Summary                         

Post-surgical implant-associated spinal infection is a devastating complication commonly caused by Staphylococcus aureus. Biofilm formation is thought to reduce penetration of antibiotics and immune
cells, contributing to chronic and difficult-to-treat infections.

In the present study, a rabbit model of a posterior-approach spinal surgery was created, in which bilateral titanium pedicle screws were interconnected by a plate at the level of lumbar vertebra L6 and inoculated with a methicillin-resistant S. aureus (MRSA)  bioluminescent strain. In vivo whole-animal bioluminescence imaging (BLI) and ex vivo bacterial cultures demonstrated a peak in bacterial burden by day 14, when wound dehiscence occurred. Structures suggestive of biofilm, visualized by scanning electron microscopy, were evident up to 56 days following infection.

Infection-induced inflammation and bone remodeling were also monitored using nanoScan PET/CT. FDG accumulated in the soft tissue and bone surrounding the implanted materials. CT imaging demonstrated marked bone remodeling and a decrease in dense bone at the infection sites.

Results also show that this rabbit model of implant-associated spinal infection provides a valuable preclinical in vivo approach to investigate the pathogenesis of implant-associated spinal infections and to evaluate novel therapeutics.

Results from nanoScan PET/CT

During the surgical procedure, L6 spinous process was removed using a small rongeur, creating a hollow self-contained defect, mimicking a partial laminectomy defect. An orthopedic-grade titanium plate was placed into the defect and over the L6 transverse processes. Self-drilling pedicle screws were used to fix the plate. Finally, an inoculum of the bioluminescent MRSA strain in 100μl PBS (1×10⁴ - 1×10⁶ CFUs) was pipetted onto the implanted screws and plate.

On post-operative days 7, 21 and 56, rabbits were placed under anesthesia and 14.04±1.7 MBq ¹⁸F-FDG was injected intravenously via the ear vein. Static images were acquired 45 min following tracer injection. PET images were obtained using a single static 15-min acquisition followed by CT imaging for attenuation correction and anatomical co-registration using the nanoScan PET/CT. 3D spherical volumes of interest were drawn to measure ¹⁸F-FDG-PET signal surrounding infected hardware and in an uninfected reference point (anteriorly to L4). SUVs were derived. Uptake into bone and soft tissue was determined based on an ROI surrounding the hardware and using CT-derived thresholds for bone (>700 HU). SUVs were derived and ratios between bone and soft tissue were calculated.

Results show:

• The ¹⁸F-FDG-PET imaging demonstrated distinct and localized uptake at the site of infection
• In agreement with in vivo BLI signals, SUV ratios of the ¹⁸F-FDG-PET imaging signals between the infected site surrounding the hardware and a non-infected reference increased from 3.99±0.23 on post-operative day 7 to 5.37±0.41 on post-operative day 21
• ¹⁸F-FDG uptake decreased by day 56 (SUV ratio=3.88±0.04), but the ¹⁸F-FDG uptake signals were still visible and localized to the site of infection, likely indicating persistent inflammation.
• During the 56-day course of infection, the ¹⁸F-FDG-PET signal from bone did not statistically

change, with bone-to-non-infected reference SUV ratios of 2.55±0.14, 2.30±0.07 and 2.21±0.13 on post-operative days 7, 21 and 56, respectively

• Bone-to-soft tissue ratios had statistically significant changes as they were 0.86±0.12 and 1.22±0.07 on postoperative days 21 and 56, respectively, suggesting an increase in bone inflammation at the later time point.

• On post-operative day 56, rabbits were euthanized and infectioninduced changes in the bone were evaluated by high-resolution ex vivo CT imaging: the infected vertebra had decreased amount of dense bone (31.9±1.5%), compared with the uninfected vertebra (36.5±1.0%)

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