3D foot type classification could improve orthotic management

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From Biomechanics November 2004

3D foot type classification could improve orthotic management

By: R.

http://biomech.com/showArticle.jhtml?articleID=52601675

A 3D objective classification method can correctly identify more than

98% of feet by foot type, according to a poster presented at the annual

meeting of the American Society of Biomechanics in Portland, OR, in

September. The authors hope this mechanism will not only improve and

standardize foot classification, but will also allow practitioners to

make better decisions about footcare, alignment, orthosis prescription,

and whether to proceed to surgery.

The information was presented by investigators from the Rehabilitation

Research and Development Center of Excellence for Limb Loss Prevention

and Prosthetic Engineering at the VA Puget Sound (WA) and the

departments of orthopedics, sports medicine, and mechanical engineering

at the University of Washington in Seattle.

As the investigators note, structural differences among feet are in

three dimensions, for that reason, current 2D foot-type classifications

may lack detail and rely too much on a practitioner's subjective

opinion. Researchers worked with engineers to create a program that

would accurately classify foot types in three dimensions and create a

standard for measurement.

An orthopedic surgeon examined the feet of 40 participants with foot

deformities and identified 65 feet by type. The researchers noted that,

architecturally, feet are divided into three categories (pes cavus,

neutrally aligned, and pes planus). Fourteen of the feet were pes cavus,

20 were neutrally aligned, and 31 were pes planus. If a participant's

foot deformity was bilateral, both feet were studied.

Computed tomography scans of the subjects' feet and ankles were then

taken (while weight-bearing). From the scans, the researchers created 3D

models of each bone. They then calculated 24 Euler angles, which are

used to describe rotation (x, y, and z coordinates of eight angles, see

table). These angles were used to create a classification tree analysis.

This analysis was then used to determine whether the Euler angle data

would allow the researchers to classify the feet into the three clinical

foot-type groups.

For example, a cuneiform-talus angle of less than 23.55 degrees caused a

foot to be classified as pes cavus. Feet with first metatarsal-talus

angles of less than 4.92 degrees are classified as pes planus. The

deciding factor between which feet were neutrally aligned and which were

pes planus was the cuneiform angle.

Feet that were inverted less than 32.14 degrees relative to the

navicular were classified as neutrally aligned, and those that were

inverted more than 32.14 degrees were classified as pes planus.

The presenters reported that this classification method matched that of

the orthopedic surgeon for 64 out of 65 feet (one pes planus foot was

incorrectly classified as neutrally aligned). Even with the above

cross-validated miscalculation error in the study, the authors state

this method would be able to classify foot types with a 98% success

rate. As the method is objective and based on three-dimensional

measurements, the researchers speculate it may be a better

classification method than current means.

EIGHT ANGLES MEASURED

First metatarsal-talus

Fifth metatarsal-talus

Calcaneus-talus

Second metatarsal-first metatarsal

Calcaneus-fibula

Cuneiforms-talus

Cuneiforms-navicular

Navicular-talus