Abstract
Background18F-FDG-PET imaging has emerged as a promising method in the diagnosis of chronic osteomyelitis commonly due to Staphylococcus aureus. The inaccuracy of 18 F-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 the18F-FDG-PET characteristics of S. aureus osteomyelitis and foreign-body-associated S. epidermidis infections under standardized laboratory conditions.MethodsTwenty-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 × 105 CFU/mL) or one of the two S. 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-associated S. epidermidis infections. At 8 weeks, the animals were imaged using 18 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.ResultsThe 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 18 F-FDG uptake of the contralateral tibias probably reflecting a systemic impact of severe osteomyelitis.ConclusionThe peri-implant inoculation of S. epidermidis, reflecting low virulence of the pathogen and limited leukocyte infiltration, was characterized by low 18 F-FDG uptake.
Highlights
18F-FDG-PET imaging has emerged as a promising method in the diagnosis of chronic osteomyelitis commonly due to Staphylococcus aureus
The purpose of this study was to determine whether the differences in the virulence of the staphylococcal strains and the severity of subsequent infections contribute to the level of 18 F-FDG uptake in PET/CT imaging of osteomyelitis and biomaterialrelated bone infection
Our results suggest that the 18 F-FDG uptake of infected bones is related to the degree of leukocyte infiltration, which in turn reflects the virulence of the pathogen
Summary
18F-FDG-PET imaging has emerged as a promising method in the diagnosis of chronic osteomyelitis commonly due to Staphylococcus aureus. The inaccuracy of 18 F-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 the18F-FDG-PET characteristics of S. aureus osteomyelitis and foreign-body-associated S. epidermidis infections under standardized laboratory conditions. Staphylococcus aureus and S. epidermidis represent the most commonly found coagulase-positive and coagulase-negative staphylococcal species, respectively [2]. S. epidermidis represents opportunistic microorganisms, which have emerged as the leading nosocomial pathogen of biomaterialrelated bone infections [4]. Related to differences in virulence, the outcome of bone implant infections caused by S. epidermidis strains is better than those caused by S. aureus strains [5]
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