## 4 Discussion  
In vivo wear of THA could be detected by 2D and 3D measurement techniques. Within this experimental study, a 2D\({}_{\text{wear}}\) approach was compared to the gold standard 3D\({}_{\text{wear}}\) methodology, the RSA method. The results indicated superior accuracy (0.017 mm) and precision (0.043 mm) of the 3D\({}_{\text{wear}}\) method in comparison to the 2D\({}_{\text{wear}}\) approach (Tables 1 and 2). No significant effect of mal-aligned pelvic positions on the 3D\({}_{\text{wear}}\) measurement outcome existed. These results of the 2D and 3D measurement techniques could be confirmed already by [PERSON] et al. (2018) and the results of [PERSON] et al. (2018). [PERSON] et al. (2018) detected in vivo wear by four different methods: (i) RSA, (ii) the Scheier-Sandel method, (iii) the Charnley-Duo method, and (iv) EBRA. The 2D EBRA approach was observed to be the most accurate 2D\({}_{\text{wear}}\) approach in comparison to the gold standard (RSA). After excluding radiographs with the presence of a pelvic tilt (rotation around the horizontal axis), the measurement error was 0.08 \(\pm\) 0.11 mm (mean \(\pm\) SD), which was superior to the investigated 2D\({}_{\text{wear}}\) approach of this study, but inferior to the RSA result. [PERSON] et al. (2018) reported precision for RSA and a 2D method (Poly Wear 3D Pro 5.10, Draftware Developers Inc., Vevay, Indiana) as 2D wear measurements of 0.078 mm and 0.076 mm and for 3D wear measures of 0.189 mm and 0.244 mm, respectively. In comparison to the outcome of this study, the 3D\({}_{\text{wear}}\) (as the RSA method using EGS models) was within 0.043 mm of the prior results of [PERSON] et al. (2018), but the 2D approach (PolyWear) performed better than the applied 2D\({}_{\text{wear}}\) method (mediCAD) in this study, with 0.206 mm (Table 1). However, for reported accuracy, the results of [PERSON] et al. (2018) for 2D measures of 0.055 mm (RSA) and 0.335 mm (Poly-Wear) and 3D measures of 0.2 mm and 0.3 mm respectively, were worse in comparison to the investigated 3D\({}_{\text{wear}}\) and 2D\({}_{\text{wear}}\) approach of this study, with 0.017 mm and 0.159 mm, respectively (Table 2). In contrast, clinical validation of 12 patients undergoing total hip replacement surgery by the same wear measurement tools showed similar repeatability of PolyWare and RSA with a LoA of \(\pm\) 0.22 mm and \(\pm\) 0.23 mm, respectively, and good concurrent validity between them with limits of agreement of \(\pm\) 0.55 mm ([PERSON] et al., 2018). LoA of this experimental study range in a worst case (high wear protocol) from \(-\)0.16 up to 0.49 mm (mean difference: 0.16 mm) to a best case (low wear protocol) from \(-\)0.06 up to 0.20 mm (mean difference: 0.07 SD) (Figure 5). There seemed to be a discrepancy in accuracy between clinical and experimental images. Based on the results of this investigation and the available literature, the 2D\({}_{\text{wear}}\) detection technique could be used to some degree as a substitute for the 3D technique to investigate in vivo wear. [PERSON] et al. (2018) reported for [PERSON]'s Hip Analysis Suite software (version 8.0.1.4.3; UCTech the University of Chicago) accuracy and precision as mean \(\pm\) SD to be 0.060 \(\pm\) 0.021 mm and 0.207 \(\pm\) 0.099 mm, respectively. A mean bias of 0.089 mm and reproducibility of 0.106 mm existed for this 2D approach. The bias of this