Introduction: Aseptic loosening is a long-term complication of many cemented arthroplasty procedures. The integrity of the fixation interface, in particular the level of interdigitation between cement and bone, is crucial to sustaining the stability of arthroplasty components[1]. Studies have shown that the viscosity of cement at the time of application is a significant parameter in determining this level of interdigitation[2]. However, the rheological properties of cement at key stages in arthroplasty procedures have not been quantified, and it is unclear if current operative techniques achieve optimum cement delivery properties. Furthermore, because the cure process of bone cement is highly dependent on environmental conditions, it is extremely difficult to accurately predict the time to curing. Oscillatory shear rheometry can be used to characterise the viscoelastic properties of bone cement. However, most commercial rheometers used for this purpose are too large, expensive and delicate for peri-operative use. The aim of this work is to develop a new laboratory method for measuring the viscoelastic properties of bone cement at the time of application and to investigate the relationship between these properties and the level of cement interdigitation.
Methods: A simple, inexpensive electromagnetic rotary actuator has been developed to provide accurate measurements of force, displacement and velocity without the use of sensors. These parameters can be used to continually monitor both viscous and elastic properties of curing bone cement. To consider subjective cementation techniques, a method has been devised where a surgeon indicates early and late doughing stages for a PMMA bone cement within a clinical environment. A computer interface has been developed to plot the real-time properties of the cement that are measured using the self-sensing device concurrently. The range of practical variability of cement delivery properties is then established. In order to investigate the effect of cement viscosity on the level of interdigitation a rig has been developed in which cement is applied to a standardised bone analogue under controlled conditions. The open pore ceramic analogue has been shown through microCT scanning to have a structure that is representative of the trabecular structure in human bone. CMC solution is used to represent back bleeding. Once set, the sample is evaluated using microCT to measure the level of interdigitation.
Results: Preliminary results show that bone cement has largely viscous properties following mixing and largely elastic properties towards setting. Values of dynamic viscosity obtained show the cement to have a low viscosity following mixing, then as polymer beads begin to dissolve in the monomer, the viscosity rapidly increases. The rate of viscosity increase then slows as polymer chains are created, before a final rapid increase in viscosity indicating the onset of setting.
Conclusion: A validated method has been developed to measure the viscoelastic properties of curing bone cement at key stages in arthroplasty procedures and to investigate the effect of these viscoelastic properties using a simple standardised bone model.
