## 2 Materials and Methods
### Measurement and Analysis Protocol  
RSA analyses of femoral stem components were performed with a commercially available software package (MBRSA 4.1, RSA Core, Leiden, The Netherlands). During the analysis with standard thresholds, condition number (\(\leq\)100) and rigid body error (\(\leq\)0.35 mm), were continuously monitored according to the recommended RSA Guidelines for producing standardized analysis procedure [11; 24]. Migration was calculated based on a reference point, which represents the center of gravity of a rigid body. After aligned the reference rigid body (e.g., bone rigid body), translation could be calculated based on the difference of the migration rigid body location (e.g., location of the reference point of implant) between two follow-up time points, rotation could be calculated based on the difference of the migration rigid body orientation between two follow-up time points. The rigid body orientation can be estimated by several shape matching methods in case of using RE/CAD model [12] or EGS model [13]. To verify the quality of the image calibration procedure, standard thresholds for calibration errors (translation \(\leq\)0.05 mm, focus error \(\leq\)0.5 mm) were used for image analysis [24]. Migration of about 6 degrees of freedom using rigid body kinematics were calculated with respect to a global coordinate system. Application of a calibration box defined translation along the medial-lateral (x) and cranial-caudal (y) axes as in-plane motion, and translation along the anterior-posterior axis (z) as out-of-plane implant to bone motion (migration). Rotation around the anterior-posterior axis (Rz) described in-plane motion and around the medial-lateral (Rx) and cranial-caudal (Ry) axes, out-of-plane implant to bone motion, respectively.  
Three independent raters participated in this study, two of which have 2 years' experience in RSA project (rater 1 and rater 3), one has half year experience in RSA analysis (rater 2). Each rater carried out RSA analyses with the marker-based RSA and mbRSA-EGS methods according to the standard analysis protocol in the user manual (MBRSA 4.1, RSA Core, Leiden, The Netherlands). When applying mbRSA-EGS, the raters themselves chose which conical portion of the contour of the distal stem to analyse. After all RSA radiographs were analyzed once by both methods, the raters took a one-week break. This process was repeated until each pair of RSA radiographs was analyzed three times by each rater (Figure 3). Once achieved, for an individual rater, calibration of RSA radiographic image pairs was kept unchanged for the remainder of the analysis sequence of each image pair for both RSA methods (marker-based RSA and mbRSA-EGS). However, for repeated analysis (three times with the same images) by different raters, each time the calibration and analysis were done repeatedly. During the analyses, raters were allowed to revise the analyses when any procedure went against the defined analysis protocol. However, they were not allowed to revise the analyses based only on suspicion of migration. Before all analyses were accomplished, raters had no information of the migration data from previous studies or any other source.