THE EFFECT OF MONOMER/POLYMER AND INITIATOR/ACTIVATOR RATIOS ON THERMAL AND MECHANICAL PROPERTIES OF ACRYLIC BONE CEMENT

Abstract

Introduction: Acrylic bone cement (ABC) manufacturers vary their products by using different proportions of the principle ingredients to optimise handling time or mechanical properties. Surprisingly, there is limited research showing the effect of varying monomer/polymer and initiator/activator ratios (independent from other constituents) on thermal and mechanical properties of ABC.

Materials and Methods: The formula for CMW (DePuy) was reproduced using original ingredients obtained from different suppliers. The commercially available CMW monomer/polymer ratio is approximately (0.6 ml/gm). Six variants of CMW bone cement were prepared by varying the monomer/polymer ratio (0.4–1 ml/gm) and eight variants were prepared by varying the initiator/activator BPO/DMPT ratio (1.71–11.25). Specimens were stored in an incubator for 7 days at 37 °C. Thermal characteristics of the polymerisation reaction such as maximum polymerisation reaction temperature (Tmax) and setting time (Ts) were recorded using a thermocouple and Picolog digital data recorder. Compressive mechanical properties were measured using Zwick Roell All Round Testing System implementing ISO5833 recommendations. SPSS software was used to perform ANOVA and calculate Pearson correlation coefficient.

Results: Increasing monomer/polymer ratio resulted in prolongation of setting time (5.3–11.3 minutes) displaying a significant (p= 0.000) correlation (r=0.988); however, there was no significant correlation with Tmax (r=−0.123, p=0.792). Increasing the monomer/polymer ratio resulted in a significant reduction in yielding compressive strength (F=110.97, p=0.000) and modulus (F=16.1, p=0.000). Pearson correlation test showed that monomer/polymer ratio had a significant correlation with yielding compressive strength (r= −0.930, p=0.002) and a significant correlation with the corresponding modulus of elasticity (r= −0.827, p=0.022). An increase in the BPO/DMPT ratio did not display a significant (p= 0.172) correlation (r=−0.535) with Tmax; however, the setting time was prolonged by increasing the BPO/ DMPT ratio with a significant (p=0.002) strong positive correlation (r=0.903). Compression tests showed a significant (F=13.45, p=0.000) reduction in yielding compressive strength with a significant (p=0.04) inverse (r=−0.729) correlation with the BPO/DMPT ratio. Modulus of elasticity followed a similar pattern to a lesser degree displaying a significant (F=5.123, p=0.001) reduction in values which was moderately correlated (r=−0.619), though insignificant (p= 0.101) with BPO/ DMPT ratio.

Discussion and Conclusions: Varying the monomer/ polymer ratio independently from other constituents in acrylic bone cement significantly affects setting time and compressive mechanical properties. Setting time can be prolonged to increase handling time; however this will occur at the expense of a reduction in compressive stiffness and strength. Similarly, varying the BPO/DMPT ratio may result in optimised handling time; however, this will also cause a reduction in compressive strength and stiffness. These finding are paramount in clinical applications where compressive strength is essential e.g. percutaneous vertebroplasty.