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 ste p to minimize, if not eliminate, cement fracture is to understand the material characteristics controlling fracture resistance. At least fou r phases of bone cement can be identified that may affect the damage z one formation: pre-polymerized beads, interbead matrix polymer, BaSO4, and porosity. Gel permeation chromatography (GPC) was used to determi ne 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. Mi crocracks defined the damage zone for fatigue fracture. The microcrack s developed in the interbead matrix and not through the pre-polymerize d beads. Light microscopy revealed evidence of craze formation on surf aces of fractured beads during rapid fracture, but not on fatigue surf aces. GPC analysis indicated an increase in MW from the bead phase alo ne to the fully cured bone cement, indicating a greater MW in the inte rbead matrix polymer. Increases of 36 and 176% were measured for two d ifferent bone cements, but the bulk of the polymer has an MW of less t han 1 x 10(6). Three factors were suggested to explain why the microcr acks seem to prefer to grow in the interbead matrix: the presence of B aSO4, shrinkage during the curing process, and the different polymeriz ation processes of the bead and the interbead polymers. Pores had an a ffect on the microcrack formation as well, and did not need to be dire ctly in front of the crack tip to interact with the damage zone. The p ores seemed to act as nucleation sites for microcracks. The porosity-m icrocrack nucleation interaction may explain and reconcile the apparen tly disparate results concerning the effect of porosity on fracture to ughness 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 effect ively blunting the crack. The crack propagation mechanisms revealed by this research indicated the importance of microstructure in the fatig ue failure of PMMA.