Effect of cementless bowed stem distal surface contour and coronal slot on femoral bone strains and torsional stability

Six pairs of unembalmed cadaver femurs were instrumented with strain gauges and prepared with flexible reamers for insertion of long bowed cementless femoral stems. A fully porous-coated cobalt chrome stem was inserted into each left femur, and a distally fluted, slotted, stem of the same implant geometry and implant material was inserted into each right femur. Bone strains were measured during stem insertion and torsional stability tested after the stems were fully seated. Distal strains were significantly higher (p < 0.05) for the fully porous compared to the distally fluted, slotted stem. Three fractures occurred in femurs with fully porous-coated stems. There was no difference in torsional stability between the two stem geometries. Our data demonstrate that a long bowed cementless stem with a distal coronal slot and flutes is associated with decreased bone strains and fracture risk during stem insertion compared to a fully porous-coated stem.