Comparison of the primary stability of revision acetabular cups with various acetabular bone defects using a cup-block model

Total hip arthroplasty is currently one of the most widely performed surgical interventions. However, surgical failures still occur and can lead to severe consequences. The main cause of implant failure is aseptic loosening, related to a deficient osseointegration of the implant. Therefore, a good primary stability is mandatory to achieve bone ingrowth and long-term stability. In press-fit acetabular cups, the initial stability is achieved through frictional forces by inserting the cup in an under-reamed cavity. In the presence of acetabular defects, the effective contact area can be extremely reduced, and the choice of the implant can drastically affect the cup stability. The aim of this Thesis is to compare the initial stability of a primary and a revision acetabular cup in three types of Sawbones® blocks characterized by different defect sizes. The influence of the defect size on primary stability was investigated both experimentally and numerically using a FE model. Preliminary insertion tests were performed to determine the seating position of the cup using a static universal testing machine under load-controlled conditions. Push-in and lever-out tests were performed under displacement-controlled conditions at a crosshead velocity of 20 mm/min with the same static testing machine. Push-in force, lever-out moment, lever-out work, and interface stiffness determined for each trial were compared to delineate the cups’ behavior in the different blocks. We found a decrease in all the parameters as the extension of the defect increased, and generally better results were achieved by the TM cup. A statistical analysis was performed to investigate whether the different types of implant and the presence and the size of the defects, determine significant differences in the parameters used to assess primary stability. Statistical significance was established between the two cups for all the larger defect parameters except the interface stiffness. Furthermore, the influence of the defect size on the primary stability of the cups was confirmed. The FE model for the intact cavity was validated using the experimental test results from another studies, performed with the SeleXys PC® acetabular cup. We found a maximum difference of 33% in the push-in forces between the previously obtained experimental data and our FE model. The push-in force, the lever-out moment, and the interface stiffness of the cup-block FE models with the two different defects sizes were compared to the ones of the intact model in order to establish the influence of defects in the primary stability. All the extracted parameters show a strong dependence on both the defect size and the blocks’ material density. Moreover, the initial cup stability decreases with an increase of the defect size and with a decrease in the density. The analysis of the contact area demonstrated that the contact between the cup and its substrate is localized on a narrow surface near the equator of the cavity. The influence of the contact area between the cup and the bone on the primary stability was also investigated. A direct correlation between the contact area and all the all parameters was determined. The results of this Thesis could be used in future works for the development of a FE model for the revision acetabular cup, in order to further investigate the influence of the type of implant on the primary stability in the presence of acetabular defects.