Background/Purpose: In advanced disease Ankylosing Spondylitis (AS) is frequently associated with a reduction in bone mineral density (BMD) which contributes to pain and predisposes to fractures. Quantifying this reduction in BMD is complicated because overgrowth of bone and loss of trabecular bone occur at the same time. Traditional methods, such as dual-energy X-ray absorptiometry (DXA), may not generate accurate estimates of BMD in AS patients. It has recently been shown that osteoporosis, osteopenia, and normal BMD can be differentiated by evaluating the L1 vertebra on abdominal Computed tomography (CT) scans. The aim of the present study was to evaluate the use of opportunistic CT screening in the diagnosis of osteporosis in patients with severe AS who had sustained vertebral fractures.

Methods: A retrospective study of patients with severe AS and bridging syndesmophytes who presented to our Level 1 trauma center with acute fractures of the spine (both high impact and low impact) and had an evaluable CT scan of the abdomen. Using a picture archiving and communication system (PACS), a region of interest (ROI) was generated to fit to the body of L1 (excluding the cortex) and a mean value for Hounsfield Units (HU) was computed. The values derived were compared against threshold values which differentiate between osteoporosis and osteopenia. For specificity of 90%, a threshold of 110 was set; for balanced sensitivity and specificity, a threshold of ≤135 HU was set, and for 90% sensitivity a threshold of ≤160 HU was set.

Results: A total of 17 AS patients fit the above criteria. Using an L1 CT-attenuation threshold of ≤135 HU balanced for sensitivity and specificity, 14/17 (82%) patients were found to have osteoporosis. Using an L1 CT-attenuation threshold of ≤160 HU to increase sensitivity, 15/17 patients (88%) were found to be osteoporotic. Even using the L1 CT-attenuation threshold of ≤110 HU to increase specificity 14/17 (82%) had osteoporosis.

Conclusion: A high proportion of AS patients who sustain fractures have osteoporosis as shown by using opportunistic CT screening. This overcomes some of the difficulties that have been encountered with the use of DXA in this unique group of patients. This simple and accessible diagnostic method saves on excess cost and exposure to radiation. It may also inform the surgeon in structuring surgical approach.