Date on Master's Thesis/Doctoral Dissertation

5-2005

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Mechanical Engineering

Committee Chair

Hnat, William Patrick

Author's Keywords

Strain sensor; Biocompatible; Telemetric strain monitoring; Capacitive bending strain sensor; MEMS

Subject

Microelectromechanical systems; Biotelemetry; Strains and stresses--Measurement

Abstract

Lumbar arthrodesis or spinal fusion is usually performed to relieve back pain and regain functionality from ruptured discs, disc degenerative disease, trauma and scoliosis. Metal rods are often fixed to the spine with screws or hooks, while fusion develops on the affected vertebrae. Fusion is determined by visual examination of radiographic images (X-ray), computed tomography (CT) scans or magnetic resonance imaging (MRI), yet these inspection procedures are subjective methods of review. They do not objectively confirm the presence of spinal fusion, which can lead to exploratory surgery to determine if fusion has occurred. Therefore, a need has arisen to develop an objective method that will offer unbiased information for the determination of fusion. Discussed herein is a housing and sensor designed to be used in conjunction with telemetric circuitry that will attach to the spinal instrumentation rods. The housing will transmit strain to an internatal capacitive MEMS-based sensor that will relay strain magnitudes via telemetry. Observed reductions of bending strain will indicate a successful fusion. These objective assessments will reduce the incidence of costly exploratory surgeries where fusion is in question. The housing design was fabricated using Polyetheretherketone (PEEK) material, which was selected for its physical properties and its ability to be implanted for long durations. The housing was tested under cyclical, static and maximum strain transfer loading configurations in the Material Testing System (MTS). Results from these tests demonstrated that the housing transferred 102% of the bending strain and successfully met the design criteria. Additionally, a MEMS-based sensor was developed to change the capacitance with detected alterations in bending strain transmitted through the housing. Sensors were fabricated using microfabrication techniques and highly doped boron silicon wafers to create a transverse comb drive or an interdigitated finger array. The sensor was tested using similar methods that were used for the housing. Results from cyclical testing demonstrated the sensor's response needed to be increased 50% and it did not exhibit any capacitance drift.

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