of Weight Bearing CT (WBCT) with Pedography Shows Relationship Between Anatomy-Based Foot Center (FC) and Force/Pressure-Based Center of Gravity (COG)

, 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



Weight bearing CTPedCATPedographyCentre of gravity (COG)Foot center (FC)


Weight bearing CT (WBCT) is a technology that allows 3D imaging with full weight bearing which is not influenced by projection and/or foot orientation [1, 2]. In the first published study, specific bone position (angle) measurements using WBCT were compared with conventional weight bearing radiographs and conventional non weight bearing CT [2]. The angles differed between radiographs, CT, and WBCT, indicating that only WBCT is able to detect the correct angles, i.e., bone position [2]. In a subsequent study, the correlation between 3D bone position and pedographic measurements, i.e., force and pressure (distribution), has been investigated [3]. In that study, 3D bone position did not correlate with force and pressure distribution under the foot sole during simultaneous WBCT scan and pedography [3]. Consequently, the bone positions measured with WBCT did not allow conclusions about the force and pressure distribution in this static configuration [3]. Vice versa, pedography parameters did not allow conclusions about the 3D bone position [3]. One conclusion was that further investigations with higher case number and more other parameters should be carried out to further validate these surprising findings [3]. Meanwhile, center of gravity (COG) and foot center (FC) were discussed to be important parameters for biomechanical assessment around foot and ankle and consequently as basis for diagnostics and planning of corrective surgeries and/or joint replacement [4, 5]. In particular, a semiautomatic system (TALAS, CurveBeam, Warrington PA, USA) designed to measure hindfoot alignment as a 3D biometric uses the anterior midline of the forefoot (which joins the FC with the midpoint between the first and the fifth metatarsal heads) as a landmark for hindfoot alignment [4]. The aim of this study was to analyze the difference between morphology- and anatomy (bone/WBCT)-based FC, calculated as the intersection of the median lines of the triangular-based pyramid model of the foot and force/pressure (pedography)-based COG. Motion of COG during WBCT/pedography scan should also be assessed as potential source for bias. For this study, a customized pedography sensor (Pliance, Novel, Munich, Germany) was inserted into a WBCT as described previously [3]. Our hypothesis was that the FC should be a good predictor of mediolateral position of the COG but not longitudinal since the anatomy of the hindfoot allows free anteroposterior movement but limited mediolateral.


In a prospective, comparative, and consecutive study starting November 28, 2016, 90 patients (180 feet) were included [6]. A WBCT scan with simultaneous pedography with full weight bearing in standing position was performed (Fig. 5.1). A customized pedography sensor (Pliance, Novel, Munich, Germany) was inserted into the WBCT and connected to a PC with the standard software installed (Expert, Novel, Munich, Germany) (Fig. 5.1) [3]. Demographic data and underlying foot and ankle pathologies were registered.


Fig. 5.1

WBCT with pedography sensor (arrow). 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

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 according to local practice as described previously [2]. These indications have recently evolved to include all the patients presenting at our institution except initial postoperative follow-up radiographs without weight bearing.

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: Foot Center (FC)

The patients walked into the device and were positioned in bipedal standing position (Fig. 5.1). 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 [2]. The acquisition time was 52 seconds. The morphology-based definition of the FC was performed with the WBCT data following the Torque Ankle Lever Arm System (TALAS) algorithm (Fig. 5.2a) [4]. The software takes four bony landmarks into consideration (lowest point of posterior calcaneal process, center of ankle joint, lowest or weight bearing points of metatarsal heads 1 and 5). These landmarks are manually pointed out by the clinician using the MPR windows. This remains necessary as part of a semiautomatic process with the early version of the TALAS (CurveBeam, Warrington PA, USA) software used for this study. Future versions of this will include automatic detection of the landmarks. This defines a 3D volume as opposed to a 2D angle and allows for precise evaluation of hindfoot alignment, given as the foot ankle offset (FAO) (Fig. 5.2b) [4]. The software includes a semiautomatic database (requiring manual input of the clinical record) which stores the 3D coordinates of the points, allowing further anonymous retrieval of the latter and secondary calculation of FC position.


Fig. 5.2

(a) The WBCT software screen view with foot center (FC) definition with TALAS (top left), axial reformation (top right, red frame), parasagittal reformation (bottom left, green frame) and coronal reformation (bottom right, blue frame). The standard view is with 1 mm slice thickness. For the definition of FC (F in image), the following landmarks are used: lowest point of posterior calcaneal process (C), center of talar dome/tibial plafond (T), lowest point of the first metatarsal head (M1), and lowest point of the fifth metatarsal head (M5). (b) The triangular-based pyramid model of the foot with foot ankle offset (where D is the projection of the center of the ankle, C the calcaneus weight bearing point, A the first metatarsal head, B the fifth metatarsal head), FC (foot center), E is the midpoint between M1 and M5

Pedography: Center of Gravity (COG)

The data of the pedography sensor was gathered during the entire WBCT scan (52 seconds). The force/pressure-based COG was defined with the pedography data using a software-based algorithm (Fig. 5.3) [3]. COG motion during data acquisition was recorded and analyzed.


Fig. 5.3

Pedography software screen view showing the center of gravity (COG) for each foot (circles)

Comparison of FC/COG

The images with the FC (Fig. 5.4a) and COG (Fig. 5.4b) were semiautomatically superimposed (Fig. 5.4c). The average position of COG during acquisition time was used for this superimposition. The distance between FC and COG (Fig. 5.4c) and the direction of a potential shift (distal-proximal; mediolateral) were measured and analyzed. The pedographic images include a raster with 10 × 10 mm squares that correspond to the different sensor fields with this exact geometric size (e.g., Fig. 5.3b, c). This raster was used as reference for the measurements.


Fig. 5.4

(a) An exported image from TALAS with FC of both feet (yellow points in red circles, labelled with G). The right foot is displayed on the left side. For the definition of FC, the following landmarks are used: lowest point of posterior calcaneal process (C, green triangles), center of talar dome/tibial plafond (T, black point), lowest point of the first metatarsal head (M1, blue rhombus), and lowest point of the fifth metatarsal head (M5, red square). (b) An exported image from the pedography software with the COG (white/blue points in red circles) of each foot. The right foot is displayed on the right side. The squares have a size of 10 × 10 mm. The numbers in some squares show the measured pressure (kPa), and the different colors are coding different pressure values. (c) The superimposition of the TALAS and pedography images (a, b). FC (red points) and COG (white/blue points) both surrounded by red circle. The TALAS image was horizontally mirrored for superimposition of the same foot side. The right foot is displayed on the right side for the TALAS and pedography image


The statistical analysis was performed with Microsoft Excel 2016 (Microsoft, Redmond, WA, USA) and SPSS 24.0 (IBM, Rochester, MN, USA). The data (distances/shift between FC and COG) was successfully tested for normal distribution with a Shapiro-Wilk test. A bilateral paired t-test was used to compare data from the left to the right foot. One-way ANOVA with potential post hoc Scheffe test was used for data comparison between different pathologies. Pearson test (two sided) was used for correlation of BMI with measured data (distances/shift between FC and COG). Correlation was defined as significant when p < 0.05 and when significant then sufficient when r > 0.5 or r <−0.5.


Mean age of patients was 53.8 on average (range, 17–84) years, and 57 (63%) were female. Height was 171 cm on average (range 169–184), weight 71.4 kg (range, 43–108), and BMI 24.3 kg/m2 (range, 15.6–34.8). Table 5.1 shows the registered pathologies. Fifty-two patients (29%) had unilateral pathologies and 128 (71%) bilateral pathologies. Maximum COG motion during the 68 seconds pedography scan was 1.2 mm on average (range, 0–4.8 mm). Table 5.2 shows measurements of position differences of COG and FC. The distance between FC and COG was 28.7 mm on average (range, 0–60). FC was distal to COG in 175 feet (97%) (mean, 27.5 mm; range, −15 to 60) and lateral in 112 feet (52%; mean, 2.0 mm; range, −18 to 20). No distal or proximal shift of FC occurred in 4 feet (2%) and proximal shift in 1 (1%). No lateral or medial shift of FC occurred in 35 feet (19%) and medial shift in 33 (18%). The variation was high as shown by high standard deviations. No difference between the right and left side occurred (t-test, each p ≥ 0.5). No difference between pathology groups occurred (One-way ANOVA, distance FC/COG, p = 0.62; mediolateral shift, p = 0.48; distal-proximal shift, p = 0.53, post hoc test not applicable). No significant correlation with BMI occurred (Pearson, distance FC/COG, p = 0.36; mediolateral shift, p = 0.91; distal-proximal shift, p = 0.20, R-value irrelevant due to missing significance).

Table 5.1

Registered foot and ankle pathologies in 180 feet in 90 patients




Isolated hallux valgus



Complex forefoot deformity



Hallux rigidus






Cavus foot



Other combined deformities



Ankle instability



Osteoarthritis without relevant deformity






Complex forefoot deformity, hallux valgus plus lesser ray deformities. Hallux rigidus, only cases without deformity, i.e., hallux valgus. Flatfoot might include hindfoot valgus. Cavus foot might include hindfoot varus. Ankle instability, only cases without relevant deformity such as hindfoot valgus/varus

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Apr 25, 2020 | Posted by in MUSCULOSKELETAL MEDICINE | Comments Off on of Weight Bearing CT (WBCT) with Pedography Shows Relationship Between Anatomy-Based Foot Center (FC) and Force/Pressure-Based Center of Gravity (COG)

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