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Anthropometrics Goes Beyond Immediate Prosthetic Benefits
By Miki Fairley Accurate measurement techniques and instruments can improve the
application of anthropometrics--the comparative study of human body
measurements and properties--not only in prosthetics practices, but
also in enabling the prosthetics profession to contribute to the
knowledge base of other rehabilitation disciplines, Geil
observed.
These measures may include information about changing amputee
anthropometrics and socket shape and fit, he said.
However, there is a problem in comparing amputee
anthropometrics: the wide range of available calipers,
rulers, and tape measures impede consistent comparisons across a
spectrum of sources.
"Anecdotal observation suggests that even traditional
hand-measurement devices vary in terms of precision, accuracy, and
ease of use," Geil noted.
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Geil works with Georgia State Graduate students to verify consistent landmark measurement of a transtibial residual limb model using (left–right) traditional calipers, an electromagnetic field-based position sensor, and an optoelectronic CAD system. Photo by Wat Limroon. |
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In addition, a variety of sophisticated tools for shape capture
have been developed, including generally accepted CAD/CAM
digitizers, along with custom-designed methods using X-ray and
cineradiography, finite element modeling, and laser and optical
capture methods. Plus, of course, you have the human element--the
precision of the individual taking the measurements.
Thus Geil and some colleagues decided to undertake a study with
the hope of bringing some consistency to the table for amputee
anthropometrics.
The study assessed the accuracy and reliability of seven
prosthetic anthropometric measurement devices. After the
instrument accuracy was independently assessed, a group of
prosthetic/orthotic practitioners and a group of
prosthetic/orthotic students measured six common anthropometric
dimensions on three foam positive models of transtibial amputee
residual limbs. Two of the models were identical, enabling
assessment of individual repeatability. Also, CAD/CAM imaging was
performed using two systems: TracerCAD Premier and Omega T-Ring,
with the same anthropometrics being recorded.
Students vs. Practitioners
The comparison was addressed in two ways. First, the standard
deviation and range of each measurement for each group was
compared. Next, the ability of each group to make consistent
measurements on identical Models A and C was assessed.
Surprisingly, the students actually performed a little better
overall than the practitioners. "The practitioner group produced
larger standard deviations and ranges than the students in a slight
majority of measurements," according to the study. The sum of
average student ranges across all measurements was 22.9cm versus
43.4cm for experienced practitioners. "Students were generally more
consistent with linear measurements--length, AP, ML--while
experienced practitioners were more consistent with measures of
circumference," Geil said. "Students produced larger standard
deviations than experienced practitioners in only 26 percent of
linear measurements as opposed to 70 percent of circumference
measurements." Students and practitioners showed similarly small
error values when repeating measurements of identical Models A and
C. While the largest error was 2cm, the average error for all
measurements across all subjects was only 2mm.
Most Accurate Instrument
The anthropometer, the U-ML Gauge, and the standard tape measure
were all tested to be accurate within 1/10 of 1mm when compared
against a single known length comparable to those measured in the
study. "The circumferential tape measure was similarly accurate,
but it should be noted that this instrument is difficult to compare
to a known linear dimension because of its built-in curvature,"
said Geil. "The TT Length Gauge also was very accurate, but does
lend itself to error in an accuracy test because the end of the
device meant to placed on the patellar tendon is a curved surface
and is difficult to compare to a discrete mark."
The only device in the study with substantial inaccuracies was
the VAPC. "Compounding practical error is the fact that the scale
on the VAPC is very difficult to use and read, with poorly
contrasting tick marks and numbers and unusual divisions," Geil
commented.
Instrument Consistency
Once outlier data from one subject were removed, the instrument
producing the least consistent results was the VAPC, followed
closely by the TT Length Gauge. The most consistent instruments
turned out to be, in each analysis, tape measures. "The spring tape
measure did produce very slightly more consistent measurements, but
the differences versus the two other tape measures were not
clinically significant," Geil said.
Measurements, Site Results
With an outlier subject excluded, the most problematic
measurement was length, which produced an average range of 1.42cm.
Measuring the AP distance produced the most consistent results.
Within the circumference category, which included all subjects,
measurers were most consistent at the site 2 inches distal to the
MPT, more variable at the MPT, and most variable at 4 inches distal
to the MPT. In measurement of Models A and C, no measurement
produced more than 1.5mm mean error.
CAD came off well in the study: "CAD results were consistent and
agreed with most accurate and reliable analog devices, exceeding
the accuracy and reliability of several of the analog devices,"
Geil reported.
What Was Learned
The study showed that improper measurement technique or poor
understanding of a measurement device can lead to clinically
significant errors, Geil pointed out. However, when technique is
proper and devices are used correctly, the general variability in
measurements is not clinically significant, he added.
The study also produced some good measurement advice for
practitioners:
- For length measurements, calipers must be held parallel to the
long axis of the residual limb. Small deviations produce large
errors.
- For circumference measurements, the tape measure should be
wrapped around the limb in a plane perpendicular to the long axis
of the residual limb. In this study, tape tension was not a
significant factor in measurement error; however, more fleshy
residual limbs likely would increase the importance of consistent
wrapping tension.
- For circumference measurements of a region at which the limb
tapers substantially, the proximal edge of the tape, which
maintains closer contact to the limb, should be used for the
measurement.
- The precision of the instrument is important. If multiple
scales are available on the same instrument (for example, inches
and centimeters), use the scale that provides greater
precision.
- The type of tape measure used did not affect the results of
this study, suggesting that tension is less important than
anticipated; however, as mentioned, more fleshy residual limbs
might produce a different result.
Improvement of measurement devices and techniques goes
beyond helping to improve amputee locomotion.
"Anthropometry has applications in ergonomics, design
of assistive devices, assessment of the impact of disease on the
growth and development of children, and understanding of specific
anatomical pathologies, such as musculoskeletal foot deformities,
to name just a few," Geil pointed out. He added. "Improvement of
measurement devices and techniques might not only improve
prosthetic service but also allow the profession of prosthetics to
contribute to a number of rehabilitative disciplines." 
Table Of Contents - January 2006
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