## 1 Introduction
In comparison to the mbRSA approach using CAD/RE models, for each implant size and design variation, one CAD/RE model is required as it is matched with the actual implant shape [18]. Exceptions exist, if, for a design variation, a reduced contour selection is possible [19]. For instance, a THA system with seven different sizes and three variations of capt-collum-diaphyseal (CCD) angle means 21 RE models in total. If each RE model costs approximately EUR 200-250, the total costs of RE models will amount to EUR 4200-5250. In contrast, the application of EGS model can save this cost, as it only demands the corresponding geometry of implants. The generality of EGS model enables the wide application to various types and sizes of implants [20]. At present, EGS model is mostly applied to regular-shaped femoral stems (with conical shaped distal part) [13,16]. However, an irregular-shaped design has been introduced into a large number of femoral stem designs for different reasons [21]: rectangular cross-sectional design for strong rotational stability, conical design with multiple splines for primary fixation, anatomical design to achieve maximum contact, short stem design without a long distal part to preserve bony tissue. However, the application of EGS model on those irregular-shaped femoral stems has not been proven. The development of computer assisted RSA approaches has turned most of the cumbersome manual procedures into automation, thereby reducing unnecessary sources of error from most of manual procedures [22]. Migration analysis using RSA software still contains some user interactions (Figure 2). Figure 1: Investigated femoral stem component. RSA analyses by marker-based RSA and mbRSA-EGS illustrated on the same set of RSA radiographs. Bone markers: Green; Implant markers: Yellow; EGS model and projection contour: Red. Figure 2: Potential user interactions during RSA image analyses. (**A**,**B**): poor image quality can lead to variations in the choosing marker center. (**C**): the blurry projection contour of a ceramic femoral head. (**D**,**E**): choosing two conical-shaped parts when applying EGS models to the irregular-shaped femoral stem. Variations existed in both the length and position of the cone model. An example for user interactions presents the implant contour detection, which required the rater to select the correct contour. However, not all the implant contours can be clearly displayed on the X-ray image. The clarity of the implant contour can be affected by the image quality, thickness of soft tissue, density of implant material, etc. For instance, the contour of ceramic femoral head is more blurred compared to metal implants (Figure 2c). In addition to the selection of implant contour, the rater needs to decide which part of distal stem is the required conical shape when applying EGS model to irregular-shaped femoral stems. The current RSA approach and available software-package for analysis is not able to standardize this procedure, considering that those user interactions may affect the resulting implant or bone rigid body definition, thereby influencing the reliability of migration measurement. Information of RSA methodology reliability is lacking. There is a need for this kind of analysis, whilst keeping in mind, that user interactions may affect migration results. Therefore, a rater reliability RSA study was designed to investigate whether the EGS model can be reliably applied to one kind of the irregularly shaped designs: an anatomically shaped stem. The aim of this study is to answer the two following questions by investigating the rater reliability: (i) which directions of migration measurements have acceptable reliability by applying mbRSA using EGS model; (ii) the reliability of which direction of measurements may be greatly affected. This was demonstrated by the following steps:  
1.