, Francois Lintz2, Cesar de Cesar Netto3, Alexej Barg4, Arne Burssens5 and Scott Ellis6
KeywordsHindfoot alignmentWeight bearing CTComputed radiologyHindfoot correction
Exact radiographic assessment of hindfoot alignment remains a challenge [1, 2]. The various measurement techniques and hindfoot views (either inclined anteroposterior (AP) or posterioranterior (PA)) reflect the lack of a standardized and accurate methodology . All current methodologies try to overcome two main inaccuracies: the superposition caused by the osseous structures in the midfoot and the rotational errors created during the positioning of the foot, as demonstrated by several recent studies [4–6]. Weight bearing CT (WBCT) of the foot and ankle has been shown to be more accurate in hindfoot measurements . This recent imaging technique offers the advantage of a standing position as with weight bearing radiographs but overcomes the disadvantages of the osseous superposition caused by the complex anatomy of the foot and ankle [8–10]. This allows for complete visualization of the hindfoot . Additionally, WBCT software settings can rotate the foot and ankle after the imaging process to acquire a standardized positioning of the hindfoot [7, 8].
Although computed tomography was introduced to orthopedic surgery in the mid-1970s , its routine clinical and 3D use only started in the early 1990s with the introduction of the spiral CT, which allowed better insight into complex fracture patterns . Further applications were lacking, which made some authors question the added value of a 3D CT . Reluctance to adopt 3D CTs was evident in foot and ankle literature where most available measurements and reference angels were still performed in 2D [8, 9, 15]. Nevertheless, the orthopedic field’s interest in 3D printing and computer-assisted surgery (CAS) has grown in recent years [16, 17]. These tools allow for more precise preoperative planning and intraoperative surgical procedures . However, in order to successfully apply them, a better understanding of 3D technology is required. Although the application of these techniques on the skeletal system is generally well understood, their potential use on and subsequent insights from the hindfoot remain unclear; most weight-bearing research of the lower limb has been focused on hip and knee joints [19–21].
The advantage of these methods is that they incorporate each plane according to the region of interest with a high measurement accuracy .
Using WBCT, the previously described hindfoot measurements allow for correct foot positioning in the coronal, sagittal, and axial plane, but the actual angles are only obtained from one CT slice in one plane [7, 10, 22, 23].
Although interobserver reliability is high, important spatial data is not used, and the manually drawn angles and foot positioning steps impose additional measurement errors .
The aim of this paper is therefore to use computed methods to convert these conventional 2D measurements to a 3D environment. This analytic process will be assessed by rater reliability and regression analysis.
Materials and Methods
Study Population, Design, and Measurement Protocol
Total (N = 48)
Age (±) SD
39.6 ± 13.2 years
MTP I fusion
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.
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.
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) .
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.
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).