Background/Purpose: 18F-fluoride uptake represents active osteoblastic bone synthesis. We investigated the relationship between the magnitude of bone formation in anterior vertebral corners, as assessed through 18F-fluoride uptake on positron emission tomography (PET) and lumbar spine bone mineral density (BMD) in patients with ankylosing spondylitis (AS).

Methods: Twelve male patients with AS who was biologics-naïve underwent whole body 18F-fluodride PET and dual energy X-ray absorptiometry (DXA). The maximum standardized uptake value (SUVmax) of 18F-fluoride uptake at the anterior vertebral corner (L1 upper to L4 lower) was analyzed to determine the degree of active bone formation; the BMD of the lumbar spine in the AP (L1 to L4) and lateral (L2 to L4) projections was measured using DXA (Figure 1). At each lumbar vertebra level, the relationship between the sum of the SUVmax in the upper and lower anterior vertebral corners and the BMD of the corresponding lumbar vertebral body was assessed using a generalized estimating equation (GEE) to adjust within-patient correlation.  

Results: A total of 48 lumbar vertebrae were analyzed. In correlation analyses, the sum of SUVmax in upper and lower anterior vertebral corners positively correlated with in both the lumbar AP (Spearman correlation coefficient=0.273, p=0.039) and lateral spine BMD (Spearman correlation coefficient=0.206, p=0.04). In the GEE model, the sum of SUVmax in upper and lower anterior vertebral corners was significantly associated with the both lumbar AP (b=0.012, p=0.002) and lateral spine BMD (b=0.013, p<0.001). This association remained significant after adjusting for disease duration and the presence of syndesmophytes. 

Conclusion: Active bone formation in the anterior vertebral corners, as assessed through 18F-fluoride uptake on PET was associated with increased BMD of corresponding vertebral bodies in patients with AS. This finding suggests that the precise measurement of lumbar spine BMD using DXA can be hampered by active bone formation in AS.        

Figure 1. 18F-fluoride positron emission tomography (PET) scan (A) and dual energy X-ray absorptiometry evaluating bone mineral density in the AP (B) and lateral (C) projection of a 42-year-old man with ankylosing spondylitis. On PET scan, the maximum standardized uptake value of 18F-fluoride uptake in the upper (arrow in A) and lower (arrowhead in A) parts of the anterior vertebral corner was measured from L1 to L4.