Can pain influence the severity of findings of an (18)F-FDG PET/CT scan in loosening hip arthroplasty?

Pain is a common symptom after loosening and/or after infection of arthroplasty. The aim of the present case report is to indicate that fluorine-18-fluorodeoxy-glucose positron emission tomography/computerized tomography((18)F-FDG PET/CT) scan can be used for the evaluation of pain at the site of arthroplasty with a semi quantitative measurement. An 84 years old male patient, with a history of papillary thyroid cancer was referred to our Nuclear Medicine Department for an (18)F-FDG PET/CT scan for evaluation of his metastatic disease. He reported right hip prosthesis thirty years ago and now suffered from significant pain in the right hip joint which on a scale from 1 to 10 could be rated as grade 8. White blood cell count, sedimentation rate and C-reactive protein were within normal limits. Four years ago he also had severe pain on the right leg that could be rated as grade 8, and underwent an (18)F-FDG PET/CT scan that showed a maximum standardized uptake value (SUV(max)) of 9.8. A year later, at a follow up (18)F-FDG PET/CT scan and under a similar severe pain, SUV(max) was 10. The patient had a hip prosthesis in the left leg 3 years ago, for which he reported pain that could be rated as grade 3 and the SUV(max) was then 3.7. The degree of (18)F-FDG uptake is related to cellular metabolic rate and to the number of glucose transporters. In inflammation, the activated inflammatory cells demonstrate increased expression of glucose transporters and the affinity of glucose transporters for deoxyglucose is increased by various cytokines and growth factors. Furthermore, when the mononuclear cells and the granulocytes are activated by certain infectious humoral stimuli, they use large amounts of glucose by the hexose monophosphate shunt and the rate of oxygen uptake is intensely increased, during the so called "respiratory burst". In the present case, it was interesting to note that the degree of (18)F-FDG uptake at the sites of loosening hip arthroplasty was related to the severity of pain. Signs of infection or inflammation although not obvious in our case, could not be excluded because sensitivity, specificity and accuracy of PET for detecting infection associated with hip prostheses has been reported to be, 90%, 89.3% and 89.5%, respectively and sensitivity and specificity for detecting periprosthetic inflammation 100% and 45.5%,respectively. In another study using as a criterion for periprosthetic infection the increased (18)F-FDG activity at the bone-prosthesis interface of the femoral component, sensitivity, specificity, positive and negative predictive values for the (18)F-FDG PET study where 85%, 93%, 80% and 95%, respectively. Inflammation, often intense, may be present in aseptic loosening and characterized by increased periprosthetic (18)F-FDG uptake. In conclusion, in our case the degree of (18)F-FDG uptake in a loosening hip arthroplasty was related to the severity of pain although inflammation or infection could also play some role. Further studies are required to prove this suggestion.