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Influence of cement compressive strength and porosity on augmentation performance in a model of orthopedic screw pull-out
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Materials in Medicine)ORCID iD: 0000-0001-7004-2853
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Materials in Medicine)ORCID iD: 0000-0003-4139-6913
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Materials in Medicine)ORCID iD: 0000-0001-6663-6536
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Materials in Medicine)
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Disease and injuries that affect the skeletal system may require surgical intervention and internal fixation, i.e. orthopedic plate and screw insertion, to stabilize the injury and facilitate tissue repair. If the surrounding bone quality is poor the screws may migrate, or the bone may fail, resulting in screw pull-out. Though numerous studies have shown that cement augmentation of the interface between bone and implant can increase screw holding strength in bone, the physical properties of cement that influence pull-out force have not been investigated. The present study sought to determine how the physical properties of calcium phosphate cements (CPCs), and the strength of the biological or synthetic material surrounding the augmented screw, affected the corresponding orthopedic screw pull-out force in urethane foam models of healthy and osteoporotic bone (Sawbones). In the simplest model, where only the bond strength between screw thread and cement (without Sawbone) was tested, the correlation between pull-out force and cement compressive strength (R2 = 0.79) was weaker than correlation with total cement porosity (R2 = 0.89). In open pore Sawbone that mimics “healthy” cancellous bone density the stronger cements produced higher pull-out force (50-60% increase). Higher strength, lower porosity, cements also produced higher pull-out forces (50-190% increase) in Sawbones with cortical fixation if the failure strength of the cortical material was similar to (bovine tibial bone), or greater than (metal shell), actual cortical bone. This result is of particular clinical relevance where fixation with a metal plate implant is indicated, as the nearby metal can simulate a thicker cortical shell and, thereby, increase the pull-out force of screws augmented with stronger cements. The improvement in pull-out force was apparent even at low augmentation volumes of 0.5 ml (50% increase), which suggest that in clinical situations where augmentation volume is limited the stronger, lower porosity CPCs may still produce a significant improvement in screw holding strength. When correlations of all the tested models were compared both cement porosity and compressive strength accurately predicted pull-out force (R2=1.00, R2=0.808), though prediction accuracy depended upon the strength of the material surrounding the Sawbone. The correlations strength was low for bone with no, or weak, cortical fixation. Higher strength and lower porosity CPCs also produced greater pull-out force (1-1.5 kN) than commercial CPC (0.2-0.5kN), but lower pull-out force than PMMA (2-3 kN). The results of this study suggest that the likelihood of screw fixation failure may be reduced by selecting calcium phosphate cements with lower porosity and higher bulk strength, in patients with healthy bone mineral density and/or sufficient cortical thickness. This is of particular clinical relevance when fixation with metal plates is indicated, or where the augmentation volume is limited.

National Category
Medical Materials
Research subject
Engineering Science with specialization in Materials Science
Identifiers
URN: urn:nbn:se:uu:diva-320157OAI: oai:DiVA.org:uu-320157DiVA: diva2:1088835
Funder
Swedish Research Council, 262948EU, FP7, Seventh Framework Programme, 262948
Available from: 2017-04-16 Created: 2017-04-16 Last updated: 2017-04-18
In thesis
1.
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2. The biological and physical performance of high strength dicalcium phosphate cement in physiologically relevant models
Open this publication in new window or tab >>The biological and physical performance of high strength dicalcium phosphate cement in physiologically relevant models
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The chemical properties of calcium phosphate cements (CPCs) are very similar to the mineral phase of bone. CPCs are, consequently, very effective substrates (scaffolds) for tissue engineering; bone and stem cells attach readily, and can proliferate and differentiate to form new bone tissue. Unlike other CPCs that may remain largely unchanged in the body for years, such as hydroxyapatite, dicalcium phosphates are remodelled by the body and rapidly converted to new bone. Unfortunately, the dicalcium phosphates are also typically too weak to support load bearing in the human body. Our laboratory has recently developed a novel, high strength brushite CPC, (hsCPC), which can reach 10-50 fold higher failure strength than many commercially available CPCs. The aim of this thesis was to investigate the physical, chemical and biological performance of hsCPCs in physiologically relevant model of drug release, load bearing, osteoconductivity, and as a scaffold for bone tissue engineering.

Multiple CPCs were compared in a model of screw augmentation to determine whether the physical properties of the cement, such as bulk strength and porosity, affected orthopedic screw holding strength. In an in vitro model of bone regeneration stem cells were grown on macroporous scaffolds that were fabricated from hsCPC. Drug releasing scaffolds were fabricated to examine whether the low porosity of hsCPC impeded drug release during a 4 week incubation period. The biological activity of an incorporated drug, Rebamipide, was examined after acute and chronic incubation periods. In the drug release study it was noted that the biological response to hsCPC was significantly better than tissue culture grade polystyrene, even in groups without drug. The mechanism underlying this biological response was further investigated by testing the effect of pyrophosphate, a common cement additive, on bone cell proliferation and differentiation. This thesis concludes that a high strength cement can produce significant improvement in screw augmentation strength, if there is sufficient cortical bone near the augmentation site. The hsCPC is also cytocompatible, and can support bone and stem cell proliferation and differentiation. hsCPC scaffolds stimulated osteogenic gene expression comparable to native bone scaffolds. hsCPC scaffolds are also capable of delivering drug for up to 4 weeks, in vitro. Finally, a cement additive, pyrophosphate, stimulated differentiation, but not proliferation of bone cells.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 78 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1502
Keyword
biomaterial, bioceramic, osteoinduction, calcium phosphate, cement, osteoblast, pyrophosphate, Rebamipide, drug delivery, screw augmentation
National Category
Medical Materials
Research subject
Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-319495 (URN)978-91-554-9885-6 (ISBN)
Public defence
2017-06-02, Å2001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, GA 621-2011-3399EU, FP7, Seventh Framework Programme, 262948
Available from: 2017-05-05 Created: 2017-04-06 Last updated: 2017-08-09Bibliographically approved

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