Acta Orthopaedica 2011; 82 (6): 639–645 643 Figure 2. Cox survival curves. Endpoint was any revision of the implant for the total material. Adjusted for age, sex, and diagnosis. Age: < 50, 50–59, 60–69, 70–79 and > 80. Diagnosis: OA, RA/inflammatory, or other. different combinations of components. In this pre-2000 period, we believe that in some instances failure to achieve solid fixation of an uncemented cup made the surgeon convert to a cemented cup. Since the year 2000, the reverse hybrid concept has been used more systematically in Norway and the number of implanted primary reverse hybrids has increased from 90 in the year 2000 to 1,735 in 2009. In Sweden, the number of hybrid THRs has declined and the total number of reverse hybrids has increased (Swedish Hip Arthroplasty Reg- ister 2007). This increase in popularity called for the evalua- tion of medium-term results using this method. Comparing the reverse hybrid group with the 10 most used cemented THRs, we found similar implant survival with 0–10 years of follow-up. The only differences found were for sub- analyses on femoral fractures, but the difference in survival at 5 years with this endpoint was only 0.25% and 5-year survival exceeded 99% for both groups. This indicates that peripros- thetic femoral fractures are an infrequent complication lead- ing to revision surgery. For the total material, the proportion of males was higher and the mean age was lower in the reverse hybrid group than in the cemented group. We had no scoring for activity level, and there could be a bias in comparing high-demand young men to a group with low-demand elderly women. However, when we limited analyses to patients less than 60 years of age, the groups were much more similar to each other. The median follow-up for total material and for cases below 60 years differed significantly between study groups, and with short follow-up it may therefore be difficult to uncover differ- ences between these 2 concepts. We found similar risk of deep infection with reverse hybrid and cemented THR. Only revisions that included removal or exchange of parts or the whole implant were reported to the register. Thus, soft tissue revisions without the exchange of Figure 3. Cox survival curves. Endpoint was any revision of the implant. Age < 60 years. Adjustment for age, sex, and diagnosis. Age: <50, 50–60. Diagnosis: OA, RA/inflammatory, or other. prosthetic parts were not reported to the NAR. For the period 2003–2007 and using data from the NAR, Dale et al. (2009) found a statistically significant difference in numbers of revi- sions due to deep infection with inferior results for uncemented THR compared to cemented THR. One explanation for our finding is that antibiotic in the cement in reverse hybrids may protect against deep infection (Engesaeter et al. 2003). In the present study, 97% of the stems had HA coating and in the medium term we found results comparable to those for cemented THR (Table 4). In 2002, the NAR reported inferior results for 2 types of HA-coated cups as compared to cemented Charnley cups (Havelin et al. 2002). In 2010, Lazarinis et al. reported increased risk of revision of acetabular cups coated with HA, and in 2009 Stilling et al. reported inferior results for an HA-coated cup compared to those for a non HA-coated cup at 15 years. Concerns have been raised about third body wear induced by HA from HA-coated implants. Røkkum et al. (2002) discussed whether thick HA coatings may delaminate, and suggested that thick HA coatings may be a reservoir for HA particles. Wear and wear-related problems may appear several years after the primary procedure. Studies with large numbers and long follow-up are thus necessary in order to be able to con- clude whether the performance of cup implants is influenced by the stem having an HA coating. Regarding this problem, ran- domized controlled trials measuring wear with precise methods are important, but registry studies collecting a large amount of data on prostheses may also reveal differences between HA- coated implants and those without any HA coating. In the NAR, femoral fractures are reported if they require revision surgery. We found a higher risk of revision for peri- prosthetic femoral fracture in the reverse hybrids than in the all-cemented THRs. Although it was more common with unce- mented stems, periprosthetic femoral fracture was uncommon in both groups. Hailer et al. (2010) found in a study from the