Preliminary In Vitro Biocompatibility of Injectable Calcium Ceramic-Polymer Composite Bone Cement

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  • Preliminary in vitro biocompatibility of injectable calcium ceramic-polymer composite bone cement

    Mervi Puska1,2,3,a, Joni Korventausta2,3,b, Sufyan Garoushi3,c,

    Jukka Seppl1,d, Pekka K. Vallittu3,e and Allan Aho3,f

    1Laboratory of Polymer Technology, Helsinki University of Technology, P.O. Box 6100 FI-02015 HUT, Finland

    2Turku Biomaterials Centre, Itinen Pitkkatu 4 B, FI-20520 Turku, Finland

    3University of Turku, Institute of Dentistry, Department of Prosthetic Dentistry and Biomaterials

    Science, Lemminkisenkatu 2, FI-20520 Turku, Finland

    amervi.puska@utu.fi, bjoni.korventausta@utu.fi, csufyan.garoushi@utu.fi, djukka.seppala@tkk.fi, epekka.vallittu@utu.fi, fallan.aho@fimnet.fi

    Keywords: bone cement composite, compressive properties, calcium ceramics, biomineralization. Abstract. In the coming decades, the need for reconstructive surgery of bones is predicted to increase with the ageing of the population as well as the increase of injuries needing traumatologic treatments. Therefore, there is still a constant search for tissue engineering and bone substitute materials. Xenografts, synthetic hydroxyapatitite, bioactive glasses and other bone substitutes have widely been studied. When bone defects are filled using bioceramics in granules, their utilization is limited to small size defects, because the injected granules do not give immediate support against the biomechanical loading of the bone. The aim of this study was to evaluate the preliminary biomineralization and the compression strength of experimental injectable bone cements modified with calcium ceramics. Our studies have focused on the development of injectable composites of bone cements, i.e. in situ curable resin systems containing impregnated Ca ceramics. The polymerized bone cement composites aspire to simulate as closely as possible the mechanical and structural properties properties of bone. The present compressive strength of our inorganic-organic bone cements are >65 up to ~180 MPa. These cements are slightly porous from their outermost surface and showed preliminarily osteoconductivity of some degree. Introduction The acrylic bone cement connecting the endoprosthesis to bone should be structurally and mechanically as close to bone as possible [1]. A disadvantage associated with the use of traditional acrylic bone cement is the anchoring effect. It happens only as a mechanical locking, because the surface of the polymerised bone cement is of a dense structure that does not allow the bone to grow into the cement. Therefore, new bone formation is limited only to the surface between the bone cement and the bone. If the fixation is only mechanical, micromotion between the bone and implant interface can lead to resorption of bone and then the failure and the final loosening of prosthesis [2]. If the acrylic layer was porous and bioactive, it would stimulate bone ingrowth into the structure [3, 4]. Therefore, the focus of our studies is to modify biostable bone cements in order to improve their biological properties [5], for example, by creating porosity and a bioactive structure in the cement matrix. Clinically, injectable and moldable bone filling materials are desired for filling bone defects. In this study, the preliminary in vitro biocompatibility vs. compression strength of injectable bone cement composites impregnated with calcium ceramics were evaluated. Materials and Methods Table 1 shows the classification of the test groups. To prepare composites (Groups 1 and 2), the inorganic component was mixed with the organic prepolymer compound. The inorganic compound (30 or 50 wt% of CaSO4) was intensively dispersed in the viscous prepolymer liquid/dough,

    Key Engineering Materials Vols. 396-398 (2009) pp 273-276Online available since 2008/Oct/21 at www.scientific.net (2009) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.396-398.273

    All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 130.15.241.167, Queen's University, Kingston, Canada-18/08/14,13:46:25)

    http://www.scientific.nethttp://www.ttp.net

  • whereafter the composite was allowed to cure under NTP conditions in the shape of cylinders. The Control A group was prepared by mixing the plain CaSO4 compound with water that allowed to cure under NTP conditions over night. The manufacturing of Control B group and Group 3 are described in the literature [3,6,7]. Cylindrical test specimens (diameter: 4.5mm and height: 9mm) were manufactured from Control A or Groups 1 and 2. The chemical structure of the composites were characterized with Fourier-transform infrared (FTIR, PerkinElmer) spectroscopy. The morphology of the composites was evaluated by scanning electron microscopy (SEM, JSM-5600, JEOL, Japan). Compression strengths were measured using Lloyds materials testing machine (Lloyds model LRX, Lloyd Instruments). Table 1. The classification of test groups for the compression test. Group Description Control A Ca ceramics based on CaSO4 (plaster of Paris) Control B PMMA, acrylic bone cement (Palacos R)

    1 PMMA containing 30 wt% of Ca ceramics 2 Crosslinked bisphenol A polymer containing 30 wt% of Ca ceramics 3 Fibre reinforced crosslinked polymer (BisGMA network)

    The cylindrical test specimens (150-200 mg) containing 50 wt% of Ca ceramics were soaked in simulated body fluid (SBF) to analyze the in vitro calcium phosphate formation on the fracture plane. The composites were placed in test tubes with 50 ml SBF and incubated in a shaking water bath at 37C. The follow-up time was 10 days. The formation of mineral layer was investigated morphologically by SEM connected to an energy dispersive X-ray analyzer (EDXA, Princeton Gamma-Tech, Prism2000). Results and Discussion

    4000,0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 650,0

    50,0

    55

    60

    65

    70

    75

    80

    85

    90

    95

    100,0

    cm-1

    %T

    Figure 1. The FTIR spectrum of Palacos R containing 30 wt% of Ca ceramics.

    According to the analysis of FTIR and SEM/EDX, Ca ceramics in the biostable polymer network seemed to be homogenously distributed (Fig. 1 and 3A). The compression strength of Palacos R containing 30wt% of Ca ceramics was ca. 80 MPa, whereas it was ca. 70 MPa if bisphenol-A-based polymer contained 50 wt% of Ca ceramics (Fig. 2). As a reference values, the compression strength of plaster of Paris (Control A) was ca. 10 MPa and of plain acrylic bone cement (Control B) was ca. 90 MPa. According to the preliminary in vitro biocompatibility studies, injectable biostable bone

    274 Bioceramics 21

  • cement composites containing Ca ceramics seem to hold some osteoconductivity. Namelly, after 10 days of soaking in SBF a uniform calcium phosphate layer was formed on the Ca ceramics composite (Fig. 3). In addition, the measured Ca/P molar ratio of the mineral, obtained from the formed calcium phosphate layer, was ca. 1.68. Actually, this Ca/P molar ratio is quite equal to 1.67, which is obtained for the biological apatite stoichiometry.

    Compression Strength [MPa]

    0

    50

    100

    150

    200

    250

    Control A Control B Group 1 Group 2 Group 3

    Figure 2. The compression strengths of Palacos R containing 30wt% of Ca ceramics (Group 1) and bisphenol-A-based polymer containing 50wt% of Ca ceramics (Group 2). The reference groups are the plaster of Paris (Control A), Palacos R bone cement (Control B) and dental fiber-reinforced BisGMA network (Group 3). The compression strength of cortical bone is reported to be between 120 and 150 MPa [8]. According to the results of this study, if biostable bone cement contain Ca ceramics, the produced composite has more biocompatible properties in vitro. These injectable composites are also possible to be utilized for the manufacturing of ex vivo implants. Therefore, these materials would most likely be very practical for bone and joint reconstruction. By utilizing these composites as ex vivo implants, the leaching of toxic substances (e.g. the residual acrylic monomers) could also be minimized or even eliminated. In addition, the mechanical properties of these composites are also remarkably good for reconstruction of cortical bone. In future, the studies as these partly biostable bone cements containing Ca ceramics will be continued to improve their biomechanical properties using fiber-reinforcing. Namely, the results obtained from the dental fiber-reinforced composites have shown that the compression strength corresponding the demands of cortical bone are possible to achieve [6,7]. In orthopaedical applications, the biomechanical properties of orthopaedic materials should be strong enough to withstand the physiological stresses of the body, but they should also have identical elasticity to bone. If the biomechanical fixation is not good enough, the bone - cement interface is prone to micromotion, which can lead to the resorption of bone and then the failure of prosthesis. Therefore, the average lifetime of total knee/hip replacement system is not long enough and several expensive reoperations are performed in every year (in Finland ca. 2000 in the year 2005).

    The approximated level of cortical bones compression strength.

    Key Engineering Materials Vols. 396-398 275

  • Figure 3. SEM graphs show the structure of experimental injectable bone cement containing Ca ceramics before (A) and after (B) biomineralization. On the left, the image shows the fracture plane of composite containing Ca ceramics (50wt%) and, on the right, the situation after 10d of soaking in SBF. During the biomineralization test, CaP was precipitated on the composite. The length of measuring rod is 20 m. Conclusion Within the limitations of these studies, the present compressive strength of injectable bone cements containing Ca ceramics are >65 MPa up to ~180 MPa. In addition, this inorganic-organic hybrid cement is slightly porous from its outermost surface and shows some osteoconductivity. In fact, after the biomineralization test, CaP was precipitated on the composite, thus preliminarily indicating a significant bioactivity of composite. Acknowledgement The research was funded by the Academy of Finland and Tekes (Finnish Funding Agency for Technology and Innovation). The Finnish Dental Society APOLLONIA is thanked for the travel grant. Mr Timothy Wilson (M.Sc.) is thanked for language proofreading of this article. References

    [1] A. Aho, T. Tirri, J. Kukkonen, N. Strandberg, J. Rich, J. Seppl, and A. Yli-Urpo: J. Mater. Sci. Mater. Med. Vol. 15 (2004), p. 1165

    [2] S. Santavirta, J. Xu, J. Hietanen, A. Ceponis, T. Sorsa, R. Kontio, and Y. Konttinen: Clinical orthopaedics and related research Vol. 352 (1998), p. 16

    [3] M. Puska, A. Kokkari, T. Nrhi, and P. Vallittu: Biomaterials Vol. 24 (2002), p. 417

    [4] M. Puska, A. Aho, T. Tirri, A. Yli-Urpo, M. Vaahtio, and P. Vallittu: Key Engineering Materials Vols. 309-311 (2006), p. 809

    [5] E. Frankenburg, S. Goldstein, T. Bauer, S. Harris, and R. Poser RD: J Bone Joint Surg Am. Vol. 80 (1998), p.1112

    [6] S. Garoushi, P. Vallittu, and L. Lassila: Dent. Mater. Vol. 23 (2007), p. 1356

    [7] S. Garoushi, L. Lassila, and P. Vallittu: J. Contemp. Dent. Pract. Vol. 7 (2006), p. 10

    [8] D. Reilly and A. Burstein: J. Bone Joint. Surg. Am.Vol. 56A (1974), p. 1001

    (A) (B)

    276 Bioceramics 21

  • Bioceramics 21 10.4028/www.scientific.net/KEM.396-398 Preliminary In Vitro Biocompatibility of Injectable Calcium Ceramic-Polymer Composite BoneCement 10.4028/www.scientific.net/KEM.396-398.273 DOI References[1] A. Aho, T. Tirri, J. Kukkonen, N. Strandberg, J. Rich, J. Seppl, and A. Yli-Urpo: J. Mater. Sci. Mater.Med. Vol. 15 (2004), p. 1165doi:10.1023/B:JMSM.0000046401.50406.9b [4] M. Puska, A. Aho, T. Tirri, A. Yli-Urpo, M. Vaahtio, and P. Vallittu: Key Engineering Materials Vols.309-311 (2006), p. 809doi:10.4028/www.scientific.net/KEM.309-311.809 [1] [ 2] [ 3] [ 4] [ 5] [ 6] [ 7] [ 8] A. Aho, T. Tirri, J. Kukkonen, N. Strandberg, J. Rich, J. Seppl, and A. Yli-Urpo: J. Mater. Sci. Mater. Med. Vol. 15 (2004), p. 1165 S. Santavirta, J. Xu, J. Hietanen, A. Ceponis, T.Sorsa, R. Kontio, and Y. Konttinen: Clinical orthopaedics and related research Vol. 352 (1998), p. 16 M.Puska, A. Kokkari, T. Nrhi, and P. Vallittu: Biomaterials Vol. 24 (2002), p. 417 M. Puska, A. Aho, T. Tirri,A. Yli-Urpo, M. Vaahtio, and P. Vallittu: Key Engineering Materials Vols. 309-311 (2006), p. 809 E.Frankenburg, S. Goldstein, T. Bauer, S. Harris, and R. Poser RD: J Bone Joint Surg Am. Vol. 80 (1998),p.1112 S. Garoushi, P. Vallittu, and L. Lassila: Dent. Mater. Vol. 23 (2007), p. 1356 S. Garoushi, L. Lassila,and P. Vallittu: J. Contemp. Dent. Pract. Vol. 7 (2006), p. 10 D. Reilly and A. Burstein: J. Bone Joint. Surg.Am.Vol. 56A (1974), p. 1001http://dx.doi.org/10.1023/B:JMSM.0000046401.50406.9b

    http://dx.doi.org/www.scientific.net/KEM.396-398http://dx.doi.org/www.scientific.net/KEM.396-398.273http://dx.doi.org/10.1023/B:JMSM.0000046401.50406.9bhttp://dx.doi.org/10.4028/www.scientific.net/KEM.309-311.809http://dx.doi.org/http://dx.doi.org/10.1023/B:JMSM.0000046401.50406.9b

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