Correlation between impact strength and fracture toughness of PMMA-based bone cements

The thrust of the work involved determining the impact strength, IS (in kJ m(-2)) [using non-ASTM-sized Charpy-type specimens and an in-house impact tester] and fracture toughness, K1c (in MPa square root(m)) [using ASTM-sized rectangular compact tension specimens] of Surgical Simplex P and three variants of Palacos R acrylic bone cements. The attractions and drawbacks of this method for determining IS are detailed. The best fit to the K1c and IS results yielded a power relationship K1c = 0.795(IS)(0.59). The usefulness and limitations of this relationship are detailed.     

Fatigue and fracture toughness of acrylic bone cements modified with long-chain amine activators

The composition of acrylic bone cement has been identified as one of the important parameters affecting its mechanical properties and may, in turn, ultimately influence the longevity of a cemented arthroplasty. Our aim in this study was to determine the influence of change of one compositional variable, the activator, on the fatigue performance and fracture toughness of specimens of the fully cured cement. To that end, three sets of cements were prepared, containing either the conventional activator, 4-N,N dimethyl p-toluidine (DMPT), or novel ones that are tertiary amines based on long-chain fatty acids, that is, 4-N,N dimethylaminobenzyl oleate (DMAO) and 4-N,N dimethylaminobenzyl laurate (DMAL). In the fatigue tests, the specimens were subjected to tension-tension loading, and the results (number of cycles to failure, Nf) were analyzed using the linearized form of the three-parameter Weibull equation. The fracture toughness (KIc) tests were conducted with rectangular compact tension specimens. All fracture surfaces were subsequently examined with scanning electron microscopy. We found that the Weibull mean fatigue lives for specimens fabricated using the DMPT, DMAL, and DMAO containing cements were 272,823, 453,551, and 583,396 cycles, respectively. The corresponding values for KIc were 1.94 +/- 0.05, 2.06 +/- 0.09, and 2.00 +/- 0.07 MPa radical m, respectively. Statistical analyses showed that for both the DMAL- and DMAO-containing cements, the mean values of Nf were significantly higher compared to the corresponding value for the DMPT-containing cement (Mann-Whitney test; alpha < 0.10). This result is attributed to the higher molecular weights of the former cements compared to the latter. The same trend was found for the mean KIc values (Mann-Whitney test; alpha < 0.05), with the trend being explained in terms of the differences seen in the crack morphologies. These results thus demonstrate that these novel amines are viable alternatives to DMPT for incorporation into acrylic bone cement formulations in the future.     

Bone bonding ability and handling properties of a titania-polymethylmethacrylate (PMMA) composite bioactive bone cement modified with a unique PMMA powder

One of the challenges of using bioactive bone cements is adjusting their handling properties for clinical application. To resolve the poorer handling properties of bioactive bone cements we developed a novel bioactive bone cement containing a unique polymethylmethacrylate (PMMA) powder, termed SPD-PMMA (40 μm in diameter), composed of cohered minute particles of PMMA (0.5 μm). The present study aimed to examine the mechanical and handling properties and the in vivo bone bonding strength of this cement. The titania content of the cement varied from 10 to 30 wt.% (Ts10, Ts20, and Ts30). The mechanical and thermal properties of Ts10 and Ts20 exceeded those of commercially available PMMA cements (PMMAc). The setting properties of Ts20, including a shorter dough time and a working time that was comparable with that of PMMAc, were adequate for clinical application. Hardened cylindrical cement specimens were inserted into rabbit femurs and the interfacial shear strengths were measured by a push-out test at 6, 12, and 26 weeks after the operation. The interfacial shear strength values (in Newtons per square millimeter) of Ts10, Ts20, and Ts30 at 12 weeks and those of Ts20 and Ts30 at 26 weeks were significantly higher than that of PMMAc (P < 0.05). These results show that a bioactive titania–PMMA composite bone cement modified by SPD-PMMA particles possesses adequate mechanical and handling properties, as well as osteoconductivity and in vivo bone bonding ability, and can be used for prosthesis fixation.     

Influence of lactose addition to gentamicin-loaded acrylic bone cement on the kinetics of release of the antibiotic and the cement properties

The purpose of this study was to characterize a poly(methyl methacrylate) bone cement that was loaded with the antibiotic gentamicin sulphate (GS) and lactose, which served to modulate the release of GS from cement specimens. The release of GS when the cement specimens were immersed in phosphate-buffered saline at 37 °C was determined spectrophotometrically. The microstructure, porosity, density, tensile properties and flexural properties of the cements were determined before and after release of GS. A kinetics model of the release of GS from the cement that involved a coupled mechanism based on dissolution/diffusion processes and an initial burst effect was proposed. Dissolution assay results showed that drug elution was controlled by a diffusion mechanism which can be modulated by lactose addition. Density values and mechanical properties (tensile strength, flexural strength, elastic modulus and fracture toughness) were reduced by the increased porosity resulting from lactose addition, but maintained acceptable values for the structural functions of bone cement. The present results suggest that lactose-modified, gentamicin-loaded acrylic bone cements are potential candidates for use in various orthopaedic and dental applications.     

Encapsulated verses hand-mixed zinc phosphate dental cement.

Zinc phosphate cements are commonly supplied as two components, powder and liquid, and the proportions of the constituents are determined by operator experience. A capsulated system which is mechanically mixed has been marketed and this study investigated the performance of the encapsulated cement system. The mean fracture strength, standard deviation and associated Weibull Moduli (m) of encapsulated cements were determined by compressive fracturing 20 cement specimens filled directly from the mixing syringe or from narrower cement tubes. Pore distribution within the cylindrical specimens was determined using image analysis to assess the influence of the method of mould filling with the cement. The strength data showed variation in magnitude and consistency ranging from 44.6+/-13.7 MPa (m = 3.18+/-0.71) for cements filled directly from the syringe to 61.0+/-7.8 MPa (m = 8.35+/-1.87) for cements filled from cement tubes. Larger pores were found in specimens consolidated directly from the cement syringe. Mechanical mixing of the encapsulated cement resulted in air entrapment in the cement mix which manifested itself as large pores (over 200 microm diameter) within the cylindrical specimens. The smaller orifice of the cement tube compared with the syringe was considered to be responsible for eliminating the majority of the air entrapped in the cement mass during mixing. Whilst mechanical mixing of encapsulated cements is quicker and more convenient, the encapsulated specimens consolidated according to the manufacturers instructions from the syringe offered no significant advantage in terms of reliability or strength over hand-mixed cements in this investigation.     

PMMA bone cement containing a quaternary amine comonomer with potential antibacterial properties

An iodinated quaternary amine dimethacrylate monomer was synthesized and incorporated as a comonomer in acrylic bone cements. Bone cement is used in orthopaedic surgery and imparting antibacterial properties to the cement can be beneficial in the lowering of bacterial infection post surgery. PMMA based bone cements were modified by copolymerising the monomer methylmethacrylate (MMA) with a quaternary amine dimethacrylate by using the redox initiator activator system as used for curing commercial bone cements. The cements were prepared using the commercial PMMA bone cement CMW and the liquid component was modified with the amine to render antimicrobial properties to the cement. The physical, mechanical, and antimicrobial properties of the modified cements were evaluated; in addition, the viability of the cement to function as a orthopaedic cement was also established, especially with an advantage of it being radiopaque, due to the inclusion of the iodine containing quaternary amine. The cytotoxicity of the modified cements were tested using a human cell model and the results indicated that the cells remained metabolically active and proliferated when placed in direct contact with the experimental cement specimens. The cements and their eluants did not evoke any cytotoxic response.     

In vivo behavior of three different injectable hydraulic calcium phosphate cements

Two dicalcium phosphate dihydrate (DCPD) hydraulic cements and one apatite hydraulic cement were implanted in epiphyseal and metaphyseal, cylindrical bone defects of sheep. The in vivo study was performed to assess the biocompatibility of the DCPD cements, using the apatite cement as control. After time periods of 2, 4 and 6 months the cement samples were clinically and histologically evaluated. Histomorphometrically the amount of new bone formation, fibrous tissue and the area of remaining cement were measured over time. In all specimens, no signs of inflammation were detectable either macroscopically or microscopically. All cements were replaced by different amounts of new bone. The two DCPD-cements showed the highest new bone formation and least cement remnants at 6 months, whereas the apatite was almost unchanged over all time periods.     

Creep properties of three low temperature-curing bone cements: a preclinical assessment

The mechanical characteristics of new bone cements should be assessed before these cements are released on the orthopedic market in great quantities. In this study, we present the deformational response of 3 relatively new, low-curing temperature bone cements (Cemex RX, Cemex System, and Cemex Isoplastic) to a dynamic compressive force in comparison to Simplex P bone cement. For this purpose, dynamic compressive creep tests were performed on cylindrical shaped specimens at a maximal load level of 20 MPa for a period of 250,000 cycles. The results showed that Cemex System and Cemex RX produced creep rates that were higher (20% and 30%, respectively) as compared to Simplex P bone cement. The creep behavior of Cemex Isoplastic was very similar to that of Simplex P. It was concluded that although Cemex RX and Cemex System produced higher creep rates than Simplex P, these differences were not considered excessive. Hence, although other tests are required to assess the safety and efficacy of these new cements, the dynamic creep properties under compression can be considered adequate for clinical use.     

The effect of water uptake on the behaviour of hydrophilic cements in confined environments

Physiological fluids will be in contact with the implant components from the first moments after a surgery. Therefore, the study of the effect of water on the properties of the bone cements that are part of the arthroplasty procedure is of critical importance to predict the long-term performance of the whole system. In our research group, we have developed a novel concept, the hydrophilic, partially degradable and bioactive cements which uptake considerably more water than standard bone cements. In this paper, we aimed to study the effect of water uptake (WU) by these cements on their behaviour. The tests were carried out in confined cavities, which represent more accurately the in vivo situation the cement will face (constrained by the bone and prosthesis surfaces). We observed that the equilibrium WU decreased up to 60% (as compared to non-confined situations), depending of the formulation. This decrease resulted in a latent tendency of the cements to swell, and the hindering of such swelling generated a swelling pressure against the constraining walls. The pressure, and consequent press-fitting effect, could be controlled by a number of mechanisms, and resulted in higher stability of the hydrophilic cements, expressed as an increase in the push-out force, required to extract the specimens from such constrained cavities. This effect was only observed in hydrophilic cements, not in commercial, hydrophobic ones used as controls. We conclude that such cements will provide an additional and very useful source of immediate adhesion in the short-term after surgery: water induced press fitting.     

VersaBond bone cement prospective randomized study of the clinical properties of a new bone cement in total knee replacement

VersaBond is a newly developed bone cement. To investigate its clinical performance, VersaBond was compared to Palacos R in a prospective randomized study in total knee replacement. Fifty-nine patients (61 knees) undergoing total knee replacement were randomized to either VersaBond or Palacos R bone cement and followed for 24 months using radiostereometric analysis (RSA). Up to 2 years there were no significant differences in clinical performance between the two cements. The mean/median values for implant migration were very similar for the two bone cements, as were the dispersion, and distribution of outliers. Also the proportion "stable" and "continuously migrating" implants was similar between the two cements. The result of this study indicates that VersaBond bone cement will perform at least equally as well as Palacos R in total knee replacement as regards as aseptic loosening.     

Relative influence of composition and viscosity of acrylic bone cement on its apparent fracture toughness

The composition and viscosity of an acrylic bone cement have both been identified in the literature as being parameters that affect the mechanical properties of the material and, by extension, the in vivo longevity of cemented arthroplasties. The objective of the present study was to determine the relative influence of these parameters on a key cement mechanical property; namely, its fracture toughness. Two sets of cements were selected purposefully to allow the study objective to be achieved. Thus, one set comprised two cements with very similar compositions but very different viscosities (Cemex RX, a medium-viscosity brand, and Cemex Isoplastic, a high-viscosity brand) while the other set comprised two cements with similar viscosities but with many differences in composition (Cemex Isoplastic and CMW 1). Values of the fracture toughness (as determined using chevron-notched short rod specimens) [K(ISR)] obtained for Cemex RX and Cemex Isoplastic were 1.83 +/- 0.12 and 1.85 +/- 0.12 MPa square root(m), respectively, with the difference not being statistically significant. The K(ISR) values obtained for Cemex Isoplastic and CMW 1 were 1.85 +/- 0.12 and 1.64 +/- 0.18 MPa square root(m), respectively, with the difference being statistically significant. Thus, the influence of cement composition on its K(ISR) is more marked relative to the influence of cement viscosity. Explanations of this finding are offered, together with comments on the implications of the results for the in vivo longevity of cemented arthroplasties.     

Influence of polymeric additives on the biological properties of brushite cements: an experimental study in rabbit

The resorbability and ability of calcium phosphate hydraulic cements to promote new bone formation was investigated in vivo. The effects of two hydrosoluble polymeric additives (hyaluronic acid, and xanthan gum,) on the biological response of two brushite cement formulations (BHC-A vs BHC-B) was investigated. The brushite cements differed in P/Ca (0.71 vs 0.98) and S/Ca (0.10 vs 0.005) atomic ratios and by the presence of calcium sulfate hemihydrate in BHC-A. Polymer-free cements were used as controls. Cement specimens were injected in cylindrical bone defects manually drilled in the distal condyle of rabbit femora. The implants were harvested at 12 and 24 weeks after implantation and subjected to quantitative histomorphometry. The study showed a significantly lower resorption rate for cement BHC-A, which induces the formation of well-mineralized bone in close apposition to the residual material. In contrast, cement BHC-B showed a significant increase of bone formation period and the formation of a thick layer of unmineralized osteoid tissue at the bone/residual cement interface. The presence of xanthan gum made the biological response even worse, particularly in the case of cement BHC-B. The presence of hyaluronic acid has little effect, except for a slight decrease in initial resorption rate, in the case of cement BHC-A.     

Novel bioactive composite bone cements based on the beta-tricalcium phosphate-monocalcium phosphate monohydrate composite cement system

Bioactive composite bone cements were obtained by incorporation of tricalcium silicate (Ca3SiO5, C3S) into a brushite bone cement composed of beta-tricalcium phosphate [beta-Ca3(PO4)2, beta-TCP] and monocalcium phosphate monohydrate [Ca(H2PO4)2.H2O, MCPM], and the properties of the new cements were studied and compared with pure brushite cement. The results indicated that the injectability, setting time and short- and long-term mechanical strength of the material are higher than those of pure brushite cement, and the compressive strength of the TCP/MCPM/C3S composite paste increased with increasing aging time. Moreover, the TCP/MCPM/C3S specimens showed significantly improved in vitro bioactivity in simulated body fluid and similar degradability in phosphate-buffered saline as compared with brushite cement. Additionally, the reacted TCP/MCPM/C3S paste possesses the ability to stimulate osteoblast proliferation and promote osteoblastic differentiation of the bone marrow stromal cells. The results indicated that the TCP/MCPM/C3S cements may be used as a bioactive material for bone regeneration, and might have significant clinical advantage over the traditional beta-TCP/MCPM brushite cement.     

Soft-tissue response to injectable calcium phosphate cements

In this study, the soft tissue reaction to two newly developed injectable calcium phosphate bone cements (cement D and W) was evaluated after implantation in the back of goats. For one of the cements (cement D) the tissue reaction was also investigated after varying the concentration of accelerator Na(2)HPO(4) in the cement liquid (resulting in cement D1 and D2). Eight healthy mature female Saanen goats were used. The cement was applied 10min after mixing while it was still moldable and plastic. The material was given a standardized cylindrical shape. Thirty-two implants of each cement formulation were inserted and left in place for 1, 2, 4, and 8weeks. At the end of the study, eight specimens of each material and healing period were available for further analysis. Two specimens were used for X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) and six specimens were used for light microscopical evaluation. XRD and FTIR showed that the cements did set as microcrystalline carbonate apatite with the disappearance of monetite from the cements during implantation. Histological analysis showed that after 8weeks of implantation around all materials a thin soft-tissue capsule was formed (thickness ranging from 5 to 15 cell layers) with almost complete absence of inflammatory cells. Only in some specimens a slightly higher inflammatory reaction was observed. This was due to cement surface defects and a zone of dispersed particles near the cement-soft tissue interface. There was almost no resorption of the material after 8 weeks of implantation. In a few 4 and 8weeks samples, small areas of calcification were found in the fibrous capsule surrounding the implants. On the basis of our observations, we conclude that the tested cements were biocompatible and can be used next to soft tissue.     

Surface characterisation of various bone cements prepared with functionalised methacrylates/bioactive ceramics in relation to HOB behaviour

This study reports the relationship between the biocompatibility and surface properties of experimental bone cements. The effect of hydroxyapatite (HA) or alpha-tri-calcium phosphate (alpha-TCP) incorporated into bone cements prepared with methyl methacrylate as base monomer and either methacrylic acid or diethyl amino ethyl methacrylate (DEAEMA) as comonomers was investigated. The in vitro biocompatibility of these composite cements was assessed in terms of the interaction of primary human osteoblasts grown on the materials over a period of 5 days and compared with a control surface. These results were related to the surface properties investigated through a number of techniques, namely Fourier transform infrared, contact angle measurements, X-ray photoelectron spectroscopy and energy dispersive analysis of X-rays. Complementary techniques of thermal analysis and ion chromatography were also performed. Biocompatibility results showed that the addition of alpha-TCP improves biocompatibility regardless of comonomer type. This is in contrast to HA-based cements where cell proliferation was significantly lower. Surface characterisations showed that structural integrity of the materials was maintained in the presence of the acid and base comonomers, and water contact angles were reduced particularly in DEAEMA containing materials. Furthermore, ion chromatography confirmed higher Ca2+ and PO4(3-) ion release by both types of ceramics, particularly for those containing DEAEMA. In conclusion, the incorporation of acidic and basic comonomers to either HA or alpha-TCP ceramics containing bone cements can have differential effects upon the attachment and proliferation of bone cells in vitro. Moreover, those cements consisting of alpha-TCP and containing DEAEMA comonomer indicated the most favourable biocompatibility.     

A radiopaque polymeric matrix for acrylic bone cements

As part of the search for an alternative to inorganic radiopaque agents, this work studies the possibility of modifying bone cement formulations by incorporating a radiopaque monomer, that is, 4-iodophenol methacrylate (IPMA), in the liquid phase. The monomer was synthesized in the laboratory, and cements were prepared by the standard method. The influence on the different cement characteristics of various monomer concentrations was studied. It was seen that the setting time decreased as the percentage of monomer increased. The radiopacity attained in the 15 vol.% IPMA formulations was about the same as that for a cement containing 10 wt.% barium sulphate. Dynamic and static mechanical properties were measured. The materials did not show significant differences in the glass transition temperature. However, static mechanical properties showed enhanced compressive strength, tensile strength, and elastic modulus with respect to conventional cements formulated with barium sulphate. Histological studies showed a good response of muscular tissue to implanted specimens.     

Gentamicin-loaded strontium-containing hydroxyapatite bioactive bone cement--an efficient bioactive antibiotic drug delivery system

Modified strontium-containing hydroxyapatite (Sr-HA) bone cement was loaded with gentamicin sulfate to generate an efficient bioactive antibiotic drug delivery system for treatment of bone defects. Gentamicin release and its antibacterial property were determined by fluorometric method and inhibition of Staphylococcus aureus (S. aureus) growth. Gentamicin was released from Sr-HA bone cement during the entire period of study and reached around 38% (w/w) cumulatively after 30 days. Antibacterial activity of the gentamicin loaded in the cements is clearly confirmed by the growth inhibition of S. aureus. The results of the amount and duration of gentamicin release suggest a better drug delivery efficiency in Sr-HA bone cement over polymethylmethacrylate bone cement. Bioactivity of the gentamicin-loaded Sr-HA bone cement was confirmed with the formation of apatite layer with 1.836 ± 0.037 μm thick on day 1 and 5.177 ± 1.355 μm thick on day 7 after immersion in simulated body fluid. Compressive strengths of the gentamicin-loaded Sr-HA cement reached 132.60 ± 10.08 MPa, with a slight decrease from the unloaded groups by 4-9%. Bending moduli of Sr-HA cements with and without gentamicin were 1.782 ± 0.072 GPa and 1.681 ± 0.208 GPa, respectively. On the contrary, unloaded Sr-HA cement obtained slightly larger bending strength of 35.48 ± 2.63 MPa comparing with 33.00 ± 1.65 MPa for loaded cement. No statistical difference was found on the bending strengths and modulus of gentamicin-loaded and -unloaded Sr-HA cements. Sr-HA bone cement loaded with gentamicin was proven to be an efficient drug delivery system with uncompromised mechanical properties and bioactivity.     

Tissue response to in situ polymerization of a new two-solution bone cement: evaluation in a sheep model

A two-solution bone cement (2-SC) was evaluated in a non-load bearing sheep model that simulated insertion of a cemented total joint replacement. A commercial powder-liquid bone cement formulation (Palacos R) was used as the control. The systemic response to the two cements was determined by monitoring changes in arterial blood pressure (ABP) and serum concentrations of methyl methacrylate monomer at intervals after insertion of the cement. The short-term tissue response to the two cements was assessed by quantifying histomorphometric parameters of new bone formation at 2, 4, and 12 weeks postsurgery. Intraoperatively, injection and pressurization of bone cement were well tolerated, with no significant changes in ABP in either group and no detectable circulating monomer in any animal. Several interesting trends were identified in the histomorphometry data. In the trabecular specimens, new bone formation immediately adjacent to the cement mantle was apparently suppressed in the first 2 weeks postsurgery, increased dramatically at 4 weeks, and then returned to baseline values by 12 weeks. This pattern was seen with both Palacos and 2-SC. In the cortical specimens, new bone formation was reduced on the endosteal surface when compared with the periosteal surface, with this effect being more noticeable at 2 and 4 weeks than at 12 weeks. There were no significant histopathological findings in either the bone or the draining lymph nodes. These data indicate that the biological response to 2-SC is substantially equivalent to that of Palacos R. Additional testing in a functional, load-bearing animal model is now recommended to more fully characterize the long-term biological response to 2-SC and to determine the mechanical performance of this new cement in vivo.     

Bone marrow modified acrylic bone cement for augmentation of osteoporotic cancellous bone

The use of polymethylmethacrylate (PMMA) cement to reinforce fragile or broken vertebral bodies (vertebroplasty) leads to extensive bone stiffening. This might be one reason for fractures at the adjacent vertebrae following this procedure. PMMA with a reduced Young's modulus may be more suitable. The goal of this study was to produce and characterize PMMA bone cements with a reduced Young's modulus by adding bone marrow. Bone cements were produced by combining PMMA with various volume fractions of freshly harvested bone marrow from sheep. Porosity, Young's modulus, yield strength, polymerization temperature, setting time and cement viscosity of different cement modifications were investigated. The samples generated comprised pores with diameters in the range of 30-250 μm leading to porosity up to 51%. Compared to the control cement, Young's modulus and yield strength decreased from 1830 to 740 MPa and from 58 to 23 MPa respectively by adding 7.5 ml bone marrow to 23 ml premixed cement. The polymerization temperature decreased from 61 to 38 °C for cement modification with 7.5 ml of bone marrow. Setting times of the modified cements were lower in comparison to the regular cement (28 min). Setting times increased with higher amounts of added bone marrow from around 16-25 min. The initial viscosities of the modified cements were higher in comparison to the control cement leading to a lower risk of extravasation. The hardening times followed the same trend as the setting times. In conclusion, blending bone marrow with acrylic bone cement seems to be a promising method to increase the compliance of PMMA cement for use in cancellous bone augmentation in osteoporotic patients due to its modified mechanical properties, lower polymerization temperature and elevated initial viscosity.     

Newly developed Sr-substituted α-TCP bone cements

New bone cements made of Sr-substituted brushite-forming α-tricalcium phosphate (α-TCP) were prepared and characterized in the present work. The quantitative phase analysis and structural refinement of the starting powders and of hardened cements were performed by X-ray powder diffraction and the Rietveld refinement technique. Isothermal calorimetry along with setting time analysis allowed a precise tracing of the setting process of the pastes. The pastes showed exothermic reactions within the first 10–15 min after mixing and further release of heat after about 1 h. An apatitic phase formed upon immersion of the hardened cements in simulated body fluid for 15 and 30 days due to the conversion of brushite into apatite confirming their in vitro mineralization capability. The compressive strength of the wet cement specimens decreased with increasing curing time, being higher in the case of Sr-substituted CPC.The results suggest that the newly developed Sr-substituted brushite-forming α-TCP cements show promise for uses in orthopaedic and trauma surgery such as in filling bone defects.