Sensitivity of a strain measurement technique to detect disc injury using radiostereometric analysis

Author: Vasanthi Pendyala

Pendyala, Vasanthi, 2018 Sensitivity of a strain measurement technique to detect disc injury using radiostereometric analysis, Flinders University, College of Science and Engineering

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The intervertebral disc (IVD) is a vital element of the spine in the human body. It helps in the spine movement and acts as a shock absorber. When there is an annular tear or any damage to the disc, there will be a change in its internal strains. One way of calculating internal strains is by placing radiographic end plate markers, circumferential markers and a grid of wires in the disc and then calculating the displacement of the markers by reconstructing a 3D image by digitising the digital radiographs of the disc using the radio-stereometric analysis (RSA). The primary objective of the experiment was to study how sensitive the internal strain measurement technique in detecting the internal strains in different stages of disc injury. Bovine tail intervertebral discs were used in this study, as they are similar to human intervertebral disc composition, geometry and are also cost effective. Five healthy bovine intervertebral discs marked with radiographic endplate markers, circumferential markers, and a wire grid were inserted into the disc, and 6 degrees of freedom (DOF) testing was performed followed by taking x-rays after each DOF. In this study, three different states of the disc are subjected to test. The three states are as follows: Case 1: Testing the disc in its uninjured state, Case 2: Testing with a minor injury (5mm width and depth rim lesion), Case 3: Testing with a significant injury (5mm depth and 10mm width rim lesion). In each case, the disc was subjected to 6DOF testing. The injury induced onto the disc in this study was a rim lesion, and the 6DOF in which the disc was stimulated are left axial rotation, right lateral bending, extension, flexion, flexion rotation, and compression at 0.1Hz. The 3D reconstructed outputs of x-rays taken after each DOF are compared with the 3D reconstructed outputs of x-rays taken at the neutral position of the disc. In total 18 test results were collated for each disc. From 6DOF Hexapod testing output data the stiffness and hysteresis loss ratio (mechanical properties) of the disc of the disc are also calculated. The mechanical properties of the disc in each DOF were compared in the three test cases. There were no significant changes within-subjects effects observed between the stiffness and hysteresis loss ratio values (p > 0.05) of the disc in its different test cases. The reconstructed 3D images of the IVD after each DOF were compared to IVD in neutral position for the three test cases. Displacement of the wire intersection points was calculated. An intra-observer variability (repeatability) test was conducted to check for the user error that might affect the final results of the specimen. Hence, it was concluded that there was no change in the mechanical properties of the disc after performing the 6DOF testing. The maximum value of user error calculated from the intra-observer repeatability study is about 0.29 mm. The change in the radius of the disc periphery after inducing an annular tear explains that the RSA technique was able to detect the changes occurring in the disc, by experimenting on different ranges of disc injuries and more number of samples gives an accurate sensitivity of the technique.

Keywords: Radiostereometric analysis, strain measurement, IVD, 6DOF.

Subject: Engineering thesis

Thesis type: Masters
Completed: 2018
School: College of Science and Engineering
Supervisor: Assoc Prof John Costi