Progressive failure analysis of laminated composite femoral prostheses for total hip arthroplasty.

In this research program, a numerical method was developed to predict the progressive failure of a thick laminated composite femoral component for total hip arthroplasty. A 3-D global/local technique was used 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 different material degradation models were incorporated as individual subroutines in the numerical method, giving it the flexibility to model a wide range of materials and structures. Numerical modeling was also conducted to design experimental test methods for component fatigue testing that closely simulate in vivo loading conditions. Parametric studies were then conducted with the numerical model of the experimental system and the results were compared to the actual experimentally determined damage behavior of fabricated laminated composite femoral component to assess which parameter set most accurately predicted the actual damage development behavior. The best fitting parameter set was then applied to the failure problem of the composite hip prosthesis implanted in an anatomically modeled femur to predict in vivo performance. This work provides a ply level understanding of the damage behavior of laminated composite femoral components and a numerical tool which can serve as a guide for the design of fatigue resistant implants made from composite material for this and other implant applications.

The effects of curing history on residual stresses in bone cement during hip arthroplasty.

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.

Effects of pre-cooling and pre-heating procedures on cement polymerization and thermal osteonecrosis in cemented hip replacements.

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.

Failure analysis of composite femoral components for hip arthroplasty.

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.

Evaluation of the effects of implant materials and designs on thermal necrosis of bone in cemented hip arthroplasty.

The exothermic polymerization of bone cement may induce thermal necrosis of bone in cemented hip arthroplasty. A finite element formulation was developed to predict the evolution of the temperature with time in the cemented hip replacement system. The developed method is capable of taking into account both the chemical reaction that generates heat during bone cement polymerization (through a kinetic model) and the physical process of heat conduction (with an energy balance equation). The possibility of thermal necrosis of bone was then evaluated based on the temperature history in the bone and an appropriate damage criterion. Specifically, we evaluate the role of implant materials and designs on the thermal response of the system. Results indicated that the peak temperature at the bone/cement interface with a metal prosthesis was lower than that with a polymer or a composite prosthesis in hip replacement systems. Necrosis of bone was predicted to occur with a polymer or a composite prosthesis while no necrosis was predicted with a metal prosthesis in the simulated conditions. When reinforcing osteoporotic hips with injected bone cement in the cancellous core of the femur, the volume of bone cement implanted is increased which may increase the risk of thermal necrosis of bone. We evaluate whether this risk can be decreased through the use of an insulator to contain the bone cement. No thermal necrosis of bone was predicted with a 3 mm thick polyurethane insulator while more damage is predicted for the use of bone cement without the insulator. This method provides a numerical tool for the quantitative simulation of the thermal behavior of bone-cement-prosthesis designs and for examining and refining new designs computationally.

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.

Static and fatigue mechanical characterizations of variable diameter fibers reinforced bone cement.

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.

Thermal characterization of PMMA-based bone cement curing.

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 degrees 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 degrees C for different mold heights (6mm vs. 40 mm), but only by 28 degrees 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.

Finite element thermal analysis of bone cement for joint replacements.

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.