The aim of this study is to define structural damage to the manubriosternal joint (MST) in axial spondyloarthritis (axSpA) by comparing its CT scan aspects between a population of patients with radiographic (axSpA) and a control population free of chronic inflammatory rheumatism.

Axial spondyloarthritis (axSpA) is, together with rheumatoid arthritis, one of the 2 major chronic inflammatory rheumatic diseases. While the lumbar spine and sacroiliac joints are at the forefront of the clinic and diagnosis of axSpA, clinical involvement of the rib cage is common and long-standing. It can lead to limitation of thoracic extension and respiratory consequences. Clinical involvement of the anterior chest wall was evaluated in the early 2010s in axSpA, especially in the newer forms.

Several anatomical structures of the thoracic cage can be the cause of pain and among these is the manubriosternal joint (MST). It is known to be a possible site of inflammatory involvement in axSpA and this is well demonstrated by MRI imaging. Bone CT scans allow precise analysis of changes in the MST joint, but unlike MRI, they do not analyse inflammation, which is itself the cause of structural changes. While inflammation is the main cause of pain, structural changes, particularly ankylosis, are the cause of potentially disabling sequelae. To date, there have been no studies of structural damage to the MST joint in patients with axSpA, including the radiographic form. Moreover, little is known about the CT scan data of the MST joint in healthy subjects. Apart from the interest in defining axSpA-related abnormalities for CT scan analysis of the MST joint in general, it is important to know the frequency and types of structural abnormalities of the MST joint in axSpA, not for early diagnosis, but in terms of assessing the severity of the disease.

The investigator's hypothesis is that there are structural abnormalities of the STD joint in the radiographic axSpA, the characteristics, prevalence and relationship to patient and disease characteristics of which must be determined. This should be analysed in the non-radiographic axSpA. This requires comparison with a control population, but as there are no validated criteria for assessing the pathological nature of the STD joint on CT, it is necessary to proceed in 2 steps:

1. - To define the structural damage to the MST joint in radiographic axSpA by comparison with control subjects. This comparison should focus first on the various basic abnormalities known to be present in radiographic axSpA, and in particular in the sacroiliac joints, such as erosions, subchondral bone condensation, pinching of the joint space, and ankylosis, which should be compared to the fusion phenomena described in controls.
2. - Study the characteristics of this structural damage in radiographic and non-radiographic axSpA.

Inclusion Criteria :

axSpA population:

• Adults aged 18 to 69 years,
• Diagnosis of axSpA according to the ASAS 2009 criteria,
• Hospitalized in the Rheumatology Department of the NOVO Hospital (Pontoise site) from 01/01/2015 to 30/10/2022 and had a Chest CT scan with images available in the hospital's computer system,
• Informed of the study and did not object.

Control population:

• Adults aged 18 to 69 years,
• Day hospitalization in the pneumology department of the NOVO Hospital (Pontoise site) from 01/01/2015 to 30/10/2022 and having had a lung scan on this occasion,
• Informed of the study and did not object.

Exclusion Criteria :

axSpA population:

• Peripheral spondyloarthritis,
• Unavailability of chest CT scan images in the hospital computer system,
• Unknown result of pelvic X-ray.

Control population:

• Lung cancer or suspected lung cancer
• Any of the following conditions: spondyloarthritis, rheumatoid arthritis, psoriatic arthritis, SAPHO syndrome, chest wall bone disease, tumour bone disease, severe scoliosis, malformative syndrome
• History of thoracic trauma or thoracic surgery,
• Unavailability of chest CT scan images in the hospital computer system.

Patients having a Chest CT scan (as part of a conventional hospitalization or Day hospitalization) between 01/01/2015 and 30/10/2022 for a population of patients with radiographic axSpA and a control population free of chronic inflammatory rheumatism