Comparative micromotion of fully and proximally cemented femoral stems

This investigation studied the differences of in vitro micromotion between two stem designs. The two stem types investigated were a proximally cemente d stem with distal press fit and a fully cemented stem. After initial micro motion testing to 2250 N in simulated single leg stance and stair climb, si x of each stem type were loaded dynamically for 1 million cycles at 950 N a t 1 Hz. Micromotion studies were repeated. The two stem types had similar m icromotion, For the single leg stance, fully cemented implant motion averag ed(+/- 95% confidence) 18 +/- 8 mu m toggle, 41 +/- 5 mu m axial, and 59 +/ - 22 mu m rotation. Proximally cemented implant motion averaged 20 +/- 6 mu m toggle, 42 +/- 6 mu m axial, and 31 +/- 15 mu m rotation. For the simula ted stair climb, fully cemented implant motion averaged 24 +/- 10 mu m togg le, 45 +/- 8 mu m axial, and 92 +/- 32 mu m rotation. Proximally cemented i mplant motion averaged 19 +/- 10 mu m toggle, 42 +/- 9 mu m axial, and 87 /- 53 mu m rotation, For both loading conditions, there were no significant differences measurable between the two systems. After dynamic testing of t he fully cemented implants, there were no significant changes in the microm otion of either the toggle or the rotation, but an average of 18 mu m incre ase of axial motion was measured in the fully cemented stem. For the proxim ally cemented implants, there were no significant changes after dynamic tes ting, This difference was not considered clinically significant because roe ntgen stereophotogrammetric analysis studies have shown that more than 4 mm of migration is required before clinical symptoms manifest. The protocol d eveloped in this study may help provide a screening process to determine th e stability of femoral stem designs before these devices are used clinicall y.