Primary rotatory stability of hip endoprostheses stems after manual and robot assisted implantation

Aim: In our study we compared the primary rotatory stability of hip endoprostheses which were implanted with the help of a robot with that of manually implanted stems. Method: We examined three different types of prosthesis-stems: Osteolock (Stryker-Howmedica), CBC (Mathys), Excia (Aesculap). Furthermore, we examined two types of prostheses which could not be implanted with a robot (Sl-Plus/Endoplus; Hipstar/Stryker-Howmedica). 10 stems of each prosthesis type were implanted in identical polyurethane foam blocks; 5 of them were implantated manually, 5 with the help of a robot (CASPAR, URS-Ortho). After implantation, a defined rotatory stress was put on the stem with the help of a torquing machine. The torsional moment which was necessary until the stem broke out of the foam was documented with the help of special software. Results: In all types of prostheses the use of the robot system led to a higher rotatory stability compared to the manual group. Among the three tested types of prosthesis stems, the CBC stem is significantly more stable for rotatory forces after robot-assisted implantation compared to the other two types. After manual implantation there was no difference in stability between the different stems. The Zweymuller stem and the Hipstar prosthesis did very well, even in comparison to the robot-implanted group. The results of the Zweymuller stem and the robot-implanted CBC Stem were similar, and a significantly higher rotatory stability of the Hipstar prosthesis compared to every other stem was observed. Conclusion: Using a robot, the rotatory stability of hip endoprostheses can be improved. However, the design of the stems seems to be even more important for the stability than the implantation technique.