to skin distances. The fourth and fifth measurements were the cuboid to floor (Fig. 17.1k) and to cuboid to skin distances [23]. The sixth and seventh measurements were the medial cuneiform to floor and medial cuneiform to skin distances [24–26]. The eighth measurement was the talus-first metatarsal angle (Fig. 17.1l). The ninth measurement was the medial cuneiform-first metatarsal angle, which was formed by the intersection of the axes of the first metatarsal and medial cuneiform (Fig. 17.1m). The tenth measurement was the navicular-medial cuneiform angle, which was also created by the intersection of the axes of the navicular and medial cuneiform (Fig. 17.1n) [23].

The axis of short bones (i.e., navicular, medial cuneiform) was defined as a line connecting the midpoint of their proximal and distal articular surfaces. Because of a limitation in the field of view of the WBCT scan used in the study, the distal aspect of the first metatarsal could not be visualized, hindering the assessment of the true axis of the first metatarsal bone. An alternative standardized definition of the axis was used, represented by a line connecting the midpoint of the proximal articular surface and the midpoint of the width of the proximal third of the first metatarsal shaft.

Statistical Analysis

We used the Shapiro-Wilk test to assess normality of the data distribution for each measurement. The intraobserver reliability of each measurement was determined using the Pearson or Spearman correlation test, depending on the normality of the data. Intraclass correlation coefficients (ICCs) were used to assess interobserver reliability. The extent to which bias and interaction factors can decrease the ICC was also considered. Correlations were categorized as excellent, >0.74; good, 0.60–0.74; fair, 0.40–0.59; and poor, <0.40 [23, 27]. We also compared the means of each measurement among the three readers using one-way ANOVA when the data distribution was normal. For non-normally distributed data, we used Kruskal-Wallis analysis. P values of less than 0.05 were considered significant.

Intraobserver Reliability

Intraobserver reliability for each of the three readers is listed in Table 17.1. All measurements showed significant intraobserver reliability (P < 0.05). Averaged values showed excellent intraobserver reliability for the board-certified foot and ankle surgeon (r = 0.87), orthopedic resident (r = 0.86), and medical student (r = 0.81).

Table 17.1

Intraobserver reliability of measurements of adult-acquired flatfoot deformity in 19 patients using weight bearing computed tomography

Measurement by view	Board-certified foot and ankle surgeon		Orthopedic surgery resident		Medical student
	Pearson/Spearman r	P	Pearson/Spearman r	P	Pearson/Spearman r	P
Axial view
Talonavicular coverage angle	0.72	0.003	0.70	0.005	0.55	0.023
Talus-first metatarsal angle	0.65	0.005	0.63	0.008	0.43	0.031
Coronal view
Subtalar horizontal angle, 25%	0.91	<0.001	0.91	<0.001	0.87	<0.001
Subtalar horizontal angle, 50%	0.93	<0.001	0.88	<0.001	0.87	<0.001
Subtalar horizontal angle, 75%	0.88	<0.001	0.85	<0.001	0.85	<0.001
Forefoot arch angle	0.99	<0.001	0.97	<0.001	0.94	<0.001
Navicular to skin distance	0.99	<0.001	0.97	<0.001	0.96	<0.001
Navicular to floor distance	0.98	<0.001	0.99	<0.001	0.96	<0.001
Calcaneal-fibular distance	0.92	<0.001	0.93	<0.001	0.88	<0.001
Medial cuneiform to skin distance	0.99	<0.001	0.98	<0.001	0.96	<0.001
Medial cuneiform to floor distance	0.99	<0.001	0.98	<0.001	0.98	<0.001
Sagittal view
Calcaneal inclination angle	0.95	<0.001	0.96	<0.001	0.85	<0.001
Navicular to floor distance	0.94	<0.001	0.96	<0.001	0.92	<0.001
Navicular to skin distance	0.92	<0.001	0.91	<0.001	0.88	<0.001
Cuboid to floor distance	0.96	<0.001	0.96	<0.001	0.90	<0.001
Cuboid to skin distance	0.95	<0.001	0.94	<0.001	0.90	<0.001
Medial cuneiform to floor distance	0.96	<0.001	0.95	<0.001	0.91	<0.001
Medial cuneiform to skin distance	0.98	<0.001	0.95	<0.001	0.90	<0.001
Talus-first metatarsal angle	0.73	0.004	0.74	0.003	0.70	0.004
Medial cuneiform-first metatarsal angle	0.41	0.032	0.33	0.040	0.33	0.034
Navicular-medial cuneiform angle	0.55	0.025	0.58	0.020	0.49	0.028
Averaged value	0.87		0.86		0.81

Correlations were categorized as perfect agreement, 0.81–1.0; substantial, 0.61–0.80; moderate, 0.41–0.60; fair, 0.21–0.40; slight, 0.10–0.20; and poor, less than 0.10

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