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We studied whether prosthetic femoral stem centralizers have a detrimental effect on the macroporosity of the cement mantle. With pressure and time monitoring identically cast, moulded prosthetic femora had Charnley C-stems implanted. The stems were removed and the cement mantle was sectioned and photographed to record the size and location of voids (macropores). Cement voids were found in 85% of femora when a centralizer was used compared to 20% of the control group (p=0.002). A modular centralizer produced cement voids of a larger volume and more frequent occurrence (p=0.002). The macroporosity of the cement mantle was independent of the rate of implantation (p=0.39). Modern distal stem centralizers therefore increase the incidence of void formation in the cement mantle; although they help guard against malposition of the implant, they may have implications for early loosening.