J Orthopaed Traumatol (2012) 13:137–143 139 Fig. 1 The coordinate system ﬁve in the greater trochanter and four in the lesser tro- chanter region. The initial RSA examination was per- formed within 1 week postoperatively, after weight bearing (Table 1). The RSA measurements were performed using UmRSA software (version 6.0, RSA Biomedical Innovation, Umea, Sweden). The tantalum balls had to be sufﬁciently scattered within each segment to ensure acceptable quality mea- surement. The condition number (CN) is a measure of the conﬁguration of the markers in the segment. The mean error of rigid body ﬁtting (ME) is a measure of marker stability. The upper limit on the ME was set to 0.25 mm, and the upper limit for the CN was set to 130, according to the guidelines in the Instructional RSA User Course Manual (RSA Biomedical). The ME and CN were met in all analyses except for 1. This patient was excluded. The results for the present study are presented in Table 2. Several RSA studies conclude that the migration of a cemented femoral stem mainly occurs at the implant– cement interface, not between the cement mantle and sur- rounding bone [15–17]. Tantalum markers that are mixed into the cement are difﬁcult to scatter and also difﬁcult to visualize on radiographs [17]. The present study was not designed to separate these two possible migrations, so all migration data refer to the movement of the implant in relation to the femoral bone. It is recommended that the measurement precision in each particular study should be deﬁned using double examinations [18]. We calculated the precision levels by performing 22 double examinations at the 2-year follow- up. Each patient rose from the X-ray table and walked between the two sequential examinations within 10 min (Table 5). The right hand side was used as the reference for the coordinate system when deﬁning the micromotions [18]. Thus, the y rotation, z rotation, and x translation values for the left hips were changed to relate to a right-handed coordinate system (Fig. 1). The patients returned for follow-up at 3, 6, 12, 24, and 60 months. The numbers of RSA and clinical examinations at different time intervals are presented in Table 3. At each follow-up, the data from standard radiographs, RSA, and Merle d’Aubigne´-Postel were recorded. The Merle d’Aubigne-Postel score (which measures pain, mobility, and ability to walk) has a maximum possible score of 18. Statistical methods The independent samples two-tailed t test was used to analyze the differences in migration between the two cement groups at 5 years postoperatively. Q–Q plots were used to verify the normality of the distribution. The mea- surement data from the two groups were subsequently pooled, and descriptive data were calculated for the whole patient population. Each precision level was calculated in two steps. First, the differences between the two examinations were calcu- lated for each patient. Second, the standard deviation (SD) of these differences (with respect to zero) was calculated. sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ Pn SD ¼ i¼1nðxiÞ2: Here, x represents the differences between the double examinations, n = 22. Finally, the precision was obtained by multiplying the SD by 2.074 (representing the 0.975 % point in a t22 distribution). Results Twenty-three patients were examined at the 5-year follow- up. The Merle d’Aubigne´-Postel score was completed for each of the 23 patients. The mean score and range were 18 (17–18) for all patients in both groups (Table 1). All 23 patients except 4 reached the maximum score. Three patients had value score 5 for pain, which indicates ‘‘pain is rare and mild’’. One patient had value score 5 for mobility, indicating a mobility of 70 –90 [19]. Examination of the 5-year follow-up conventional radiographs showed no osteolysis or radiological signs of loosening. 123