Date Published: June 4, 2012
Publisher: Informa Healthcare
Author(s): Rüdiger J Weiss, Nils P Hailer, André Stark, Johan Kärrholm.
Monoblock acetabular cups represent a subtype of uncemented cups with the polyethylene liner molded into a metal shell, thus eliminating—or at least minimizing—potential backside wear. We hypothesized that the use of monoâ€‹block cups could reduce the incidence of osteolysis and aseptic loosening, and thus improve survival compared to modular designs.
We identified all 210 primary total hip arthroplasty (THA) procedures in the Swedish Hip Arthroplasty Register that used uncemented monoblock cups during the period 1999–2010. Kaplan-Meier and Cox regression analyses with adjustment for age, sex, and other variables were used to calculate survival rates and adjusted hazard ratios (HRs) of the revision risk for any reason. 1,130 modular cups, inserted during the same time period, were used as a control group.
There was a nearly equal sex distribution in both groups. Median age at the index operation was 47 years in the monoblock group and 56 years in the control group (p < 0.001). The cumulative 5-year survival with any revision as the endpoint was 95% (95% CI: 91–98) for monoblock cups and 97% (CI: 96–98) for modular cups (p = 0.6). The adjusted HR for revision of monoblock cups compared to modular cups was 2 (CI: 0.8–6; p = 0.1). The use of 28-mm prosthesis heads rather than 22-mm heads reduced the risk of cup revision (HR = 0.2, CI: 0.1–0.5; p = 0.001). Both cups showed good medium-term survival rates. There was no statistically significant difference in revision risk between the cup designs. Further review of the current patient population is warranted to determine the long-term durability and risk of revision of monoblock cup designs.
We found good medium-term survival rates for both cup designs and no statistically significant difference in the risk of revision. The concept of monoblock cups is attractive, with encouraging results published in the literature and theoretical advantages over modular acetabular components. Excellent 10-year results were described after the use of the non-modular porous-coated Morscher cup. Of 335 THAs performed with this cup, none had required cup revision due to aseptic loosening after a mean follow-up of 10 years, and with cup revision for any reason, the 10-year survival rate was 99% (Garavaglia et al. 2011). In that study, no radiolucencies were seen around the cups, whereas osteolytic defects were found around 8% of the stems investigated. Similarly encouraging results were described after a minimum of 9 years follow-up of 125 THAs using the Morscher cup, where none of the cups were revised because of aseptic loosening or osteolysis, and 3 were revised for other reasons (Gwynne-Jones et al. 2009). Berli et al. (2007) reported the 15-year results of 280 hips implanted with the Morscher cup, quoting a survival of 98% for aseptic loosening and 95% overall. A different monoblock implant, the titanium-coated RM acetabular component, showed 94% survival after 20 years with cup revision due to aseptic loosening as endpoint (Ihle et al. 2008). A series of 127 THAs using a hydroxyapatite-coated version of the RM cup had a 98% 10-year survival with cup revision for any reason as the endpoint (Ali and Kumar 2003).