80 Part I Chapter 3 At the same time, it should be noted that thorough understanding by the nuclear medicine physician of the surgical procedures and used components are crucial to achieve this high accuracy. This means that concerning component design the nuclear medicine physician needs to know whether a posterior cruciate retaining, a posterior stabilized, rotating hinged or total stabilized TKA was implanted, with different fixation points of the components and different effect on the remaining structures in terms of biomechanical loading [1]. Correct information whether cement was used for fixation of the components is also of importance, as this influences the bone turnover pattern as well. Confirmation from other groups on the feasibility and reproducibility of the results (especially the high accuracy) is dearly needed. Subsequent implementation in daily clinical practice would then require specialized software that is able to display and take measurements, which is not available at this time. Secondly specialized nuclear medicine physicians together with orthopedic surgeons dedicated to imaging need to get well acquainted with a wide range of findings and patterns, of which the most crucial are mentioned in this current review. In a prospective study, including 62 consecutive patients who underwent primary TKA without patellar resurfacing and valgus alignment of the femoral TKA, SPECT/ CT was used to evaluate bone turnover in the patella [35]. SPECT/CT revealed that increased BTO at the lateral patellar regions (p < 0.05) and increased BTO at the lateral superior joint adjacent part (p < 0.05) correlated with external rotation of the tibial TKA. Thus SPECT/CT revealed clinically relevant post-operative patellar maltracking due to femoral TKA position in the coronal plane and very likely explains anterior knee pain in the majority of TKA patients with femoral valgus alignment [35]. Another study retrospectively evaluated SPECT/CT-arthrography in 38 patients (hip, 21; knee, 17) compared with reference standards of surgical evaluation, spontaneous resolution of symptoms without revision, or a minimum of 1-year clinical and radiographic follow-up. This yielded excellent accuracy of SPECT/CT-arthrography with a sensitivity of 100%, specificity of 96%, PPV of 93%, NPV of 100%, and accuracy of 97% for detecting aseptic loosening of TKA [57]. Infected TKA Where evidence for the use of RNI in PJI is still maturing, large prospective studies specifically aiming at suspected TKA infection are even more infrequent [44, 45, 47]. Plain radiographs, CT, and MRI often fail to diagnose PJI, especially in early-onset disease [45, 47]. Therefore, SPECT/CT may be of additional value in selected cases. When performing SPECT/CT for PJI, one should remember that the most important contribution of SPECT/CT is its very high negative predictive value (NPV), but this