Robotically-milled bone cavities: a comparison with hand-broaching in different types of cementless hip stems

We performed an experimental study to compare the effectiveness of robotic bone milling (Robodoc and CASPAR) with hand-broaching as regards primary rotational stability of 7 different cementless stems. Using 48 synthetic femora and a specially-designed apparatus, we compared the implant stability of proximal and distal rotational stem displacement (slip) in relation to the cortex. We also measured stem deformation (twist) and the location of torque transfer from stem to cortex (i.e., fixation pattern). 5-ROM, Antega, and ABG stems were more stable in hand-broached femora. Osteolock stems showed no difference between CASPAR and hand preparation, but rotational stability was better in the Robodoc group. G2, VerSys ET and Vision 2000 stems gave increased rotational stability in the robotic groups. When placed too laterally, Vision 2000 showed a pattern of more distal fixation. The findings emphasize the current difficulties in creating a perfect match of robotically-milled cavity and stem geometry to achieve enhanced primary rotational stability. The pattern of fixation seems to depend not only on stem design, but also on canal preparation and stem positioning.