Biometrics for Hindfoot Alignment Using Weight Bearing Computed Tomography: A Prospective Assessment of 249 Feet

, Francois Lintz2, Cesar de Cesar Netto3, Alexej Barg4, Arne Burssens5 and Scott Ellis6

Department for Foot and Ankle Surgery, Hospital Rummelsberg, Schwarzenbruck, Germany

Foot and Ankle Surgery Centre, Clinique de l’Union, Toulouse, France

Department of Orthopedics and Rehab, University of Iowa, Iowa City, IA, USA

University Orthopedic Center, University of Utah, Salt Lake City, UT, USA

Department of Orthopedics and Trauma, University Hospital of Ghent, Ghent, OVL, Belgium

Department of Orthopedic Surgery, Hospital for Special Surgery, New York, NY, USA



Foot ankle offsetPedCATTALASHindfoot alignmentWeight bearing computed tomography


Hindfoot malalignment is a common cause of foot and ankle disability [1, 2]. A reliable and precise measurement of hindfoot alignment (HA) is paramount for the diagnosis and treatment of various foot and ankle pathologies, although the best way to reach a correct assessment is still a matter of debate. Traditionally, HA can be assessed from weight bearing measurements on plain radiographic views, such as the calcaneal long axial view and HA view. These methods are used worldwide for preoperative planning and postoperative evaluations [25]. However, radiography has many flaws such as perspective distortion due to lower limb rotation, bone superimposition because of two-dimensional (2D) visualization and some operator related bias, which all affect the method of measurement making it imprecise and at times unreliable [58].

The recent development of new radiographic imaging equipment such as 3D weight bearing computed tomography (WBCT) has the potential to provide more accurate anatomical information on full bipedal weight bearing scans, allowing for more precise measurements with a similar total amount of radiation [913]. Recent studies have shown how the different angles used to assess HA may differ substantially between radiographs, traditional CT, and WBCT, suggesting that 3D weight bearing measurements represent a major step forward to detect the “true alignment” [2, 14, 15]. In addition, this technology can provide a digitally reconstructed radiograph, similar to traditional radiography with the advantage of no rotation bias [9, 16]. Common measurements, such as the long axial view (termed HACT), can be taken using this reconstruction to evaluate HA.

A novel parameter used to assess HA on WBCT images is foot ankle offset (FAO) [5, 17]. Technically, this is a semiautomatic 3D biometrics and is calculated using semiautomatic software: Torque Ankle Lever Arm System or TALAS™ (CurveBeam®, Warrington, PA). This measurement considers the forefoot rather than the tibia as the reference since the forefoot interacts with the ground like the hindfoot and contributes to ankle biomechanics [5, 18]. In a previous retrospective study, Lintz et al. found that FAO had excellent inter- and intraobserver reliability and concluded that it was a precise and discriminating tool for measuring HA [5]. However, the clinical efficiency of FAO and the reference values for normal, varus, and valgus hindfoot have not yet been confirmed.

The aim of this study was to assess FAO and HACT on a large population of patients. We hypothesized that FAO would be a clinically relevant and reproducible method and that the values would be comparable to previously published data in normal, varus, and valgus cases.

Materials and Methods

Study Population and Design

Between September 2017 and April 2018, all consecutive patients undergoing a WBCT investigation in our department as part of their standard care were enrolled in this prospective comparative study.

Based on a standard physical examination, the patients were divided into three groups: clinically normally aligned hindfoot group (G1); valgus alignment group (G2); and varus alignment group (G3). This division was based on clinical examination by two experienced foot and ankle specialist orthopedic surgeons (one of the two). When the assessment of the hindfoot alignment was unclear, a goniometer was applied on the posterior aspect of the ankle and the hindfoot, measuring the standing tibiocalcaneal angle (angle between the bisectors of the calf and the calcaneus). Values between 0 and 7 degrees of valgus were considered normal. If greater than 0 degrees, the hindfoot was varus. If lower than −7, it was valgus.

Clinical indications to perform bilaterally WBCT scans were the same as with a conventional radiographic setup (obtaining comparative images).

The study was approved by the relevant Ethics Committee and was compliant with the Health Insurance Portability and Accountability Act (HIPAA) and the Declaration of Helsinki. Informed consent was signed by all participants to be included in the study.

Patient Assessment

WBCT images were obtained from scans performed using a PedCAT® unit (CurveBeam®, Warrington PA, USA) installed in the outpatients department of the Foot and Ankle Orthopaedic Surgery and Paedorthic Centre of our institution. The data sets were obtained using the following cone beam scanner settings: voxel size, 0.37 mm; field of view diameter, 350 mm; field of view height, 200 mm; exposure time, 9 seconds; and total scan time, 54 seconds. Using WBCT patient datasets, 3D coordinates (x, y, z) required for the built-in software to process and calculate FAO were manually collected for the following landmarks: first metatarsal head WB point, fifth metatarsal head WB point, calcaneus WB point, and talus centermost and highest point. To identify WB points, images were scrolled through until the smallest contact area can be observed on the axial slice closest to the ground plane. This was usually possible until the last voxel of bone. When it was not, the center of the smallest area of contact was chosen. The software processed and calculated FAO automatically, as described previously (Fig. 7.1) [5].


Fig. 7.1

PedCAT® software screen view with the Torque Ankle Lever Arm System (TALAS™) (top left). For the definition of FAO, three-dimensional coordinates (x, y, z planes) were harvested for the following landmarks: first metatarsal head WB point (Met1 or M1), fifth metatarsal head WB point (Met5 or M5), calcaneus WB point (C), and talus centermost and highest point (T). F represents the ideal position of the center of rotation of the ankle joint that lies on a bisecting line of the tripod

HACT was measured on digitally reconstructed radiographs provided by the software, selecting the sagittal view where the foot was aligned with the second ray and inclined 45 degrees (Fig. 7.2). The same method used to measure the long axial view on standard radiographs was adopted [19]. Angular measurement of FAO and HACT was performed on the same day by two experienced investigators (in two independent settings) and repeated 14 days later.


Fig. 7.2

Measurement method for the long axial view on digitally reconstructed radiographs. (a) Dorsoplantar radiograph from weight bearing CT (HACT); (b) foot aligned with the second ray and inclined towards 45 degrees, and (c) then visualized from the back to assess the hindfoot alignment. The mid-diaphyseal axis of the tibia was identified by bisecting the tibia into two mid-diaphyseal points (lines A and B) 30 mm apart (line E). The mid-diaphyseal axis of the calcaneus was defined by a line through two points in the calcaneus. At a distance of 7 mm from the most distal part of the calcaneus, a horizontal line was drawn (line D) and divided into a 40%: 60% ratio (where the length of the 40% line was measured from the lateral side). Another line (line C) was drawn horizontally, 30 mm from the posterior edge of the calcaneus. The calcaneus axis (line F) was drawn by connecting the 40% mark at line D and the bisected line C. The hindfoot alignment (G) was defined by the angle between lines E and F

Our analysis included only patients who had a bilateral WBCT, in order to minimize the risk of a nonphysiological static loading of the foot. Among these, feet which had a previous surgery with some important metal artifacts were excluded.

Statistical Analysis

Quantitative variables are reported as mean ± standard deviation (SD), range, and 95% confidence intervals [95% CIs]. The Kolmogorov-Smirnov test was performed to investigate the distribution of the normal foot group (G1). Regression analysis was performed to calculate the correlation between FAO and HACT using Spearman’s coefficient and visualization of a corresponding scatterplot.

To assess the reproducibility of FAO and HACT, inter- and intraobserver variability expressed by the intraclass correlation coefficient (ICC) was calculated. ICC values range from 0 to 1, with a higher value indicating better reliability. An ICC value >0.7 indicates good reliability and a value >0.9 excellent reliability. All analyses were carried out using SPSS 20.0 software (Standard version; IBM, Armonk, NY, USA). The significance level was set at p < 0.05.


Overall, 249 feet (126 patients) were included in the study: G1 = 115, G2 = 78, and G3 = 56 feet. Three feet from three patients were excluded from the analysis. The mean values for FAO and HACT in the three groups are reported in Table 7.1 (Fig. 7.3).
Apr 25, 2020 | Posted by in MUSCULOSKELETAL MEDICINE | Comments Off on Biometrics for Hindfoot Alignment Using Weight Bearing Computed Tomography: A Prospective Assessment of 249 Feet
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