644 Swedish Arthroplasty Register that uncemented stems were more frequently revised due to periprosthetic fracture than cemented stems during the first 2 postoperative years. We used the Kaplan-Meier method to estimate prosthesis survival, censoring death and emigration. Both death and emigration are competing risks regarding revision. In a study from the Australian Orthopaedic Association National Joint Replacement Registry, Gillam et al. (2010) found that the Kaplan-Meier method overestimated the risk of revision com- pared to a method called the cumulative incidence function. The latter method uses competing risk methods in the analy- ses. With a short- to medium-term follow-up and a rather low incidence of death, we assumed that the Kaplan-Meier method would be appropriate to use in this study. Regarding revision due to deep infection, dislocation, aseptically loosened stem, and aseptically loosened cup, we did not find any statistically significant difference between cemented and reverse hybrid THRs. In planning the study, we aimed to do subanalyses with the endpoints revision due to deep infection, dislocation, aseptically loosened stem, and aseptically loosened cup for the different combinations of cup/ stem (different brands) in the reverse hybrid group. We found that the number of revisions and the number of procedures in some groups were quite small (Table 2). Thus, 1 single revi- sion would have a large effect on the survival calculations for certain implant combinations. Although our register has a high completeness of data, we do not know for certain that all revisions of the primary THRs included were reported to the register. 1 or 2 missing revisions in 1 study group may offset the results quite dramatically when the groups are small. Fur- thermore, the accuracy of registry results is not known; the surgeon may type the data into the wrong box on the form, or the register may enter wrong data into the database. It is therefore difficult to make conclusions about the performance of the different components used in the reverse hybrid group. Subtle differences between study groups, if found, should be interpreted with caution—even if they are statistically signif- icant. Factors other than the implant itself, such as surgical technique, revision policy, incorrect registration, or unknown patient factors may bias the results. In summary, we found no statistically significant differ- ences in survival between reverse hybrid and all-cemented THRs in this population-based registry study. Both groups performed well, with 95–96% survival after up to 7 years of follow-up. Thus, there were no early signs of warning against the reverse hybrid method according to our findings. Due to the small number of revisions in the present study, we cannot make any conclusions regarding the results for the different cup/stem combinations of reverse hybrid THR. With a short- to medium-term follow–up, it appears that the reverse hybrid method might be a promising alternative in THR surgery using UHMWPE. We emphasize that long-term follow-up will be required to evaluate whether the concept has any advantage over all-cemented THR. Acta Orthopaedica 2011; 82 (6): 639–645 EL: conception of study, planning, interpretation of data, and writing of the manuscript. All the others contributed to planning, interpretation of data, and critical review of the manuscript. ED and AMF: statistical analysis. We thank all the orthopedic surgeons in Norway who contribute to the regis- tration of joint implants, and the NAR for their continued work. No competing interests declared. Alho A, Lepisto J, Ylinen P, Paavilainen T. 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