## 2 In Vivo Wear Testing  
It has just been described how the need to eliminate or reduce wear in joint replacements is of great importance for the reduction of revision rates. This can be achieved by deeply understanding the mechanisms of implant tribology using in vitro and in vivo experiments. In vivo (word derived from Latin meaning \"within the living\") refers to the experimentation developed in living organisms as opposed to those conducted on parts of the organism or in corpses. In vivo tests are often accompanied by in vitro tests to observe the overall effects of an analysis. Translating these concepts into the orthopedic field, many wear assessment procedures are used to evaluate the in vivo performance of a prosthesis [9]. The most used diagnostic methodology for the monitoring of wear and osteolysis is the radiographic one [10,11]. Radiographs are usually used to determine clinical wear by assessing the degree of penetration of surfaces in contact. For example, in the assessments concerning hip prostheses, the penetration of the femoral head into the acetabular cups can be assessed using a compass necessary to identify the position of the minor eccentricity found. Hence, linear wear is the difference between the initial postoperative radiography and the most recent radiography measured in millimeters [9,12].  
However, traditional imaging techniques such as conventional radiographs, are limited by poor contrast resolution and specificity [13]. In addition, magnetic resonance imaging (MRI) or radiostereometric analysis (RSA) could be considered as additional techniques in the orthopedic field. In particular, MRI is commonly used in body composition research to measure whole body skeletal muscle mass [14,15]. MRI calculation methods can vary by analyzing the images at different slice intervals (or interval gaps) along the length of the body [15]. The RSA is used in the orthopedic field to measure the translations/migration of joint replacement components with respect to the host bone in vivo [16, 17, 18]. This kind of application is able to detect early stages of particle disease before osteolysis is apparent on radiographs. Obviously, the precision and accuracy of the method should be determined from time to time when a new anatomical area or a new prosthetic design is considered. In particular, precision is equal to repeatability and defined as the agreement between two test results under the same conditions; the accuracy, instead, is the \"trueness\" of a measurement, and it is defined as the closeness of agreement between a test result and a known value [17, 18, 19]. In particular, [PERSON] and colleagues [20] suggest that RSA is an alternative method for in vivo measurement of implant migration; they found an accuracy that ranged from \(-0.048\) to \(0.037\) mm, and \(-0.057^{\circ}\) to \(0.078^{\circ}\) or translation and rotation respectively. In agreement, [PERSON] et al. [18] found a precision of \(\pm 0.045\) mm by using the RSA technique in the measurement of THR femoral component translations.  
These kinds of applications, including the evaluation of growth plate integrity, joint kinematics, and implant stability, may be successfully undertaken in patients following total hip/knee replacement, and are considered as the most accurate in which the magnitude of relative displacements of prosthesis is determined from radiographic images [21; 22].