71 SPECT/CT in the post-operative painful knee In reality, often a combination of these mentioned causes will be responsible for post-operative knee pain after TKA. Although many patients (40% in a large recent study) experience persistent pain 6 months post-operatively, the majority of these patients will become pain-free spontaneously within 12 months [29]. This is not to be considered as a complication, but rather prolonged pain due to delayed healing and this justifies waiting to perform SPECT/CT until at least one year post-operatively in most cases (depending on the type of surgery). Aseptic loosening after TKA is one of the most frequent reasons of late failure and revision [1, 29]. The main cause of aseptic loosening is accelerated wear of early generation polyethylene inlays (especially the thin and low-conformity types) [1]. Additionally, surgical factors such as mal-alignment, malposition, and uncorrected instability may contribute to aseptic loosening [1]. Malpositioning of the prosthesis is a leading cause resulting in post-operative pain. Femoral and/or tibial TKA component malpositioning are known to cause loosening, e.g. early loosening or varus collapse of the bone in position of the tibial component in varus [34]. Malpositioning does not only cause loosening, it also causes altered loading, patellofemoral maltracking, instability, soft-tissue pain and finally loosening. Especially in patients with femoral valgus alignment, clinically relevant post-operative patellofemoral maltracking after TKAmay lead to pain complaints [35]. However, due to variation in anatomy it is still difficult what the knee surgeon should aim for. A recent systematic review of 10,309 cementless TKA procedures reported incidences of prosthesis failure during prolonged follow-up: the extrapolated 5-year, 10-year and 15-year failure of 2.3%, 4.6% and 7.0%, respectively [36]. Also, in this study, aseptic loosening contributed the most to implant failure and this amount was comparable to the findings of another study: in cementless TKA 25% from the failures were aseptic loosening, being 1.8% out of all TKAs after 15 years [36]. Periprosthetic knee joint infection is a growing problem in TKA. The incidence of infection associated with primary TKA has been estimated to be 0.39%-2.5%. Prosthetic joint infection (PJI) is the most serious complication after TKA and is notorious because of its potentially severe morbidity [29, 37, 38]. PJI can be divided into early (within 3 months), delayed (312 or 24 months, depending on definition), and late onset (12 or 24 months and afterwards) [39]. The early infections are acquired during surgery or the following days, the delayed infections are also still predominantly acquired during surgery, and the late infections occur predominantly by hematogenous seeding from remote infections [39]. The patient’s history, subjective symptoms, and biochemical and physical findings are often inconclusive, particularly in the early stages. 3