Foot & Ankle International - June 2017 - 686

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Foot & Ankle International 38(6)

Figure 3. Gaussian distribution of Foot and Ankle Offset values
in the normal population.

Figure 2. Schematic representation of Foot and Ankle Offset in
the transverse plane (see the text for abbreviations).

D was the orthogonal projection of D1 on the ABC
plane (i.e., the position of the center of the talus in
the ground plane);
the perpendicular line to CE, which included D, was
found (i.e., the line from the center of the talus to the
midline of the foot on the ground plane);
F was the intersection of these 2 lines and thus DF
was found, being positive when D was medial to F
and negative when lateral;
FAO was DF/CE × 100 and was given as a
percentage.

FAO therefore corresponds to the offset between the hindfoot-to-forefoot midline and the talus. It was therefore given
as a percentage of foot-length and was theoretically representative of the torque, which would be produced in the
ankle by this offset. This conversion to percentage of foot
length was done to normalize the FAO value to foot size so
that it was comparable between feet of different sizes, since
the FAO was a metric, not an angle.

Statistical analysis
FAO values in 3 groups were compared using a 1-way analysis of variance. Normality test was performed using the
Kolmogorov-Smirnov test method on the normal foot
group. An intraclass correlation coefficient (ICC) 2-way
mixed model was used to assess the intraobserver and
interobserver reliability of FAO values. ICCs were calculated with the measurements performed twice by 2 independent observers on data sets in the 3 different groups: normal,

varus, and valgus. ICC values range from 0 to 1, with a
higher value indicating a better reliability. ICCs below 0.40
were considered poor, 0.40 to 0.59 fair, 0.60 to 0.74 good,
and 0.75 to 1.00 excellent.
Data are presented as mean, standard deviation, range,
and 95% confidence intervals (CIs). All analyses were carried out using Stata statistical software package (version
12.0, StataCorp, 2011). A P value <.05 was considered to
indicate statistical significance.

Results
In normal cases, mean value for FAO was 2.3% ± 2.9%
(95% CI, 1.5%-3.1%; range, -3.2% to 9.0%), whereas in
varus and valgus cases the mean was −11.6% ± 6.9% (95%
CI, -13.9% to −9.4%; range, -26.4% to −1.0%) and 11.4%
± 5.7% (95% CI, 9.6%-13.3%; range, 2.0%-24.5%), respectively. The difference was statistically significant among
the 3 groups (P < .001). FAO values distribution in the normal population was normal (P > .81) (Figure 3).
The inter- and intraobserver reliability of FAO measure
were excellent, with small standard deviations; the global
mean ICC was 0.99 ± 0.00 (95% CI, 0.99-1.00) and 0.97 ±
0.02 (95% CI, 0.92-1.02) for the inter- and intraobserver
assessment, respectively. ICC values for the normal, varus,
and valgus groups are shown in Table 1.

Discussion
This study confirmed our hypothesis that in a series of anonymized and independently reviewed 3D WBCT data sets,
the FAO was successful in discriminating clinically normal
from varus and valgus hindfoot alignment cases using a
novel 3D biometric computerized, semi-automatic measurement. The distribution of FAO in the normal population

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