The material properties of bone-particle impregnated PMMA

Mechanical properties of acrylic bone cement containing PMMA-SiO2 hybrid sol-gel material

An organic-inorganic hybrid material, poly(methyl methacrylate) (PMMA)-SiO2 (SiO2 content of 72 wt%), was prepared by incorporating PMMA structure units covalently into an SiO2 glass network via the sol-gel approach. The hybrid sol-gel material PMMA-SiO2 was subsequently used as the solid powder component of bone cement and its mechanical properties were evaluated. The effects of the addition of tricalcium phosphate (TCP), hydroxyethyl methacrylate (HEMA), and ethylene glycol dimethacrylate (EGDMA) on the properties of the sol-gel hybrid bone cement were also investigated. The influence of these components on the temperature rise during polymerization was discussed. It was found that the new bone cement containing PMMA-SiO2 hybrid sol-gel material had higher modulus than that of Simplex-P bone cement. The addition of TCP in the new bone cement increased the Young's modulus and the polymerization time; the inverse was observed for the tensile, bending, and compressive strengths, and the polymerization temperature. The addition of HEMA and EGDMA in the new bone cement had the opposite effect of TCP. The comparison between the new sol-gel bone cement and the commercial Simplex P bone cement was discussed.

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.

Characterization of acrylic bone cement using dynamic mechanical analysis

Dynamic mechanical analysis (DMA) was used to characterize the properties of acrylic bone cement with the addition of tricalcium phosphate (TCP), hydroxyethyl methacrylate (HEMA), and ethylene glycol dimethacrylate (EGDMA). The glass transition temperature of acrylic bone cement is >100 degrees C; the cement has a flat modulus response near human body temperature. The height of the damping peak decreases and becomes broader with increasing TCP content. Thus, TCP is incompatibile with acrylic bone cement. When the frequency is changed from high to low, the damping peak shifts to low temperature. The shift in damping peak with frequency indicates that this relaxation is time-dependent. When acrylic bone cement contains TCP with HEMA and EGDMA, the incompatibility between acrylic bone cement and TCP can be ameliorated.

Bioactive bone cements

Poly(methylmethacrylate) (PMMA) bone cement, used to fix implants into the bone, produces good surgical results if used correctly. However, prostheses do eventually become loose and the breakdown of the cement mantle is a factor in this failure. Limitations of PMMA cement, which lead to problems with the fixation of the implant, include its mechanical characteristics and its influence upon surrounding bone, associated with the polymerization reaction. A bioactive bone cement is particularly designed to produce a better interface between the cement and bone. However, an improvement in mechanical properties, especially fatigue, creep and fracture toughness, are an added necessary requirement to increase the lifetime of a cemented implant. The development of a bioactive cement has been conducted mainly in two ways; firstly, to improve existing PMMA cement by the addition of various bioactive agents and secondly, to design an alternative matrix for the bioactive material to be combined with. The most promising investigations which have been conducted, along with their relative benefits and drawbacks, are discussed.

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.

Calorimetric characterization of the formation of acrylic type bone cements

The formation of acrylic bone cements upon heating was investigated by differential scanning calorimetry (DSC). The effects of the contents of initiators, accelerator, biocompatibilizer, and crosslinking agents on the rate and the heat of polymerization during DSC heating were studied. The rate and the heat of polymerization (delta H) were characterized by the peak temperature and the area of the DSC exotherm, respectively. It was found that both the rate and heat of polymerization decreased with increasing heating rate. The delta H was increased considerably with increasing benzoyl peroxide (BPO) initiator concentration from 1 to 10% (w/v), whereas the rate of polymerization was reduced significantly. An increase in azobisisobutyronitrile (AIBN) initiator concentration also induced an increase in delta H, but the rate of reaction was not affected considerably. The addition of accelerator promoted the rate of reaction but resulted in a drop in delta H. The rate of polymerization for the system containing BPO initiator was increased quite significantly with the addition of hydroxyethyl methacrylate (HEMA) biocompatibilizer, while the delta H was slightly increased. For the system using AIBN as the initiator, the rate of polymerization was decreased slightly and the delta H dropped significantly with the addition of HEMA. The effect of ethylene glycol dimethacrylate (EGDMA) crosslinking agent was also examined. Polymerization became more rapid with the addition of EGDMA in the bone cement using BPO initiator, while it remained approximately constant for the system using AIBN as the initiator. No systematic change in delta H was observed with the addition of EGDMA in both systems. This study demonstrated that DSC is a potential tool to measure the amount of heat released and also the rate of polymerization for bone cements.

Stress relaxation in native and EDTA-treated bone as a function of mineral content

The relaxation Young's modulus, epsilon(t), of bovine femoral bone was measured as a function of mineral content. Five different specimens with different mineral contents were prepared by the EDTA treatment. The relaxation curves for specimens of different mineral contents were superimposable upon one another by shifting along the log t axis as well as the log epsilon(t) axis. A well-specified master curve of stress relaxation was obtained from a set of relaxation curves for samples with different mineral contents. The mineral content dependence of vertical shift factors, along the log epsilon(t) axis, accorded well with the mineral content dependence of Young's modulus itself, the accordance indicating the plausibility of the vertical shifting procedure. The stress relaxation in bone has been reported to be related to the viscoelastic properties of the collagen matrix. It is considered that the reinforcement of the matrix around the mineral, by the mineral as fillers, increases the average modulus of the matrix and lengthens the characteristic time of the relaxation. This consideration explains the existence of a shift factor along the logt axis and its mineral content dependence. As the structures of bone specimens treated by EDTA are expected to be different from that of normal bone, the conclusion drawn from this experiment cannot be immediately applied to normal bone. It can be, however, concluded that the size of the mineral and also the interface area between mineral and collagen matrix play an important role in the viscoelastic properties of normal bone and the bone specimen treated by EDTA.

Bone-particle-impregnated bone cement: an in vivo weight-bearing study

To evaluate an experimental inorganic-bone-particle-impregnated bone cement, canine hip prostheses were implanted in dogs using a regular bone cement on one side and the experimental bone cement on the other. In a preliminary feasibility study, bone ingrowth into the resorbed bone-particle spaces was established 3 months after implantation in three dogs. In a more detailed study, twenty-eight (28) dogs were divided in four groups to delineate the effects of time on the phenomena of bony ingrowth. One month after implantation, active bone ingrowth into the bone cement was obvious. By 3 months postimplantation, the ingrowth appeared to have traversed the thickness of the bone-particle-impregnated cement. By the fifth month, most of the interconnected inorganic bone particles were replaced by new bone. At the end of a year, the ingrown bone was mature and negligible new bone activity was present. Biomechanical pushout tests closely corroborated the histologic observations. The maximum shear strength of the cement/bone interface of the experimental side reached 3.6 times that of the control side at 5 months postimplantation. No further improvements were seen at 12 months postimplantation. A viable bone/cement interface may result in a better orthopedic implant fixation system by combining the advantages of both cement for immediate rigidity and biological ingrowth for longterm stability.

Mechanical properties of hydroxyapatite-reinforced gelatin as a model system of bone

The elastic Young's modulus of hydroxyapatite-reinforced gelatin as a mechanical model system of bone was measured as a function of the volume fraction of hydroxyapatite, phi h. Initially, the Young's modulus gradually increased with an increase in phi h and then increased rapidly in the vicinity of phi h approximately 0.2. The phi h dependence of the Young's modulus was analysed by means of the theory of composite materials. It was found that with the increase in phi h the initial uniform stress deformation mode of the sample changed to the uniform strain deformation mode. The non-linear character in phi h dependence of the Young's modulus of this system was considered to reproduce well the novel behaviour of the mechanical properties of bone as a function of the mineral fraction. The situation was considered to be similar to a percolation problem. A preliminary analysis revealed that the critical exponent about the viscosity of the system accorded with the theoretically expected value. The result may present the evidence that the discontinuous point in mechanical properties of bone would be originated from an interaction such as a percolation of mineral particles on a matrix protein.

Bone-particle-impregnated bone cement: an in vitro study

Bone-particle-impregnated bone cement specimens (up to 30% by weight) were characterized by various test methods. The experimental bone cement showed decreased crack propagation rates and increased Young's modulus, while the ultimate tensile strength and impact strength were decreased. The viscosity could be adjusted by adding initiators lost when substituting the PMMA powder with bone particles. The present study warranted further in vivo experiments on the possibility of tissue ingrowth for which the new bone cement was developed.