THE DYNAMIC VOLUMETRIC BEHAVIOUR OF POLYMERISING POLYMETHYL METHACRYLATE BONE CEMENT

Abstract

Polymerisation of PMMA results in a volume change resulting from molecular rearrangement. The calculated maximal volume reduction is approximately 7.6%; however, void growth reduces this to 3–6%. The significance of volume reduction is controversial, in particular with reference to void elimination techniques. Whilst the impact of mixing technique on overall volume change is known, little is understood about the dynamic volumetric changes occurring during the crucial time of cement-bone micro-interlock formation. This study aimed to investigate the volumetric behaviour of bone cement during polymerisation.

Polyethylene tubes were modified to simulate the physical and dimensional constraints of the human femoral medullary canal. The tubes were filled with either hand or enhanced vacuum mixed cement and suspended in a water bath. The residual weight of the cement specimen in water was recorded at 60sec intervals for 30 minutes. The dry weight of the cement is known and the immersed weight can be calculated. Archimedes principal allows calculation of the density and thus the volume of the cement mass throughout polymerisation. The specimens were sectioned, stained and analysed to assess sectional porosity.

In no specimen was it possible to demonstrate overall net expansion, however, hand mixed specimens demonstrated a temporary period of expansion during the early exothermic period. Vacuum mixed specimens demonstrated progressive contraction only. Overall volume change correlated closely with sectional porosity.

The overall volume reduction is strongly influenced by porosity. The temporary expansion observed in porous cement specimens must result from temperature driven growth of voids. This expansion occurs during the crucial period of cement-bone micro-interlock formation, and may therefore enhance attempts at pressurisation. Conversely, progressive volume reduction, as seen with low porosity cement, may impede micro-interlock formation. Successful cementation using vacuum mixed cement may therefore be solely dependent on adequate cement pressurisation.