The most important finding of the present case was twofold:
Firstly, we reported the case of a painful knee due to a foreign body reaction and degradation under pressure after ACL graft fixation using biodegradable interference screws. According to Frosch et al. 18 months after ACL reconstruction one could expect that the used interference screws were entirely degraded and partially incorporated [1, 10, 15]. The authors investigated the in vivo degradation and biocompatibility of the same interference screw as in our case report. In a series of 12 patients after arthroscopic ACL reconstruction using hamstring tendon grafts serial MR imaging was performed at 3, 6 and 12 months after surgery [1, 10, 15]. At 12 months post-operatively the average volume loss of the tibial and femoral screws were 83 ± 17% and 92 ± 6%, respectively [1, 10, 15]. The screws were entirely degraded and only remants were identifiable [1, 10, 15]. In contrast, Ma et al. found no full degradation of PLLA interference screws at at 2-4 years after surgery in patients after hamstring ACL reconstruction . Fink et al. showed similar results with complete degradation at 12 months using CT scans on PGA screws 
Although the Milagro™ screw is considered to be biocompatible and osteoconductive the femoral and to a lesser degree the tibial screw in the presented case did not degrade and caused a foreign body reaction, which was confimed by histology. This is in line with others describing inflammatory or foreign body reactions after use of biodegradable screws [1, 7, 10, 13]. In which patients and under which condition and environment the degradation of this biodegradable interference screw causes a foreign body reaction and related arthrofibrosis remains unclear. A number of different factors such as the polymers and copolymers, the size of the screw, the position of the interference screw and the sterilization are considered to be key factors for the degradation of biodegradable screws . Also the angulation of the tunnel, location of tunnel entrance, the angle and depth of graft fixation and the joint metabolism might be of influence not only for the degradation process but also for the graft tension [21, 31, 32].
In the case presented it seems to be a combination of improper graft placement, delayed degradation of the used interference screws and biological factors, which could not be elucidated. The too shallow and high graft placement might have led to impingement and thus chronic intraarticular inflammation.
Secondly SPECT/CT may facilitate the establishment of diagnosis, process of decision making and further treatment in patients with knee pain after ACL reconstruction. Due to the combination of structural, functional and metabolic information the patient's cause of the pain was established [25, 26].
Marchant et al. investigated the intra- and inter-observer reliability in identifying bone tunnels and assessing tunnel widening on radiographs, MRI and CT . They showed that CT was most reliable. Radiographs and MRI were not reliable, even for simply identifying the presence of a bone tunnel . Without the use of SPECT/CT the decision to debride the ACL graft and remove the interference screws would have not been made.
Three dimensional reconstructions of CT provide the surgeon with a better 3D understanding of patient's anatomy . The position of the tunnels, the orientation of the graft and the localization of attachment area can be assessed, which have been reported to be decisive for outcome after ACL reconstruction [33, 34]. In the presented case 3D-CT out of the SPECT/CT data was used to accurately evaluate the tunnel placement in relation to standardized frames of reference.
Hogervorst et al. found that scintigraphic tracer uptake correlated significantly with tibial tunnel enlargement and location of the tibial tunnel . They concluded that bone scintigraphy is a reliable method assessing the complex biology of graft remodelling, tunnel enlargement and osseous homeostasis . In contrast to scintigraphy or SPECT alone SPECT/CT is able to accurately allocate the tracer uptake to anatomical areas (e.g. tibiofemoral or patellofemoral joint as well as tibial and femoral tunnels).