140 Table 1 Patient demographic data Number of patients (hips) 35 (36) Females:males 24:12 Merle d’Aubigne´-Postel 10 (7–13) score, mean (range) Reasons for not attending (n) At inclusion At 5-year follow-up 23 (23) 18:5 18 (17–18) Deceased (4) Did not want to attend (4) Missed the RSA postop measurement (4) Condition number exceeded 130 (1) SmartSet HV Palacos R p values Discussion J Orthopaed Traumatol (2012) 13:137–143 Table 4 Migration data at the 5-year follow-up (mean values) along with corresponding p values for the two-tailed independent samples t test comparing the two cements Axes Rotation ( ) Translation (mm) x -0.08 -0.17 0.61 y 1.39 1.90 0.10 z -0.23 -0.18 0.75 x 0.00 -0.03 0.45 y -0.19 -0.22 0.12 z -0.05 -0.24 0.12 Table 2 Mean error of rigid body ﬁtting and condition number Femur Mean error of rigid body ﬁtting (ME) Mean 0.17 Range 0.10–0.24 Condition number (CN) Mean 56 Range 31–115 Markers visible Mean 6 Range 3–8 Implant 0.08 0.03–0.21 34 34–35 4 4–4 Table 3 Number of RSAs and clinical examinations at different time intervals Follow-up time Postop 3 months 6 months 12 months 24 months (precision) 60 months Number of examinations 32 27 20 32 28 (22) 23 Number in parenthesis represents the number of double examinations First, the data from both cement groups were compared. No statistically signiﬁcant differences in migration were observed between the two groups at 5 years postopera- tively (Table 4). The data from the two cement groups were then pooled. The migration measurement results for the merged data are presented in Fig. 2a–f. The measurement data were found to be normally distributed for all 6 data sets. 123 This follow-up study describes the migration pattern of the Charnley ﬂanged 40 femoral stem. A total of 35 patients (36 hips) were enrolled, and data from 23 patients (23 hips) at 5 years were obtained. The patients were followed and evaluated by RSA as well as by clinical examination. The Merle d’Aubigne´-Postel score at 5 years indicated that the patient group was functioning well. None of the patients lost to follow-up had their hips revised. Originally the study was designed to compare stem stability upon using SmartSet HV and Palacos R bone cement in a prospective randomised 2-year follow-up study. No differences in stem stability were found [13]. In the present study at 5-year follow-up, both cements also performed equally well. The most striking ﬁnding at this point was the observation of the continuous migration of a composite construction. Stem subsidence (translation along the y-axis) and stem retroversion (rotation around the y-axis) continued progressively up to 5 years postopera- tively. Consequently, the main focus was then to describe the overall stem migration at 5 years regardless of the cement used. The precision levels found in the present study (Table 5) are comparable to those seen in other studies—somewhat poorer than those presented by Hallan et al. [16], but somewhat better than those presented by Onsten et al. [20]. As seen in Table 5, the precision levels vary consider- ably between the 3 rotational migrations measured. The tantalum markers mounted on the implant deﬁne a polygon. Evaluating the scattering of the markers is vital in order to determine the migration of the implant (polygon) in rela- tion to the rotational axes. The method is more sensitive to rotations when the markers are located far from the rota- tional axes. Good scattering of the markers improves the level of precision accordingly. This phenomenon is gen- erally seen in RSA measurements of hip implants. Our results show that most of the rotations and trans- lations take place initially. The translation along and the rotation around the y-axis represent stem subsidence and retroversion, respectively (Fig. 2b, e). The resultant joint force that is transferred by the prosthetic head and acts on