A comparative 18F-FDG PET/CT imaging of experimental Staphylococcus aureus osteomyelitis and Staphylococcus epidermidis foreign-body-associated infection in the rabbit tibia

Abstract

AbstractBackground18F-FDG-PET imaging has emerged as a promising method in the diagnosis of chronic osteomyelitis commonly due toStaphylococcus aureus. The inaccuracy of18 F-FDG-PET in the detection of periprosthetic joint infections may be related to the predominance of low-virulentS. epidermidisstrains as the causative pathogen. We have compared the18F-FDG-PET characteristics ofS. aureusosteomyelitis and foreign-body-associatedS. epidermidisinfections under standardized laboratory conditions.MethodsTwenty-two rabbits were randomized into three groups. In group 1, a localized osteomyelitis model induced with a clinical strain ofS. aureuswas applied. In groups 2 and 3, a foreign-body-associated infection model induced with a clinical or laboratory strain ofS. epidermidiswas applied. A small block of bone cement was surgically introduced into the medullary cavity of the proximal tibia followed by peri-implant injection ofS. aureus(1 × 105 CFU/mL) or one of the twoS. epidermidis(1 × 109 CFU/mL) strains with an adjunct injection of aqueous sodium morrhuate. In group 1, the cement block was surgically removed at 2 weeks but left in place in groups 2 and 3 in order to mimic foreign-body-associatedS. epidermidisinfections. At 8 weeks, the animals were imaged using18 F-FDG PET/CT. The presence of bacterial infection was confirmed by cultures, and the severity of bone infections was graded by means of radiography, peripheral quantitative CT, and semi-quantitative histology.ResultsTheS. aureusstrain caused constantly culture-positive osteomyelitis. The clinicalS. epidermidisstrain resulted in foreign-body-associated infections, while the laboratoryS. epidermidisstrain (ATCC 35983) induced only occasionally culture-positive infections. There was a correlation (r = 0.645;P = 0.013) between semi-quantitative score of leukocyte infiltration and the18 F-FDG uptake in animals with positive cultures. Standardized uptake value (SUV) of the infected bones was twofold (P < 0.001) inS. aureusanimals compared withS. epidermidisanimals, but there was only a trend (P = 0.053, ANOVA) in the differences of the corresponding SUV ratios. This was due to the altered18 F-FDG uptake of the contralateral tibias probably reflecting a systemic impact of severe osteomyelitis.ConclusionThe peri-implant inoculation ofS. epidermidis, reflecting low virulence of the pathogen and limited leukocyte infiltration, was characterized by low18 F-FDG uptake.