Survival of monoblock RM vitamys compared with modular PINNACLE cups: mid-term outcomes of 200 hips performed by a single surgeon

Introduction: Monoblock cups have theoretical advantages over modular cups; however, their superiority in terms of survival has not been confirmed in comparative studies.

Methods: We compared the mid-term survivorship of 107 RM Pressfit vitamys monoblock cups (Mathys Ltd., Bettlach, Switzerland) with 93 modular pressfit Pinnacle cups (DePuy Synthes, Warsaw, IN, USA). All cases were registered in the Arthroplasty Registry Thessaloniki and performed by a senior surgeon through the same approach between 2013 and 2014. The groups were comparable in sex distribution, preoperative diagnosis, follow-up, cup diameter, head diameter, type and use of screws, HOOS and HSS preop scores; the recipients of RM cup were significantly younger.

Results: 3 sockets were revised. The 6-year survival for any reason was 99.1% for the RM and 97.8 % for the PINNACLE group. There was no difference in survival for aseptic loosening and any reason between groups (log-rank test p = 0.921 and p = 0.483, respectively). The age (95% CI, 0.79-1.1), sex (95% CI, 0.2-45.0), cup diameter (95% CI, 0.18-1.1), head diameter (95% CI, 0.004-6.2), preoperative diagnosis and use of screws (95% CI, 0.02-4.3), did not influence hazard ratio for revision between groups. HHS and HOOS were comparable at the last follow-up.

Conclusions: Our study demonstrated that both cup designs had similar revision rates at mid-term follow-up, regardless the fact that the RM vitamys was used in a cohort of younger patients; which according to the literature would have led to earlier failures. Further long-term data are needed to evaluate the superiority of RM vitamys in the clinical setting, especially in the young.