Summary Statement. In vivo microCT allows monitoring of subtle bone structure changes around infected implants in a rat model. Introduction. The principal causes of orthopedic implant revisions are periprosthetic bone loss and infections. Immediately after implantation, a dynamic process of bone formation and resorption takes place around an orthopedic implant, influencing its mechanical fixation. Despite its importance, the effect of bacteria on the temporal pattern of periprosthetic remodeling is still unknown. The aim of this study was to evaluate the morphological changes of bone adjacent to an implant in the presence and absence of infection using micro computed tomography (microCT). Materials and methods. Twenty-four three-month-old female Wistar rats were used in this study. Twelve rats received a single control screw (sterile) in the proximal part of the right tibia while the other twelve received an infected screw (1×10. 4. CFU Staphylococcus aureus). The self-tapping cancellous bone screws, custom made of PEEK and coated with 30µm of titanium, were 2mm in outer diameter and 5mm in length. Bone changes around the screws were assessed using in vivo microCT with a nominal isotropic resolution of 12mm (at 70 kV, 300 ms integration time, 1000 projections) at days 0, 3, 6, 9, 14, 20 and 27. Each measurement took approximately 30 min while the animal was anesthetised via isoflurane inhalation. After reconstruction, these data were registered in space. The screw was segmented and dilated to define a region surrounding the coating. Bone-implant contact (BIC) was defined as the bone volume fraction (BV/TV) within this region. The changes in bone structure were computed from the differences between two consecutive time points. After sacrifice, in each group six tibiae were prepared for histology and six were used for mechanical pullout of the screw from the tibia, then quantitative microbiological analysis was carried-out after homogenization of the bone sample and sonication of the screw. Results. In the control group, no animal showed an infection, while all animals in the infected group developed an infection. In the uninfected group, BIC increased from 35±5% to 55±10% between day 0 and day 27 (p<0.05); at day 27 pullout stiffness was 220±48 N/mm and the maximal force 120±16 N. The microstructural changes were most prominent between day 0 and day 14. In the infected group, BIC dramatically dropped to zero within 14 days and the animals were sacrificed. Histology revealed that in the infected group there was marked osteolysis, purulent inflammation and a fibrous capsule around the screws. The pullout stiffness and maximal force were not significant (respectively 39±54 N/mm and 12±16 N). While 1×10. 4. CFU were introduced at day 0, at day 27, microbiological analysis revealed 1×10. 6. CFU on the screws and 5×10. 5. CFU in the neighboring bone. Conclusion. High-resolution in vivo microCT shows in the current model a rapid progression of osteolysis. This new approach allows a better understanding of the changes in bone structure around S. aureus infected implants. It may be particularly useful in detecting low-grade infections, such as S. epidermidis infections in the same model
