Date of Award

5-2010

Document Type

Thesis

Degree Name

Master of Science (MS)

Program

Biomedical Engineering

Research Advisor

Denis J. DiAngelo, Ph.D.

Committee

Brian P. Kelly, Ph.D. Gladius Lewis, Ph.D.

Keywords

center of rotation, instantaneous axis of rotation, load control, lumbar spine, pure moment, spine biomechanics

Abstract

The pure moment protocol is the accepted standard for performing in-vitro biomechanical testing of spinal devices. Published studies predominantly report range of motion and flexibility data, but information regarding the segment center of rotation is also relevant. Most current pure moment platforms are not sensitive enough to accurately calculate the instantaneous axis of rotation (IAR) for a segment throughout a bending motion. The purpose of this study was to simulate a pure moment protocol using a programmable spine robot, and use the data gathered to calculate the IAR for harvested specimen and those implanted with a constrained total disc replacement (TDR) device.

Six human lumbar single-level motion segment units (MSUs) at the L4-L5 level were dissected and potted. The average age of the spines was 47 ± 11.4 years. The robot was programmed to rotate the specimen in flexion and extension and left and right lateral bending in 0.25 degree increments, minimizing shear and axial loading after each rotation, thereby finding a quasistatic rotational path of minimal loading. The specimens were rotated to 8Nm of sagittal moment during flexion and extension and 6Nm of lateral moment during lateral bending. During lateral testing, the specimens were unconstrained axially. Once harvested testing was completed, specimens were implanted with a constrained ProDisc-L implant (Synthes Inc., West Chester, PA) by a spine surgeon under fluoroscopy. All pure moment testing was repeated on the implanted specimen.

Throughout testing, the specimens underwent an average off-axis force of 1.51N. With an average perpendicular distance of 0.062mm, this force value contributed 0.000094Nm to the maximum bending moment, meaning the test platform was 99.99% free of off-axis loading. During flexion and extension tests the specimens rotated an average of 9.90 ± 2.23 degrees and 3.40 ± 1.43 degrees respectively. During left and right lateral bending tests the specimens rotated an average of 6.21 ± 1.34 degrees and 5.64 ± 1.77 degrees respectively. These values are in agreement with other published studies of lumbar spinal biomechanics. Range of motion comparisons between the harvested and implanted specimen showed a significant difference in right lateral bending and combined lateral bending (one-way repeated measures ANOVA with SNK test, p<0.05). No significant differences were observed for flexion or extension motions.

IAR values were calculated for the harvested and implanted specimen for flexion and extension testing and normalized based on the height and anterior-to-posterior (A-P) width of the disc. These values were compared with a One-Way ANOVA with Dunn's comparison test between locations of X and Y coordinates for each IAR within and between conditions (p<0.05). All comparisons save for the position of Y-coordinates in harvested testing between flexion and extension showed significance.

Future work will be to allow for a user-inputted axial load to simulate a net muscle vector, use of the protocol with other constrained as well as unconstrained TDR devices, and use of the protocol within multi-body studies.

DOI

10.21007/etd.cghs.2010.0098

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