69 SPECT/CT in the post-operative painful knee Knee arthroplasty Continued pain due to end-stage osteoarthritis (OA) is a good indication to undergo knee arthroplasty, if non-operative treatment has failed for at least 3-6 months [25]. However, the decision always takes into account the patient’s symptoms and the individual burden caused by the disease. Generally, one can distinguish between partial and total knee arthroplasty. Partial knee arthroplasty has its place when only one compartment of the knee is affected by OA. Unicondylar knee arthroplasty (UKA) replaces either the medial or lateral tibiofemoral joint. It is a common procedure, the cemented medial Oxford UKA’s being the most used UKA [26]. From 2000 to 2012, 14,496 cemented medial Oxford III UKA’s were performed in Scandinavia [26]. Patellofemoral knee arthroplasty (PFA) only replaces the trochlea and the patellar facet. The most common complications of partial knee arthroplasty are progression of OA in other knee compartments, loosening, infection, mechanical maltracking, prosthetic malposition and persistent pain. Total knee arthroplasty (TKA) replaces all knee compartments. If the patellar facet is not resurfaced at least the medial and lateral tibiofemoral joint and the trochlea is resurfaced (Figure 2). Four major component designs have been developed for primary and revision surgery of the knee: the posterior cruciate retaining, the posterior stabilized, the rotating hinged and the total stabilized TKA [1]. The different techniques and materials, articular surface geometries of the implant and remaining soft tissues (either the anterior or both cruciate ligaments are resected) in the knee after surgery determine biomechanical loading on the structures. This biomechanical loading, which for example is around 3 to 4 times the body weight during walking and can reach up to 8 times the body weight for axial loading when descending downstairs, largely determines uptake on bone scintigraphy. To interpret both physiologically increased bone turnover (BTO) around postsurgical knee structures as well as pathological uptake, sound knowledge of at least the most commonly used components and techniques is required. The incidence of TKA has increased from 600,000 TKAs per year in the US in 2010 to 700,000 in the US in 2015, mostly due to the aging population [27-30]. A comparable trend is seen in the UK with an annual increase to 85,019 TKAs in 2015 [31]. Although TKA is considered to be a very successful orthopedic procedure with satisfying outcome, approximately 20-30% of patients are not pain-free or satisfied after TKA [32]. The most common reasons for persistent, recurrent or new onset pain after TKA are aseptic loosening of a TKA component, malposition / malalignment leading to polyethylene wear, instability, recurrent hemarthrosis, stiffness, periprosthetic fracture, patellofemoral maltracking, tendon rupture and infection. 3