Introduction and Aims: The scientific ‘need’ for porosity reduction of polymethylmethacrylate (PMMA) bone cement due to the influence of PMMA porosity with respect to femoral stem subsidence in total hip arthroplasty (THA) is not well understood. Therefore, we investigated the influence of bone cement porosity upon subsidence in physiologically simulated THA models.

Method: Twelve Exeter femoral components were implanted into mechanical analog femora – six with PMMA mixed under vacuum-mixed (VM) conditions (0.26 atms.), the others with PMMA under atmospheric (AM) conditions. Each specimen was subjected to cyclic loading in an MTS from 0.2 kN to 2.0 kN at 2Hz for at least two million cycles. Specimens were subjected to 0.5 hrs ‘on’ (3600 cycles), and 0.5 hrs ‘off’, where ‘off’ represents no cyclic loading but a static rest period at 0.1 kN. Radiographs were acquired during the testing regime. Femurs were sectioned and PMMA porosity determined; subsidence was measured from radiographs.

Results: Even though stem subsidence was greater in AM PMMA than in the VM PMMA, implant subsidence in our study between air and vacuum mixed was not significantly different (p> 0.05), nor was total percent porosity (p> 0.05). Evaluating porosity regardless of mixing method, distal porosity correlated with subsidence. The rates of stem subsidence were influenced by PMMA porosity near the distal tip (r = 0.62). Specifically, during the first six months at level 0 (distal tip), increased porosity correlated with increased subsidence. After the initial six months, this correlation (level 0) ceased. The stems with the greatest distal tip porosity were the first stems to fail (range 16 to 20 months, average 18 months).

Conclusion: We concluded, air or vacuum mixed cement does not alter porosity, but subsidence is related to the level of porosity located about the distal stem. Exeter subsidence depends upon PMMA porosity and pore location in this model.