65 SPECT/CT in the post-operative painful knee Introduction The knee joint consists of three articulations: the medial and lateral tibio-femoral joint, the patello-femoral joint and the tibio-fibular joint. It is the largest joint in the human body, which permits flexion and extension in the sagittal plane, internal and external rotation in the transverse plane and varus-valgus in the coronal plane, resulting in a ‘six-degrees-of-freedom’ range of motion [1]. Establishing the true cause of postoperative knee pain is mandatory to guide optimal treatment. Often it is difficult, however, to achieve a comprehensive understanding of the reasons for persistent, recurrent or newly onset postoperative knee pain. In addition to the patient’s history and a detailed clinical examination radiologic and radionuclide imaging (RNI) complement the diagnostic workup in this challenging group of patients. This review aims to report the current evidence-based imaging strategy in patients with a painful postoperative knee. It emphasizes on the current value of bone scintigraphy using Technetium-99m-hydroxymethylene diphosphonate ([99mTc]-HDP) or Technetium-99m-methylene diphosphonate ([99mTc]-MDP), both hereafter called bone scan (BS). The evidence for Single Photon Emission Tomography combined with Computed Tomography (SPECT/CT) over planar imaging and directions how to achieve optimal image quality will specifically be discussed in depth. For all indications its relation to radiological techniques (conventional radiographs, MRI and CT) is discussed. In addition, the role of Technetium-99m-sulesomab, Technetium-99m- and Indium-111labeled leukocytes will be addressed and compared with that of 2-deoxy-2-[18F] fluoroD-glucose Positron Emission Tomography (FDG-PET), Fluorine-18-sodium fluoride Positron Emission Tomography (NaF-PET), and conventional radiological techniques. There is no single imaging modality to date (both in the field of radiology or nuclear medicine) that is able to diagnose all possible knee disorders with satisfying accuracy in one visit. A combination of modalities will therefore remain necessary in many cases. This review presents the most common clinical diagnostic problems in patients with knee pain and provides solutions for an optimal imaging strategy in text, pictorial examples and flowcharts. Knee Surgery and Arthroplasty When non-operative treatment for knee pathology fails, knee surgery might be carefully considered. One can differentiate between knee joint preserving surgery and knee arthroplasty, which resurfaces the worn parts or the entire knee joint. Knee joint preserving surgery includes osteotomy aiming for alignment correction, 3