LAUSR

Structural damage in axial SpA usually refers to the sacro-iliac (SI) joints or the spine. The classic order of the progress of structural damage is in the SI joints, followed by the spine. Consequently, in early disease assessment of structural damage should be focused on the SI joints and in established disease (i.e. in patients with radiographic sacroillitis) evaluation of the spine becomes more useful. The most striking abnormality representing structural damage in axial SpA is bone formation, although in early phases bone destruction can also be seen. Typically, structural damage is assessed on radiographs. For radiographs there are two frequently used scoring methods: grading of the SI joints according to the modified New York (mNY) criteria and assessment of the spine by the modified Stoke Ankylosing Spondylitis Score (mSASSS). The mNY grades range from 0–4 per SI joint, 0–8 in total. Although it is well known that reliability of mNY grading is poor, this continuous grading has recently been successfully used as an outcome measure. The mSASSS is mainly based on bone formation in the anterior vertebral corners in the cervical and lumbar spine (range 0–72). It is a reliable measure but the progression observed by mSASSS is rather modest, resulting in a minimum follow-up of two years. Imaging of structural damage by MRI is another option. Due to the imaging of the whole spine and the three-dimensional technique this seems an attractive alternative. However, especially bone formation is very hard to assess. Fatty deposition is a typical MRI abnormality, which can be seen as an intermediate step between inflammation visible on MRI and bone formation on radiographs. However, this finding is insufficiently validated to be able to consider it as a true surrogate for structural damage. Finally, a CT scan is a method having the advantages of MRI (whole spine, tomographic technique) but also the best capabilities of bone imaging. The big disadvantage of CT scan is the radiation dose. Recently, it became possible to make images with good image quality but acceptable radiation, the so-called low-dose CT. A validated scoring system for CT exists: the CT scoring system (CTSS). Only bone proliferation is assessed in this method. In a direct comparison with mSASSS more progression was observed over a 2 year period. This was mainly due to the progression observed in the thoracic spine. Only structural damage assessed by the mSASSS has shown a clear relationship with outcomes that are important for patients such as function and quality of life. No data on MRI or CTSS exist. However, it is hard to describe what defines a clinically relevant progression. The interval to assess progression to assess a treatment effect is typically 2 years. Only small changes can be assessed over this period, normally about 1–2 mSASSS units. It is hard to define that this is clinically relevant. The most important for treatment is to show that there is inhibition of structural progression in comparison to untreated patients, especially as axial SpA is a lifelong disease and 1 unit over 30 years still leads to severe ankylosis of the spine. Disclosure of Interest None declared