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

Background: F-18-FDG-PET imaging has emerged as a promising method in the diagnosis of chronic osteomyelitis commonly due to Staphylococcus aureus. The inaccuracy of 18F-FDG-PET in the detection of periprosthetic joint infections may be related to the predominance of low-virulent S. epidermidis strains as the causative pathogen. We have compared the F-18-FDG-PET characteristics of S. aureus osteomyelitis and foreign-body-associated S. epidermidis infections under standardized laboratory conditions. Methods: Twenty-two rabbits were randomized into three groups. In group 1, a localized osteomyelitis model induced with a clinical strain of S. aureus was applied. In groups 2 and 3, a foreign-body-associated infection model induced with a clinical or laboratory strain of S. epidermidis was applied. A small block of bone cement was surgically introduced into the medullary cavity of the proximal tibia followed by peri-implant injection of S. aureus (1 x 10(5) CFU/mL) or one of the two S. epidermidis (1 x 10(9) 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-associated S. epidermidis infections. At 8 weeks, the animals were imaged using F-18-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. Results: The S. aureus strain caused constantly culture-positive osteomyelitis. The clinical S. epidermidis strain resulted in foreign-body-associated infections, while the laboratory S. epidermidis strain (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 the 18 F-FDG uptake in animals with positive cultures. Standardized uptake value (SUV) of the infected bones was twofold (P < 0.001) in S. aureus animals compared with S. epidermidis animals, but there was only a trend (P = 0.053, ANOVA) in the differences of the corresponding SUV ratios. This was due to the altered F-18-FDG uptake of the contralateral tibias probably reflecting a systemic impact of severe osteomyelitis. Conclusion: The peri-implant inoculation of S. epidermidis, reflecting low virulence of the pathogen and limited leukocyte infiltration, was characterized by low F-18-FDG uptake.