## 4 Discussion
The intra-rater variability of cranial-caudal rotation measurements also exceeded 1.15 deg (Table 3). One of the reasons for this large variance was considered as the limitation of its working principle using pose-estimation technique. It was demonstrated that mbRSA using CAD or RE performed less accurately than marker-based RSA on the cranial-caudal rotational measurements of femoral stem implant [14]. The implant projection contour did not change much with a slight rotation around this longitudinal axis, which provided too little information for migration measurement. Until now, research question (ii) can be answered: the rater reliability of out-of-plane migration measurement (cranial-caudal rotation) by applying EGS model were harmed. The mismatch of EGS model and actual stem shape also played an important role on the poor reliability of this rotation measurement [16]. During the analyses, raters found that the distal part of this anatomically shaped stem was the most difficult part to determine. Rater 1 and 2 tended to choose a longer contour of the distal stem, while rater 3 chose a shorter contour. As the selection of shorter contour may provide less information about the stem axis, it led to a larger intra-rater variability as well as worse ICC of rater 3 (Figure 4), especially in the cranial-caudal rotation measurement. However, it is worth noting that the rater's choice of contour length (rater 1 \(\geq\) rater 2 \(>\) rater 3) is not consistent with their experience of RSA analysis (rater 1 \(\approx\) rater 3 \(>\) rater 2), as the results supported that the choice of contour length was associated with the reliability of out-of-plane migration measurement. It is recommended to clearly define the standard operating protocol for mbRSA-EGS with the region of interest (length and position) for establishing a standardized template for the contour cone segment to represent the femoral stem component. Considering that two of the three virtual markers in the EGS model depended on the position of the stem central axis, the results presupposed that the irregular-shape of investigated stems would have an impact on the reliability of mbRSA-EGS. On the other hand, these results supported that choosing a longer stem contour could help to improve the reliability harmed by the mismatch of EGS model and actual irregular stem shape of the investigated stem. Additionally, this situation can be improved when applied to stem implants which has the shape matched with available EGS models (likewise cones or cylinders). A study showed better measurement precision of mbRSA-EGS on an hip stem with a strictly conical shaped stem (precision on cranial-caudal rotation measurement:0.614 deg) [13]. For other irregular stem designs, it could be recommended to do a proof of concept study in advance of the clinical application of mbRSA-EGS for these designs. Therefore, these results showed that the user interaction can affect the reliability of some migration measurements, especially the choice of stem contour length when using EGS stem model. In general, the reliability of mbRSA-EGS is more sensitive to user interaction compared with marker-based RSA, which should be carefully considered before applying to clinical implant migration measurements. If considering measuring the out-of-plane migration of irregular-shaped stem with mbRSA-EGS, it is better to validate the measurement accuracy by in vitro experiments, double examination in advance, or choosing other validated methods as marker-based RSA or RSA using CAD/RE models. In addition, as the EGS stem model considers the head-stem as a rigid body compared with the CAD/RE model that considers the stem only, it can violate the definition of a rigid body when head-taper motion exists in the actual clinical situation, and consequently may cause deviations in translation measurements. Additional attention should be paid to possible head-taper motion when applying the EGS stem model [17]. A uniplanar RSA set up was used.