## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Protocol**} & \multicolumn{2}{c}{**Tilt**} & \multirow{2}{*}{**Tilt**} & \multirow{2}{*}{**Rotation**} & \multirow{2}{*}{**Rotation**} & \multirow{2}{*}{**Tilted**} & \multirow{2}{*}{**Lateral**} \\  & & **Forward** & & & & **Left** & **Right** \\ \hline \multirow{2}{*}{Low wear} & LoA (\(\uparrow\)) & 0.29 & 0.37 & 0.29 & 0.27 & 0.38 & 0.42 \\  & mean & 0.07 & 0.15 & 0.11 & 0.12 & 0.14 & 0.18 \\  & LoA (\(\downarrow\)) & \(-\)0.15 & \(-\)0.07 & \(-\)0.07 & \(-\)0.02 & \(-\)0.10 & \(-\)0.06 \\ \hline \multirow{2}{*}{Medium wear} & LoA (\(\uparrow\)) & 0.54 & 0.41 & **0.25** & 0.32 & 0.34 & 0.39 \\  & mean & 0.07 & 0.10 & 0.00 & 0.07 & 0.03 & 0.11 \\  & LoA (\(\downarrow\)) & \(-\)0.40 & \(-\)0.21 & \(-\)**0.24** & \(-\)0.17 & \(-\)0.28 & \(-\)0.17 \\ \hline \multirow{2}{*}{High wear} & LoA (\(\uparrow\)) & 0.76 & 0.75 & 0.61 & 0.82 & 0.52 & **1.36** \\  & mean & 0.37 & 0.32 & 0.23 & 0.33 & 0.17 & 0.15 \\  & LoA (\(\downarrow\)) & \(-\)0.03 & \(-\)0.11 & \(-\)0.16 & \(-\)0.17 & \(-\)0.18 & \(-\)**1.06** \\ \hline \hline \end{tabular}  
* **Remark: Bold values indicate LoA with the highest and lowest ranges.**  
\end{table} 
Table 3: Mean value with upper and lower LoA for six changed pelvic positions. investigation is a quarter less of the value for the 3D\({}_{\text{wear}}\) approach. 2D\({}_{\text{wear}}\) bias is, for the medium wear protocol, approximate to the results of [PERSON] et al. [21] for high and low wear protocols of inferior accuracy. Derbyshire & Barkatali [10] reported for their customized software (programmed in C++ language) a mean error of \(-0.002\) mm. The precision was 0.055 mm and the performed comparison tests with the RSA method showed similar accuracy. [PERSON] et al. [30] investigated the outcome of two in vivo methods (PolyWare and Martell Hip Analysis Suite) with the direct wear measurement of the articulating surface of the retrieved polyethylene liners. Wear was detected by a coordinate measuring machine. Results suggested that both 2D methods accounted for the majority of 3D ex vivo results. [PERSON] et al. [18] analysed radiographs of 140 patients using both 3D and 2D techniques and showed that 3D analysis detected 10% more wear, but its repeatability was four times worse than 2D. The decrease in repeatability was explained by the authors by poor quality of the lateral radiographs. Taking into account that 2D\({}_{\text{wear}}\) methods were used originally to measure wear rates less than 0.2 mm/year for UHMWPE cup inserts [10] and that a decreased rate with vitamin E-diffused or crosslinked polyethylene to 0.04 mm or less [12; 15] is possible, the gold standard RSA is certainly to be given preference in the clinical measurement of wear in vivo. In clinical practice, anteroposterior pelvic radiographs are standard to monitor the performance of THA. This radiological image material has the advantage that it can be widely applied at any follow-up point in place of RSA. However, the pelvis is often not in a neutral position at the time of X-ray generation due to patient or radiology technician factors. Patient factors are usually due to lumbar spine disease or hip pain. The technician factor is usually a lack of compliance with standard specifications or quality control for radiographs. This affects the projection of the implant component within the X-ray, resulting in errors in wear measurements for the applied 2D\({}_{\text{wear}}\) technique. It must be taken into account that wear measurements of the investigated 2 Dwear or 3 Dwear approach could be influenced by subjective error by the clinician or technician performing the analysis. A possible error source could be point detection defining the centre point of the ball head via a circle over three detected points within the 2 Dwear approach. Similarly, within the 3D\({}_{\text{wear}}\) method, a user interaction is also necessary and may result in a subjective error.