Study Population, Design, and Measurement Protocol

The contralateral unaffected foot was used for each analysis. The measurements were performed on the images retrieved from the weight bearing pedCAT® cone beam CT, using the incorporated Cubevue® software for the 2D analysis (CurveBeam, Warrington, PA, USA). The 3D analysis was obtained after segmentation of the images using Mimics® 19.0 and analysis using 3-matic® software (Materialise, Leuven, Belgium). The patient records were anonymized and deidentified prior to processing in accordance with the standard data release procedures of the hospital involved in the study. All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. The Institutional Review Board of AZ Monica approved this study (OG10601102015), and formal consent was not required for this type of study. The following imaging protocol was used: radiation source was set at 4 mAs and 50 kV, with a focus distance of 100 cm, and the beam pointed at the ankle joint. PedCAT used the following settings: tube voltage, 96 kV; tube current, 7.5 mAs; CTDIvol 4.3 mGy; matrix, 160,160,130; pixel size, 0.4 mm; and slice interval, 0.4 mm.

At the department of radiology, patients were asked to stand naturally with both feet parallel to each other, shoulder width apart. Hindfoot measurements were performed in 2D by authors AB and MP. Each measurement was repeated three times. After the set of measurements was complete, the average of these three measurements was used for further analysis. A similar test/retest methodology was performed in other studies concerning hindfoot measurements [3, 5, 8, 25–27]. The hindfoot angle was determined based on the inferior point of the calcaneus (HA2D), as described previously [8]. In brief, the foot was first positioned in line with the collinear axis of the shaft of the second metatarsal, which is considered as the longitudinal axis of the foot in the axial plane (Fig. 10.1). The hindfoot angle was defined as the intersection between the anatomical tibia axis (TA2D) and the talocalcaneal axis (TC2D), which connects the inferior point of the calcaneus and the middle of the upper surface of the talus in the coronal plane (Fig. 10.1c, d).

The varus and valgus alignment of the hindfoot was respectively defined as when the TCA runs medial from the vertical axis and when the TCA runs lateral from the vertical axis, which is often considered as a reference axis [28, 29]. Authors RM and TL determined the 3D hindfoot angle (HA3D) by the use of computer-aided design (CAD) operations (Fig. 10.2a–d).

The anatomical tibia axis (TA3D) was calculated by a best fit centroidal axis along the diaphysis marked above the incisura fibularis (Fig. 10.2a). The talocalcaneal axis (TC3D) was computed by connecting the inferior calcaneus point (ICP) with central talus point (CTP). The ICP was obtained after the calculation of an extrema analysis of the calcaneus (function to determine the most outer point of a structure in the direction of a given axis) (Fig. 10.2b). The CTP was determined by the calculated centroid of the talus (mean position of all the points in a given structure) (Fig. 10.2c). The computed intersection of both the TA3D and the TC3D became the HA3D (Fig. 10.2d). The TA and the TC were measured separately in the hindfoot angle when comparing the 2D and the 3D angles in order to emphasize possible inconsistencies attributable to either the tibial or talocalcaneal component.

The talocrural angle (TCr) was used as a radiographic parameter to assess the ankle in the coronal plane [30]. TCr was measured in 2D (TCr2D) as the angle between the intersection of the intermalleolar axis (obtained after connecting the interior point of the medial with the most inferior point of the lateral malleolus) and the horizontal axis of tibial joint line (Fig 10.3a).

In 3D (TCr3D), this measurement is performed in the same manner by the intersecting angle of the intermalleolar axis (the most inferior points of the malleoli were computed using an extrema analysis) and the computed best fitted axis through the horizontal contour of the tibial joint line (Fig. 10.3b).

Characteristics in the tibiotalar joint were measured as the inclination of the tibial joint surface towards the vertical axis perpendicular to the floor (TI2D) and the tilt of the talus towards the vertical axis perpendicular to the floor (TT2D) as described previously (Fig. 10.3c) [8].

The TI3D and TT3D were similarly analyzed by reconstructing the joint surface respective of the tibia and the talus in the coronal plane. This reconstruction allows for the computation of the horizontal axis of both surfaces and the intersection with the vertical axis perpendicular to the floor resulted in the TI3D and TT3D.

In the subtalar joint (STJ), the middle subtalar vertical angle (SVA2D) was determined in the coronal plane according to the method described by Colin et al. [9]. This measurement required the length of the posterior facet of the STJ to be measured in the sagittal plane. In the midpoint of this distance, the inclination of the STJ surface towards the vertical axis perpendicular to the floor in the coronal plane was determined (Fig. 10.3e). The SVA3D was analyzed similarly to the SVA2D with the same methods as applied in the TI3D and TT3D, which is generalized in Fig. 10.3f and detailed in Fig. 10.4.

By applying goniometric functions built into the software, commonly used measurements from a 2D radiograph can be translated into a 3D angle and its subsequent projection. The general sequence is depicted and explained for the talocrural angle as an example (Fig. 10.4a–d).

The coronal plane in these methods was derived from the Cartesian coordinate system with the inferior calcaneus point as the origin. The z-axis was defined as running through the origin perpendicular to the ground floor. The x-axis runs through the origin perpendicular to the z-axis and lies in the sagittal plane, formed through the center of the second metatarsal head and the origin perpendicular to the ground floor. The y-axis goes through the origin, perpendicular to the x-axis and z-axis (Fig. 10.5e–f).