Primary Stability Of Cementless Anchored Primary And Revision Cup Designs In Various Acetabular Bone Defect Situations

Background The primary stability of cementless anchored acetabular cups is essential for the osseointegration of the implant. Factors influencing primary stability include diametral press fit, design and surface topography of the implant [1-3], bone density [4], and acetabular defect situation [5]. Objectives Our present experimental study aims to evaluate the primary stability of primary vs. revision cup designs in various acetabular bone defects. To evaluate primary implant stability, both lever-out and torsional moments are determined in artificial bone models. Study Design & Methods A primary cup (Plasmafit Plus) and a revision cup design (Plasmafit Revision, both Aesculap, Tuttlingen, Germany) were tested. These were pressed into bone models (Sawbones 20 pcf, Malmö, Sweden) in a position-controlled manner using a universal testing machine (Z050, ZwickRoell GmbH & Co. KG, Ulm, Germany). The bone models represented an intact acetabular bone situation, a "medium defect" and a "large defect" situation according to Schulze et al. [5]. The specimens were evenly divided for further lever-out or torsional testing (n = 5 per group). The lever-out test was performed using a universal testing machine by applying a force to a rod that was centrally connected to the cups. Accordingly, the bone-implant interface was loaded with a tilting moment in inferior direction. Furthermore, torsional testing was carried out with a servo-hydraulic biaxial testing machine with a constant axial load of 500 N. The torsional moment was applied to a rod, which was also centrally fixed to the cups. The lever-out and torsion tests were performed until failure of the bone-implant interface, i.e. until complete loosening of the implants occurred. Results The lever-out moments of the primary cup were 13.8 ± 0.8 Nm, 7.3 ± 0.6 Nm, and 5.0 ± 0.3 Nm and for the revision cup 13.7 ± 0.9 Nm, 8.6 ± 0.8 Nm, and 7.0 ± 0.7 Nm in case of the intact bone model, medium defect, and large defect situation, respectively. Hence, the revision cup achieved 17 % (medium defect) and 40 % (large defect) higher maximum lever-out moments than the primary cup. The maximum torsional moments of the primary cup were 67.3 ± 4.3 Nm, and 51.0 ± 3.8 Nm while the revision cup showed 68.9 ± 2.3 Nm, and 56.9 ± 1.3 Nm in the case of the medium defect, and large defect situation, respectively. Within the intact bone model, the torsional moments were so high that the connection between the cup and the rod fractured. Therefore, no values were detected. Hence, the revision cup achieved 2 % (medium defect) and 12 % (large defect) higher torsional moments than the primary cup. Conclusions In both the lever-out and torsion tests, both cup designs showed differences in primary stability depending on the acetabular defect situation. The revision cup design led to higher lever-out and torsional moments than the primary cup in all load scenarios. References 1 Amirouche et al. 2014, ClinBiomech, 29(10), S. 1177-1185 2 Weißmann et al. 2018, Metals, 8(10), S. 839 3 Saleh et al. 2008, AmJOrthop, 37 (10), S. 519–522 4 Schulze et al. 2019, ComputMethodsBiomechBiomedEngImaging, 22(1), S. 25–37 5 Schulze et al. 2020, J.Arthroplasty, 35(6), S. 1720-1728