75 SPECT/CT in the post-operative painful knee usually high starting one year after TKA depending on the procedure performed and pathology suspected. Knee joint preserving surgery In ACL reconstruction procedures, the surgeon may either use metallic or biodegradable interference screws, both behaving differently on bone-SPECT/CT [53]. Typically, failure of degradation of the interference screws shows an increased BTO in SPECT/CT. Biodegradable interference screws sometimes contain hydroxyapatite and tricalcium phosphate, leading to an increased BTO within the tunnel. As degradation may last up to 18months depending on the specific composition of screws, increased BTO within this period is a normal finding. Increased BTO of the tunnel may be related to failure of degradation of the biodegradable interference screw and may be associated with a foreign body reaction, fracture, and screwmigration (Figure 3). This failure of degradation may not be diagnosed on MRI [7]. Tunnel widening is a frequently observed finding in the follow-up of asymptomatic patients and may be related to the degradation process of the biodegradable screw [7]. SPECT/CT can be used to evaluate this tunnel widening and tunnel position. An increased BTO can be seen with cyst formation in the tunnel and accompanied tunnel widening. It might also reflect a non-union of the screw and the adjacent bone [7]. SPECT/CT is able to depict the in vivo loading of the knee joint as well as bone remodeling and the graft incorporation process after ACL reconstruction. In addition, SPECT/CT can detect ACL graft insufficiency by displaying increased femoral and tibial bone turnover [54]. SPECT/CT is limited by the fact that it is not able to detect graft tear, arthrofibrosis and ACL laxity, where MRI is usually able to visualize these conditions [7, 55]. Also, in SPECT/CT radiation is involved, albeit delivering a modest dose of 300 to 360 mrem (3,0 to 3,6 mSv), being and effective dose of 0,0057 mSv/MBq and 16mrem (0,16 mSv) for full-dose CT of the knee, in contrast to no radiation using MRI [40, 46]. This may also influence the method of choice in younger patients. 3