Anthropometrics Goes Beyond Immediate Prosthetic Benefits

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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.

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.
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.

 

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."