Method and System for Measuring Deflection at Multiple Locations
Figure 1. Schematic illustration of an antenna for detecting structural member deflections (e.g., beams, rods, or orthopedic hardware) in multiple discrete locations.
At A Glance
Researchers at Colorado State University have developed a monitoring system to determine the relative displacement of orthopedic implants as the result of an applied load using radiofrequency at multiple locations. The unique selective antenna coiling employed here aligns regions of maximum and minimum sensitivity to deflection to allow fracture monitoring at multiple discrete regions.
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Currently, after orthopedic surgery, radiographs are used to monitor fracture healing. X-rays use attenuation to provide images of the bone and healing tissue, as attenuation is related to the density of the material. X-rays, however, are unable to discern if a fracture is healing properly in the early post-operative period because there is little to no mineralized tissue in the healing fracture callus (tissue).
Diagnostic monitoring and prediction of bone fracture healing is critical for the detection of delayed union or non-union and provides the requisite information as to whether therapeutic intervention or timely revision are warranted. A promising approach to monitor fracture healing is to measure the mechanical load-sharing between the healing callus and the implanted hardware used for internal fixation.
The monitoring system includes an antenna which can detect the displacement of implanted hardware relative to the antenna as the result of an applied load. A source connected to the antenna, such as a network analyzer, generates an electromagnetic field which is emitted by the antenna over one or more pre-determined frequency bandwidth sweeps. The antenna receives signals from the electromagnetic field, and thus can detect changes to the pattern resulting from changes to the distance between the antenna and hardware and/or deflection of the hardware. Thus, shifts in the coupling between the hardware and antenna during mechanical loading are indicative of deformation of the hardware.
Analysis of the electrical signals coming from the antenna characterize the electromagnetic coupling between the antenna and the hardware. Electrical signal data are collected over a pre-determined period of time so that temporal changes to the coupling ratio may be quantified. Thus, repeated measurements during the collection period quantify temporal changes in the hardware deflection due to a given mechanical loading.
The coiled shape of the antenna increases the signal strength by aligning multiple locations of maximum sensitivity. The relationship between the location along the antenna and the resultant sensitivity to hardware deflection is harmonic, and thus there are equal numbers of locations with maximum sensitivity or minimum sensitivity for a given resonant frequency harmonic number. The antenna may thus be coiled so that multiple discrete regions are formed with each region being highly sensitive at some harmonic numbers while simultaneously being minimally sensitive within the same region at other harmonic numbers. Such arrangement of the antenna allows for a single antenna to discretely measure hardware deflection at multiple spatial locations by simultaneously interrogating frequency bandwidths featuring multiple resonant frequency harmonics with a single network analyzer port.
- No internal sensor required (implant itself provides the signal)
- Requires no alterations or additions to existing implants
- Eliminates any regulatory approval hurdles
- Can measure deflection/displacement at multiple locations
- Cost effective
- Orthopedic fracture healing monitoring
- Detection of loosening orthopedic implants (e.g., joint replacements)
- Research development
Last updated: June 2021
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#CSUInvents – #TechTuesday! Currently, there is no way to accurately monitor fracture healing in the early post-operative period. Researchers at CSU Walter Scott, Jr. College of Engineering have developed a novel method and apparatus to solve this problem. Check out this #patentpending telemetric load-sensing #orthopaedics device invented by Christian Puttlitz, Kirk McGilvray, and Kevin Labus in the Orthopaedic Bioengineering Research Laboratory at Colorado State University.