BACKGROUND AND PURPOSE: Under physiological loads, debonded cemented femoral stems have been shown to move within their cement mantle and generate a fluid pump that may facilitate peri-prosthetic osteolysis by pressurizing fluid and circulating wear debris. The long-term physiological loading of rough and polished tapered stems in vitro has shown differences in performance, with greater interface pressures generated by the rough stems. In this study we investigated the individual effects of stem surface finish, degree of mantle wear, and mode of loading on the stem pump mechanism. METHOD: Rough and polished stems were loaded under different regimes in artificially worn cement mantles that permitted either 2 or 5 degrees of rotational stem movement, and the interface pressures were compared. RESULTS: The pressures generated by the rough and polished stems were similar in either type of mantle. The pattern of pressure generation in the 2-degree mantles was similar to the pressures generated by rough stems after long-term loading, but the high posterior wall pressures fell and the tip pressures increased in the 5-degree mantles. The torsional loads were principal drivers of pressure generation in all areas of the interface other than the implant tip, where axial loading predominated. INTERPRETATION: Femoral stems with rotational instability under cyclic torsional loads generate elevated interface fluid pressures and flows independently of stem surface finish. The rough surface finish is only important in creating this instability in tapered stems.