Introduction

Hindfoot alignment has classically been determined using a long axial or hindfoot alignment view [1]. Studies using these radiographic methods in normal asymptomatic feet report values between 2° and 5° of valgus in the general population [2]. Clinical measurements of the hindfoot are situated between 5.61° and 6.50° of valgus [3]. These findings give the impression of a physiological valgus alignment of the hindfoot. However, results are based on small cohorts [2, 4], lack a clear correlation between clinical/radiographical data [5], and impose important measurement errors due to bony superposition present in plane weight bearing radiographs [6]. The latter is currently overcome by the use of weight bearing CT which provides an accurate bone position and allows a natural stance of the patient [7]. Various methods now have been described to determine hindfoot alignment using weight-bearing CT [7, 8]. This study will use a method composed out of the anatomical axis of the tibia and the talocalcaneal axis based on the inferior point of the calcaneus as described previously [9]. To investigate not only the radiological relevance of this point but also a possible biomechanical role, a density analysis will be performed. An increased ossification around the inferior point would indicate a higher load application as stated by Wolff’s law [10]. Currently the measurement method was only used in malalignments of the hindfoot and lacks reference values. Therefore, the goal of this study is to obtain measurements from a population with clinical and radiological absence of hindfoot pathology. These will be compared to hindfoot measurements obtained from the long axial view based on the anatomical axis of the tibia and the calcaneal axis, to point out possible differences attributed to the measurement method [1]. Although surgical hindfoot corrections are frequently performed either extra-articular by osteotomies or intra-articular by arthrodesis, still numerous debate exists on the amount of correction and the ideal foot position after arthrodesis [11, 12]. Per-operative tools are already used to obtain a more accurate correction [13] or a physiological load distribution [14], but a preoperative planning remains paramount. This study will contribute to the preoperative planning by providing further insights into a physiological hindfoot alignment. The null hypothesis is the existence of an overall physiological valgus alignment in the hindfoot.

Materials and Methods

Patient Characteristics

Each time the contralateral not affected foot was used for analysis. This was performed using CurveBeam® software applied on the images retrieved from the weight bearing CT (pedCAT®). Ethical committee gave permission in performing the study (OG10601102015). Following imaging protocol was used: radiation source was set at 4 mAs and 50 kV, with a focus distance of 100 cm, with 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 attain a natural stance with both feet parallel to each other and straight ahead at shoulder width.

Hindfoot measurements were performed by two authors AB and EDV. Each measurement was repeated three times; after the complete set of measurements, the mean out of three measurements was used for further analysis. The hindfoot angle was determined based on the inferior point of the calcaneus (HA_IC) as described previously [9]. In brief the foot is positioned according to the second ray, and the angle is composed out of the intersection between the anatomical tibial axis (TA_x) and the talocalcaneal axis (TCA_x) (Figs. 9.1a and 9.2a). The latter is formed by connecting the inferior point of the calcaneus with the middle of the upper surface of the talus (Fig. 9.3a, b). This will be compared to the hindfoot angle measured on the long axial view (HA_LA), for which firstly the foot needed to be aligned with second ray and inclined 45° by applying the reconstruction mode built in to the used software (Figs. 9.1b and 9.2b, c). Secondly the calcaneus needed to be divided 50–50% in the upper part and 40–60% in the lower part to determine the calcaneal axis (CA_x) as described by van Dijk et al. [1] (Fig. 9.3c). The HA_LA was composed out of the intersection between the TA_x and the CA_x on the inclined foot (Fig. 9.3d).

To investigate the relevance of the inferior calcaneus point, a bone density analysis was performed by calculating the pixel density of this region of interest (ROI) in the coronal plane and comparing it to a regional area with the same surface window by using an OsiriX®-based plug in software (Fig. 9.4a, b). A higher pixel density would concur with an increased calcium/bone density, and by applying Wolff’s law, this would indicate a higher load exposure [10]. To avoid the influence of traction exerted by the fascia plantaris on the bone formation in this inferior calcaneal region, the ROI was set at a distance of 5 mm from the medial calcaneal tuberosity in the sagittal plane (Fig. 9.4c, d).

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