The present study demonstrates that osteoporosis is common in Swedish patients with AS, but is often undiagnosed and thus untreated. BMD was below the expected range for age in 5% of patients under the age of 50 years. Osteoporosis, as defined by the WHO, was diagnosed in 21% of patients aged 50 years or older (in 30% of the women and 14% of the men). Most patients with osteoporosis were undiagnosed before the study. The prevalence of osteoporosis found in the present study is comparable with the prevalence in the Swedish general population aged 50 to 80 years, where 21.2% of women and 6.3% of men have been identified as having osteoporosis [25
]. The results indicate that the increased risk of osteoporosis in AS compared with the general population is especially accentuated in male patients, which is in accordance with findings in another study [26
In the current study osteoporosis and osteopenia in the lumbar spine were more common in women than in men in the age-group of 50 years or older when measured with AP lumbar DXA, and thus more women were diagnosed as osteoporotic. The prevalence of osteoporosis and osteopenia was equal among women and men at all other locations. The high prevalence of syndesmophytes in the men may have resulted in falsely elevated BMD causing an underestimation of male osteoporosis at the lumbar spine.
The results from the present study indicate that osteoporosis and osteopenia affect both the central and the peripheral skeleton. We found almost as many patients with BMD below the expected range for age or osteoporosis at the radius (n
= 17, 8%) as in the lumbar spine (n
= 24, 12%). Results from earlier studies indicate that osteoporosis in AS is predominantly confined to the central skeleton [27
]. In one study no correlation was found between bone density at the forearm measured with peripheral quantitative computed tomography (pQCT) and DXA and quantitative CT (QCT) measurements of BMD at the lumbar spine or hip [29
]. Quantitative ultrasound studies of the heel in patients with AS have inconsistent findings, with normal results in one study, and signs of peripheral osteoporosis in another [30
]. The theory that osteoporosis is a general process affecting the whole skeleton in AS was supported by a study of bone biopsies from the iliac crest, showing trabecular thinning and low trabecular peripheral bone volume strongly correlated with lumbar spine BMD measured using QCT [32
There is uncertainty about how to treat osteoporosis in patients with AS. Bisphosphonates have been studied in respect to their effect on disease activity in AS, but their effects on fractures, BMD and the new bone formation in AS needs to be further investigated. Pamidronate has been reported to hamper disease activity in AS [33
]. In a recent placebo-controlled study of alendronate 70 mg weekly, no improvement of AS symptoms or disease activity was found [35
In the current study we found that low BMD was associated with older age, longstanding disease, syndesmophyte formation, impaired back mobility, history of coxitis, use of glucocorticoids and laboratory signs of inflammation. Menopause was a strong risk factor for women. No connection was found between low BMD and the disease indices BASDAI, BASFI, BAS-G or ASDAS. BASMI and ASDAS were associated with inflammatory parameters (ESR, CRP), but BASDAI, BASFI and BAS-G were not.
The association between extensive syndesmophyte formation, restriction of spinal movement and osteoporosis has been demonstrated previously [26
]. One study found significant correlation between low lumbar spine BMD and elevated ESR and CRP [37
]. Two follow-up studies have shown that patients with AS and persistent inflammation, that is, with elevated ESR or CRP, developed significant decreases in BMD, whereas patients with low inflammatory activity did not [38
In men, who had significantly higher mSASSS than women, increasing mSASSS and BASMI were significantly associated with decreasing vBMD and lateral BMD at the lumbar spine, along with lower BMD at the hip and radius, while AP lumbar BMD had a non-significant tendency to increase. Our interpretation of the results is that in comparison with AP spine BMD, lumbar spine vBMD and lateral lumbar spine BMD are less affected by the new bone formation in AS and hence are more adequate tools in assessing osteoporosis and osteopenia, especially in male patients in AS.
Lateral lumbar spine BMD is usually lower than AP BMD, because the lateral DXA scan measures only the trabecular-rich vertebral body, whereas the AP scan includes both the vertebral body and the posterior part of the vertebra, mainly consisting of dense cortical bone. AP scanning is also affected by artefacts due to to osteophytes, aortic calcifications and degenerative changes in the facet joints of elderly people and from hyperostosis in AS. The trabecular bone is more metabolically active than the cortical bone, therefore a decrease in BMD is first found in the trabecular bone. Consequently lateral lumbar spine BMD declines faster than AP BMD in early osteoporosis [40
The current study demonstrates that lateral lumbar spine DXA is more sensitive than AP DXA in detecting osteoporosis and osteopenia in AS. The same results have been reported in two studies using lateral DXA of vertebra L3 in patients with AS [42
]. Other studies have shown that lateral DXA is more sensitive than AP DXA in detecting osteopenia and osteoporosis in degenerative spinal disease [44
]. In one study of 100 AS patients and 58 healthy controls assessed with both AP and lateral lumbar DXA using a scanner similar to the one used in the current study, the authors reported that lumbar spine BMD was significantly lower in AS patients compared with healthy controls when measured by lateral projection DXA, but not when measured by AP DXA [45
]. However, to apply lateral DXA in clinical practice, reference intervals based on measurements on large populations of healthy men and women are required. Most likely, new threshold values for definition of osteoporosis have to be defined to avoid overestimation of osteoporosis with lateral DXA. The current WHO definition of osteoporosis is based on the PA projection and according to the Official Positions of the International Society for Clinical Densitometry 2007, the lateral spine should not be used for diagnosis of osteoporosis, but it may have a role in monitoring [23