142 Axes Precision Table 5 Precision of the RSA measurements Rotation x 0.43 y 1.01 z 0.12 Translation x 0.10 y 0.10 z 0.22 migration. The rotation into varus also increases, and was 0.18 at 5 years postoperatively (Fig. 2f). All 6 migrations at 5 years in the present study are equal to or smaller than those noted in the 2-year follow-up Charnley study by Hallan et al. [16] (a longer follow-up period is not reported). Our 5-year data conﬁrm this inclination. We found the 5-year translation values to be consider- ably lower than what Onsten et al. described in a 2-year follow-up of 65 Charnley stems, where most of the migration took place during the ﬁrst 3 months. Mean migration along the y-axis was 0.24 mm, along the x-axis 0.31 mm, and along the z-axis 0.67 mm. The rotations were not reported [20]. Different stem designs are expected to migrate differ- ently [8, 15]. Ka¨rrholm et al. [6] believe that the quantity of subsidence after 2 years is the best predictor of medium- to long-term loosening. Subsidence of the femoral head of 1–2 mm during the ﬁrst 2 postoperative years indicated an increased risk of loosening and revision. The postoperative examinations were performed over a time interval stretch- ing from the ﬁrst day to 2 months after surgery. The patient population consisted of both primary THR and revisions. The implant was the cemented Lubinus SP I prosthesis [6]. Several factors—type of implant, follow-up time, time for baseline RSA, and precision level—mean that Ka¨rrholm et al.’s study and results are not directly comparable to those from our study. Ryd et al. argues that a migration exceeding 0.2 mm at 2 years implies an increased risk of implant loosening. This study on knees evaluated several brands of (cemented and uncemented) knee prostheses. Mechanical loosening occurred exclusively in prostheses which migrated con- tinuously [7]. Our study is not straightforward comparable to their study, since there are fundamental differences between hip and knee replacement, and Ryd et al. used maximum total point motion (MTPM) to identify migra- tion, while we used orthogonal translations and rotations to describe segment motions. The study of Ryd et al. dealt with knee prostheses, while that of Ka¨rrholm et al. evaluated primary and revision hips. This may explain the large disparity between their two different limits for increased risk of loosening, which differ by factors of 10, 1–2 mm, and 0.2 mm in the two studies, respectively [6, 7]. This exempliﬁes the importance of more implant-speciﬁc migration data that can be used to 123 J Orthopaed Traumatol (2012) 13:137–143 predict long-term implant survival based on short-term migration data. The Exeter stem is constructed as a loaded, double- tapered, collarless, polished stem, and is designed to sub- side, in contrast to the Charnley stem [15]. A study with the Exeter stem found a mean subsidence of 1.25 mm at the 2-year follow-up. The mean internal rotation was 0.6 and the varus alignment was 0.21 [21]. Another follow-up study of the Exeter stem showed continuing subsidence (1.77 mm, median) and retroversion (1.6 , median) up to 5 years. This may be expected with this type of ﬁxation [17]. The Charnley stem is not supposed to migrate but to show immediate stability due to its composite construction [15]. In spite of this, studies have conﬁrmed that the Charnley stem does migrate and rotate [13, 16, 20]. Our study supports these ﬁndings and also demonstrates that migration continues until at least 5 years. A number of earlier RSA studies on hips differ from current guidelines in the standardization of RSA investi- gation. The different measuring methods, follow-up inter- vals, and type of implants used constitute an obstacle to any comparison of these heterogeneous results. Planned RSA studies ought to act in accordance with published guidelines for the standardization of RSA [18] so that it is easier to compare them. Since several factors appear to inﬂuence the level of migration, it may be difﬁcult to deﬁne acceptable limits on implant short-term migration with respect to long-term sur- vival [4]. More implant-speciﬁc RSA background data are needed to predict what can be considered to be acceptable micromotion. Migration data are continuous. In conclusion, we have found that the Charnley stem continued to subside and to rotate in retroversion up to 5 years after surgery in a well-functioning patient group. More implant-speciﬁc RSA data are needed to predict long-time implant survival based on short-term observa- tions. The migration data presented herein could contribute to this, serving as reference data for a normal migration pattern of a Charnley ﬂanged 40 prosthesis. Conﬂict of interest None. Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, dis- tribution, and reproduction in any medium, provided the original author(s) and the source are credited. References 1. Garellick G, Ka¨rrholm J, Rogmark C, Herberts P (2009) Swedish Hip Arthroplasty Register; annual report 2009. Swedish Hip Arthroplasty Register, Gothenburg