## 1 Introduction  
The success of total hip arthroplasty (THA) depends on good interaction between three factors: (i) the surgeon/surgical technique; (ii) the implant and (iii) the patient [1]. THA replaces the bio-tribological system of the patient's native hip joint. The soft human cartilage of the femoral head and the acetabulum is replaced by some kind of harder material, such as ultra-high molecular weight polyethylene (UHMWPE), cobalt chrome molybdenum alloy or ceramic, which now represents the new \"artificial\" bearing situation for the patient. The materials used in the THA are subjected to mechanical stress, especially in the area of the bearing couple [2, 3]. Each material replacing something within the native human joints is, without exception, susceptible to material wear. Out of a technical perspective, it could be stated that if two materials articulate against each other under enormous stresses, tribological wear results [3]. The applied material, ball head size, or a combination of both within the \"artificial\" bearing influences the clinical outcome and long-term survival rate [4, 5, 6]. However, THA failure is a multifactorial problem. Registriesidentify aseptic loosening as one of the five main reasons for revision surgery of primary THA [6; 7]. Aseptic loosening also has a multifactorial etiology, and its mechanism is not explained by a single theory [8]. One cause of implant loosening is the combination of tribological and biological processes [9]. A major factor leading to aseptic implant loosening is the destruction of the periprosthetic bone. Wear particles are incorporated by macrophages, stimulating osteoclasts and degrading periprosthetic bone. The relative movements of two materials against each other under high compressive forces always lead to tribological abrasion. The material properties of the sliding partners, as well as their quality, determine the degree of wear and the shape of the wear products (so-called particles), which, in each case, determine the concrete biological behaviour in type and extent [3]. Wear detection of artificial joints can be performed within an experimental in vitro or a clinical in vivo setting. Tribological testing of THA bearing couples is essential for the evaluation of wear properties and the resulting debris. This is an important part of the preclinical approval of new medical materials or devices. However, experimental testing is associated with limitations, while the \"real world\" for the biological behaviour within the human body is not one-to-one exchangeable onto a testing bench. To validate tribological material behaviour under \"real world\" conditions, an in vivo detection of wear is necessary and required. There is a high priority and relevance for clinical acetabular cup wear measurements to assess common or new types of polymers that implant manufacturers introduce [10]. To detect in vivo wear of THA, several radiographic techniques are available [11], including conventional standard a.p. radiographs of the hip joint, special radiographs using the Roentgen stereophotogrammetric analysis (RSA) method [12], computer tomography [13], and magnetic resonance imaging [14]. These mentioned imaging techniques can be divided into two-dimensional wear detection methods, including standard a.p. radiographs (2D\({}_{\text{wear}}\)), and three-dimensional methods (3D\({}_{\text{wear}}\)), including multi-plane radiographs (RSA) within a specialized radiological setup or a multilayer stack of radiographs (CT, MRI) of the artificial hip. However, modern 2D\({}_{\text{wear}}\) methods are developed and used to detect standard UHMWPE cups with a wear rate between 0.1 mm and 0.2 mm/year [10]. [PERSON] et al. [15] reported 13 year results and found a wear rate of 0.08 \(\pm\) 0.03 mm/year for a conventional polyethylene and of 0.04 \(\pm\) 0.02 mm/year for a crosslinked polyethylene. Wear was measured using RSA as a 3D\({}_{\text{wear}}\) method.