MICROSTRUCTURAL PATHWAY OF FRACTURE IN POLY(METHYL METHACRYLATE) BONE-CEMENT

Mechanical failure of poly(methyl methacrylate) (PMMA) bone cement is linked to failure of cemented total joint prostheses. An essential step to minimize, if not eliminate, cement fracture is to understand the material characteristics controlling fracture resistance. At least four phases of bone cement can be identified that may affect the damage zone formation: pre-polymerized beads, interbead matrix polymer, BaSO4, and porosity. Gel permeation chromatography (GPC) was used to determine the molecular weight (MW) distributions of the two polymer phases. Mechanical testing, scanning electron microscopy and light microscopy were used to analyse fracture mechanisms. Fatigue crack propagation of bone cement was distinctly different from rapid crack propagation. Microcracks defined the damage zone for fatigue fracture. The microcracks developed in the interbead matrix and not through the pre-polymerized beads. Light microscopy revealed evidence of craze formation on surfaces of fractured beads during rapid fracture, but not on fatigue surfaces. GPC analysis indicated an increase in MW from the bead phase alone to the fully cured bone cement, indicating a greater MW in the interbead matrix polymer. Increases of 36 and 176% were measured for two different bone cements, but the bulk of the polymer has an MW of less than 1 x 10(6). Three factors were suggested to explain why the microcracks seem to prefer to grow in the interbead matrix: the presence of BaSO4, shrinkage during the curing process, and the different polymerization processes of the bead and the interbead polymers. Pores had an affect on the microcrack formation as well, and did not need to be directly in front of the crack tip to interact with the damage zone. The pores seemed to act as nucleation sites for microcracks. The porosity-microcrack nucleation interaction may explain and reconcile the apparently disparate results concerning the effect of porosity on fracture toughness and fatigue life. Porosity may, however, also provide positive contributions to the fracture properties of bone cement by dispersing the energy at the crack tip, forming a larger damage zone, and effectively blunting the crack. The crack propagation mechanisms revealed by this research indicated the importance of microstructure in the fatigue failure of PMMA.