Effect of curing time on mechanical properties of PMMA bone cement

Abstract

Bone cement is commonly used in orthopedic surgeries of human and animal. The most common bone cement material used is poly methyl methacrylate, or PMMA. Most commercially available PMMA bone cements for human are Cobalt (Biomet, Inc.), Simplex (Stryker, Inc.), and Palacos (Heraeus Company) while for animal, available PMMA bone cements are Biomedtrix, Jorgensen, etc. One of the major drawbacks of PMMA cements that are used for human and animal orthopedic surgeries is strong exothermic reaction that happens during the curing of PMMA cement. The increased temperature can cause damage to the surrounding bone cells as well as the tissues. There are significant differences in the maximum temperature rise of the bone cement samples for different curing time of the PMMA. The thermal expansion of the cement would have generated large residual stress which would affect the cement stress distributions at the stem cement interface. The residual stresses, caused by the exothermic temperature difference, can influence the local strain energies and the fracture at the bone-cement interface. Properties of bone cements are investigated by many researchers but the effects of curing time on the mechanical properties remain unaddressed. In addition, the effects of curing time on the interfacial strength between implant and cement interface due to pull out tensile force under static and cyclic loading are not known yet. Such knowledge has clinical importance since the clinicians will know the time period requires for the cement to reach to its maximum mechanical capabilities after surgery. The goal of this study is to understand how the mechanical behavior of implant-cement interface is influenced by the change of mechanical properties of two different bone cements, which are differentiated by curing time. This study hypothesizes that the curing temperature and time influence the mechanical properties of the cement adjacent to the implant, which resulted in the variability in bonding strength between the implant and cement. To test this hypothesis, this study measured the flexural strength, hardness, and morphology of the human bone cement (HBC) and veterinary bone cement (VBC) at different curing times. In addition, this study measured the shear strength at the interfaces of implant/HBC and implant/VBC samples during static and stepwise cyclic tests at different curing times. Stryker Simplex P and BioMedtrix 3 poly methyl methacrylate (PMMA) cements were used as an HBC and VBC, respectively. The HBC and VBC cement samples were cured for 10, 30, and 60 min and then conducted flexural, hardness, and interface fracture tests to evaluate the curing effect on mechanical behavior of each of the cements. It has been found that the curing time significantly increases the values of flexure and hardness properties of each cement and shear strength of implant/HBC and implant/VBC.