FDG-PET, Tc-99m-HMPAO white blood cell SPET and bone scintigraphy in the evaluation of painful total knee arthroplasties

Fluorine-18 fluorodeoxyglucose positron emission tomography (FDG-PET), tech netium-99m hexamethylpropylene amine oxime (HMPAO)-labelled white blood cel l (WBC) scintigraphy and bone scintigraphy were used in the evaluation of t otal knee arthroplasties (TKAs). We prospectively included 21 patients who had a three-phase bone scan for exclusion of infection of TKAs. Four hours after injection of 185 MBq Tc-99m-HMPAO-labelled WBCs, planar and single-ph oton emission tomographic (SPET) imaging was performed. Planar imaging was repeated at 24 h p.i. Consecutively images of the knees were obtained with a dedicated PET system 60 min following the injection of 370 MBq of FDG. Fo cal tracer uptake was scored on SPET and PET visually (0=no uptake, 4=inten se uptake). In addition., SUV (standardised uptake value) per voxel was cal culated from attenuation-corrected PET images using the MLAA algorithm. Foc al uptake at the bone-prosthesis interface was used as the criterion for in fection before and after correlation with the third phase of the bone scan. Final diagnosis was based on operative findings, culture and clinical outc ome. In the infected TKAs. the WBC scan showed focal activity of grade 2 (n =2), 3 (n=1) or 4 (n=2). PET scan revealed focal activity of grade 4 (n=5) or 3 (n=1). WBC scan alone had a specificity for infection of 53% [positive predictive value (PPV) 42%, sensitivity 100%], compared with 73% for PET s can (PPV 60%. sensitivity 100%). Considering only lesions at the bone-prost hesis interface that were also present on the third phase of the bone scan, we found a specificity of 93% (PPV 83%) for WBC scan. Using these criteria , a specificity of 80% (PPV 67%) was obtained for PET scan. Two out of thre e false-positive PET scans were due to loosening of the TKA. It is conclude d that WBC scintigraphy in combination with bone scintigraphy has a high sp ecificity in the detection of infected TKAs. FDG-PET seems to offer no addi tional benefit.