Impacts of mixing techniques and vacuum levels on the structural and mechanical properties of PMMA bone cement: A comparative study

The mechanical integrity of polymethyl methacrylate (PMMA) bone cement is crucial for the long-term fixation of orthopaedic implants, yet it is often compromised by porosity introduced during mixing. This study investigates the influence of mixing technique and vacuum level on the structural and mechanical properties of PMMA bone cement. Three clinically relevant mixing approaches, open bowl, vacuum bowl and vacuum cartridge were evaluated at vacuum levels ranging from 0 to 650 mm Hg. Cylindrical PMMA specimens were produced and their porosity assessed using high-resolution micro-computed tomography (µCT), with corresponding mechanical properties determined through compressive testing in accordance with ISO 5833:2002. µCT analysis revealed significant reductions in porosity at higher vacuum levels, particularly with the vacuum cartridge system. A strong and significant negative correlation was observed between compressive strength and porosity (R2 = 0.864), while specimen mass showed no predictive value for mechanical performance. Although vacuum mixing reduced porosity, no consistent changes in Young's modulus were detected across the mixing groups. These findings emphasise the importance of porosity control in bone cement preparation and highlight the limitations of current ISO standards. The study advocates for improved testing protocols that more accurately reflect clinical conditions to enhance the predictive value of in vitro assessments of PMMA cement performance.