Quantification of aging effects upon global knee inflammation by 18F-FDG-PET

OBJECTIVE: The goal of this study was to quantify aging effects upon the global knee joint and surrounding capsule and soft tissue inflammation using fluorine-18 fluorodeoxyglucose (18F-FDG) PET imaging.

METHODS: This reanalysis of a prospective study included 64 patients who had undergone 18F-FDG-PET for evaluation of hip joint prostheses, and whose scans included the knee joints in the field of view. Mean patient age was 53 years (range: 33-84 years). A fixed-sized three-dimensional region of interest was placed around each knee joint, paying close attention to exclude the popliteal vessels. 18F-FDG-avid regions in each knee joint were then segmented using an adaptive contrast-oriented thresholding method, and metabolically active volume (MAV), mean standardized uptake value (SUV mean), partial volume-corrected SUV mean (cSUV mean), and partial volume-corrected mean metabolic volumetric product (cMVP mean = cSUV mean × MAV) of the segmented regions were calculated. Finally, global knee inflammation (GKI) for each knee joint was calculated as the sum of cMVP mean in all segmented regions. Association of GKI with age was assessed with Pearson's correlation and linear regression methods, and GKI was compared between patients at different ages - between patients younger than 55 years and those older than 55 years - using the unpaired t-test.

RESULTS: The correlation coefficient of GKI with advancing age was 0.57 (P = 0.02). In the linear regression model, considering GKI as the dependent variable and age and sex as independent covariates, the β coefficient of age was 2.1 (95% confidence interval: 1.1-3.2). For patients aged younger than 55 years versus those aged older than 55 years, the mean GKI was 157 and 190 cm3, respectively (P = 0.01).

CONCLUSION: Through the use of novel quantitative techniques, we were able to calculate GKI and demonstrate a significant increase in the entity of joint inflammation with advancing age. As degenerative disease is age-related and inflammation is implicated in its pathogenesis, our findings further support this association. These preliminary data suggest that this approach can potentially provide a means to objectively quantify the degree of inflammation in various joint disorders, and possibly in other knee degenerative/inflammatory diseases.