Development of a Strain Transferring Sensor Housing for a Lumbar Spinal Fusion Detection System

[+] Author and Article Information
J. W. Aebersold1

Mechanical Engineering Department, J.B. Speed School of Engineering, University of Louisville, Louisville, KY 40292julia.aebersold@louisville.edu

W. P. Hnat

Mechanical Engineering Department, J.B. Speed School of Engineering, University of Louisville, Louisville, KY 40292

M. J. Voor, R. M. Puno

Department of Orthopaedic Surgery, School of Medicine, University of Louisville, Louisville, KY 40292

D. J. Jackson, J. T. Lin, K. M. Walsh, J. F. Naber

Department of Electrical Engineering, J.B. Speed School of Engineering, University of Louisville, Louisville, KY 40292


Corresponding author.

J. Med. Devices 1(2), 159-164 (Sep 05, 2006) (6 pages) doi:10.1115/1.2735971 History: Received January 27, 2006; Revised September 05, 2006

Lumbar arthrodesis or spinal fusion is usually performed to relieve back pain, and regain functionality from degenerative disc disease, trauma, etc. Fusion is determined from radiographic images (X-ray) or computed tomography scans, yet these inspection procedures are subjective methods of review. As a result, exploratory surgery is performed if the presence of fusion cannot be confirmed. Therefore, a need exists to provide objective data to determine the presence of fusion that could avoid the cost, pain, and risk of exploratory surgery. One method to achieve this objective is to observe bending strain from spinal rods implanted during surgery. A system has been developed that will attach to the spinal instrumentation rods, transmit strain information wirelessly, and without the use of batteries. Major components of the system include a strain transferring sensor housing, a microelectromechanical (MEMS)-based strain sensor, telemetry circuitry, and antennae. Only discussed herein are the design, testing, and results of the housing without a cover and its ability to transfer strain from the rod to an internal surface where a foil strain gage is attached to characterize strain transfer efficiency. Strain gauges rather than the MEMS sensor were employed for housing characterization due cost and limited availability. Design constraints for the housing are long-term implantation, small size, greater than 95% transfer of bending strain from the spinal rods to the internal strain sensor, and ease of installation. ABAQUS finite element modeling software was employed to develop a working model that was fabricated using polyetheretherkeytone. The housing underwent cycle testing in a material testing system to simulate long-term implantation along with static testing to determine if creep was present. Both series of tests showed that the housing’s response did not degrade over a period of time and there was no indication of creep. The experimental results also validated the results of the ABAQUS finite element model.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Illustrations of spinal fusion instrumentation implemented on a demonstration spine model

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Figure 2

(a) Initial housing designs with a thin wall section next to the rod surface and a narrow cable guide; (b) a housing with a thick wall section near the rod surface and a wide cable guide; and (c) an extensometer design with a thin wall section that is elevated away from the rod surface

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Figure 3

ABAQUS finite element analysis (FEA) model of the housing and rod

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Figure 4

Illustration of the meshed assembly

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Figure 5

(a) The PEEK housing manufactured as one piece and with a single opening to accommodate clearance space for the (b) spherical bearings of the Medtronic Sofamor Danek Atlas cable system

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Figure 6

(a) The MTS fixture used to achieve a constant magnitude of strain on the surface of the rod; and (b) an illustration of loads applied by the MTS and placement of the strain gauges on the rod

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Figure 7

(a) ABAQUS longitudinal strain graphical results from the FEA model; and (b) strain transfer values for each of the housing designs at 10MPa clamping pressure

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Figure 8

(a) Percentage strain data levels for increasing pressure values applied to the cable guides, which indicate the node of highest amplification; and (b) strain transfer percentages averaged across the sensor surface

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Figure 9

(a) Strain values collected during static testing; and (b) maximum strain values collected from each 20s cycle during the MTS cycle test




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