Patients
This was a cross-sectional study of patients diagnosed with AS and treated at the rheumatology department of Beijing Jishuitan Hospital between January 2013 and December 2019. The inclusion criteria were: 1) >18 years of age; 2) met the modified New York criteria (1984) for AS classification; and 3) duration of disease ≥1 year. The exclusion criteria were: 1) incomplete data; 2) bone tumor, bone metastasis, or hematological cancer; or 3) other rheumatic diseases, such as rheumatoid arthritis, gouty arthritis, or infectious arthritis.
The study was approved by the Ethics Committee of Beijing Jishuitan Hospital (No: 202003-13). The need for individual consent was waived by the committee because of the retrospective nature of the study.
Data collection
The variables included sex, age, age at onset, duration of disease, smoking history, family history, BMI (classified into <18.5 kg/m2 and ≥18.5 kg/m2) [7], Schober's test, peripheral arthritis (physician found swelling or tenderness in peripheral joints or joint effusion and synovitis on imaging examination), and iritis (diagnosed by an ophthalmologist). The BASDAI and BASFI scores of all patients were recorded [8,9]. The erythrocyte sedimentation rate (ESR) was recorded (Italian ALIFAX Test-1 automatic rapid erythrocyte sedimentation rate analyzer; normal reference range, 0-20 mm/h), as well as C-reactive protein (CRP) (Beckman IMAGE800 analyzer and matching kit, immunoturbidimetry; normal reference range, 0-8 mg/L), HLA-B27 status, and bone mass. Bone mass was determined at the calcaneus using an Ultrasonic bone intensity meter (GE Healthcare, Waukesha, WI, USA). A Z-value ≤-2 was defined as bone mass lower than that of peers. The drugs (anti-TNF, non-steroid anti-inflammatory drugs (NSAIDs), sulfasalazine, methotrexate, thalidomide, and glucocorticoids) were recorded. All indicators were collected by the specialists at the department of rheumatology and immunology.
Outcomes
The outcome index was the radiographic hip joint involvement assessed by the BASRI-hip score [10]. The BASRI-hip score ranges from 0 to 4 points: 0) normal, i.e., no radiological hip joint damage; 1) suspected hip joint damage, i.e., limited joint space stenosis; 2) mild hip joint damage, i.e., with an obvious hip joint lesion, but the hip joint space >2 mm; 3) moderate hip joint damage, with a definite hip joint lesion, hip joint space ≤2 mm and articular bony interface ≤2 cm; and 4) severe hip joint damage, i.e., with hip joint fusion or articular bony interface ≥2 cm, or indication for total hip replacement. A BASRI-hip score ≥2 was defined as radiological hip joint involvement.
Statistical analysis
SPSS 22.0 (IBM, Armonk, NY, USA) was used for all analyses. Continuous data were tested with the Kolmogorov-Smirnov test for normal distribution. Normally distributed continuous data are expressed as means ± SD, and non-normally distributed continuous data are expressed as medians (Q1, Q3). Categorical variables were presented as frequencies. Normally distributed continuous data were tested using the Student t-test, while non-normally distributed continuous data were analyzed by the Mann-Whitney U test. Univariable and multivariable (forward (LR) method) logistic regression analyses were performed to analyze the factors associated with radiological hip joint involvement. The variables with P<0.1 in the univariable analyses were included in the multivariable analysis. Two-sided P-values <0.05 were considered statistically significant.