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Research Papers

Numerical Methodology to Evaluate the Effects of Bone Density and Cement Augmentation on Fixation Stiffness of Bone-Anchoring Devices

[+] Author and Article Information
Yan Chevalier

Department of Orthopedic Surgery,
Physical Medicine and Rehabilitation,
University Hospital of Munich (LMU),
Campus Grosshadern,
Marchioninistrasse 15,
Munich D-81377, Germany
e-mail: yan.chevalier@med.uni-muenchen.de

1Corresponding author.

Manuscript received November 21, 2014; final manuscript received June 16, 2015; published online July 14, 2015. Assoc. Editor: Kristen Billiar.

J Biomech Eng 137(9), 091005 (Sep 01, 2015) (10 pages) Paper No: BIO-14-1574; doi: 10.1115/1.4030943 History: Received November 21, 2014; Revised June 16, 2015; Online July 14, 2015

Bone quality is one of the reported factors influencing the success of bone anchors in arthroscopic repairs of torn rotator cuffs at the shoulder. This work was aimed at developing refined numerical methods to investigate how bone quality can influence the fixation stiffness of bone anchors. To do that bone biopsies were scanned at 26-μm resolution with a high-resolution microcomputer tomography (micro-CT) scanner and their images were processed for virtual implantation of a typical design of bone anchor. These were converted to microfinite element (μFE) and homogenized classical FE models, and analyses were performed to simulate pulling on the bone anchor with and without cement augmentation. Quantification of structural stiffness for each implanted specimen was then computed, as well as stress distributions within the bone structures, and related to the bone volume fraction of the specimens. Results show that the classical method is excellently correlated to structural predictions of the more refined μFE method, despite the qualitative differences in local stresses in the bone surrounding the implant. Predictions from additional loading cases suggest that structural fixation stiffness in various directions is related to apparent bone density of the surrounding bone. Augmentation of anchoring with bone cement stiffens the fixation and alters these relations. This work showed the usability of homogenized FE (hFE) in the evaluation of bone anchor fixation and will be used to develop new methodologies for virtual investigations leading to optimized repairs of rotator cuff and glenoid Bankart lesions.

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Figures

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Fig. 1

Combined methodology for the structural investigation of anchoring fixation stiffness. The creation of μFE models at 26 μm element size was done from modified micro-CT images of trabecular bone merged with a scanned bone anchor. The creation of hFE models was, in parallel, done after volumetric meshing of a filled implanted bone cube and subsequent bone material mapping from the micro-CT images. Simulation of augmented fixation with cement was done by modification of the material mapping procedure within the cementing volume described as a dilation of various thicknesses around the voxelized bone anchor. For clarity, the images show a cut-view of the bone structures through one of the symmetry planes of the bone structures (mirroring axis).

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Fig. 2

Load cases used to compute the 6D structural stiffness matrix of the implanted bone anchor with the hFE models. The highlighted area shows the upper nodes of the bone anchor rigidly connected to the central superior point at which displacements and rotations were applied. Elements are shown with local bone volume fraction ρ mapped from the modified micro-CT images.

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Fig. 3

Correlations between the hFE and μFE predictions for stiffness coefficients in the six orthogonal load cases

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Fig. 4

Relations between BV/TVglobal and the main stiffness components predicted by the μFE and hFE approaches

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Fig. 9

The effect of cement augmentation on peak stresses within bone, as predicted by the hFE models

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Fig. 8

Relations between the predicted hFE axial pull-out stiffness Sf3δ3 and BV/TVglobal for different cement thicknesses

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Fig. 7

Correlation coefficients between stiffness components of the: (a) Sfδ, (b) Sfθ, and (c) Smθ submatrices with axial pull-out stiffness Sf3δ3 for the hFE models without cement (white) and with different cement thicknesses (light gray = 0.8 mm; dark gray = 1.6 mm; and black = 2.4 mm)

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Fig. 6

Correlation coefficients between stiffness components of the: (a) Sfδ, (b) Sfθ, and (c) Smθ submatrices and BV/TVglobal for the hFE models without cement (white) and with different cement thicknesses (light gray = 0.8 mm; dark gray = 1.6 mm; and black = 2.4 mm)

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Fig. 5

Similarities observed in the stress distributions obtained for the μFE: (a) and the hFE models (b) for the five modeled specimens in simulated axial pull. Models are shown cut through one of the symmetry planes of the bone structures.

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