0
Design Innovation

A Handheld Computer as Part of a Portable In Vivo Knee Joint Load-Monitoring System

[+] Author and Article Information
J. A. Szivek, V. S. Nandakumar, C. P. Geffre

Orthopaedic Research Laboratory, Department of Orthopaedic Surgery and Arizona Arthritis Center, University of Arizona, Tucson, AZ 85724

C. P. Townsend

 MicroStrain Inc., 310 Hurricane Lane, Suite 4, Williston, VT 05495

J. Med. Devices 2(3), 035001 (Aug 06, 2008) (9 pages) doi:10.1115/1.2952815 History: Received June 24, 2007; Revised February 14, 2008; Published August 06, 2008

In vivo measurement of loads and pressures acting on articular cartilage in the knee joint during various activities and rehabilitative therapies following focal defect repair will provide a means of designing activities that encourage faster and more complete healing of focal defects. It was the goal of this study to develop a totally portable monitoring system that could be used during various activities and allow continuous monitoring of forces acting on the knee. In order to make the monitoring system portable, a handheld computer with custom software, a USB powered miniature wireless receiver, and a battery-powered coil were developed to replace a currently used computer, ac powered benchtop receiver, and power supply. A Dell handheld running Windows Mobile operating system programed using LABVIEW was used to collect strain measurements. Measurements collected by the handheld-based system connected to the miniature wireless receiver were compared with the measurements collected by a hardwired system and a computer based system during benchtop testing and in vivo testing. The newly developed handheld-based system had a maximum accuracy of 99% when compared to the computer based system.

FIGURES IN THIS ARTICLE
<>
Copyright © 2008 by American Society of Mechanical Engineers
Your Session has timed out. Please sign back in to continue.

References

Figures

Grahic Jump Location
Figure 2

Computer based measurement system showing a Macintosh G3 power book (right) connected to the ac powered receiver via a serial cable and the power supply for the coil that inductively powers the transmitter

Grahic Jump Location
Figure 3

(a) A Dell handheld computer (right) connected to the ac powered receiver via a serial cable. Also shown (top right) is the power coil. (b) A Dell handheld computer connected to the portable wireless receiver via a serial cable.

Grahic Jump Location
Figure 4

An implantable scaffold with one uniaxial strain gauge showing. The scaffolds used in this study were approximately 11.3mm in length and 8.9mm in diameter.

Grahic Jump Location
Figure 5

Measurements from a single channel collected by hardwired (upper line) and telemetry (lower line) setups at 1Hz. The point of intersection of the vertical lines and the curves represent the peak strain value. Note that strains collected with the PC were offset by 3000μ strain to allow easier comparison of the two curves.

Grahic Jump Location
Figure 6

Strain values collected by the two systems corresponding to one of the measurement channels. The first strain peak of both PC and handheld recorded measurements coincided exactly. The final strain peak of the measurements collected with the handheld is shifted by 0.69s. Note that strains were recorded with an offset. Actual strain magnitudes are the difference between the base line (approximately 4100 μstrain) and peak values.

Grahic Jump Location
Figure 7

Strain values collected by the two systems corresponding to one of the measurement channels. The first strain peak of both PC and handheld recorded measurements coincided exactly. The final strain peak measured with the handheld is shifted by 0.38s. Note that magnitudes are the difference between the base line and peak values (approximately 3200 μstrain).

Grahic Jump Location
Figure 8

Strain values collected with the PC and handheld systems. All strain peaks collected with both the PC system and the handheld system running program SMV3 coincided exactly when scaffolds were loaded at 1Hz. Note that magnitudes are the difference between the base line and peak values.

Grahic Jump Location
Figure 9

Strain values collected with the PC and handheld systems. All strain peaks of both the PC and handheld systems running (SMV3 ) coincide exactly when scaffolds were loaded at 3Hz. Note that magnitudes are the difference between the base line (approximately 4200 μstrain) and peak values (approximately 3400 μstrain).

Grahic Jump Location
Figure 10

Strain values collected with the PC and handheld systems. All strain peaks of both the PC and handheld systems running (SMV3 ) coincide exactly when scaffolds were loaded at 10Hz. Note that magnitudes are the difference between the base line and peak values.

Grahic Jump Location
Figure 11

(a) Strain values corresponding to one of the four channels collected by the two Dell handheld-based systems (benchtop receiver and wireless receiver) at 1Hz. Note that magnitudes are the difference between the base line and peak values. (b) Strain values corresponding to one of the four channels collected by two Dell handheld systems (one with a benchtop receiver and one with a portable wireless receiver) at 3Hz. Note that magnitudes are the difference between the base line and peak values. (c) Strain values corresponding to one of the four channels collected by the two handheld systems (one with a benchtop receiver and one with a portable wireless receiver) at 10Hz. Strains were recorded with an offset and actual strain magnitudes are the difference between the base line and peak values. Note that an occasional signal spike is observed with the wireless receiver.

Grahic Jump Location
Figure 12

These in vivo strain recordings were collected while test animals ran at three different speeds on a treadmill. They were recorded with an offset and the actual strain magnitudes are the difference between the base line and peak values. Note that the strain measurements collected with the PC and those collected with the handheld computer overlap perfectly.

Grahic Jump Location
Figure 13

This recording shows two artifacts caused by the slipping of the power coil from the transmitter. These artifacts are easily detected because they are generally very short in duration and large in magnitude. As such they can be identified without difficulty and removed from a data set.

Grahic Jump Location
Figure 14

These two curves overlap up to data point 400, which shows an artifact produced by switching off the power supply. There is a time shift between the two curves beyond the artifact. This is easily detected when a second artifact is produced by switching off the power supply a second time.

Grahic Jump Location
Figure 1

The rf digital transmitter (approximately 25mm diameter), which has a built-in multiplexer, differential amplifier, A/D converter, and pulse code modulator shown next to a dime

Tables

Errata

Discussions

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles
Related eBook Content
Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In