Cements based on poly(methyl-methacrilate) (PMMA) are used for anchoring the prosthetic components to contiguous bones. The aim of this work is to characterize experimentally two different bone cement formulations through DMA in a water chamber.
CMW-1 and CMW1-G (from DePuy) bone cements were tested. In order to investigate the dependence of material properties on water uptake and monomer release, each cement sample was randomly divided in two groups conditioned in a water chamber at 37oC for 2 hours (sample 1) and 1 week (sample 2) respectively. Mechanical tests were carried out using the 3 point bending method (ASTM D790) in a water chamber at 37oC.
The radical polymerization of PMMA does not proceed to completition. Thus, beside water absorption, conditioned specimens (samples 2) are characterized also by the release of unreacted monomer.
Our results indicate that at low frequency sample 1 dissipate more energy than sample 2, while at high frequency no difference was observed between the samples. The higher damping factor values of sample 1 at 0.01Hz clearly indicate that the unreacted monomer plays an important role on the main chain mobility. As the ureacted monomer is released through a prolonged conditioning process (sample 2), the dissipating capability at low frequency drastically drops, while dissipation values similar to sample 1 are measured at high frequency. This effect at high frequencies may be related to the small role played by the solvent thermodynamic quality on the time-scale and activation energy for local conformational transitions. Thus the residual unreacted monomer, in vivo applications of bone cements, mainly affect the relaxation or creep in a transient manner, while water absorption mainly increase the damping capability at high frequency, thus affecting the side chain mobility.