70 Part I Chapter 3 The frequencies of these causes and typical bone-SPECT/CT findings are described in Table 1 [7, 29] and can be divided into intra-articular and extra-articular causes, which has been described in detail by Lim, et al [29]. Vaz, et al. recently published a comprehensive overview of scintigraphic findings in ten different extra-articular causes of post-operative pain after THA and TKA [33]. Figure 2 SPECT/CT of the knee after TKA, displaying a “hot patella.” (A) SPECT, (C) fused SPECT/CT, indicative of severe (secondary) OA of the patella after TKA. Also note the large subchondral cyst formation in the patella on localization CT (B). Image courtesy of Prof. Dr K. Strobel (Lucerne, Switzerland). Table 1 Epidemiology of Intra-articular Aseptic Causes in Postoperative Knee Pain 12 Months After TKA Based on literature [7, 29] Condition %* Key BS SPECT/CT Characteristics Aseptic loosening of the TKA components (malalignment,† post-traumatic) 48 See Table 3 for detailed criteria. Polyethylene wear 19 Subsequent malalignment and aseptic loosening as a result of polyethylene wear is noticeable on BS SPECT/CT. Instability 12 BS SPECT/CT is rarely required for diagnosis. Recurrent hemarthrosis 6 BS SPECT/CT is not primarily indicated and may show increased perfusion and blood pool around the TKA. Stiffness 6 BS SPECT/CT is not sensitive for this indication. Patellar maltracking 5 Increased patellar BTO‡, most often at the lateral or craniolateral facet, less medial Tendon rupture 4 BS SPECT/CT is not primarily indicated andmay show increased perfusion and blood pool in the trajectory of the rupture. * Percentages based on Lim, et al. [29], including 2534 patients with TKA, of which 83 were described for intra-articular aseptic causes. † Malalignment can occur in the coronal or sagittal plane, or can be rotational [1]. ‡ BTO on SPECT/CT.