Calorimetric characterization of the formation of acrylic type bone cements

Elastomeric high-mineral content hydrogel-hydroxyapatite composites for orthopedic applications

The design of synthetic bone grafts that mimic the structure and composition of bone and possess good surgical handling characteristics remains a major challenge. We report the development of poly(2-hydroxyethyl methacrylate) (pHEMA)-hydroxyapatite (HA) composites termed "FlexBone" that possess osteoconductive mineral content approximating that of human bone yet exhibit elastomeric properties enabling the press-fitting into a defect site. The approach involves crosslinking pHEMA hydrogel in the presence of HA using viscous ethylene glycol as a solvent. The composites exhibit excellent structural integration between the apatite mineral component and the hydroxylated hydrogel matrix. The stiffness of the composite and the ability to withstand compressive stress correlate with the microstructure and content of the mineral component. The incorporation of porous aggregates of HA nanocrystals rather than compact micrometer-sized calcined HA effectively improved the resistance of the composite to crack propagation under compression. Freeze-dried FlexBone containing 50 wt % porous HA nanocrystals could withstand hundreds-of-megapascals compressive stress and >80% compressive strain without exhibiting brittle fractures. Upon equilibration with water, FlexBone retained good structural integration and withstood repetitive moderate (megapascals) compressive stress at body temperature. When subcutaneously implanted in rats, FlexBone supported osteoblastic differentiation of the bone marrow stromal cells pre-seeded on FlexBone. Taken together, the combination of high osteoconductive mineral content, excellent organic-inorganic structural integration, elasticity, and the ability to support osteoblastic differentiation in vivo makes FlexBone a promising candidate for orthopedic applications.

Structured Drug-eluting Bioresorbable Films: Microstructure and Release Profile

Bioresorbable drug-eluting films can be used in many biomedical applications. Examples for such applications include biodegradable medical support devices which combine mechanical support with drug release and antibiotic-eluting film coatings for prevention of bacterial infections associated with orthopedic implants or during gingival healing. In the current study, bioresorbable drug-loaded polymer films are prepared by solution processing. Two film structures are studied: A polymer film with large drug crystals located on its surface (A-type) and a polymer film with small drug particles and crystals distributed within the bulk (B-type). The basic mode of drug dispersion/location in the film (A or B-type) is found to be determined mainly by the process of film formation and depends mainly on the solvent evaporation rate, whereas the drug's hydrophilicity has a minor effect on this structuring process. Most release profiles from A-type films exhibit a burst effect of ~30% and a second release stage that occurs at an approximately constant rate and is determined mainly by the polymer weight loss rate. An extremely high burst release is exhibited only by a very hydrophilic drug. The matrix (monolithic) nature of the B-type film enables release profiles that are determined mainly by the host polymer's degradation profile, with a very low burst effect in most of the studied systems. In addition to the drug location/ dispersion in the film, the host polymer and drug type also strongly affect the drug's release profile from the film. It has been demonstrated that appropriate selection of the process parameters and film components (polymer and drug) can yield film structures with desirable drug release behaviors. This can lead to the engineering of new bioresorbable drug-eluting film-based implants for various applications.

Gentamicin-loaded bioresorbable films for prevention of bacterial infections associated with orthopedic implants

Adhesion of bacteria to biomaterials and the ability of many microorganisms to form biofilms on foreign bodies are well-established as major contributors to the pathogenesis of implant-associated infections. Treatment of bone infection remains problematic, due to the difficulty of systemically administered antibiotics to locally penetrate bone. The current research addresses this issue by focusing on the development and study of novel gentamicin-loaded bioresorbable films designed to serve as "coatings" for fracture fixation devices and prevent implant-associated infections. Poly(L-lactic acid) and poly (D,L-lactic-co-glycolic acid) films containing gentamicin were developed through solution processing. The effects of polymer type, drug content, and processing conditions on the drug release profile were studied with respect to film morphology. The examined films generally exhibited a burst effect followed by a moderate approximately constant rate of release. The drug contents in the surrounding medium exceeded the required minimal effective concentration. Various gentamicin concentrations that were released from the films with time exhibited efficacy against bacterial species known to be involved in orthopedic infections. The developed systems can be applied on the surface of any metallic or polymeric fracture fixation device, and may therefore comprise a significant contribution to the field of orthopedic implants.

Synthesis and properties of cyclic acetal biomaterials

There is an increasing need to develop new biomaterials as tissue engineering scaffolds. Unfortunately, many of the materials that have been studied for these purposes are polyesters that hydrolytically degrade into acidic products, which may harm the surrounding tissue, and lead to accelerated degradation of the biomaterial. To overcome this disadvantage, a novel class of biomaterials based on a cyclic acetal unit has been created. Specifically, materials based upon the monomer 5-ethyl-5-(hydroxymethyl)-beta,beta-dimethyl-1,3-dioxane-2-ethanol diacrylate (EHD) is examined. This study investigates the effects of fabrication parameters, including initiator content, volume of diluent, and volume of accelerant, on several properties of EHD networks. Twelve different formulations were fabricated by varying the three parameters in a factorial design. The effects of the fabrication parameters on properties of the EHD networks were examined. Results show that the volume of accelerant most affected the EHD network gelation time, while the volume of diluent most affected the maximum reaction temperature, sol fraction, and degree of swelling. Cell viability on the EHD networks varied between (18 +/- 6)% and (57 +/- 10)% of the control at 4 h, and between (36 +/- 14)% and (140 +/- 50)% of the control at 8 h. These results indicate that it is possible to control the properties of the EHD networks by varying the fabrication parameters, and that EHD networks support a viable cell population.

Drug/device combinations for local drug therapies and infection prophylaxis

Combination devices-those comprising drug releasing components together with functional prosthetic implants-represent a versatile, emerging clinical technology promising to provide functional improvements to implant devices in several classes. Landmark antimicrobial catheters and the drug-eluting stent have heralded the entrance, and significantly, routes to FDA approval, for these devices into clinical practice. This review describes recent strategies creating implantable combination devices. Most prominent are new combination devices representing current orthopedic and cardiovascular implants with new added capabilities from on-board or directly associated drug delivery systems are now under development. Wound coverings and implantable sensors will also benefit from this combination enhancement. Infection mitigation, a common problem with implantable devices, is a current primary focus. On-going progress in cell-based therapeutics, progenitor cell exploitation, growth factor delivery and advanced formulation strategies will provide a more general and versatile basis for advanced combination device strategies. These seek to improve tissue-device integration and functional tissue regeneration. Future combination devices might best be completely re-designed de novo to deliver multiple bioactive agents over several spatial and temporal scales to enhance prosthetic device function, instead of the current 'add-on' approach to existing implant device designs never originally intending to function in tandem with drug delivery systems.

Preparation of epoxy-SiO2 hybrid sol-gel material for bone cement

An organic-inorganic hybrid material, epoxy-SiO(2), was prepared by incorporating epoxy structure units covalently into a SiO(2) glass network via the sol-gel approach. The precursor was obtained by the reaction of diglycidyl ether of bisphenol A (DGEBA) with 3-aminopropyl trimethoxysilane (APTS). The precursor was then hydrolyzed and co-condensated with tetraethyl orthosilicate (TEOS) in tetrahydrofuran (THF) at room temperature to yield epoxy-SiO(2) hybrid sol-gel material having a 50 wt % SiO(2) content. Thermal properties of the hybrid material were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The hybrid sol-gel material epoxy-SiO(2) was the solid, powder component of bone cement. The liquid component contains bis-phenol-A glycidyl methacrylate (Bis-GMA), triethyleneglycol dimethacrylate (TEGDMA), and methyl methacrylate (MMA) with 25, 55, and 20 vol %, respectively. We discuss the comparison between the new epoxy-SiO(2) bone cement and the commercial Simplex P bone cement. Mechanical properties such as Young's modulus, compressive strength, hardness, and impact strength of the new epoxy-SiO(2) bone cement exceeded those of Simplex P bone cement. The tensile and bending strengths of the new epoxy-SiO(2) bone cement were approximately the same as those of Simplex P bone cement. In order to evaluate the biocompatibility of the new bone cement, an MTT test and optical microscopy were conducted in cell culture. Results indicated that the new epoxy-SiO(2) bone cement exhibits very low cytotoxicity compared with Simplex P bone cement.

Effect of triphenyl bismuth on glass transition temperature and residual monomer content of acrylic bone cements

Self-curing acrylic bone cements are widely used in the fixation of prosthetic implants in orthopaedic surgery. Commercial bone cements are rendered radiopaque by the addition of heavy metal salts of barium and zirconia. The addition of barium sulphate adversely affects the mechanical strength and fracture toughness of bone cement and despite the fact that it has low solubility in water; its slow release and subsequent toxicity have caused concern. In an earlier study triphenyl bismuth (TPB) was found to be a viable alternative as a radiopaque agent in acrylic bone cements, which provided enhanced homogeneity. In this study we report the effect of the inclusion of TPB on the thermal properties of PMMA-based bone cements using both conventional DSC and Modulated Temperature DSC. Furthermore, analysis of the residual monomer contents is reported analysed by NMR spectroscopy in order to ascertain the influence of TPB on the polymerisation reaction. The glass transition temperature (Tg) determined by DSC showed that the values decreased with the addition of increasing amounts of TPB through both blending and dissolution methods; however, the method of incorporating TPB did not influence Tg. The magnitude of reduction was dependent of the amount of TPB and was greatest in the case of highest concentration of TPB used. A TPB melting peak was observed in the 25 wt% TPBBC, suggesting a limit to the solubility of TPB. The residual monomer analysis showed that at 10 and 15% by weight of TPB in the cement caused no significant changes in the residual monomer content but 25 wt% of TPB exhibited a significantly higher residual monomer content.

Effect of MMA-g-UHMWPE grafted fiber on mechanical properties of acrylic bone cement

Ultrahigh molecular weight polyethylene (UHMWPE) fibers were treated with argon plasma for 5 min, followed by uv irradiation in methyl methacrylate (MMA)-chloroform solution for 5 h to obtain MMA-g-UHMWPE grafted fiber. The grafting content was estimated by the titration of esterification method. The grafting amount of 5280 nmol/g was the largest for the MMA concentration at 18.75 vol%. To improve the mechanical properties of acrylic bone cement, pure UHMWPE fiber and MMA-g-UHMWPE fiber were added to the surgical Simplex. P radiopaque bone cement. The mechanical properties including tensile strength, tensile modulus, compressive strength, bending strength, and bending stiffness were measured. Dynamic mechanical analysis was also performed. By comparing the effect of the pure UHMWPE fiber and MMA-g-UHMWPE grafted fiber on the mechanical properties of acrylic bone cement, it was found that the acrylic bone cement with MMA-g-UHMWPE grafted fiber had a more significant reinforcing effect than that with untreated UHMWPE fiber. This might be due to the improvement of the interfacial bonding between the grafted fibers and the acrylic bone cement matrix.

Study of polymerization of acrylic bone cement: effect of HEMA and EGDMA

The polymerization reaction of standard surgical Simplex-P radiopaque bone cement was investigated by differential scanning calorimetry to determine the influence of hydroxyethyl methacrylate (HEMA) and ethylene glycol dimethacrylate (EGDMA) on the polymerization reaction. From the kinetic analysis, the polymerization reaction of the modified acrylic bone cement was found to be approximately a first-order reaction. The reaction rate constants (k) were determined. It was found that the effects of HEMA and EGDMA contents on the rate and the heat of polymerization can be explained by the frequency factor and the activation energy. An increase in HEMA content tends to result in an increase in the values of both frequency factor and activation energy, whereas an increase in EGDMA content tends to induce a decrease in the frequency factor and activation energy.

Covalent bonding of PMMA, PBMA, and poly(HEMA)to hydroxyapatite particles

In our earlier study, we showed that the surface hydroxyl groups of hydroxyapatite have the ability to react with organic isocyanate groups. In this study, the feasibility of grafting poly(methyl methacrylate) (PMMA), poly(n-butyl methacrylate) (PBMA), and Poly(hydroxyethyl methacrylate) [poly(HEMA)] by using the reaction of isocyanate groups with the hydroxyl groups on the surface of HA was investigated. Double bonds were introduced to the surface of HA via the coupling reaction of isocyanateoethyl methacrylate (ICEM) with HA, or through hexamethylene diisocyanate (HMDI) with hydroxyethyl methacrylate (HEMA) and HA, followed by radical polymerization in MMA, BMA, or HEMA. Infrared spectra indicated the existence of polymers on the surfaces of HA. Thermogravimetric analysis also confirmed the presence of grafted polymers on the surface of HA powder particles (20-26 wt%). The polymers gave typical PMMA, PBMA, or poly(HEMA) infrared spectra, with the exception of amide bands, a result of the coupling reaction of ICEM or HMDI with hydroxy groups of HA or HEMA. Therefore it is concluded that the polymers were chemically bonded to the surface of HA through the isocyanate groups of ICEM or HMDI.

Polymerization of acrylic bone cement using differential scanning calorimetry

The polymerization of acrylic bone cement using differential scanning calorimetry (DSC) was investigated. The polymerization reaction of the acrylic bone cement was found to be an approximately first order reaction. Two kinds of reaction rate constants for the polymerization reaction were observed. Both rate constants were calculated before and after the peak time. The effects of the addition of tricalcium phosphate (TCP) on the polymerization reaction of standard Surgical Simplex-P Radiopaque Bone Cement have been investigated by DSC. The TCP content had a strong retardation effect on the rate constants. The thermal stability of the acrylic bone cement was also studied by thermogravimetric analysis.