## 1 Introduction
However, besides the mentioned additional roentgen equipment, additional X-ray images are also required, as the standard a.p. radiographs cannot be used for wear detection using the RSA approach. These additional X-ray images mean an additional radiation exposure for the patient, which is challenging in the context of radiation hygiene for the patient. [PERSON] et al. [24] reported that it was difficult to determine small amounts of wear in vivo if the RSA method was not used. The accuracy of RSA, irrespective of the prosthetic component studied or the direction of wear, was reported to be about 0.4; mean error was reported to be 0.010 mm. These were compared to two 2D approaches, which demonstrated an accuracy of 1.3 for each and a mean error of 0.19 mm (Charnley-Duo method) and 0.13 mm (Imagika). However, the authors reported the effect of image quality as well and applied RSA technology. The results indicated that a digital RSA analysis decreased the mean measurement error from 0.011 to 0.004 mm. The authors therefore hypothesized that using a digital RSA measurement setup would probably culminate in better results [24]. Preoperative planning software enables, in addition to the preoperative planning of total joint arthroplasty, the measurement of THA wear using standard a.p. radiographs of the pelvic region. Wear measurement using these kinds of radiographs is conducted for two reasons. Firstly, wear along the \(x\)-axis and \(y\)-axis on the coronal plane is extensively considered to be the site of occurrence of the most wear [25]. Secondly, lateral radiographs are of low quality and do not meet the requirements for wear measurement [26]. The main question, therefore, is whether a simple measurement method, such as the mentioned 2D\({}_{\text{wear}}\) method used in the manuscript, which is integrated within commercially available preoperative planning software can determine the in vivo wear of THA bearing couples in a manner that is equivalent to the gold standard. The current gold standard measurement method could be associated with some challenges; likewise, its complexity in the acquisition of RSA image pairs has been mentioned above. Therefore, if a clinic has an in vivo wear measuring tool available, e.g., already integrated in a software solution for preoperative planning, the assessment of its accuracy is essential before its clinical application. To the authors' knowledge, to date, there have not been many studies comparing 2D\({}_{\text{wear}}\) with 3D\({}_{\text{wear}}\) methods to detect acetabular cup insert wear. There is no data available within literature which validates the investigated 2D\({}_{\text{wear}}\) approach. Testing the interchangeability of a 2D\({}_{\text{wear}}\) method next to the gold standard RSA is rarely described in the literature; the potential influence of the pelvic orientation onto wear measurement outcomes is also rarely described in the literature. Therefore, pelvic positioning could be added as an influencing factor within this investigation. The aims of this experimental study are:  
1. to evaluate the interchangeability of 2D\({}_{\text{wear}}\) next to the gold standard 3D\({}_{\text{wear}}\) method by comparing the accuracy and reproducibility of both methods;
2. to determine further the extent to which the two different methods are influenced by changing pelvic orientation. The authors hypothesize that the 3D\({}_{\text{wear}}\) detection method would show higher precision and accuracy than the 2D\({}_{\text{wear}}\) detection method and be insensitive to pelvic misalignment.