Chaodi Li

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.

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.

Improved mechanical properties of acrylic bone cement with short titanium fiber reinforcement.

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.

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.

Improvement of the mechanical properties of bone cement by shape optimization of short 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.

Strengthening of Bone Cement by Shape Optimization of Short Fibers

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.Copyright

Residual Stresses in Self-Curing Bone Cement During Hip Arthroplasty

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.Copyright