Surface treated polyethylene fibres as reinforcement for acrylic resins

The use of poly(methyl methacrylate) as a bone and dental cement material over several decades has provided us with experience related to processing and performance. A recognized disadvantage of such cements is their mechanical behaviour, expressed by low crack propagation resistance, impact strength, fatigue resistance and reduced fracture toughness. Many attempts have been made to resolve these problems either by modifying the poly(methyl methacrylate) chemical structure via copolymerization or incorporating reinforcing additives. The latter is of great importance, because it can lead to the preparation of composite materials with considerably improved performance. Besides reinforcement, the incorporation of additives, such as fibres, results in better processing characteristics, e.g. lowers polymerization exotherm. In this work, poly(methyl methacrylate) reinforced with Tekmilon ultra high modulus polyethylene fibres was studied, focussing on the interfacial bonding between matrix and reinforcement. Some finishing agents were used to treat the fibres and their efficiency was mainly evaluated through the effect on the mechanical properties of the composite material prepared.     

The combined effects of crosslinking and high crystallinity on the microstructural and mechanical properties of ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene (PE) has been used for more than forty years as the bearing surface in total joint replacements. In recent years, there have been numerous advances in processing conditions that have improved the wear resistance of this material. In particular, crosslinking has been shown to dramatically improve the wear behavior of this orthopedic polymer in simulator studies. This benefit to wear resistance, however, is accompanied by a decrease in mechanical properties such as ultimate tensile strength, ductility, toughness and fatigue resistance. This degradation to mechanical properties may have serious implications for devices with high stress concentrations or large cyclic contact stresses. Tailoring microstructure for improved structural performance is essential for implant design. In this work we examined the role of crystallinity and crosslinking on the microstructure and mechanical properties of PE. Crystallinity was increased with a high pressure process and crosslinking was obtained with gamma irradiation. Crystallinity was beneficial to fatigue crack propagation resistance and when coupled with crosslinking a polymer with both wear and fatigue resistance was obtained.     

Microstructural pathway of fracture in poly(methyl methacrylate) bone cement

Mechanical failure of poly(methyl methacrylate) (PMMA) bone cement is linked to failure of cemented total joint prostheses. An essential step to minimize, if not eliminate, cement fracture is to understand the material characteristics controlling fracture resistance. At least four phases of bone cement can be identified that may affect the damage zone formation: pre-polymerized beads, interbead matrix polymer, BaSO₄, and porosity. Gel permeation chromatography (GPC) was used to determine the molecular weight (MW) distributions of the two polymer phases. Mechanical testing, scanning electron microscopy and light microscopy were used to analyse fracture mechanisms. Fatigue crack propagation of bone cement was distinctly different from rapid crack propagation. Microcracks defined the damage zone for fatigue fracture. The microcracks developed in the interbead matrix and not through the pre-polymerized beads. Light microscopy revealed evidence of craze formation on surfaces of fractured beads during rapid fracture, but not on fatigue surfaces. GPC analysis indicated an increase in MW from the bead phase alone to the fully cured bone cement, indicating a greater MW in the interbead matrix polymer. Increases of 36 and 176% were measured for two different bone cements, but the bulk of the polymer has an MW of less than 1 × 10⁶. Three factors were suggested to explain why the microcracks seem to prefer to grow in the interbead matrix: the presence of BaSO₄, shrinkage during the curing process, and the different polymerization processes of the bead and the interbead polymers. Pores had an affect on the microcrack formation as well, and did not need to be directly in front of the crack tip to interact with the damage zone. The pores seemed to act as nucleation sites for microcracks. The porosity-microcrack nucleation interaction may explain and reconcile the apparently disparate results concerning the effect of porosity on fracture toughness and fatigue life. Porosity may, however, also provide positive contributions to the fracture properties of bone cement by dispersing the energy at the crack tip, forming a larger damage zone, and effectively blunting the crack. The crack propagation mechanisms revealed by this research indicated the importance of microstructure in the fatigue failure of PMMA.     

Fatigue crack propagation path across the dentinoenamel junction complex in human teeth

The human tooth structures should be understood clearly to improve clinically used restorative materials. The dentinoenamel junction (DEJ) plays a key role in resisting crack propagation in teeth. The aim of this study was to determine the fracture toughness of the enamel-DEJ-dentin complex and to investigate the influence of the DEJ on the fatigue crack propagation path across it by characterizing fatigue-fractured enamel-DEJ-dentin complexes using optical and scanning electron microscopy. The results of this study showed that the fracture toughness of the enamel-DEJ-dentin complex was 1.50 +/- 0.28 Mpa x m(1/2). Based on the results of this investigation, it was concluded that the DEJ complex played a critical role in resisting crack propagation from enamel into dentin. The DEJ complex is, approximately, a 100 to 150 microm broad region at the interface between enamel and dentin. The toughening mechanism of the DEJ complex may be explained by the fact that crack paths were deflected as cracks propagated across it. Understanding the mechanism of crack deflection could help in improving dentin-composite as well as ceramic-cement interfacial qualities with the aim to decrease the risk of clinical failure of restorations. Both can be viewed as being composed from a layer of material of high strength and hardness bonded to a softer but tougher substratum (dentin). The bonding agent or the luting cement layer may play the critical role of the DEJ in improving the strength of these restorations in clinical situations.     

Toward standardization of methods of determination of fracture properties of acrylic bone cement and statistical analysis of test results

A succinct critical review of the literature on the fatigue, fatigue crack propagation, and fracture toughness (herein collectively termed "fracture properties") of acrylic bone cement is presented, whereby it is pointed out that a plethora of test conditions have been used. This situation precludes meaningful interstudy comparisons and mitigates against a definitive delineation of the effect of a named variable on a specified fracture property. A case for standardization of test conditions is thus made, culminating in the presentation of a recommended set of such conditions. In addition, it is shown that many literature parametric studies employed inappropriate statistical methods for performing pairwise comparisons, and all these studies have not addressed the issue of possible interactions between the parameters being investigated. A methodology for addressing these deficiencies is presented in the present report, and its use is illustrated with a set of notional fatigue test results.     

Physicochemical, mechanical, and biological properties of bone cements prepared with functionalized methacrylates

Bone cements prepared with methyl methacrylate (MMA) as a base monomer and either methacrylic acid (MAA) or diethyl amino ethyl methacrylate (DEAEMA) as comonomers were characterized in terms of curing behavior, mechanical properties, and their in vitro biocompatibility.The curing time and setting temperature were found to be composition dependent while the residual monomer was not greatly affected by the presence of either acidic or alkaline comonomers in the bone cements. For samples with MAA comonomer, a faster curing time and higher setting temperature were observed when compared to the cement with DEAEMA comonomer.In terms of mechanical properties, the highest compressive strength was exhibited by formulations containing MAA, while the highest impact strength was shown by the formulations prepared with DEAEMA. There were no differences observed between the two formulations for tensile, shear, and bending strength values. Similarly, fatigue crack propagation studies did not reveal differences with the addition of either DEAEMA or MAA.No differences were observed in the initial number of attached primary rat femur osteoblasts on the different bone cements and positive controls. However, after 48 h there was a reduced proliferation in the cells grown on bone cements containing MAA.     

Effect of porosity and environment on the mechanical behavior of acrylic bone cement modified with acrylonitrile-butadiene-styrene particles: I. Fracture toughness.

The elastomeric copolymer acrylonitrile-butadiene-styrene (ABS) was added to a conventional acrylic bone cement matrix. The results obtained show that although strength and stiffness decreased with an increasing second phase volume fraction, ductility and toughness both increased. The crack propagation became stable for specimens containing over a 5% volume fraction of the second phase. The fracture toughness increased up to 60% when the amount of ABS reached 20% (v/v). For larger amounts linear elastic fracture mechanics techniques could not be used properly. The effects of porosity and environmental conditions on the mechanical behavior were also studied. The mechanisms that control the fracture process were investigated by means of scanning electron microscopy.     

Reinforcement of bone cement using zirconia fibers with and without acrylic coating

Acrylic (polymethylmethacrylate or PMMA) bone cement was modified by the addition of high-strength zirconia fibers with average lengths of 200 microm and diameters of 15 microm or 30 microm. A novel emulsion polymerization process was developed to encapsulate individual fibers in PMMA. Improvements in tensile and compressive properties as well as in fracture toughness were investigated upon incorporation of uncoated and acrylic coated zirconia fibers. Bone cements were reinforced with 2% by volume of the 15 microm diameter and 5% by volume of the 30 microm fibers. Results indicate that elastic modulus and ultimate strength of bone cements reinforced with zirconia fibers were higher than controls, being the largest for cements reinforced with 30 microm diameter fibers. The fracture toughness of the cement increased by 23% and 41% by the addition of 15 microm and 30 microm fibers, respectively. Coating of individual zirconia fibers did not result in improved material properties of bone cements. The use of uncoated or acrylic coated 30 microm fibers is recommended based on the significant increases in ultimate strength and fracture toughness of the cements.     

Mechanisms of decrease in fatigue crack propagation resistance in irradiated and melted UHMWPE

Adhesive/abrasive wear in ultra-high molecular weight polyethylene (UHMWPE) has been minimized by radiation cross-linking. Irradiation is typically followed by melting to eliminate residual free radicals that cause oxidative embrittlement. Irradiation and subsequent melting reduce the strength and fatigue resistance of the polymer. We determined the radiation dose dependence and decoupled the effects of post-irradiation melting on the crystallinity, mechanical properties and fatigue crack propagation resistance of room temperature irradiated UHMWPE from those of irradiation alone. Stiffness and yield strength, were largely not affected by increasing radiation dose but were affected by changes in crystallinity, whereas plastic properties, ultimate tensile strength and elongation at break, were dominated at different radiation dose ranges by changes in radiation dose or crystallinity. Fatigue crack propagation resistance was shown to decrease with increase in radiation dose and with decrease in crystalline content. Morphology of fracture surfaces revealed loss of ductility with increase in radiation dose and more detrimental effects on ductility at lower radiation doses after post-irradiation melting.     

The effect of strain rate on fracture toughness of human cortical bone: a finite element study

Evaluating the mechanical response of bone under high loading rates is crucial to understanding fractures in traumatic accidents or falls. In the current study, a computational approach based on cohesive finite element modeling was employed to evaluate the effect of strain rate on fracture toughness of human cortical bone. Two-dimensional compact tension specimen models were simulated to evaluate the change in initiation and propagation fracture toughness with increasing strain rate (range: 0.08–18 s⁻¹). In addition, the effect of porosity in combination with strain rate was assessed using three-dimensional models of micro-computed tomography-based compact tension specimens. The simulation results showed that bone’s resistance against the propagation of a crack decreased sharply with increase in strain rates up to 1 s⁻¹ and attained an almost constant value for strain rates larger than 1 s⁻¹. On the other hand, initiation fracture toughness exhibited a more gradual decrease throughout the strain rates. There was a significant positive correlation between the experimentally measured number of microcracks and the fracture toughness found in the simulations. Furthermore, the simulation results showed that the amount of porosity did not affect the way initiation fracture toughness decreased with increasing strain rates, whereas it exacerbated the same strain rate effect when propagation fracture toughness was considered. These results suggest that strain rates associated with falls lead to a dramatic reduction in bone’s resistance against crack propagation. The compromised fracture resistance of bone at loads exceeding normal activities indicates a sharp reduction and/or absence of toughening mechanisms in bone during high strain conditions associated with traumatic fracture.     

Mixed-mode stress intensity factors for kink cracks with finite kink length loaded in tension and bending: Application to dentin and enamel

Fracture toughness resistance curves describe a material’s resistance against crack propagation. These curves are often used to characterize biomaterials like bone, nacre or dentin as these materials commonly exhibit a pronounced increase in fracture toughness with crack extension due to co-acting mechanisms such as crack bridging, crack deflection and microcracking. The knowledge of appropriate stress intensity factors which depend on the sample and crack geometry is essential for determining these curves. For the dental biomaterials enamel and dentin it was observed that, under bending and tensile loading, crack propagation occurs under certain constant angles to the initial notch direction during testing procedures used for fracture resistance curve determination. For this special crack geometry (a kink crack of finite length in a finite body) appropriate geometric function solutions are missing. Hence, we present in this study new mixed-mode stress intensity factors for kink cracks with finite kink length within samples of finite dimensions for two loading cases (tension and bending) which were derived from a combination of mixed-mode stress intensity factors of kink cracks with infinitely small kinks and of slant cracks. These results were further applied to determine the fracture resistance curves of enamel and dentin by testing single edge notched bending (SENB) specimens. It was found that kink cracks with finite kink length exhibit identical stress fields to slant cracks as soon as the kink length exceeds 0.15 times the initial straight crack or notch length. The use of stress intensity factor solutions for infinitely small kink cracks for the determination of dentin fracture resistance curves (as was done by other researchers) leads to an overestimation of dentin’s fracture resistance of up to 30%.     

Comparative fatigue behavior and toughness of remelted and annealed highly crosslinked polyethylenes

Highly cross-linked polyethylenes (HXLPEs) have been incorporated into the hip replacement armamentarium based on their improved wear resistance. However, two different methods of thermal treatment separate the orthopedic community as strategies to control potential long-term oxidation, and controversy remains with problems in the long-term use of acetabular liners (long-term oxidation, rim fracture after impingement, etc.). Meanwhile, the mechanical properties of HXLPEs that may alleviate these problems are still unclear. On the other hand, HXLPEs are scarcely used in knee replacements, as there exists concern about the probably reduced fatigue and fracture performances of these materials. Thus, our aim was to compare the effects of both thermal treatment regimes on mechanical properties and to associate these findings with the material microstructure. The fatigue behavior of annealed and remelted HXLPEs was characterized using short-term cyclic stress-strain, long-term fatigue, and fatigue crack propagation tests. On the other hand, impact tests, tensile experiments, and the J-integral multispecimen method allowed us to assess toughness. Microstructure features such as crosslink density, crystallinity percentage, and lamellar thickness were investigated by swelling measurements, differential scanning calorimetry, and transmission electron microscopy, respectively. This study confirms that annealing preserves mechanical properties better than remelting from both fatigue and fracture resistance points of view, and it remarks that a suitable selection of irradiation and stabilization conditions is needed to achieve optimal mechanical performances of ultra high molecular weight polyethylenes for each specific total joint replacement.     

Calculation of porosity and osteonal cement line effects on the effective fracture toughness of cortical bone in longitudinal crack growth

Based on the microscopic analyses of cracks and correlational studies demonstrating evidence for a relationship between fracture toughness and microstructure of cortical bone, an equation was derived for bone fracture toughness in longitudinal crack growth, using debonding at osteonal cement lines and weakening effect of pores as main crack mechanisms. The correlation between the measured and predicted values of fracture toughness was highly significant but weak for a single optimal value of matrix to cement line fracture toughness ratio. Using fracture toughness values and histomorphometrical parameters from an available data set, matrix to cement line fracture toughness ratio was calculated for human femoral bone. Based on these calculations it is suggested that the effect of an osteon on fracture toughness will depend on the cement line's ability to compensate for the pore in an osteon. Matrix to cement line fracture toughness ratio significantly increased with increasing age, suggesting that the effectiveness of osteons in energy absorption may be reduced in the elderly due to a change in cement line properties.     

Effect of initiation chemistry on the fracture toughness, fatigue strength, and residual monomer content of a novel high-viscosity, two-solution acrylic bone cement.

Porous-free, two-solution bone cements have been developed in our laboratory as an alternative to commercial powder/liquid formulations. Each pair of solutions consist of poly(methyl methacrylate) (PMMA) powder dissolved in methyl methacrylate (MMA) monomer, with benzoyl peroxide (BPO) added to one solution as the initiator and N,N-dimethyl-p-toluidine (DMPT) added to the other as the activator. When mixed, the solutions polymerize via a free radical reaction, which is governed by the concentrations of initiator and activator and their molar stoichiometry. Previous work by the authors has demonstrated that these two-solution cement compositions are comparable to Simplex P bone cement in polymerization exotherm, setting time, and flexural mechanical properties. This study was designed to evaluate the effect of BPO and DMPT concentrations, along with their molar ratio, on the fracture toughness, fatigue strength, and residual monomer content of the experimental compositions. The results showed that fracture toughness and fatigue strength for the solution cements were comparable to Simplex P and were not significantly affected by the BPO concentration or the BPO:DMPT molar ratio; however, the highest DMPT concentration yielded significantly lower values for both variables. Residual monomer content was significantly affected by both the individual concentrations of BPO and DMPT and their molar ratios. The two-solution cements had significantly higher residual monomer contents versus Simplex P; however, this can be attributed to their higher initial monomer concentration rather than a lower degree of conversion.     

Toughness,bonding and fluoride-release properties of hydroxyapatite-added glass ionomer cement

Improving the mechanical strength of glass ionomer cement while preserving its favorable clinical properties such as fluoride release, bonding to tooth structure and biocompatibility is desirable. In this study, hydroxyapatite was incorporated into chemically setting glass ionomer cement and its effect on the fracture toughness, bonding to dentin and fluoride release was identified. Commercial glass ionomer cement (Fuji IX GP((R)) ) was the control and base material. Eight weight percent of hydroxyapatite was added into the glass ionomer powder. Specimens were fabricated and the fracture toughness, shear bond strength and eluted fluoride ion concentration were measured. Adding hydroxyapatite into the glass ionomer cement led to significantly higher fracture toughness after 15min and 24h from mixing. The hydroxyapatite-added cement also exhibited bond strength to dentin similar to that of the control from 15min to 56 days and consistent fluoride release for 13 weeks. SEM findings showed a cohesive type of fracture in the material for all specimens in both groups. These results indicate that hydroxyapatite-added glass ionomer cement has a potential as a reliable restorative material with improved fracture toughness, long-term bonding to dentin and unimpeded ability of sustained fluoride release.     

The relative effects of radiation crosslinking and type of counterface on the wear resistance of ultrahigh-molecular-weight polyethylene

The lifetime of total joint replacement prostheses utilizing ultrahigh-molecular-weight polyethylene (UHMWPE) components has historically been determined by their wear resistance. It has been discovered that radiation crosslinking of UHMWPE can substantially increase its wear resistance. However, it is also well recognized that there is a radiation-dose-dependent decrease in several important mechanical properties of UHMWPE, such as fracture toughness and resistance to fatigue crack propagation. In this study, the effect of radiation crosslinking (followed by remelting) on the morphology, tensile properties and wear resistance of UHMWPE was investigated. Wear tests were conducted against both the commonly used cobalt-chromium counterface polished to implant grade smoothness as well as a smoother ceramic (alumina) counterface. The results showed that 50kGy dose radiation crosslinking increased the wear resistance of UHMWPE against the cobalt-chromium counterface 7-fold, but the coupling of remelted, crosslinked UHMWPE against the smoother alumina counterface led to a 20-fold increase in wear resistance. This study shows that the use of an alumina counterface would circumvent the need to use a high radiation dose in crosslinking UHMWPE, associated with poor mechanical properties, without compromising wear resistance.     

A fractographic investigation of PMMA bone cement focusing on the relationship between porosity reduction and increased fatigue life.

Fracture surfaces of both monotonic and fatigue loaded bone cement samples were examined to investigate the fractographic characteristics of PMMA. Classic cleavage step river patterns were observed on all monotonically loaded samples, running downstream in the direction of crack propagation. All fatigue cracks initiated at internal pores and the direction of crack propagation of many cracks was discernible. Porosity, pore size, and pore size distribution were found to affect the crack initiation and fatigue behavior of bone cement. Statistical analysis revealed a strong negative correlation between two-dimensional porosity present on the fracture surfaces and the cycles to failure. The fractographic observations of these fatigue samples elucidate one reason why porosity reduction by centrifugation or vacuum mixing increases the fatigue life of PMMA bone cement.     

A Qualitative Analysis of Crack Propagation in Articular Cartilage at Varying Rates of Tensile Loading

A custom-built miniature tensile testing apparatus was used to study the propagation of cracks through the articular cartilage matrix at various loading rates and initial crack lengths. The crack propagation mechanism was observed to be significantly dissimilar to that normally seen in traditional fracture mechanics opening mode, where fracture propagates through the thickness of samples or perpendicularly to the applied load. Instead, an artificially initiated microcrack in the surface layer of an articular cartilage sample grew laterally in the direction of the applied load, stretching about the crack tip, whose initial position remained unchanged throughout the fracture process. A progressive upward pull of the bottom layer toward the surface, which resulted in necking of the specimen, was observed. Our analysis revealed that the rate of necking was the same as that of the lateral stretch of the growing crack. We hypothesize that necking is due to the response of the collagen meshwork especially in the deep zones of the matrix to the tensile load. Our samples exhibited unstable fracture growth immediately after each microcrack grew to the base of the articular surface layer, with very fast crack propagation to failure, thereby indicating that the fracture toughness of the articular cartilage matrix is significantly determined by the toughness of its articular surface.     

A qualitative analysis of crack propagation in articular cartilage at varying rates of tensile loading

A custom-built miniature tensile testing apparatus was used to study the propagation of cracks through the articular cartilage matrix at various loading rates and initial crack lengths. The crack propagation mechanism was observed to be significantly dissimilar to that normally seen in traditional fracture mechanics opening mode, where fracture propagates through the thickness of samples or perpendicularly to the applied load. Instead, an artificially initiated microcrack in the surface layer of an articular cartilage sample grew laterally in the direction of the applied load, stretching about the crack tip, whose initial position remained unchanged throughout the fracture process. A progressive upward pull of the bottom layer toward the surface, which resulted in necking of the specimen, was observed. Our analysis revealed that the rate of necking was the same as that of the lateral stretch of the growing crack. We hypothesize that necking is due to the response of the collagen meshwork especially in the deep zones of the matrix to the tensile load. Our samples exhibited unstable fracture growth immediately after each microcrack grew to the base of the articular surface layer, with very fast crack propagation to failure, thereby indicating that the fracture toughness of the articular cartilage matrix is significantly determined by the toughness of its articular surface.     

Mechanical behaviour of a new acrylic radiopaque iodine-containing bone cement

In total hip replacement, fixation of a prosthesis is in most cases obtained by the application of methacrylic bone cements. Most of the commercially available bone cements contain barium sulphate or zirconium dioxide as radiopacifier. As is shown in the literature, the presence of these inorganic particles can be unfavourable in terms of mechanical and biological properties. Here, we describe a new type of bone cement, where X-ray contrast is obtained via the introduction of an iodine-containing methacrylate copolymer; a copolymer of methylmethacrylate and 2-[4-iodobenzoyl]-oxo-ethylmethacrylate (4-IEMA) is added to the powder component of the cement. The properties of the new I-containing bone cement (I-cement) are compared to those of a commercially available bone cement, with barium sulphate as radiopacifier (B-cement). The composition of the I-cement is adjusted such that similar handling properties and radiopacity as for the commercial cement are obtained. In view of the mechanical properties, it can be stated that the intrinsic mechanical behaviour of the I-cement, as revealed from compression tests, is superior to that of B-cement. Concerning the fatigue behaviour it can be concluded that, though B-cement has a slightly higher fatigue crack propagation resistance than I-cement, the fatigue life of vacuum-mixed I-cement is significantly better than that of B-cement. This is explained by the presence of BaSO4 clumps in the commercial cement; these act as crack initiation sites. The mechanical properties (especially fatigue resistance) of the new I-cement warrant its further development toward clinical application.