Value of 18 F-FDG PET/CT to Identify Occult Infection in Presumed Aseptic Pseudarthrosis after Spinal Fusion: Correlation with Intraoperative Cultures

Abstract

Objective Fluorine-18 fluorodeoxyglucose ( ¹⁸ F-FDG) positron emission tomography/computed tomography (PET/CT) has gained attention as an emerging tool in case of suspicion of infection on spine, whether native or instrumented. However, the diagnostic performance of ¹⁸ F-FDG PET/CT in clinically occult low-grade surgical site infection (SSI) after spinal fusion, an important risk factor for pseudarthrosis, remains unknown. Methods We retrospectively identified all the presumed aseptic patients with pseudarthrosis confirmed by revision surgery who underwent preoperative ¹⁸ F-FDG PET/CT scans performed between April 2019 and November 2022. These patients were presumed aseptic because they did not have clinical signs or laboratory tests suggestive of SSI, preoperatively. The PET/CT images were analyzed in consensus by two nuclear medicine physicians blinded to the clinical, biological, and imaging information. Visual assessment of increased uptake around cage/intervertebral disk space (and/or hardware) higher than background recorded from the first normal adjacent vertebra was interpreted as positive. Image data were also quantitatively analyzed by the maximum standardized uptake value as an index of ¹⁸ F-FDG uptake, and the ratio between the uptake around cage/intervertebral disk space (and/or hardware) and background recorded from the first normal adjacent vertebra was calculated. The final diagnosis of infection was based on intraoperative cultures obtained during pseudarthrosis revision surgery. Results Thirty-six presumed aseptic patients with surgically confirmed pseudarthrosis after spinal fusion underwent preoperative ¹⁸ F-FDG PET/CT scans. Cultures of samples from revisions found that 20 patients (56%) were infected. The most frequent isolated bacterium was Cutibacterium acnes ( C. acnes ) in 15 patients (75%), followed by coagulase-negative staphylococci (CNS) in 7 patients (33%). Two patients had co-infections involving both C. acnes and CNS. Of the 36 PET/CT studied in this study, 12 scans were true-negative, 10 true-positive, 10 false-negative, and 4 false-positive. This resulted in sensitivity, specificity, positive predictive value, negative predictive value, and diagnostic accuracy of 50%, 75%, 71%, 55%, and 61%, respectively. Conclusion In presumed aseptic pseudarthrosis after spinal fusion, ¹⁸ F-FDG PET/CT offers good specificity (75%) but low sensitivity (50%) to identify occult SSI. The high prevalence (56%) of SSI, mostly caused by C. acnes (75%), found in our presumed aseptic cohort of patients supports the utility of systematic intraoperative cultures in revision cases for pseudarthrosis.

Keywords: coagulase-negative staphylococci; Cutibacterium acnes; fluorine-18 fluorodeoxyglucose; positron emission tomography/computed tomography; pseudarthrosis; spinal fusion; surgical site infection.

Fig. 1

Fluorine-18 fluorodeoxyglucose ( ¹⁸ F-FDG) positron emission tomography/computed tomography (PET/CT) fusion, noncontrast CT, and PET images in a true-negative patient 16 months after spinal fusion. Sagittal PET/CT showed no uptake of the tracer around cage at L5-S1 ( red arrow ). The four intraoperative cultures obtained during pseudarthrosis revision were negatives.

Fig. 2

Fluorine-18 fluorodeoxyglucose ( ¹⁸ F-FDG) positron emission tomography/computed tomography (PET/CT) fusion, noncontrast CT, and PET images in a true-positive patient 17 months after spinal fusion. Coronal PET/CT showed increased uptake around cage at L4-L5 ( red arrow , SUVmax = 6.3, uptake ratio = 2). Four of four intraoperative cultures taken during pseudarthrosis revision grew Cutibacterium acnes .

Fig. 3

Fluorine-18 fluorodeoxyglucose ( ¹⁸ F-FDG) positron emission tomography/computed tomography (PET/CT) fusion, noncontrast CT, and PET images in a false-negative patient 38 months after spinal fusion. Sagittal PET/CT showed no uptake of the tracer around cages at C4-C5, C5-C6, and C6-C7 ( red arrows ). Five of five intraoperative cultures obtained during pseudarthrosis revision grew Cutibacterium acnes .

Fig. 4

Fluorine-18 fluorodeoxyglucose ( ¹⁸ F-FDG) positron emission tomography/computed tomography (PET/CT) fusion, noncontrast CT, and PET images in a false-positive patient 16 months after spinal fusion. Coronal PET/CT showed increased uptake around cage at L3-L4 ( red arrow , SUVmax = 7.3, uptake ratio = 2.1). The three intraoperative cultures obtained during pseudarthrosis revision were negatives.