Outcomes - Amputee Anthropometrics: Reliability and Accuracy of Analog and CAD/CAM Measurement Devices

Mark D. Geil1, Ph.D., Matthew Tate2, Ph.D., Kyle Elliot
Georgia State University 2Emory University 3Georgia Institute of Technology
Atlanta, Georgia

Introduction and Methods

Prosthetic practice faces a growing need for outcome measures and evidence of the benefits of service. The most direct outcome measures in prosthetics might be associated with movement, such as walking velocity or success in activities of daily living (1). However, outcome measures can also provide a common language for the comparison of treatment protocols, and may include information about changing amputee anthropometrics and socket shape and fit (2). One problem with the comparison of amputee anthropometrics is the wide range of available calipers, rulers, and tape measures (3). In addition, a variety of tools for shape capture have been developed, including generally accepted CAD/CAM digitizers (4-6) along with customdesigned methods using x-ray and cineradiography (7), finite element modeling (8), and laser and optical capture methods (9). Anecdotal observation suggests that even traditional hand-measurement devices vary in terms of precision, accuracy, and ease of use. This 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. In addition, CAD/CAM imaging was performed using two systems: TracerCAD Premier and Omega T-Ring and the same anthropometrics were recorded.

Results

Data were analyzed to address several questions:

  • Which group was more consistent?

  • Which instrument was most accurate?

  • Which instruments produced inconsistent measurements?

  • Which measurements and measurement sites produced the most inconsistent measurements?

Which group (students vs. practitioners) was more consistent?

This question 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.

The practitioner group produced larger standard deviations and ranges than the students in a slight majority of measurements. The sum of average student ranges across all measurements was 22.9 cm versus 43.4 cm for experienced practitioners. Students were generally more consistent with linear measurements (Length, AP, ML) while experienced practitioners were more consistent with measures of circumference. Students produced larger standard deviations than experienced practitioners in only 26% of linear measurements as opposed to 70% of circumference measures.

Students and practitioners showed similarly small error values when repeating measurements of identical Models A and C. The single largest difference in a measurement of model A vs. C was 2 cm, when an experienced practitioner measured the ML distance with an anthropometer as 15 cm for Model A and 13 cm for Model C. While the largest error was 2 cm, the average error for all measurements across all subjects was only 2 mm.

Which instrument was most accurate?

The anthropometer, the U-ML Gauge, and the standard tape measure were all tested to be accurate within one tenth of one millimeter when compared against a single known length comparable to those measured in this 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. The TT Length Gauge was also very accurate, but does lend itself to error in an accuracy test because the end of the device meant to be 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. Although the scale of the tested VAPC extends from 0 to 5 inches, measurements of a known standard of 5 inches were offthe scale. Consequently, the VAPC was compared to a known standard of approximately 4 inches (3.9975 inches). Errors on the ML end exceeded 0.25 inches. 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.

Which instruments produced the most inconsistent measurements?

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 (Figure 1). The most consistent instruments in each analysis were the 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.

Which measurements and measurement sites produced the most inconsistent measurements?

Analysis of problematic measurements again saw data skewed by the subject who produced errors when using the anthropometer. These outliers caused all measurement sites that included the anthropometer to be abnormally variable. With the outlier subject excluded, the most problematic measurement was length, which produced an average range of 1.42 cm. Measurers were most consistent when measuring the AP distance. Within the circumference category (and including data from all subjects), measurers were most consistent at the site two inches distal to the MPT ( ó = 0.15 cm). Measurers were more variable at the MPT ( ó = 0.20 cm), and most variable at four inches distal to the MPT ( ó = 0.23 cm). In comparing measurements for A vs. C error, no measurement produced more than 1.5 mm mean error.

CAD results were consistent and agreed with most accurate and reliable analog devices, exceeding the accuracy and reliability of several of the analog devices. Additional research should address efficiency of measurement using each category of device.

Discussion

This study provides evidence that improper measurement technique or poor understanding of a measurement device can lead to clinically significant errors. However, when technique is proper and devices are used correctly, the general variability in measurements is not clinically significant. The following summarizes the study's observations concerning measurement techniques and measurement devices.

Techniques:

  • 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 would likely 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.

Instruments:

  • 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.

  • The VAPC used in this study was both inaccurate and unreliable.

Improvement of measurement devices and techniques has impact beyond the direct possibility of improving 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. 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.

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