Although clinicians recognize hip involvement, which frequently leads to hip replacement surgery, as an important feature of AS, data on the epidemiology, nature of the disease and therapeutic strategies are scarce. We aimed to describe the epidemiology of clinical and radiological hip involvement and define the risk factors for the hip replacement surgery in AS patients. Data from 3 datasets were merged, including 847 Belgian (ASPECT database), 1405 Spanish (REGISPONSER database) and 466 Ibero-American (RESPONDIA database) AS patients. The ASPECT and REGISPONSER database (Dataset A) are used for exploratory analysis; the RESPONDIA database (Dataset B) is used for confirmative analysis. Factors associated with hip involvement and the hip replacement surgery were analysed. Twenty four (REGISPONSER) to 36% (RESPONDIA) of AS patients under rheumatologist's care presented clinical hip involvement, including the 5% (Dataset A) of AS patients who needed hip replacement surgery. Patients with hip involvement had significantly worse overall Bath Ankylosing Spondylitis Functional Index (BASFI) scores compared with patients without hip involvement (mean difference = 1.6, P < 0.001) (Dataset A, confirmed in B). Corrected for disease duration, patients with early disease onset, enthesial and axial disease needed most frequently hip replacement surgery (Dataset A, confirmed in B). Hip involvement is commonly recognized by rheumatologists in AS patients, and involves about one out of the three to four patients with AS and is associated with impaired functioning reflected by higher overall BASFI scores. Early onset of disease, axial and enthesial disease are associated with the hip replacement surgery in AS.
