Abstract
Threaded cups show good clinical results when implanted correctly. In clinical use, multiple cases with an incomplete placement of the EcoFit threaded cup (implantcast) were observed. This behaviour could not be explained intra- and postoperatively. The aim of this study was to compare and optimise the drill-in-behaviour of the EcoFit cup in a biomechanical investigation. EcoFit cup sizes 46, 50 and 54 mm were compared with the SC cup (Aesculap) size 50 mm. Foam blocks (Sawbones) of density 0.16 g/ml (pcf 10), 0.32 g/ml (pcf 20) and 0.48 g/ml (pcf 30) were used. After standardised placement using a universal testing system (n = 8 per group), the primary stability, the overhang of the cups and the drill-in behaviour were measured. Overreamings of 1 and 2 mm were performed (pcf 20, n = 8) for the EcoFit cup size 50 and the primary stability as well as the overhang measurements were examined. Measurements of the cup diameter, thread depth and thread pitch were performed on three-dimensional (3D) images of the cup size 50 mm. The drill-in behaviour was different between the EcoFit and the SC cups. Even with maximum torque, the EcoFit cup could not be positioned as deep as the SC cup in standard reaming conditions (overhang of 1.1 ± 0.4 mm for the EcoFit size 50 in pcf 20 and of −0.01 ± 0.2 mm for the SC cup). The primary stability was lower for the EcoFit cup in comparison to the SC cup (128.8 ± 3.2 Nm vs. 138.6 ± 9.1 Nm, p = 0.0291). With overreaming to 51 mm, a deeper positioning of the EcoFit was possible (overlap of −0.3 ± 0.1, comparable to the SC cup). The overreaming of the cavity also led to a significantly higher primary stability of 143.4 ± 3.7 Nm (p < 0.001) comparable to the unaltered condition (128.8 ± 3.2 Nm). Overreaming to 52 mm had no further advantage in terms of primary stability or overhang. The geometric measurements showed significant differences as well. The previously clinically observed difficulties in inserting the cup were confirmed by this study. By overreaming to 51 mm, the drill-in behaviour, the primary stability and the measured overhang were comparable to the reference cup. The obtained results suggest that the extension of the acetabulum cavity to 51 mm while using the implantcast EcoFit size 50 should be implemented in clinical applications.
Acknowledgements
The authors would like to thank implantcast for the financial support and the supplied instruments and implants. In addition, we thank Marco Köpff and Christopher Johnson for the measurements and data editing.
Research funding: Authors state no funding involved.
Conflict of interest: Implantcast (Buxtehude, Germany) supported this study by a grant and provided the prostheses and foams for the tests. This did however not influence the study design or the collection, analysis and interpretation of the data. It also did not influence the decision to submit the paper for publication. The company had no influence on study design nor on any results or interpretation of data and no involvement in the writing of the report and the decision to submit the article in this journal. Christoph Thorwächter: No conflict of interest in this study. Matthias Woiczinski: No conflict of interest in this study. Inês Santos: No conflict of interest in this study. Florian Schmidutz: No conflict of interest in this study. Alexander Paulus: No conflict of interest in this study. Volkmar Jansson: Consultant for Medacta, B. Braun Aesculap, implantcast. Patrick Weber: Consultant to Merete Germany.
Informed consent: Informed consent is not applicable.
Ethical approval: The conducted research is not related to either human or animals use.
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