Experimental and theoretical investigations of high-energy shifts of single InAs, InGaAs, InAlAs and InP quantum dot (QD) emission lines induced by contact pressure exerted by a near-field optical fiber tip are reported. “Pressure” coefficients of 0.65–3.5 meV/nm have been measured for ground state emission lines in agreement with numerical calculations. We show that the observed increase of the tip-induced energy shift with increasing aperture diameter is caused by a decrease of the uniaxial strain component. We also report the effect of emission instability of single QD emission intensity under tip-induced pressure.

Fibers can be used to improve the mechanical properties of bone cement for the long-term stability of hip prostheses. However, debonding of the fibers from the matrix due to the poor fiber/matrix interface is a major failure mechanism for such fiber reinforced bone cements. In this study, a novel fiber (variable diameter fibers or VDFs) technology for reinforced bone cement was studied to overcome the interface problem of short-fiber composites. These fibers change their diameters along their length to improve the fiber/matrix interfacial bond by the mechanical interlock between the VDFs and the matrix. A novel composite made from novel ceramic VDFs incorporated in PMMA matrix was developed. Both static and fatigue tests were carried out on the composites. Conventional straight fiber (CSF) reinforced bone cement was also tested for comparison purposes. Results demonstrated that both the stiffness and the fatigue life of VDF reinforced bone cement are significantly improved (P < 0.05) compared with the unreinforced bone cement. VDF contents of 10% by volume increased the fatigue life over unreinforced bone cement by up to 100-fold. Also, the fatigue life and modulus of toughness of VDF reinforced cement were significantly greater than those of CSF reinforced cement (P < 0.05 and P < 0.001, respectively). Scanning electron microscopy (SEM) micrographs revealed that VDFs can bridge the matrix cracks effectively and pullout of VDFs results in much more extensive matrix damage than pullout of CSFs increasing the resistance to fatigue. Therefore, VDF reinforced cement was significantly tougher, having a greater energy dissipation capacity than CSF reinforced cement. VDFs added to bone cement could potentially avoid implant loosening due to the mantle fracture of bone cement and delay the need for revision surgery.

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.

Acrylic bone cements are widely used in total joint arthroplasties to grout the prosthesis to bone. The changes in the tensile properties and fracture toughness of polymethylmethacrylate (PMMA) bone cements obtained by the addition of control and heat treated short titanium fibers are studied. Heat treatment of titanium fibers is conducted to precipitate titania particles on the fiber surface to improve the biocompatibility of the metal. Control and heat treated short titanium fibers (250 μ long and 20 μ diameter) were used as reinforcements at 3 volume %. X-ray diffraction indicated the presence of a rutile form of titania due to the heat treatments. The tensile and fracture properties were improved by the addition of fibers. Bone cements reinforced with titanium fibers heated at 550∘C for 1 h followed by 800∘C for 30 minutes show the largest increase in fracture toughness along with the smallest changes in elastic modulus and needs to be further investigated.

AbstractBone cement has been widely used in orthopedic surgeries since Sir John Charnley first introduced the self‐curing bone cement for the fixation of artificial joints in 1958. Several important aspects of material characteristics of bone cement, including its material compositions, short‐ and long‐term mechanical properties are discussed in this article. This article aims to provide comprehensive information for the orthopedic community on cement applications.The article is arranged in seven sections and begins with a brief introduction and description of bone cement compositions, followed by a section on cement setting procedure and cementing technique. Then, the thermal aspect (polymerization heat and potential bone thermal damage) and volumetric change effects (porosity and residual stress) are discussed. The final section focuses on the main mechanical properties of cement, including the static strength, fatigue failure, and viscoelasticity of bone cement. Finally, the article ends with a brief summary of potential research directions.

Composite materials reinforced by fibers with enlarged ends are known to have significantly better strength and toughness than those reinforced by flat-end fibers. The objective of this study is to develop an analytical model to determine the importance of deformation of the enlarged end on the reinforcement performance of ellipsoidal enlarged-end fibers. The resisting pullout load of the fiber is composed of a component due to interfacial bond at the fiber/matrix interface and a component due to mechanical anchorage at the embedded enlarged end of the fiber. The component due to mechanical anchorage at the enlarged end is due to both mechanical interlock and deformation of the enlarged end. In the past, little has been done to account for the deformation of the enlarged end. To account for this component of the mechanical anchorage resistance at the embedded enlarged end, a spring component is introduced to connect the embedded fiber with the enlarged ellipsoid. Analytical solutions were derived to predict the effects of the rigid enlarged end shape on the pullout load and stress distribution. These solutions were then compared to finite element solutions. It is shown that the shape of enlarged end has a significant influence on the stress distribution of the short fiber. Specially, the model demonstrates that the enlarged ends deform significantly for some shapes and are not effective for long fibers.

Poor interfacial properties between reinforcement fibers and a polymethylmethacrylate (PMMA) matrix may result in debonding between them, which is an important failure mechanism for fiber-reinforced bone cement. Optimization of the shape of the fibers can improve load transfer between the fibers and the PMMA matrix, thereby providing maximum overall strength performance. This article presents a procedure for structural shape optimization of short reinforcement fibers using finite-element analyses. The composite is modeled by a representative volume element composed of a single short fiber embedded in the PMMA matrix. In contrast to most previous work on this subject, contact elements are employed between the fiber and the matrix to represent a low-strength interface. Most previous models assume a perfect bond. Residual stress, due to matrix cure shrinkage and/or thermal stresses, is also included in the model. The design objective is to improve the mechanical properties of the composite. The effects of two different loading conditions and objective functions, stiffness-based and fracture toughness-based, are examined. The general trend in design optimization is to produce a threaded end short (TES) fiber. Owing to the mechanical interlock between the fibers and the PMMA matrix, the TES fiber can bridge matrix cracks effectively and improve the stiffness of the composite.

Fibers can be used to improve the mechanical properties of bone cement for the long-term stability of hip prosthesis. However, debonding of the fibers from the matrix due to the poor fiber/matrix interface is a major failure mechanism for such fiber-reinforced bone cements. Optimization of the shape of the fibers can improve load transfer between the fibers and the matrix, thereby providing improved overall mechanical performance. This paper presents a procedure for structural shape optimization of short reinforcement fibers using finite element analyses. The effects of fiber orientation and interfacial bond were investigated to obtain the optimal fiber shape for bone cement reinforced with randomly oriented fibers. The composite is regarded as an array of unit cells containing a tilting fiber embedded in the matrix. This method provides proper boundary conditions for the center unit cell. The design objective based on the center unit cell is then optimized to maximize the stiffness of the reinforced bone cement. As opposed to aligned fiber composites, where the optimum shape is an enlarged-end fiber, the general optimal fiber shape for randomly oriented fibers is a variable diameter fiber (VDF). Due to the mechanical interlock between the fibers and the matrix, the VDF can both bridge matrix cracks effectively and improve the stiffness of the composite when fibers are randomly oriented.

During cement curing in total hip arthroplasty, residual stresses are introduced in the cement mantle as a result of curing shrinkage, thermal shrinkage, and geometrical constraints. These high residual stresses are capable of initiating cracks in the mantle of cemented hip replacements. The purpose of this study was to determine the residual stresses in the cemented hip replacements. The finite element method was developed to predict the residual stresses built up in joint arthroplasties. Experimental tests were then performed to validate the numerical methodology. Then the effects of curing history on the residual stress distribution were investigated with finite element simulations. Results showed that the predictions of the thermal shrinkage residual stresses by the developed method agreed with the experimental tests very well. The residual stress buildup was shown to depend on the curing history. By preheating the prosthesis stem prior to implantation, a desired low-level residual stress at the critical prosthesis-cement interface was obtained. As a result, this article provides a numerical tool for the quantitative simulation of residual stress and for examining and refining new designs computationally.

Fractures in the bone-cement mantle (polymethyl methacrylate) have been linked to the failure of cemented total joint prostheses. The heat generated by the curing bone cement has also been implicated in the necrosis of surrounding bone tissue, leading to loosening of the implants. The addition of reinforcements may improve the fracture properties of bone cement and decrease the peak temperatures during curing. This study investigates the changes in the fracture properties and the temperatures generated in the ASTM F451 tests by the addition of 316L stainless steel fibers to bone cement. The influence of filler volume fraction (5-15% by volume) and aspect ratios (19, 46, 57) on the fracture toughness of the acrylic bone cement was assessed. Increasing the volume fraction of the steel fibers resulted in significant increases in the fracture toughness of the steel-fiber-reinforced composite. Fracture-toughness increases of up to 2.63 times the control values were obtained with the use of steel-fiber reinforcements. No clear trend in the fracture toughness was discerned for increasing aspect ratios of the reinforcements. There is a decrease in the peak temperatures reached during the curing of the steel-fiber-reinforced bone cement, though the decrease is too small to be clinically relevant. Large increases in the fatigue life of acrylic bone cement were also obtained by the addition of steel fibers. These results indicate that the use of steel fibers may enhance the durability of cemented joint prostheses.

In this research, a numerical method was developed for predicting the progressive failure of thick laminated composite femoral components. A three-dimensional (3-D) global/ 3-D local technique was developed to capture the overall structural response of this system, while also enabling the 3-D ply-level stress state to be determined efficiently and accurately. Different failure criteria and material degradation models were incorporated in the method, giving it the flexibility to model a wide range of materials and structures. Numerical modeling was also conducted to design experimental test methods to simulate in vivo loading conditions for component fatigue tests. Parametric studies were then conducted with the numerical model of the experimental system. Next, we compared the results to the damage behavior of the experimentally determined laminated composite femoral component to assess which parameter set most accurately predicted the actual damage development behavior. We then applied the best-fitting parameter set to analyze simulated in situ composite femoral components. Results showed that this methodology efficiently and accurately predicted damage initiation and propagation. This research demonstrates how analytical and numerical models may be used before conducting extensive experimental tests as initial tools to evaluate components for the design of composite hip implants that possess a high level of damage resistance and damage tolerance.

The prevalence of bone fractures increases markedly with age, which is correlated to osteoporosis, low bone mass and structural deterioration of trabecular bone. Infiltration of bone cement into vertebral bodies (vertebroplasty) has been shown to substantially increase the strength and stiffness of mechanically compromised trabecular bone. It was expected that a similar benefit might be realized in the proximal femur [Heini, et al, 2001]. Before clinical trails, the effects of this treatment technique should be investigated. The objective of this study was to investigate the effects of cement augmentation on the mechanical properties of the proximal femur. In the present paper, the relative effects of bone osteoporosis, femoral neck fracture and cement augmentation on the femur stiffness were evaluated using finite element simulations.

Bone cements are widely used to fix prostheses into bones for joint arthroplasty. During cement curing in total hip arthroplasty, residual stresses are introduced in the cement mantle. A finite element method was developed to predict such residual stress built-up. The effects of curing history on the residual stress distribution were investigated. Results showed that the predictions of the residual stresses agreed with the experimental tests very well. The residual stress build-up was shown to depend on the curing history. By preheating the prosthesis stem prior to implantation, a desired low level residual stress at the critical interface was obtained.

Numerical studies were performed to investigate bone cement polymerization, temperature history and thermal osteonecrosis in cemented hip replacements with finite element methods. In this paper, the effects of pre-cooling and pre-heating of the prosthesis and/or the cement prior to implantation were simulated. It was found that the cement polymerization initiated near the bone-cement interface and progressed toward the prosthesis when both the cement and prosthesis were initially at room temperature. When the prosthesis and/or cement were pre-cooled, a reduction of the peak temperature at the bone-cement interface resulted, and this may reduce thermal osteonecrosis. However, this also slowed the polymerization process, and may result in a weaker bone cement. If the prosthesis was significantly initially heated, bone cement polymerization reversed reaction direction, started from the cement-prosthesis interface and proceeded toward the bone. Such polymerization direction may reduce or eliminate the formation of voids at the cement-prosthesis interface. Numerical results also showed that pre-heating seemed unlikely to produce significant thermal damage to the bone. The method of pre-heating the prosthesis prior to implantation may decrease the likelihood of cement-prosthesis loosening and increase the life of total hip arthroplasty.

A finite element technique was developed to investigate the thermal behavior of bone cement in joint replacement procedures. Thermal tests were designed and performed to provide the parameters in a kinetic model of bone cement exothermic polymerization. The kinetic model was then coupled with an energy balance equation using a finite element formulation to predict the temperature history and polymerization development in the bone-cement-prosthesis system. Based on the temperature history, the possibility of the thermal bone necrosis was then evaluated. As a demonstration, the effect of cement mantle thickness on the thermal behavior of the system was investigated. The temperature profiles in the bone-cement-prosthesis system have shown that the thicker the cement, the higher the peak temperature in the bone. In the 7 mm thick cement case, a peak temperature of over 55 degrees C was predicted. These high temperatures occurred in a small region near the bone/cement interface. No damage was predicted in the 3 mm and 5 mm cement mantle thickness cases. Although thermal damage was predicted in the bone for the 7 mm mantle thickness case, the amount of thermal necrosis predicted was minimal. If more cement is used in the surgical procedure, more heat will be generated and the potential for thermal bone damage may rise. The systems should be carefully selected to reduce thermal tissue damage when more cement is used. The methodology developed in this paper provides a numerical tool for the quantitative simulation of the thermal behavior of bone-cement-prosthesis designs.

In thermal characterization tests of polymethylmethacrylate bone cement performed according to the ASTM Standard Specification for Acrylic Bone Cement, time–temperature profiles of bone cement were observed to be sensitive to the thickness of the cement patty and the mold material. Due to the heat transfer from cement to the surrounding mold, such tests might underestimate the exothermic temperature of bone cement. Developing test methods to better characterize cement thermal behavior is necessary for accurate cement curing simulations. In this paper, the effects of the mold material and geometry on experimental measurements of bone cement setting temperature and setting time were evaluated by conducting the polymerization in different test molds. Finite element (FE) numerical simulations were also performed to provide a further understanding of these effects. It was found that the mold material and geometry significantly influence the values of the parameters measured using the ASTM standard. Results showed that the setting temperature measured was about 50 °C lower in a polytetrafluoroethylene (PTFE) mold than in a polyurethane (PU) foam mold for the 6 mm thickness cement. The measured peak temperature using PTFE molds varied about 75 °C for different mold heights (6 mm vs. 40 mm), but only by 28 °C with PU molds. The measured setting time with PTFE molds varied by about 740 s for different mold heights (6 mm vs. 40 mm), while only by about 130 s for PU molds. Using PU foam materials for the test mold decreases cement heat transfer effects due to the poor heat conductivity of PU foam and provides more consistent measured results. FE parametric studies also support these observations. Poor conductivity materials, like PU foam, make better molds for the characterization of bone cement thermal behavior.

Poor interfacial properties between reinforcement fibers and a Polymethylmethacrylate (PMMA) matrix may result in debonding between them, which is a major failure mechanism for fiber reinforced bone cement. Optimization of the shape of the fibers can improve load transfer between the fibers and PMMA matrix, thereby providing maximum overall strength performance. This paper presents a procedure for structural shape optimization of short reinforcement fibers using finite element analyses. The composite is modeled by a representative element composed of a single short fiber embedded in PMMA matrix. In contrast to most previous work on this subject, contact elements are employed between the fiber and the matrix to model a low strength interface. Most models assume a perfect bond. Residual stress, due to matrix cure shrinkage and/or thermal stresses, is also included in the model. The design objective is to improve the stiffness of the composite. The results presented show that a threaded end, short fiber results in mechanical interlock between the fibers and the PMMA matrix, which helps to bridge matrix cracks effectively and improve the stiffness of the composite.