Inclusion and Exclusion Criteria: Ethics

The inclusion criteria were age ≥18 years, presentation at the local foot and ankle outpatient clinic, and indication for WBCT. The indication for WBCT was defined following the local standard [9]. For example, no indication for 3D imaging with WBCT was given for isolated forefoot deformities, whereas indication was given for deformities in the midfoot and/or hindfoot region.

The exclusion criteria were age <18 years, no indication for WBCT imaging and participation in other studies.

Approval from the local ethical committee was granted based on the indications as described above. Informed consent was obtained from all subjects.

Image Acquisition

The patient walked into the device and was positioned in bipedal standing position as shown in Fig. 4.1a. Technically, an X-ray emitter and a flat panel sensor on the opposite side are rotating horizontally around the feet. Resolution and contrast which are the principal parameters for image quality are comparable with modern conventional CT [9]. The scanning time was 68 seconds.

Pedography

The data of the pedography sensor (Fig. 4.1b) was gathered for the first 30 seconds of the WBCT scan.

Measurements of Bone Position (Angles and Distances)

The bone positions (angles and distances) were digitally measured with standard WBCT software (Cubevue, CurveBeam, Warrington, USA).

The following angles and distances were measured for the right foot by three different investigators three times: lateral talo-1st metatarsal angle (TMT), calcaneal pitch angle, minimum height of 5th metatarsal base, and 2nd–5th metatarsal heads and medial sesamoid. The medial sesamoid was chosen instead of the first metatarsal head because it is regularly closer to the foot sole/ground. The medial sesamoid was chosen instead of the lateral sesamoid because it is less likely to completely dislocate from underneath the 1st metatarsal head in forefoot deformities such as hallux valgus [19, 20].

The lateral TMT angle was defined as the angle created between the axis of the first metatarsal and the talus (Fig. 4.3a) [9, 21]. The plane for the measurement was virtually rotated within the 3D dataset to achieve an exact congruency to the bone axis of talus and first metatarsal.

The calcaneal pitch angle was defined as the angle created between a straight line, a line between the lowest part of the posterior calcaneal process and the lowest part of the anterior calcaneal process (Fig. 4.3b) [9]. The plane for the measurement was virtually rotated within the 3D dataset to achieve an exact congruency to a parasagittal plane.

Bone axes (talus, first metatarsal) were defined as the straight line between the centers of the bones proximally and distally. These bone centers were defined by linear measurements (Fig. 4.3a). The TMT angles were defined to be negative for angle corresponding to a dorsiflexion [21].

The minimum height of 5th metatarsal base, 2nd–5th metatarsal heads, and medial sesamoid was defined as the minimum distance between the footplate and the 5th metatarsal base (Fig. 4.3c), medial sesamoid (Fig. 4.3d), and 2nd–5th metatarsal heads and (Fig. 4.3e). The plane for the measurement was virtually shifted within the 3D dataset to display the lowest part of the relevant bone.

Measurement of Pedographic Parameters

A standard computerized mapping to create a distribution into the following foot regions was performed with the standard software (Automask, Novel, Munich, Germany): hindfoot, midfoot, 1st metatarsal head/sesamoids area, 2nd metatarsal head, 3rd metatarsal head, 4th metatarsal head, 5th metatarsal head, 1st toe, 2nd toe, and 3rd–5th toe (Fig. 4.4) [22]. This mapping process does not include manual determination of landmarks [22]. The outlines of the foot and the different regions are determined by the software using an algorithm as reported [17]. This software algorithm is based on geometric characteristics of a maximum pressure picture using an individual sensing threshold [22]. The following parameters were registered within the defined foot regions: midfoot contact area, maximum force midfoot, maximum force midfoot lateral, maximum force entire foot, and maximum pressure 1st to 5th metatarsal head area. The parameter maximum force midfoot was defined as the maximum force in the entire midfoot region (Fig. 4.4). The parameter maximum force midfoot lateral was defined as maximum force in the lateral sensor row of the midfoot region (Fig. 4.4).

Correlation Analysis of WBCT Parameters with Pedography Parameters

Lateral TMT, calcaneal pitch angle, and minimum height of fifth metatarsal base were each correlated with midfoot contact area, maximum force midfoot, maximum force midfoot lateral, and maximum force entire foot (Fig. 4.5). The minimum height of 2nd–5th metatarsal heads and medial sesamoid was correlated with the maximum pressure of the corresponding 1st to 5th metatarsal head area.

Statistics

The statistical analysis was performed in cooperation with the Institute for Biometry and Statistics of the affiliated university with IBM® SPSS® Statistics (Version 22.0.0.0, IBM, Armonk, NY, USA). The WBCT parameters were compared for intra- and interobserver (ANOVA with post hoc Scheffe test). The correlation of the WBCT parameters with the pedography parameters was performed with Pearson test. Significant correlation was considered as p < 0.05. Sufficient correlation was considered as r > 0.8 or <−0.8.