## 4 Discussion  
Reliability of marker-based RSA and mbRSA EGS approaches and their application in the irregular-shaped femoral stem was assessed for the first time from clinical data. Results of the analyses  
Figure 4: Mean cranial-caudal rotations of femoral stem along follow-up time analyzed by 3 raters with marker-based RSA (**A**) and mbRSA-EGS (**B**). Intra-rater variability was presented as the shaded area. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Translation (mm)**} & \multicolumn{4}{c}{**Rotation (deg)**} \\ \cline{3-8}  & & **m.l.1\({}^{\text{1}}\)** & **c.c.2\({}^{\text{2}}\)** & **a.p.3\({}^{\text{3}}\)** & **m.l.1\({}^{\text{1}}\)** & **c.c.2\({}^{\text{2}}\)** & **a.p.3\({}^{\text{3}}\)** \\ \hline \multirow{4}{*}{RSA-marker \({}^{\text{4}}\)} & Rater 1 & 0.07 \(\ast\)\(\dagger\) & 0.07 \(\ast\)\(\dagger\) & 0.15 \(\ast\) & 0.19 \(\ast\) & 0.50 \(\ast\) & 0.07 \(\ast\) \\  & Rater 2 & 0.08 \(\ast\)\(\dagger\) & 0.09 \(\ast\)\(\dagger\) & 0.15 \(\ast\) & 0.15 \(\ast\) & 0.65 \(\ast\) & 0.08 \(\ast\) \\  & Rater 3 & 0.09 \(\ast\)\(\dagger\) & 0.07 \(\ast\)\(\dagger\) & 0.18 \(\ast\) & 0.17 \(\ast\) & 0.69 \(\ast\) & 0.09 \(\ast\) \\  & Rater 1 & 0.09 \(\ast\)\(\dagger\) & 0.09 \(\ast\)\(\dagger\) & 0.17 \(\ast\) & 0.28 \(\ast\) & 0.99 \(\ast\) & 0.11 \(\ast\) \\ \multirow{2}{*}{mbRSA-EGS} & Rater 2 & 0.11 \(\ast\)\(\dagger\) & 0.11 \(\ast\)\(\dagger\) & 0.21 \(\ast\) & 0.31 \(\ast\) & 1.27 & 0.14 \(\ast\) \\  & Rater 3 & 0.15 \(\ast\)\(\dagger\) & 0.13 \(\ast\)\(\dagger\) & 0.32 \(\ast\) & 0.37 \(\ast\) & 1.81 & 0.18 \(\ast\) \\ \hline \hline \end{tabular} \({}^{\text{1}}\) Medial-Lateral; \({}^{\text{2}}\) Cranial-Caudal; \({}^{\text{3}}\) Anterior-Posterior; \({}^{\text{4}}\) Marker-based RSA; \({}^{\text{*}}\) Significantly less than 0.5 mm or 1.15 deg (\(p<0.05\)); \(\dagger\) Significantly less than 0.15 mm (\(p<0.05\)).  
\end{table} 
Table 3: Intra-rater variability estimated by root of WMS. are very encouraging, suggesting that RSA can deliver reliable and valid migration data, as confirmed within a clinical setting. Results revealed that both marker-based RSA and mbRSA-EGS have acceptable rater equivalence for the migration measurement of the anatomically designed femoral stem (Table 1). Therefore, according to the definition of CIA, the measurements between different raters can be regarded as the repeated measurements of the same rater. Thus, the intra-rater reliability was further explored. For in-plane migration measurements, mbRSA-EGS showed as good an intra-rater reliability as the gold standard marker-based RSA, with 66.7% of the measurements having \"excellent\" reliability and 33.3% of the measurements having \"good\" reliability (Table 2). The intra-rater variability of in-plane migration of mbRSA-EGS (<0.15 mm, <0.18 deg) was much lower compared with the upper-limits of RSA accuracy (0.5 mm, 1.15 deg) (Table 3). So far, research question (i) can be answered: the rater reliability of in-plane migration measurements by applying EGS model can be accepted. Moreover, the in-plane translation, subsidence of stem, has certain clinical value in predicting future aseptic loosening [29; 33]. Systematic reviews and meta-analyses showed that femoral stem subsidence was associated with long term aseptic loosening [31]. Additionally, the results of the investigated THA design proved that the intra-rater variability of this in-plane translation measurement was significantly less than 0.15 mm (the threshold of risk implants of loosening), which means that the EGS model also has a considerable application value for predicting loosening. However, it is known that the accuracy and precision of mbRSA method in general are prosthesis design-dependent. Conclusions cannot be generally applied to each investigated THA system [18]. However, the intra-rater reliability of out-of-plane migration measurements were worse than the in-plane migration, especially in cranial-caudal rotation measurements (\"poorly\" reliability).