Finite Element Modeling of Lumbar Interbody Fusion

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Abstract

To help reduce complications following lumbar interbody fusion, such as subsidence, there is a need for patient specific selection of device components. This thesis investigated the effects of age-related changes and the spacer material on the stress and strain in the lumbar spine after interbody fusion with posterior instrumentation. An exploratory addition compared a normal spacer with a custom fit spacer that conformed to the vertebral endplates. Finite element models of the L4-L5 unit were developed from computed tomography of two cadaveric lumbar spines. The models were loaded under compression and strain and stress were observed in various locations. The models were validated with experimental testing.

In one study, cortical thickness and trabecular bone material properties were altered to simulate age-related changes of cortical thinning and the loss of volumetric trabecular bone mineral density. In both models, stress increased with age at the L4 and L5 anterior bone, and in the posterior rods. One model had a similar correlation at all analyzed locations, while the other model experienced smaller changes at the bone-spacer interfaces and spacer. The stress at the L4 and L5 anterior locations, at a minimum, doubled between the 20 and 90 year old model in both subjects.

In the other study, the effects of spacer material were investigated. The effects of polyetheretherketone, titanium, self-reinforced polyphenylene (SRP), and SRP simulated 70% porosity were tested. SRP with 70% porosity produced a load shift in stress from the spacer to the rods. Though titanium is approximately 25 times stiffer than PEEK, differences in stress levels at all locations were relatively small when the spacer material was changed from titanium to PEEK. However, when a factor of stiffness was reduced by 244 in SRP with 70% porosity, stress at the bone-spacer interfaces decreased (8-15%) in one model. This study demonstrated variation in subject response to different spacer materials, and the changes in stress distributions between human subjects. The custom spacer implant significantly decreased stress at the bone-spacer interfaces. These findings show the effects of age-related changes and spacer material on the mechanical environment in the bone following lumbar interbody fusion.

Cited Works (9)

Year	Entry
2003	Liebschner MAK, Kopperdahl DL, Rosenberg WS, Keaveny TM. Finite element modeling of the human thoracolumbar spine. Spine. March 15, 2003;28(6):559-565.
2002	Kopperdahl DL, Morgan EF, Keaveny TM. Quantitative computed tomography estimates of the mechanical properties of human vertebral trabecular bone. J Orthop Res. July 2002;20(4):801-805.
1998	Mosekilde L. The effect of modelling and remodelling on human vertebral body architecture. Technol Health Care. December 1998;6(5-6):287-297.
1990	Mosekilde L, Mosekilde L. Sex differences in age-related changes in vertebral body size, density and biomechanical competence in normal individuals. Bone. 1990;11(2):67-73.
2007	Bessho M, Ohnishi I, Matsuyama J, Matsumoto T, Imai K, Nakamura K. Prediction of strength and strain of the proximal femur by a CT-based finite element method. J Biomech. 2007;40(8):1745-1753.
2004	Riggs BL, Melton LJ III, Robb RA, Camp JJ, Atkinson EJ, Peterson JM, Rouleau PA, McCollough CH, Bouxsein ML, Khosla S. Population-based study of age and sex differences in bone volumetricdensity, size, geometry, and structure at different skeletal sites. J Bone Miner Res. December 2004;19(12):1945-1954.
1984	Armstrong CG, Lai WM, Mow VC. An analysis of the unconfined compression of articular cartilage. J Biomech Eng. May 1984;106(2):165-173.
2006	Imai K, Ohnishi I, Bessho M, Nakamura K. Nonlinear finite element model predicts vertebral bone strength and fracture site. Spine. July 15, 2006;31(16):1789-1794.
1998	Keyak JH, Rossi SA, Jones KA, Skinner HB. Prediction of femoral fracture load using automated finite element modeling. J Biomech. February 1998;31(2):125-133.