Introduction
Ankylosing spondylitis (AS) is a chronic, inflammatory disease mainly affecting the sacroiliac joints and the spine and is characterized by pathological spinal new bone formation [
1]. Patients with AS also have higher prevalence of low bone mineral density (BMD) than controls, with prevalences varying between 4 and 58% in different cohorts [
2]. Furthermore, patients with AS have almost doubled risk of vertebral fractures (VFs) compared to non-AS individuals in a meta-analysis which identified risk factors for prevalent VFs to be low BMD at the total hip and femoral neck, male sex, longer duration of AS, and radiographic AS-related spinal alterations [
3]. The risk for non-VFs for patients with AS is less studied but was also increased [
3].
The standard method to measure BMD is dual-energy x-ray absorptiometry (DXA) which assesses areal BMD (aBMD). Limitations with DXA are the inabilities to separate trabecular from cortical bone and to evaluate the microarchitecture. Bone strength is determined not only by BMD but also by the degree of mineralization, trabecular and cortical microarchitecture, and bone geometry [
4]. With high-resolution peripheral quantitative computed tomography (HRpQCT), separate measurements of cortical and trabecular volumetric BMD (vBMD) as well as evaluation of the microarchitecture and geometry can be obtained at radius and tibia [
5,
6]. HRpQCT-derived vBMD at tibia is highly correlated with bone strength at femur and lumbar spine [
7], and HRpQCT measurements of tibia and radius have been shown to predict clinical fractures independent of aBMD in older men [
8‐
10]. Previous HRpQCT studies have in comparisons with controls shown reduced cortical and total vBMD both at radius and tibia in AS patients [
11] and at radius in non-radiographic axial spondyloarthritis (nr-axSpA) patients (tibia was not examined) [
12]. Baseline data from the current cohort showed lower cortical vBMD at radius and lower trabecular vBMD at tibia than in controls [
13]. To our knowledge, longitudinal HRpQCT data in AS patients have not been published before. Our aims with this prospective study were (1) to evaluate changes over 5 years in trabecular and cortical vBMD, cortical area, and trabecular microarchitecture at tibia and radius in a cohort of men with AS and (2) to assess factors associated with changes in vBMD and cortical area.
Patients and methods
Patients
Patients were recruited at baseline from three rheumatology clinics in western Sweden and were part of a larger study on osteoporosis [
14] with inclusion criterion AS according to the modified New York criteria [
15]. Exclusion criteria were psoriasis, inflammatory bowel disease, dementia, ongoing pregnancy, and difficulties in understanding the Swedish language. In total, 69 men, out of 204 patients with AS, were randomized in an age-adjusted algorithm to take part in the HRpQCT study [
13]. These patients were invited to the 5-year follow-up. Approval by the regional ethics committee in Gothenburg was given both at baseline and at follow-up, and the study was performed in accordance with the Declaration of Helsinki. Written informed consent was obtained from all patients.
The patients were assessed with the same methods at baseline and the 5-year follow-up. Questionnaires included medical history, lifestyle factors, and medications. Disease activity was assessed by the Bath AS Disease Activity Index (BASDAI) and the AS Disease Activity Score based on C-reactive protein (ASDAS_CRP) [
16]. Physical function was assessed by the Bath AS Functional Index (BASFI) [
16]. Physical examination included evaluation of back and hip mobility by the Bath AS Metrology Index (BASMI) [
16]. The dose of prednisolone was estimated from the medical records and dichotomized into having used < or ≥ 450 mg prednisolone during follow-up (dose equivalent of 5 mg/day ≥ 3 months). Non-steroidal anti-inflammatory drug (NSAID) consumption during follow-up was quantified according to the recommendation by the Assessment of SpondyloArthritis international Society [
17]. Use of tumor necrosis factor inhibitors (TNFi), conventional synthetic disease-modifying anti-rheumatic drugs (csDMARD), and bisphosphonates during follow-up was estimated from the medical records.
Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) were analyzed by standard laboratory techniques. Time-averaged ESR and CRP for the follow-up period were calculated using results obtained from the medical records.
High-resolution peripheral quantitative computed tomography
vBMD, cortical area, and microarchitecture in the non-dominant distal radius and tibia were assessed using the same HRpQCT machine (Xtreme CT, Scanco Medical AG, Brüttisellen, Switzerland), software, and operator at baseline and follow-up. The standard protocol provided by the manufacturer was used. To reduce motion artefacts, the extremities were immobilized in a carbon-fiber shell. For measurement of the volume of interest, a reference line was manually placed at the end plate of the distal radius and tibia, and the first CT slice started 9.5 mm and 22.5 mm proximal of this line for radius and tibia, respectively. A total of 110 parallel slices (voxel size 82 μm) were obtained in the proximal direction at each measuring site, resulting in an approximately 9 mm 3D representation of the bone. Separation of cortical and trabecular regions was done automatically, and borders were thereafter inspected and corrected manually by the operator if necessary. An automated matching procedure was applied to ensure common region of interests for the repeated measurements [
18]. Previously described methods to process the data [
5,
19‐
22] were used to obtain the following parameters: trabecular vBMD (Tb.vBMD; mg/cm
3), trabecular number (TbN; per mm), trabecular thickness (TbTh; mm), trabecular separation (TbSp; mm), cortical vBMD (Ct.vBMD mg/cm
3), and cortical area (Ct.Ar; mm
2). Tb.vBMD, Ct.vBMD, Ct.Ar, and TbN were measured directly, and the other parameters were derived. Tb.vBMD and Ct.vBMD assess bone density, and TbN, TbTh, and TbSp assess microarchitecture. Each scan was graded with a 5-point quality scale recommended by the manufacturer (1 = best, 5 = worst). Examinations with quality 1–3 and common region ≥ 80% were used for evaluation of microarchitecture (TbN, TbTh, and TbSp), whereas quality 1–4 was used for measurements of vBMD and Ct.Ar [
23]. Coefficient of variation (CV) for repeated measurements from our clinic was as follows: for tibia Tb.vBMD, 0.5%; TbN, TbTh, and TbSp, 3.6%; Ct.vBMD, 0.3%; and Ct.Ar, 0.5% and for radius Tb.vBMD, 0.9%; TbN and TbSp, 4.8%; TbTh, 4.1%; Ct.vBMD, 0.6%; and Ct.Ar, 1.5%. The least significant change (LSC), change recognized with 95% confidence, was calculated: 2.77 × CV (
https://www.iscd.org/resources/faqs/precision-assessment/).
The stability over time for the HRpQCT measurements was assessed with data from repeated scans of a phantom, containing five different densities, during the study period. The means and standard deviations from repeated yearly scans of each density were stable over time and are presented in Supplementary Figure
1.
Dual-energy x-ray absorptiometry
aBMD (g/cm2) was measured at the lumbar spine anteroposterior (AP) (vertebra L1–L4) and lateral (L2–L4) projection, the left hip (total hip and femoral neck), and the non-dominant forearm (total radius) using the same device (Hologic Discovery A, Hologic Inc., Bedford, MA, USA) at baseline and follow-up. CV for repeated measurements was 0.3 % for AP lumbar spine, 1.3 % for lateral lumbar spine, 0.6 % for total hip, 0.8 % for femoral neck, and 3.1 % for total radius. The T-score (compared to the young normal mean) and Z-score (compared to the age- and sex-matched mean) reference values were provided by the DXA scanner manufacturer and were not available for lumbar lateral spine.
Radiography
Lateral spinal radiographs were obtained at baseline and follow-up for grading of AS-related spinal alterations with modified Stoke Ankylosing Spondylitis Spine Score (mSASSS). With mSASSS, anterior vertebral corners of the cervical and lumbar spine are graded between 0 and 3, and the total score ranges from 0 to 72 [
24]. The radiographs were also assessed for vertebral fractures with the semiquantitative method Genant score. Based on the percentage of height reduction, vertebrae T4-L4 were graded 0 (normal), 1 (mild, 20–25 % height reduction), 2 (moderate, >25–40 % height reduction), or 3 (severe, > 40 % height reduction) [
25]. All radiographs were assessed by the same radiologist with known chronological order but blinded to clinical data.
Statistics
Statistical analyses were performed using SPSS version 22 (IBM Corp., Armonk, NY, USA). Descriptive statistics are presented as numbers (percentage), mean (SD), or median (25th (Q1) to 75th (Q3) percentile). To compare continuous variables at baseline and follow-up, the paired t-test and the Wilcoxon signed rank test were used as appropriate. For categorical comparisons, McNemar’s test was used. A one-sided t-test was used to compare the Z-score in patients to the test value 0. Changes (∆-values) between baseline and follow-up were calculated. ∆-values in percent for vBMD, Ct.Ar, and aBMD were divided by time in months between examinations and multiplied by 60 to get a time-standardized value. Correlations were calculated using Spearman’s rank correlation coefficient (rs). To compare ∆-vBMD between groups, the Mann Whitney U-test was used. To analyze factors associated with ∆-vBMD and ∆-Ct.Ar, univariate and standard multivariable linear regression analyses were used. Variables with a univariate p-value ≤ 0.1 were considered for the multivariable models. All models were adjusted for age. Multicollinearity was checked for using variance inflation factor (VIF), and correlations between independent variables considered for the models were analyzed; high correlation was found for age and symptom duration, and age was kept in the models. High correlation was found for ∆-ASDAS and ∆-ESR. In this case and if a variable was part of another variable (ASDAS_CRP and BASDAI or ∆-ASDAS_CRP and ∆-CRP), the variable with the lowest univariate p-value was used. All models were initially adjusted for baseline HRpQCT measurement at the same site, and models with ∆-ASDAS_CRP were adjusted for baseline ASDAS_CRP. However, if that variable was not significant or did not affect the significance of the other independent variables, the variable was excluded. For ∆-cortical vBMD, more than five variables were eligible for the models. For cortical vBMD at radius, selection was based on lowest univariate p-value. For cortical vBMD at tibia, several eligible variables had similar univariate p-value, and some alternative models were tested. Variables were kept based on significance and contribution to the model with increases in the adjusted R2. Residual plots were assessed. There were very few missing data among the independent variables and none for the dependent variables. If missing data occurred, pairwise deletion was used. To test the robustness of the multivariable linear regression analyses, sensitivity analyses were performed; 10 % of the patients were randomly excluded, and then the multivariable regression analyses were repeated in the smaller sample size. Interactions and subgroups were not analyzed due to relatively small number of participants. We did not analyze factors associated with ∆-values for microarchitecture (TbN, TbTh, and TbSp) due to large CVs for these variables (presented in the method section “High-resolution peripheral computed tomography”), and since data from fewer patients were analyzed based on the higher quality of measurements needed for these variables. A p-value ≤ 0.05 was considered significant, and tests were two-tailed.
Discussion
We investigated 5-year changes in vBMD, cortical area, and microarchitecture at tibia and radius in men with AS and found mean decreases in cortical and trabecular vBMD exceeding LSC at tibia, whereas worsening of tibia trabecular microarchitecture did not exceed LSC. An increase in ASDAS_CRP and use of ≥ 450 mg prednisolone during follow-up were associated with decreases in cortical vBMD at tibia.
There are no previous longitudinal studies evaluating changes in HRpQCT measurements in patients with AS for comparison. Cross-sectional data are sparse but have shown patients with AS to differ from controls, especially in cortical bone. Haroon et al. found in their mixed gender AS cohort that AS was associated with lower cortical vBMD at tibia and radius, but not with trabecular vBMD or microarchitecture [
11]. Also, in men and women with nr-axSpA, cortical vBMD, cortical area, and cortical thickness were reduced compared to controls at radius (tibia was not measured) [
12]. In our baseline study on men with AS, patients had lower trabecular vBMD at tibia and lower cortical vBMD at radius than controls [
13], whereas Caparbo et al. reported decreased trabecular vBMD, trabecular thickness, and trabecular separation at tibia and no differences at radius in AS patients vs controls [
26].
We have no control group for longitudinal comparisons in this current study, but there are two longitudinal studies on general population assessing changes in HRpQCT measurements that we relate to for indirect comparison [
27,
28]. These studies showed similar results with decrease in cortical vBMD for men > 70 years old at tibia and radius in the Canadian study [
28] and at tibia in the Danish study [
27], whereas trabecular vBMD was stable over time for all age groups. For a more in-depth comparison, the Canadian study presents mean annual percent change in bone parameters for baseline age groups in decades. One has to bear in mind though that we report percent change over 5 years, that other factors that might affect vBMD such as weight, diet, and physical exercise can differ, and that they have a rather low number of participants in each age group. Our patients are mean 48 years at baseline. In the age-span 40–49 years in the Canadian study, mean annual increases in cortical vBMD were 0.1 % and in trabecular vBMD 0.5 % at tibia. Trabecular changes were significant and exceeded LSC [
28]. Our patients decreased significantly at tibia with mean 1.0 % in cortical vBMD and 2.7 % in trabecular vBMD over 5 years. At radius in the Canadian study, mean annual decrease in cortical vBMD was 0.1 %, whereas trabecular vBMD increased with 0.1 %. However, changes at radius were not significant, neither in this group nor in our group of patients [
28]. Thus, the decrease in cortical vBMD at tibia found in our study seems to start earlier in AS men than in the general population. Moreover, the decreases in trabecular vBMD found at tibia in AS men, and especially in younger men, were not seen in the general population and might be related to the AS disease. However, we could not find any disease-related factor to be associated with the decrease in trabecular vBMD in the regression analyses, and Riggs et al. studied longitudinal changes in vBMD at radius and tibia using QCT in the general population and found decreases of trabecular vBMD throughout life [
29]. The sole independent factor associated with changes in trabecular vBMD at tibia found in this present study was age. The level of explanation was low with a low adjusted R
2. In the sensitivity analysis, age was no longer significantly associated with the changes in vBMD, and it is difficult to draw any firm conclusions why the patients decrease in trabecular vBMD.
For cortical vBMD at tibia, regression analyses showed that an increase in ASDAS_CRP from baseline to follow-up had a negative impact on the cortical vBMD, which was also found for cortical area at tibia. At radius, an increase in ASDAS_CRP affected cortical vBMD negatively and showed a trend to affect cortical area negatively. Also, a high time-averaged ESR was associated with a decrease in cortical area at radius. There are limited data regarding factors affecting the different bone compartments in AS patients. The negative associations between inflammation and changes in cortical measurements are in line with previously reported predominantly cortical alterations found in AS and nr-axSpA [
11‐
13]. Further, Haroon et al found high ESR to correlate with lower cortical and trabecular tibia vBMD [
11]. Previous longitudinal studies on AS patients assessing changes in aBMD by our group and others have shown persistent inflammation measured by ESR [
30‐
32] or CRP [
30,
33] to be associated with decreases in aBMD at the femoral neck [
30‐
33] and lumbar spine [
30,
32].
A high baseline BMI was found to predict increases in cortical vBMD and cortical area at tibia. High BMI and weight have been positively associated with aBMD in multiple studies, especially at the hip and lumbar spine [
34]. That meta-analysis did not include radius. A study of elderly men and women found weight and BMI to affect aBMD at the weight-bearing sites femur and spine but not radius in men in line with our results [
35]. One potential mechanism for BMI and weight affecting BMD at weight-bearing sites is through mechanical loading [
36]. The association for BMI and HRpQCT measurements is less studied in men; however, one study found obese adults having higher cortical and trabecular vBMD as well as cortical area at both tibia and radius compared to normal weight adults [
37].
Concerning treatments, use of TNFi during ≥ 4 years was associated with increase in tibia cortical bone. Several longitudinal studies have shown treatment with TNFi in patients with AS to increase aBMD at the lumbar spine [
33,
38,
39], total hip [
38,
39], and femoral neck in line with our results [
40]. The underlying mechanisms for increases in BMD by TNFi are not fully elucidated, but we hypothesize that it is related to reduction in systemic inflammation. To our knowledge, the effect of TNFi on HRpQCT measurements has not been studied longitudinally in AS patients before.
A limited number of patients were exposed to bisphosphonates during follow-up. Nonetheless, such exposure was associated with increases in cortical vBMD and cortical area at radius. Data concerning the effect of bisphosphonates on BMD in AS are limited, and large randomized controlled studies on this matter are lacking. We have previously, in the total AS group, shown that exposure to bisphosphonates was associated with increases in aBMD at lumbar spine and hip [
33]. Effects of alendronate on HRpQCT measurements are reported for postmenopausal women and thus not directly comparable to our AS men. Based on these studies, one would expect an impact of bisphosphonates also on tibia measurements [
41,
42].
Corticosteroids have a negative impact on BMD and fractures [
43], and use of ≥ 450 mg prednisolone during follow-up had a negative effect on tibia cortical bone, coherent with lower trabecular vBMD in prednisolone-treated patients compared with prednisolone-naïve patients in the cross-sectional study on nr-axSpA [
12]. However, no firm conclusions of our results can be drawn based on only four patients with this medication.
Associations for disease-related variables and treatments with changes in bone parameters somewhat differ between radius and tibia in this study. The precision of the measurements at radius is lower than at tibia, shown in larger LSC. The greater measurement error for radius could possibly affect the results in this rather small sample size with few patients exposed to the treatments. Further, the explanatory level of the models is at best 54 % (for tibia cortical vBMD). With much of the variation unexplained, it is difficult to determine why the patients decrease in vBMD at tibia but not at radius.
The precision errors for the assessment of microarchitecture are by far exceeding the observed changes in trabecular separation, trabecular thickness, and trabecular number. This makes it difficult to draw firm conclusions on the differences found at radius and tibia.
This study is performed on men with longstanding AS, and whether results apply to women and patients with short symptom duration need to be elucidated.
Limitation with this study is the lack of a control group, although the measurements of the machine were stable over time. Another limitation is a relatively small number of patients, which hampers precision of change over time, and preclude interaction analyses and subgroup analyses of for example those exposed to different treatments. In addition, multiple statistical tests were performed in the study, and the increase in familywise error rate across the statistical analyses was not controlled, which increases the risk of false-positive results. We consider this study exploratory, and our results need to be confirmed in independent studies. We also lack data about physical activity during follow-up, a factor that could possibly influence measurements at tibia. However, we do have information about baseline occupation, and when patients were categorized into blue-collar workers (manual labor), white-collar workers (less physical activity), or no work, there were no associations between changes in vBMD or cortical area at tibia and type of occupation (data not shown). Strengths are the long follow-up time with well-characterized patients in this first longitudinal HRpQCT study in men with AS.
In conclusion, HRpQCT is not a method for use in clinical practice yet, however, to improve the knowledge about the complex mechanism of skeletal involvement in AS, information from HRpQCT measurements is valuable to further understand how different bone compartments are affected. We have here shown that over 5 years, AS men decreased in trabecular and cortical vBMD at tibia and that inflammation-related factors and medications were associated with changes in cortical bone. We could not find any significant change in vBMD at radius, and changes in microarchitecture did not exceed the precision of the method. No disease-related variables or treatments were found to be associated with changes in trabecular vBMD. The findings strengthen the importance of controlling disease activity in patients with AS to maintain bone density also in the peripheral skeleton.
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