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Research Papers

Porous Biodegradable Lumbar Interbody Fusion Cage Design and Fabrication Using Integrated Global-Local Topology Optimization With Laser Sintering

[+] Author and Article Information
Heesuk Kang

Department of Mechanical Engineering,
University of Michigan,
Ann Arbor, MI 48109;
Department of Biomedical Engineering,
University of Michigan,
Ann Arbor, MI 48109

Scott J. Hollister

Department of Mechanical Engineering,
University of Michigan,
Ann Arbor, MI 48109;
Department of Biomedical Engineering,
University of Michigan,
Ann Arbor, MI 48109;
Department of Surgery,
University of Michigan,
Ann Arbor, MI 48109

Frank La Marca

Spine Research Laboratory,
Department of Neurosurgery,
University of Michigan,
Ann Arbor, MI 48109;
Department of Biomedical Engineering,
University of Michigan,
Ann Arbor, MI 48109

Paul Park

Spine Research Laboratory,
Department of Neurosurgery,
University of Michigan,
Ann Arbor, MI 48109

Chia-Ying Lin

Spine Research Laboratory,
Department of Neurosurgery,
University of Michigan,
Ann Arbor, MI 48109;
Department of Orthopaedic Surgery,
University of Michigan,
Ann Arbor, MI 48109;
Department of Biomedical Engineering,
University of Michigan,
Ann Arbor, MI 48109
e-mail: lincy@umich.edu

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received September 26, 2012; final manuscript received June 10, 2013; accepted manuscript posted July 29, 2013; published online September 23, 2013. Assoc. Editor: James C. Iatridis.

J Biomech Eng 135(10), 101013 (Sep 23, 2013) (8 pages) Paper No: BIO-12-1442; doi: 10.1115/1.4025102 History: Received September 26, 2012; Revised June 10, 2013; Accepted June 29, 2013

Biodegradable cages have received increasing attention for their use in spinal procedures involving interbody fusion to resolve complications associated with the use of nondegradable cages, such as stress shielding and long-term foreign body reaction. However, the relatively weak initial material strength compared to permanent materials and subsequent reduction due to degradation may be problematic. To design a porous biodegradable interbody fusion cage for a preclinical large animal study that can withstand physiological loads while possessing sufficient interconnected porosity for bony bridging and fusion, we developed a multiscale topology optimization technique. Topology optimization at the macroscopic scale provides optimal structural layout that ensures mechanical strength, while optimally designed microstructures, which replace the macroscopic material layout, ensure maximum permeability. Optimally designed cages were fabricated using solid, freeform fabrication of poly(ε-caprolactone) mixed with hydroxyapatite. Compression tests revealed that the yield strength of optimized fusion cages was two times that of typical human lumbar spine loads. Computational analysis further confirmed the mechanical integrity within the human lumbar spine, although the pore structure locally underwent higher stress than yield stress. This optimization technique may be utilized to balance the complex requirements of load-bearing, stress shielding, and interconnected porosity when using biodegradable materials for fusion cages.

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Figures

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Fig. 1

(a) Ligamentous FE models of mini-pig lumbar spine segments (L2–L5) and (b) design domain for global topology optimization at L4–L5 level

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Fig. 2

Global density maps (left) and segmentations (right) obtained using global topology optimization, under (a) flexion, (b) extension, (c) lateral bending, and (d) torsion. (e) Combination of all loading modes used for the final integrated design.

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Fig. 3

All property pairs of microstructures are on the cross-property upper bounds, indicating the microstructures are optimal. (a) and (c) were designed using microstructural topology optimization, and (b) and (d) were designed using primitive pore geometry (cylindrical holes).

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Fig. 4

(a) Pore architecture and final design of the cylindrical pore fusion cage. (b) Pore architecture and final design of topology optimized pore fusion cage. (c) A prototype fabricated using SFF. (d) Prototypes scaled to fit the minipig (upper) and human (lower) intervertebral disk spaces. (e) The customized cage height was checked in domestic pig lumbar intervertebral disk space.

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Fig. 5

For compression tests, fusion cages with (a) cylindrical pore microstructures, (b) optimized microstructures, and (c) the conventional TLIF cage were fabricated without detailed features to eliminate the initial yield caused by teethlike geometric features

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Fig. 6

Line graph showing compression test results, confirming superior stiffness and strength of the optimized designs over conventional TLIF design

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Fig. 7

Stress–strain curve obtained from compression test of a bulk cylindrical specimen to determine Young’s modulus and yield stress for the FE analysis

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Fig. 8

(a) von Mises stress level for optimal fusion cage without pore structures is below the yield stress (8.5 MPa). With initial pore structures (b) and (c), the stress level increased over the yield compared to (a). However, after initial bony fusion inside the pores (d and e), the stress level decreased below the yield (9 MPa). These results indicate that the majority of loading support is provided by the outer wall. Although local yield at the microstructures increases initially, ingrown bone will take over the loads from the fusion cage, alleviating the load burden at the microstructures.

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