We performed an experimental study to determine the effectiveness of computer assisted robotic bone preparation with regard to primary rotational stability in comparison to hand broaching. Forty-five synthetic femora were prepared by one of two robotic systems (Robodoc n = 12 and CASPAR n = 12) or by one experienced surgeon (n = 21). Seven different types of cementless femoral components were implanted using a standard protocol and measured in a specially designed testing machine with displacement in six degrees of freedom. For each implant at lease 3 measurements were taken for the handbroached and the robotic milled group, respectively. In addition the contact areas between the stems and the bone were visualised.

S-ROM, Antega and ABG stems were lightly more stable in hand broached femora. Osteolock (prepared by both robotic systems) and Vision 2000 stems were more stable in the robot group without changing the movement pattern. G2 and Versys ET performed higher stability with a change to more proximal fixation in the robotic group. Finally four of seven stems had an increase in rotational stability with the robotically milled cavities. The findings highlight the current difficulties in creating a perfect match of robotically milled cavity and stem geometry to achieve enhanced stability. The contact areas differed in some prosthesis in the way of preparation. In some stem geometries area of fixation and the movement pattern of the stem differ with the mode of preparation.