N. D. Watkins

Some failure modes of four clinical bone cements

Abstract The fracture or failure behaviours of four commercial acrylic-based bone cements have been examined in tensile, bending and compression modes, and their mechanical properties are reviewed. It was found that Palacos R-40 bone cement had high radiopaque agent concentration, with high surface hardness. It exhibited a much lower bending strength and bending modulus compared with the other three bone cements (CMW1, CMW2000 and Simplex P). The textures of tensile fracture surfaces produced were similar for the four bone cements studied. The fracture surface was fragmented by crevices, which developed through the matrix and around large undissolved polyme-thylmethacrylate (PMMA) beads. Three bands with different features existed on the bending fracture surfaces, with an abrupt transition between them. It appears that the agglomerates of zirconium dioxide particles are implicated in Palacos R-40 bone cement fracture surface. The examination of compressive failed specimens revealed that a ‘yielded crack band’ existed across the transverse section. Plastic deformation resulted in the PMMA beads being squashed in the longitudinal direction and dilated in the transverse direction.

Creep behavior comparison of CMW1 and palacos R-40 clinical bone cements

The restrained dynamic creep behaviors of two clinical bone cements, Palacos R-40 and CMW1 have been investigated at room temperature and body temperature. It was found that the two cements demonstrated significantly different creep deformations, with Palacos R-40 bone cement demonstrating higher creep strain than CMW1 bone cement at each loading cycle. For both cements, two stages of creep were identified with a higher creep rate during early cycling followed by a steady-state creep rate. The test temperature had a strong effect on the creep performance of the bone cements with higher creep rate observed at body temperature. The relationship between creep deformation and loading cycles can be expressed by single logarithmic model. The SEM examinations revealed that CMW1 bone cement is more sensitive to defects within the specimen especially to the defects at the edges of the specimen than Palacos R-40 bone cement. However, in the absence of micro-cracks or defects within the inner surface layer, the dynamic loading (at less than 10.6 MPa) is unlikely to produce micro-cracks in the CMW1 bone cement. The different behaviors between the two bone cements may be attributed to differences in chemical compositions and molecular weight distributions.

A preliminary hip joint simulator study of the migration of a cemented femoral stem

Abstract A hip joint simulator that can be used to evaluate the outcome of the cemented total hip replacement has been designed, manufactured and evaluated. The simulator produces motion of a cemented hip construct in the extension/flexion plane, with a socket to rotate internal/externally. At the same time a dynamic loading cycle is applied to the construct. A validation test was performed on a cemented femoral stem within a novel composite femur. The study demonstrates the value of using a hip joint simulator to evaluate the outcome of the cemented hip construct. A complex migration pattern of the cemented hip prosthesis with respect to load cycling was observed, demonstrated in vitro comparable prosthesis migration behaviour, both the stem migration and migration patterns, to that found clinically.

Effect of restraint on the creep behavior of clinical bone cement

Bone cement creep behavior was studied using commercial available clinical bone cement. Creep deformation was measured using a micrometer with a resolution of 1 μm. It was demonstrated that the restraint has a very strong effect on the creep of bone cement, half-restrained specimens creep more than that of fully restrained specimens. The creep of the investigated bone cement increased with the dynamic loading cycles, and two creep stages, a high initial creep rate followed by a steady creep stage, can be identified. The relationship between the creep deformation and the loading cycles was expressed by a single logarithmic model.

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].