Thermal analysis of bone cement polymerisation at the cement-bone interface

The two major problems that have been reported with the use of polymethylmethacrylate (PMMA) cement are thermal necrosis of surrounding bone due to the high heat generation during polymerisation and chemical necrosis due to unreacted monomer release. Computer models have been used to study the temperature and monomer distribution after cementation. In most of these models, however, polymerisation is modelled as temperature independent and cancellous bone is modelled as a continuum. Such models thus cannot account for the expected important role of the trabecular bone micro-structure. The aim of this study is to investigate the distribution of temperature and monomer leftover at the cancellous bone–cement interface during polymerisation for a realistic trabecular bone—cement micro-structure and realistic temperature-dependent polymerisation kinetics behaviour.

A 3-D computer model of a piece of bovine cancellous bone that underwent pressurization with bone–cement was generated using a micro-computed tomography scanner. This geometry was used as the basis for a finite element model and a temperature-dependent problem for bone cement polymerisation kinetics was solved to simulate the bone cement polymerisation process in the vicinity of the interface. The transient temperature field throughout the interface was calculated, along with the polymerisation fraction distribution in the cement domain.

The calculations revealed that the tips of the bone trabeculae that are embedded in the cement attain temperatures much higher than the average temperature of the bone volume. A small fraction of the bone (10%) is exposed to temperatures exceeding 70°C, but the exposure time to these high temperatures is limited to 50 s. In the region near the bone, the cement polymerisation fraction (about 84%) is less than that in the centre (where it is reaching values of over 96%). An important finding of this study thus is the fact that the bone tissue that is subjected to the highest temperatures is also subjected to high leftover monomer concentration. Furthermore the maximum bone temperature is reached relatively early, when monomer content in the neighbouring cement is still quite high.