(18)F-FDG and (68) Ga-FAPI PET/CT for the evaluation of periprosthetic joint infection and aseptic loosening in rabbit models

PURPOSE: We built a joint replacement loosening model based on the original rabbit model of infection and evaluated the performance characteristics of (18)F-FDG and (68) Ga-FAPI in evaluating infection and loosening. METHODS: After surgery, the rabbits were divided into four groups, with six individuals in the control group and 10 each in the aseptic loosening, S. aureus and S. epidermidis groups. PET/CT and serological examination were performed three times at two-week intervals. After the rabbits were euthanized, micro-CT, tissue pathology, pullout tests and scanning electron microscopy (SEM) were performed. RESULTS: The pullout test and SEM showed the feasibility of the aseptic loosening model. (18)F-FDG showed similar performance in the control and loosening groups. The SUVmax of the S. aureus group was consistently higher than that of the S. epidermidis group. As for (68) Ga-FAPI, the SUVmax of the control group was lowest in the second week and gradually increased over subsequent weeks. The SUVmax of the loosening group began to exceed that of the control group after the second week. The SUVmax of the S. aureus group in the second week was the lowest among the four groups and increased as the number of weeks increased. The pathology results showed concordance with the performance of PET/CT. Linear regressions between SUVmax and serology showed that (18)F-FDG was positively correlated with CRP and IL-6, while (68) Ga-FAPI revealed negative correlations with CRP and IL-6 in the second week and positive correlations in the sixth week. In addition, the SUVmax and MT(target)V of both (18)F-FDG and (68) Ga-FAPI were negatively correlated with bone volume/trabecular volume (TV) and bone surface area/TV. CONCLUSION: In this longitudinal observation, (68) Ga-FAPI showed greater sensitivity than (18)F-FDG in detecting diseases, and (68) Ga-FAPI had no intestinal or muscular uptake. The MT(target)V of (68) Ga-FAPI was larger than that of (18)F-FDG, which meant that (68) Ga-FAPI had the potential to define the scope of lesions more accurately. Finally, the SUVmax of (68) Ga-FAPI could not differentiate between loosening and infection; further study of the diagnostic criteria is warranted. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12891-022-05537-w.