Table 1 Characteristics of the study population, both overall and stratified by the presence or absence of cervical facet joint ankylosis during any period of follow-up

Baseline parameters

Age, mean ± SD years

1106

31.4 ± 9.4

34.8 ± 8.2

30.5 ± 9.5

< 0.001

Male sex, n (%)

1106

976 (88.2%)

220 (95.7%)

756 (86.3%)

< 0.001

Symptom duration, years

892

7.0 (3.0–13.0)

11.5 (6.0–17.0)

6.0 (2.0–12.0)

< 0.001

HLA-B27 positivity, n (%)

1101

1062 (96.5%)

225 (97.8%)

837 (96.1%)

0.288

ESR, mm/h

865

22.0 (7.0–46.0)

40.0 (19.0–60.0)

18.0 (5.0–41.5)

< 0.001

CRP, mg/dL

705

1.2 (0.8–2.7)

1.9 (1.3–3.3)

1.0 (0.8–2.3)

< 0.001

BASDAI (0–10), mean ± SD

263

5.1 ± 2.7

5.5 ± 2.6

5.0 ± 2.7

0.162

Sacroiliitis grade (0–4)^a, mean ± SD

1052

2.8 ± 0.9

3.5 ± 0.6

2.6 ± 0.9

< 0.001

Hip involvement^b, n (%)

1084

110 (10.1%)

41 (18.6%)

26 (3.1%)

< 0.001

Cervical facet joint score (0–15)

1106

0.0 (0.0–0.0)

6.0 (2.0–13.0)

0.0 (0.0–0.0)

< 0.001

Cervical mSASSS (0–36)

1106

6.0 (4.0–8.7)

14.4 (7.6–28.0)

5.5 (3.9–7.1)

< 0.001

Total mSASSS (0–72)

1106

7.5 (5.0–15.8)

27.0 (11.8–51.8)

6.7 (5.0–10.0)

< 0.001

≥ 1 bridging syndesmophyte at baseline, n (%)

1106

167 (15.1%)

110 (47.8%)

11 (1.3%)

< 0.001

Longitudinal parameters

Duration of follow-up, mean ± SD years

1106

8.3 ± 2.8

9.0 ± 2.9

8.1 ± 2.8

< 0.001

No. of spinal radiographs, mean ± SD

1106

4.5 ± 1.2

4.8 ± 1.3

4.4 ± 1.2

< 0.001

Smoking ever, n (%)

1052

639 (60.7%)

159 (72.3%)

480 (57.7%)

< 0.001

Uveitis ever, n (%)

930

352 (37.8%)

98 (47.1%)

254 (35.2%)

0.002

Peripheral arthritis ever, n (%)

920

391 (42.5%)

68 (33.0%)

323 (45.2%)

0.002

Use of TNF inhibitors ever, n (%)

1106

580 (52.4%)

153 (66.5%)

427 (48.7%)

< 0.001

≥ 1 bridging syndesmophyte during any period of follow-up, n (%)

1106

299 (27.0%)

186 (80.9%)

34 (3.9%)

< 0.001