Foot & Ankle International - June 2017 - 685

Lintz et al

685

Figure 1. 3D views obtained by the weightbearing computed tomography built-in software.

tool would be as follows: physically meaningful, designed
for 3D WBCT, computerized, and verifiable by data.
In this study, we investigated the efficacy of a new system: Torque Ankle Lever Arm System (TALAS; CurveBeam
LLC, Warrington, PA) designed to satisfy those requirements and used to calculate a computerized, semiautomatic
3D biometric: the Foot and Ankle Offset (FAO). It represents the lever arm or the torque generated in the ankle from
the combined actions of body weight and ground reaction
force. This technique had been previously validated in a
simulated 3D weightbearing (WB) environment on 2D
radiographs, at a time when 3D WBCT was not readily
available.22 The objective of this study was to describe the
distribution of FAO in a series of anonymized data sets
from clinically normal, varus, and valgus cases. We hypothesized that normal, varus, and valgus cases would be significantly different and that the distribution should be
Gaussian in the normal population. We also assessed the
interobserver and intraobserver reliability of the FAO
measurement.

Methods
This was a comparative study looking retrospectively at
existing data captured as part of routine clinical care. The
study was conducted with institutional review board
approval.
We analyzed 135 anonymized consecutive data sets, of
which 57 were from patients with normal hindfoot alignment (42%), 38 from patients with varus hindfoot alignment (28%), and 40 from patients with valgus hindfoot
alignment (30%). All scans were obtained from WBCT
scans using the PedCAT unit (CurveBeam) installed in the
outpatients sector of an orthopaedic foot and ankle surgery
referral center. 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, total scan time, 54 seconds. The data sets were extracted from the existing database, containing the 3D image data (Figure 1) and patient
demographics limited to side and clinical morphotype.
Data sets were screened by 2 independent observers using
the built-in software, CubeView (CurveBeam), and collected the 3D coordinates of specific anatomical landmarks required for the software to process and calculate
FAO. Two independent observers collected the 3D coordinates (x, y, z), for the following landmarks: first metatarsal
head WB point (A), fifth metatarsal head WB point (B),
calcaneus WB point (C), and the talus centermost and
highest point, respectively, in the coronal and sagittal
planes (D1). The WB point was defined in the ground
plane by its x and y coordinates as the lowest point (z
coordinate nearest to 0) on the surface of the calcaneus
and metatarsal WB surfaces. The center of the talus was
defined as the highest point (z coordinate furthest to 0) on
the talus in the centermost sagittal plane.
All these values were collected and stored anonymously in a spreadsheet, which a third investigator then
ran the through a beta version of the Torque Ankle Lever
Arm System, which calculated FAO values using an
algorithm based on the inverted 3D pyramid model22
(Figure 2):
1.

2.
3.

A, B, C, and D1 were defined as, respectively, the
WB points of the first metatarsal head, the fifth
metatarsal head, the calcaneus and the talus;
the midpoint E between A and B;
the CE line was found, and the CE distance was
called "foot length";

Table of Contents for the Digital Edition of Foot & Ankle International - June 2017