Biomet Cement and Cementing Systems

Vacuum Mixing & Delivery

Bone cement porosity

Bone cement porosity and mechanical properties
Mechanical or aseptic loosening is the major causes of revision. Loosening may originate at a number of places of the cement–bone or cement–implant interface or in the mantle itself. High cement porosity compromizes the cement’s mechanical strength and lessens its fatigue life. Pores or other inclusions concentrate stress in the material, often initiating fatigue cracks. These cracks ultimately lead to mechanical or aseptic loosening and implant failure. Reports show that fatigue fractures always occur at the largest macropores.^16,17

The sources of porosity in the mixing and delivery stages

Trapped air initially surrounding the powder and between the powder beads as the powder becomes wet.
Trapped air in the cement during mixing.
Trapped air in the cement during transfer from mixing container to application device.

Hand mixing bone cement in an open bowl allows for the greatest possibility of these occurrences, which is why hand-mixed cement contains a substantial number of pores, which weaken the cement. Modern cement mixing systems were developed to reduce cement porosity and enhance cement strength by eliminating air entrapment.

Porosity classification
Two types of pores are classified in fully polymerized bone cement:
Macropores (pore diameter > 1.0 mm)
Micropores (pore diameter of 0.1–1 mm)^6,19

Influence of vacuum level on porosity
Vacuum-mixed cement refers to bone cement mixed at approximately 0.15 bar (85% vacuum). This level of vacuum has been found optimal for porosity reduction, with lower levels resulting in less porosity reduction.⁶ Note, however, that the vacuum level of a hospital wall unit provides only around 30% vacuum. Note also that monomer will begin to boil when vacuum exceeds 94%, an occurrence that does not compromize the cement quality but does give the illusion of air bubbles in the cement.²⁰