Mechanical Properties Of Bone Cement Research Articles

Shape optimization of randomly oriented short fibers for bone cement reinforcements

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.

Addition of Hand-Blended Generic Tobramycin in Bone Cement: Effect on Mechanical Strength

This study compared the mechanical strength of commercially prepared antibiotic bone cement (Simplex With Tobramycin; Stryker, Mahwah, NJ), cement with generic tobramycin (Pharma-Tek, Huntington, NY) blended in by the orthopedic nursing staff, and standard nonantibiotic bone cement. The results showed an approximate 36% decrease in the strength of the cement with hand-mixed generic tobramycin, while the commercial antibiotic cement remained unchanged relative to the nonantibiotic control. These results indicate the mechanical properties of bone cement can be severely compromised by hand-mixing antibiotics into bone cement at the time of surgery.

On the particle size and molecular weight distributions of clinical bone cements

Currently, polymethylmethacrylate (PMMA) or acrylic bone cement is the most commonly used adhesive material in total hip replacement (THR) surgery [1, 2]. The mechanical properties of acrylic bone cement are very influential in determining successful long-term performance of THR [3‐7]. A large number of commercial formulations are available, differing in chemical composition and physical properties of both powder and monomer constituents. Studies on the mechanical properties of commercial available bone cements revealed that the different cements behave differently [8‐11]. Harper and Bonfield [12] investigated ten commercial bone cements under the same test regimes, and found that handling characteristics of each bone cement varied, and significant differences in both static and dynamic behaviors between the various bone cements. The reasons for the differences obtained in mechanical properties can be attributed to variations in both compositions of polymer and monomer, particle size, morphology, and molecular weight of powder, strength of polymer bead-matrix interface, and powder-to-liquid ratios. A large number of studies reporting the mechanical properties of bone cement and the factors that affect these properties have been reported in the literature [1, 13‐16].

The preparation of bone cement.

The hip joint is subjected to large, repetitive loads. It is therefore clear that the bone cement, which allows the transfer of load across the new joint, must be able to withstand the everyday loads that it will be subjected to. Improving the mechanical properties of the cement to withstand high stresses, fatigue and creep loading will reduce the chances of failure, ultimately increasing the longevity of the joint replacement. To date, work in this area has concentrated on improving the mechanical properties of bone cement through improved bone cement mixing techniques. In the next issue we will be covering the effect that the design of the mixer and vacuum mixing has on improving the mechanical properties, such as the strength, fatigue and creep resistance, of the bone cement.

Tensile properties of a bone cement containing non-ionic contrast media.

The addition of contrast media such as BaSO4 or ZrO2 to bone cement has adverse effects in joint replacements, including third body wear and particle-induced bone resorption. Ground PMMA containing particles of the non-ionic water-soluble iodine-based X-ray contrast media, iohexol (IHX) and iodixanol (IDX), has, in bone tissue culture, shown less bone resorption than commercial cements. These water-soluble non-ceramic contrast media may change the mechanical properties of acrylic bone cement. The static mechanical properties of bone cement containing either IHX or IDX have been investigated. There was no significant difference in ultimate stress between Palacos R (with 15.0 wt % of ZrO2) and plain cement with 8.0 wt % of IHX or IDX with mass median diameter (MMD) of 15.0 or 16.0 microm, while strain to failure was higher for the latter (p < 0.02). The larger particles (15.0 or 16.0 microm) gave significantly higher (p < 0.001) ultimate tensile strengths and strains to failure than smaller sizes (2.4 or 3.6 microm). Decreasing the amount of IHX from 10.0 wt % to 6.0 wt % gave a higher ultimate tensile strength (p < 0.001) and strain to failure (p < 0.02). Scanning electron microscopy (SEM) showed the smaller contrast media particles attached to the surface of the polymer beads, which may prevent areas of the acrylate bead surface from participating in the polymerization. In conclusion, the mechanical properties of bone cement were influenced by the size and amount of contrast medium particles. By choosing the appropriate amount and size of particles of water-soluble non-ionic contrast media the mechanical properties of the new radio-opaque bone cement can be optimized, thus reaching and surpassing given regulatory standards.

The effect of ultra-high molecular weight polyethylene fiber on the mechanical properties of acrylic bone cement

The effects of the addition of ultra-high molecular weight polyethylene fiber (UHMWPE) on the mechanical properties of standard surgical Simplex-P radiopaque bone cement have been investigated. It was found that the tensile strength and tensile modulus were apparently not improved by the incorporation of UHMWPE in the acrylic bone cement. The results of bending strength and bending modulus indicated that a reinforcing effect is obtained at UHMWPE contents as low as 1 wt%, and then levelled off with increasing UHMWPE contents. When the UHMWPE contents as low as 2 wt%, the values of compressive strength and modulus seemed approximate the same; whereas the values of compressive strength and modulus decreased with increasing UHMWPE contents. From the results of dynamic mechanical analysis (DMA), the values of dynamic storage modulus of bone cement increased at UHMWPE fiber as low as 2 wt%, but beyond that UHMWPE content the value of the dynamic storage modulus decreased with increasing UHMWPE contents. The same results were also found for the dynamic loss modulus. When methyl methacrylate was grafted onto UHMWPE by plasma and UV irradiation treatment, it was found that by adding the treated UHMWPE fiber in acrylic bone cement had a significant reinforcing effect on the mechanical properties of bone cement.

Detrimental effect of aging on the endurance of bone cement

We investigated the effect of aging on the compressive strength of bone cement under in vivo and in vitro conditions. Cement molds were implanted in the dorsum of rabbits and other molds were kept under stable conditions and in darkness. Comparative measures taken at 15 days and 1, 3, 6, 12 and 24 months showed lower endurance of the implanted molds (p < 0.001). A reactive capsule surrounded the bone cement in vivo up to the 3rd month, its cellularity increased, and then almost disappeared by 1 year. Macrophages and foreign body cells reappeared at 2 years, indicating a "chemical aging" effect in the in vivo environment. Our findings suggest that aging may play an important role in the amelioration of the mechanical properties of bone cement.

Mechanical Properties of Bone Cement

The compressive and shear strength and the modulus of elasticity were studied in three different bone cements (CMW, Simplex P, and Zimmer low-viscosity cement). One hundred four specimens were prepared by either a conventional manual mixing technique or mechanical vibration. Mechanical mixing gave a significant increase in shear strength for CMW and Simplex P. The compressive strength of Simplex P and Zimmer low-viscosity cement was significantly improved by mechanical mixing. The scatter of the results was significantly reduced by mechanical mixing. The modulus of elasticity was significantly increased by mechanical mixing for the three cements. The uniformity of the bone cement mass seems to be increased by a standardized mechanical mixing technique.

Centrifugation as a method of improving tensile and fatigue properties of acrylic bone cement.

In this study centrifugation dramatically reduced the porosity and substantially increased the mechanical properties of bone cement. Monotonic tensile tests to failure of centrifuged specimens of cement demonstrated an increase of 24 per cent in the mean ultimate tensile strength compared with the control value. Mean ultimate tensile strain was improved by 54 per cent. In fully reversed tension-compression fatigue-testing, centrifugation resulted in a mean increase in fatigue life of 136 per cent. These strong advantages in mechanical properties were obtained without any detrimental changes. There was no change in elastic modulus, setting time, or peak temperature. Handling properties were improved. There was no increase in systemic toxicity as demonstrated in dogs by assessment of arterial blood-pressure response and peak levels of monomer in the serum during simulated total hip arthroplasty. We also present a practical system of cement centrifugation and delivery that is suitable for use in the operating room.

Mechanical properties of bone cement: a review.

Many authors have examined the mechanical properties of bone cement and the various factors that affect its mechanical behavior. This article presents a comprehensive survey on the reported mechanical properties of bone cement. Variables that influence the mechanical properties, such as handling characteristics, strain rate, loading modes, additives, porosity, blood inclusion, in vivo environment, temperature, etc. have also been reviewed. The importance of specifying these variables in reporting test results on the mechanical properties of bone cement is pointed out. Previous attempts to improve the mechanical properties of bone cement are also summarized. Future research areas important for fully characterizing the physical properties of PMMA are also suggested.

The mechanical properties of bone cements.

The mechanical properties of a number of commercially available bone cements have been investigated. Tests were carried out on specimens in compression, in bending and in tension. Using the compression test as a standard, the effects of the following variables were studied: the addition of antibiotics, strain rate, environmental temperature, and age. It was concluded that age, temperature and rate of straining have a marked effect on the strength of the cement, while the addition of small quantities of antibiotics only marginally weakens the cement.

Mechanical properties of bone cements containing large doses of antibiotic powders

The addition of up to 10 g gentamicin sulfate antibiotic powder to 60 g units of Simplex-P acrylic bone cement caused gradual, proportional decreases in the bulk muchanical properties of compressive and diametral tensile strengths. Water leaching of the antibiotic from the cement did not significnatly decrease these strenghts. Scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS) showed the antibiotic to reside in the acrylic matrix as discrete particles not usually associated with internal porosity. The surface-sensitive flexural strength of a proprietary bone cement was lowered immediately by small quantities of antibiotic powder, and continued to decrease as doses of up to 10 g/unit were admixed. Water leaching caused channeling as the antibiotic was removed from the surface, but it did not create further changes in flexural strength.

Anchorage of the femoral head prosthesis to the shaft of the femur.

Anchorage of the femoral head prosthesis to the shaft of the femur.