Bone microarchitecture in ankylosing spondylitis and the association with bone mineral density, fractures, and syndesmophytes

In total, 69 male AS patients who had been included in a larger study on osteoporosis in the west of Sweden were randomized in an age-adjusted algorithm also to take part in the current study on HRpQCT and QCT. All AS patients registered at the Rheumatology Clinic at Sahlgrenska University Hospital in Gothenburg, and the Rheumatology Clinics at Borås and Alingsås county hospitals had been invited to take part in the initial study. The inclusion of the 204 patients in the initial study has been described in detail [13]. In summary, all included patients met the modified New York criteria for AS [21] .Exclusion criteria were psoriasis, inflammatory bowel disease, dementia, and difficulties in understanding Swedish. Written informed consent was obtained from all patients. The study was approved by the Regional Ethical Review Board in Gothenburg and carried out in accordance with the Helsinki declaration.

The HRpQCT results regarding trabecular and cortical vBMD of the AS patients were compared with the results of a control group consisting of 68 healthy individuals measured with the same type of XtremeCT at the Mayo Clinic in Rochester, Olmsted County, Minnesota, USA. The healthy controls were matched for age, height, weight, and race. No control could be found for the youngest patient, who thus was excluded from these calculations.

Bone microarchitecture was examined by using an HRpQCT device (XtremeCT; Scanco Medical AG, Brüttisellen, Switzerland) in the nondominant ultradistal radius and tibia. The patient’s forearm and leg were immobilized in especially designed carbon-fiber shells (Scanco Medical) to prevent movement during the procedure.

The quality of the measurements was assessed by using a 5-point scale recommended by the manufacturer (1, excellent; 2, good; 3, acceptable; 4, unacceptable; 5, poor). Only examinations with quality grades 1 through 3 were included in the study, whereas grades 4and 5 were excluded, mostly because of motion artefacts. Totally 12 examinations of the radius had to be repeated because of to movement artifacts. Seven patients had measurements of the ultradistal radius with unacceptable quality and were thus excluded, whereas all tibia measurements were included.

The volumes of interest (VOIs), 9-mm sections of radius and tibia, were examined in 110 parallel slices (voxel size, 82 μm), generating a 3D representation of the bone. The first CT slices started 9.5 mm and 22.5 mm proximal to a reference line manually placed at the center of the end plate of the distal radius and tibia, respectively, and continued proximally.

The VOIs were automatically separated into a trabecular and a cortical region. With previously described data-extracting procedures, the following parameters for the trabecular and cortical bone were obtained: trabecular volumetric BMD (DTrab; mg/cm³), trabecular bone volume/total volume (BV/TV; %), trabecular number (TbN; per mm), trabecular thickness (TbTh; μm), trabecular separation or spacing (TbSp; μm), cortical volumetric BMD (DCort; mg/cm³), cortical bone cross-sectional area (CortCSA; mm²), cortical periosteal circumference (CortPm; mm), and cortical thickness (CortTh; μm) [22‐26]. DTrab, DCort, TbN, CortCSA, and CortPm were measured directly, and the other parameters were derived.

The coefficients of variation (CVs) for repeated measurements by using the XtremeCT apparatus in Gothenburg ranged between 0.3% and 3.9% of the radius and from 0.1% to 1.6% of the tibia. The same device, software, and operator were used throughout the study.

The software Autocontouring and Eval Crtx 6× software, provided by Scanco Incorporated in the manufacturer’s Image Processing Language (IPL) software (μCT Evaluation Program v6; Scanco Medical AG, Brüttisellen, Switzerland) was used to assess cortical bone microstructure of the ultradistal radius and tibia. The cortical compartment in the VOI was detected automatically by identifying the endosteal and periosteal contours. All void voxels within the cortical compartment were identified, and the images were digitally superimposed, generating a refined cortical compartment region in the VOI. The Haversian canals were distinguished from artefacts because of surface roughness, transcortical foramens, or erosions. With this method, cortical porosity (CtPo; %) and mean cortical pore diameter (CtPoDiam; μm) were obtained [22, 23, 25]. The CVs for porosity were 15.9% at the radius and 5.5% at the tibia, and the CVs for mean cortical pore diameter were 6.0% at the radius and 3.9% at the tibia.

The healthy control group was assessed with the same type of XtremeCT at the Mayo Clinic in Rochester. A scan phantom for the vBMD measurements was sent from Gothenburg to Rochester for cross-calibration of the XtremeCT devices. Linear regression between the phantom measurements in Gothenburg and Rochester generated a formula

by which the values from Rochester were subsequently adjusted. The phantom was not designed for cross-calibration of microarchitectural parameters; hence those parameters could not be compared in the patients and controls.

Lumbar volumetric BMD was measured in the vertebrae L1 through L4 by using a QCT scanner (Siemens Somatom Sensation 16 with application Syngo Osteo CT; Siemens AG, Munich, Germany). Volumetric BMD (vBMD; mg/cm³) was assessed separately in the cortical and the trabecular bone in 10-mm-thick slices of each vertebra. All patients were scanned together with a water- and bone-equivalent calibration phantom placed below the patients along with an interpositioned gel pad to prevent artefacts and air gaps. The BMD results of the patients were compared with the reference population database of the CT scanner software, including 135 male and 139 female European subjects, 20 through 80 years of age.

BMD was measured with DXA (Hologic Discovery A; Hologic Inc., Bedford, MA, USA) in the nondominant forearm (total radius, radius 1/3) and hip (total hip, femoral neck) and in the lumbar spine (in AP L1 through L4 and lateral L2 through L4 projection) with estimation of lumbar vBMD.

Lateral radiographs of the spinal column were taken to study the presence of vertebral fractures in the thoracic and lumbar spine by using the Genant score, which scores vertebrae on visual inspection as normal, mildly, moderately, or severely deformed (grades 0 to 3) [27]. All vertebral fractures (Genant score, 1 through 3) were included in the calculations. The presence of chronic AS changes in the cervical and lumbar spine was assessed by using the modified Stoke Ankylosing Spondylitis Spine Score (mSASSS) [28] .The score grades the anterior vertebral corners with 0 through 3 points each (0, normal; 1, erosion, sclerosis, or squaring; 2, syndesmophyte; or 3, bridging syndesmophyte). The scoring scale ranges from 0 to 72.

Statistical analyses were performed by using PASW Statistics 18.0 (SPSS Inc., IBM, Chicago, IL, USA). Descriptive statistics are presented as median and range and/or mean and standard deviation (SD). The Mann–Whitney U test, t test, or the χ² test was used to compare variables as appropriate. Correlations were calculated by using the Spearman correlation (r_s). Logistic regressions with a forward conditional method were run with the presence of a syndesmophyte (yes/no) and vertebral fracture (yes/no) as the binary outcome. All tests were two-tailed, and P < 0.05 was considered statistically significant.

In total, 69 male AS patients were included in the study. The characteristics of the patients are presented in Table 1.

The AS patients had, in comparison with the controls, significantly lower vBMD in cortical bone of the ultradistal radius (P = 0.007) and in trabecular bone of the ultradistal tibia (P = 0.033) (Table 2). The AS patients also had lower weight and body mass index (BMI) than the controls, but the differences did not reach a level of significance. Weight and BMI were, however, not correlated with trabecular or cortical vBMD in the ultradistal radius or tibia. (Correlation coefficients ranged from -0.082 to 0.100, and P values ranged from 0.245 to 0.853).

Strong correlations were found between trabecular vBMD in the lumbar spine, ultradistal radius (r_S = 0.762; P < 0.001) and tibia (r_S = 0.712; P < 0.001), but the cortical vBMD of the spine and peripheral skeleton were not significantly correlated (Figure 1 and Table 3). Low-lumbar trabecular vBMD was also significantly correlated with parameters indicating poor bone microarchitecture, such as thinner trabeculae, lower trabecular number, thinner cortex, lower cortical vBMD, and increased cortical porosity (Figure 2 and Table 3). The results thus indicated a link between trabecular bone loss in the axial and peripheral skeleton in AS.

Trabecular and cortical vBMD were significantly correlated in the ultradistal radius (r_S = 0.466; P < 0.001) and tibia (r_S = 0.308; P < 0.010), but the correlation did not reach the level of significance (r_S = 0.230; P = 0.061) in the lumbar spine.

Vertebral fractures were diagnosed in eight (12%) patients, the youngest patient being 31, and the oldest, 71 years old. Twelve vertebral fractures with Genant score grade 1 and two with grade 2 were diagnosed in the study group. Age, disease duration, and mSASSS score were not significantly different between patients with or without a vertebral fracture.

Each patient with a vertebral fracture was compared with two age-matched AS controls from the same cohort (Table 4). The patients with a vertebral fracture had significantly lower cortical lumbar vBMD measured with QCT compared with the age-matched nonfractured patients. The following DXA parameters were also significantly lower in the patients with a vertebral fracture: AP and lateral lumbar BMD, lumbar vBMD, and BMD of the hip (femoral neck and total hip).

No significant difference in BMD was found in the forearm measured by DXA between the groups. When measured with HRpQCT, the patients with a vertebral fracture, however, displayed significantly lower trabecular and cortical vBMD in the ultradistal radius and, in addition, lower trabecular thickness, cortical thickness, and cross-sectional area in both the ultradistal radius and tibia, thus indicating deteriorated peripheral bone microarchitecture in the fractured patients in comparison with the age-matched AS controls. The greatest differences between the fractured and nonfractured patients were found in cortical thickness and cortical cross-sectional area (Table 4).

Multiple logistic regression was run with vertebral fracture as binary outcome. In the first model, in which age, mSASSS, and trabecular and cortical lumbar vBMD were entered as covariates, decreasing cortical lumbar vBMD (B = -0.023; P = 0.015; OR, 0.977; 95% CI, 0.959 to 0.996), and increasing mSASSS (B = 0.042; P = 0.049; OR, 1.04; 95% CI, 1.0003 to 1.087) were independently associated with the presence of a vertebral fracture.

Increasing mSASSS correlated significantly with increasing age (r_S = 0.546; P < 0.001) and decreasing trabecular vBMD in the lumbar spine (r_S = -0.620; P < 0.001). In addition, mSASSS correlated with increasing cortical porosity and decreasing trabecular thickness and vBMD of trabecular and cortical bone in the periphery (Table 3).

In 39 patients, radiographs of the cervical and lumbar spines revealed at least one syndesmophyte or bridging syndesmophyte, whereas nine patients had a disease restricted to the sacroiliac joints (mSASSS = 0), and 21 patients had only the presence of erosions, sclerosis, or squaring at the vertebral corners.

The patients with at least one syndesmophyte had, in comparison with patients without syndesmophytes, significantly older age (54 ± 12 versus 42 ± 15 years; P = 0.001), thinner trabeculae in the ultradistal radius (0.071 ± 0.016 versus 0.085 ± 0.014, P < 0.001) and tibia (0.072 ± 0.013 versus 0.081 ± 0.008; P = 0.001) and lower trabecular vBMD in the lumbar spine (100 ± 36 versus146 ± 33; P < 0.001), ultradistal radius (171 ± 42 versus 196 ± 29; P = 0.013) and tibia (175 ± 37 versus 205 ± 27; P < 0.001).

In a multiple logistic regression model with the presence of at least one syndesmophyte as the binary outcome and adjusting for age, decreasing lumbar trabecular vBMD (B = -0.058; P < 0.001; OR = 0.943; 95% CI, 0.917 to 0.970), but increasing lumbar cortical vBMD (B = 0.019; P = 0.016; OR, 1.019; 95% CI, 1.004 to 1.035) remained independently associated with syndesmophyte formation. None of the HRpQCT parameters was significantly associated with presence of syndesmophytes after adjusting for age in logistic regression. Covariates in the regression model were age, trabecular and cortical lumbar vBMD, and the HRpQCT parameters, which were directly measured and not derived (DTrab, DCort, TbN, CortCSA, and CortPm).

QCT revealed significantly more cases with T score ≤2.5 SD (n = 26; 38%) and T score ≤1.0 SD (n = 21; 30%) than AP DXA, showing considerably less osteoporosis (n = 4; 6%) and osteopenia (n = 13; 19%; P < 0.001). No reference database values were available for lateral lumbar DXA and DXA vBMD lumbar spine.

Correlation analyses between lumbar QCT and lumbar DXA demonstrated that QCT trabecular vBMD had the strongest correlation with DXA vBMD (r_S = 0.636; P < 0.001) followed by lateral BMD (r_S = 0.537; P < 0.001) and AP BMD (r_S = 0.380; P = 0.002). QCT cortical vBMD correlated with DXA in the following way: lateral BMD (r_S = 0.595; P < 0.001), AP BMD (r_S = 0.541; P = 0.002) and vBMD (r_S = 0.431; P < 0.001).