Lateral DXA More Effective in Detecting Osteoporosis than Conventional DXA in Axial Spondyloarthropathy


Gillian Fitzgerald (1,2), Jason Wyse (3), Tochukwu Anachebe (1), Ronan Mullan (4), David Kane (4), Kevin McCarroll (5), Finbar O’ Shea (1).


(1) Department of Rheumatology, St. James’s Hospital. (2) Department of Medicine, Trinity College Dublin (3) Department of Statistics, Trinity College Dublin (4) Department of Rheumatology, Tallaght University Hospital (5) Department of Medicine for the Elderly, St. James’s Hospital, Dublin, Ireland.


The consequences of osteoporosis are well outlined. In axial spondyloarthropathy (axSpA), osteoproliferation of the spine means posterioanterior (PA) dual-energy x-ray absorptiometry (DXA) can’t discriminate between new bone formation and vertebral body, potentially overestimating BMD. Lateral DXA of the spine avoids spinal osteoproliferation and is an attractive option.


The aim of this study is to compare lateral and PA DXA of the lumbar spine and determine patient variables that render conventional DXA unreliable.


Patients fulfilling modified New York (mNY) or Assessment of Spondyloarthritis International Society (ASAS) criteria were consecutively recruited in this twin-centre cross-sectional study. A detailed assessment of patients included demographics, clinical exam, laboratory assessment and validated measures of disease severity (BASDAI, ASDAS-CRP, BASMI, mSASSS). BMD of the spine was assessed using DXA in the lateral and PA projections. R software was used for statistical analysis.


One hundred and ten patients were assessed, 100 of whom had paired AP and lateral DXAs: 76% (n=84) male, 92% Caucasian, 81% mNY criteria. Median (IQR) age was 52 (17) years, disease duration 23.5 (20) years, delay to diagnosis 7 (12) years, body mass index (BMI) 27.6 (6.3) kg/m2, BASDAI 3.9 (2.1-5.6), BASMI 4.1 (2.8-5.8), ASDAS-CRP 2.2 (1.5-3), mSASSS 8.5 (2-36).
Lateral spine BMD is significantly lower than PA BMD (mean difference between AP and lateral lumbar spine DXA of 0.337 g/cm2, 95% CI 0.3-0.37) and detects more cases of both osteopenia (27% versus 17%) and osteoporosis (16% versus 2%) at the spine (p<0.01).
The following variables correlate with a larger difference between the measurement of AP and lateral DXA: BASMI (r=0.54), disease duration (r=0.37), BMI (r=0.32), mSASSS (r=0.52).
In multiple regression analysis, a model with disease duration, BMI, BASMI and shorter time to diagnosis predicts a greater difference between AP and lateral BMD (p<0.05).


Lateral DXA detects more cases of osteoporosis than PA DXA, particularly in patients with higher BASMI and BMI and longer disease. Relying on AP DXA may miss low BMD in axSpA patients. Lateral DXA is a practical and alternative to PA DXA when measuring BMD in the spine of axSpA patients.

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