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Elastic properties and strain-to-crack-initation of calcium phosphate bone cements: Revelations of a high-resolution measurement technique
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Materials in Medicine)ORCID iD: 0000-0002-1524-2059
Istituto Ortopedico Rizzoli, Laboratorio di Tecnologia Medica.ORCID iD: 0000-0001-5676-2846
Istituto Ortopedico Rizzoli, Laboratorio di Tecnologia Medica.ORCID iD: 0000-0002-6903-7245
Biomaterials, Biomechanics and Tissue Engineering Group, Dept. of Materials Science and Metallurgy, Technical University of Catalonia (UPC).ORCID iD: 0000-0002-4700-5621
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2017 (English)In: Journal of The Mechanical Behavior of Biomedical Materials, ISSN 1751-6161, E-ISSN 1878-0180, Vol. 74, p. 428-437Article in journal (Refereed) Published
Abstract [en]

Calcium phosphate cements (CPCs) should ideally have mechanical properties similar to those of the bone tissue the material is used to replace or repair. Usually, the compressive strength of the CPCs is reported and, more rarely, the elastic modulus. Conversely, scarce or no data are available on Poisson's ratio and strain-to-crack-initiation. This is unfortunate, as data on the elastic response is key to, e.g., numerical model accuracy. In this study, the compressive behaviour of brushite, monetite and apatite cements was fully characterised. Measurement of the surface strains was done using a digital image correlation (DIC) technique, and compared to results obtained with the commonly used built-in displacement measurement of the materials testers. The collected data showed that the use of fixed compression platens, as opposed to spherically seated ones, may in some cases underestimate the compressive strength by up to 40%. Also, the built-in measurements may underestimate the elastic modulus by up to 62% as compared to DIC measurements. Using DIC, the brushite cement was found to be much stiffer (24.3 ± 2.3 GPa) than the apatite (13.5 ± 1.6 GPa) and monetite (7.1 ± 1.0 GPa) cements, and elastic moduli were inversely related to the porosity of the materials. Poisson's ratio was determined to be 0.26 ± 0.02 for brushite, 0.21 ± 0.02 for apatite and 0.20 ± 0.03 for monetite. All investigated CPCs showed low strain-to-crack-initiation (0.17–0.19%). In summary, the elastic modulus of CPCs is substantially higher than previously reported and it is concluded that an accurate procedure is a prerequisite in order to properly compare the mechanical properties of different CPC formulations. It is recommended to use spherically seated platens and measuring the strain at a relevant resolution and on the specimen surface.

Place, publisher, year, edition, pages
2017. Vol. 74, p. 428-437
National Category
Ceramics Medical Materials Biomaterials Science
Identifiers
URN: urn:nbn:se:uu:diva-316718DOI: 10.1016/j.jmbbm.2017.06.023ISI: 000410253500046PubMedID: 28735216OAI: oai:DiVA.org:uu-316718DiVA, id: diva2:1083740
Funder
The Swedish Foundation for International Cooperation in Research and Higher Education (STINT), IG2011-2047Swedish Research Council, 621-2011-6258Available from: 2017-03-22 Created: 2017-03-22 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Can Bone Void Fillers Carry Load?: Behaviour of Calcium Phosphate Cements Under Different Loading Scenarios
Open this publication in new window or tab >>Can Bone Void Fillers Carry Load?: Behaviour of Calcium Phosphate Cements Under Different Loading Scenarios
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Calcium phosphate cements (CPCs) are used as bone void fillers and as complements to hardware in fracture fixation. The aim of this thesis was to investigate the possibilities and limitations of the CPCs’ mechanical properties, and find out if these ceramic bone cements can carry application-specific loads, alone or as part of a construct. Recently developed experimental brushite and apatite cements were found to have a significantly higher strength in compression, tension and flexion compared to the commercially available CPCs chronOS™ Inject and Norian® SRS®. By using a high-resolution measurement technique the elastic moduli of the CPCs were determined and found to be at least twice as high compared to earlier measurements, and closer to cortical bone than trabecular bone. Using the same method, Poisson's ratio for pure CPCs was determined for the first time. A non-destructive porosity measurement method for wet brushite cements was developed, and subsequently used to study the porosity increase during in vitro degradation. The compressive strength of the experimental brushite cement was still higher than that of trabecular bone after 25 weeks of degradation, showing that the cement can carry high loads over a time span sufficiently long for a fracture to heal. This thesis also presents the first ever fatigue results for acidic CPCs, and confirms the importance of testing the materials under cyclic loading as the cements may fail at stress levels much lower than the material’s quasi-static compressive strength. A decrease in fatigue life was found for brushite cements containing higher amounts of monetite. Increasing porosity and testing in a physiological buffer solution (PBS), rather than air, also decreased the fatigue life. However, the experimental brushite cement had a high probability of surviving loads found in the spine when tested in PBS, which has previously never been accomplished for acidic CPCs. In conclusion, available brushite cements may be able to carry the load alone in scenarios where the cortical shell is intact, the loading is mainly compressive, and the expected maximum stress is below 10 MPa. Under such circumstances this CPC may be the preferred choice over less biocompatible and non-degradable materials.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. p. 67
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1492
Keywords
Calcium phosphate, bone cement, brushite, apatite, monetite, porosity, solvent exchange, degradation, compressive strength, diametral tensile strength, flexural strength, elastic modulus, Poisson’s ratio, fatigue
National Category
Ceramics Medical Materials Biomaterials Science
Research subject
Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-316656 (URN)978-91-554-9865-8 (ISBN)
Public defence
2017-05-12, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
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Supervisors
Available from: 2017-04-19 Created: 2017-03-22 Last updated: 2017-04-19

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