Imaging of Musculoskeletal Disorders Spondyloarthropathy
Key features
Sacroiliitis
Romanus lesions
Syndesmophytes
Ankylosis
Ankylosing spondylitis has characteristic radiological features in a symmetric distribution
The other spondyloarthropathies have similar features but are usually more asymmetric
The seronegative spondyloarthropathies most commonly affect the sacroiliac joints and the spine (seronegative = negative for rheumatoid factor and other autoimmune antibodies).
Ankylosing spondylitis is the most common seronegative spondyloarthropathy. It has characteristic radiological features. Similar radiological features may be seen in patients with other spondyloarthopathies such as psoriatic arthritis, reactive arthritis (formerly known as 'Reiter’s disease'), or enteropathic spondyloarthropathy (spondyloarthropathy in patients with inflammatory bowel disease). Ankylosing spondylitis tends to be symmetrical whereas the other spondyloarthropathies tend to be more asymmetric.
All these diseases are characterised by inflammation of the entheses; the point at which ligaments and tendons connect to bone. The axial skeleton is most commonly affected, but the small and large joints of the appendicular skeleton may also be affected.
Sacroiliitis
Ankylosing spondylitis most commonly affects the sacroiliac joints first. Early changes of sacroiliitis (inflammation of the sacroiliac joints) are not visible on plain X-rays and so MRI is frequently employed in the early diagnosis of seronegative spondyloarthropathies.
These MRI images show decreased fat signal (T1 image) and increased fluid signal (STIR image) due to bone oedema adjacent to the sacroiliac joints bilaterally
These are typical features of an active sacroiliitis
MRI evidence of sacroiliitis supports the diagnosis of ankylosing spondylitis if this diagnosis is suspected clinically
The plain X-rays were completely normal in this patient
On the lower image the sacroiliac joints are not visible due to ankylosis (joint fusion)
Spondyloarthropathy of the spine
'Romanus lesions’ – which correspond to enthesitis at the insertion points of the longitudinal spinal ligaments – are the earliest sign of spondyloarthropathy affecting the spine. They can be detected with MRI much earlier than with X-ray.
On MRI Romanus lesions manifest as foci of bone oedema at the corners of the vertebral bodies. Over time the overlying cortical bone surface becomes sclerotic and so are visible as ‘shiny corners’ on plain X-ray images.
A T2 weighted (water sensitive) MRI image is shown next to the plain X-ray
The plain X-ray shows sclerosis at the corners of two vertebral bodies
These ‘shiny corners’ are chronic Romanus lesions – they are not clearly visible on the MRI as sclerosis appears black on all types of MRI images
The MRI image shows multiple small foci of high signal (fluid) in the bone marrow of the adjacent vertebral body corners (arrowheads)
This fluid represents bone marrow oedema caused by enthesitis at the point of insertion of the longitudinal spinal ligaments
Several other foci of bone oedema seen at the corners of other vertebral body corners are due to developing Romanus lesions which are not yet visible on the plain X-ray
Ankylosis
Ankylosis (fusion of bones at a joint) is a late manifestation of ankylosing spondylitis.
Chronic inflammation at the entheses of the spine – the point of attachment of the ligaments of the spine on the vertebral bodies – results in formation of syndesmophytes. These have a different appearance from that of osteophytes seen in osteoarthritis; syndesmophytes form a smooth layer of calcification, whereas osteophytes are sharp bone spurs which stick out from their point of origin.
Syndesmophytes are sometimes referred to as ‘flowing’ as they flow smoothly across the surface of the vertebral bodies affected.
Flowing syndesmophytes are seen fusing the cervical spine vertebral bodies anteriorly leading to the classic ‘bamboo spine’ sign – the fused spine resembles bamboo
In this patient the facet joints of the spine have also fused
Large and small joint involvement
Large and small joints may also be involved in patients with seronegative spondyloarthropathies.
Severe erosions may be seen in joints of the fingers and toes such that joints can be destroyed resulting in severe deformity
The severe joint erosion in this patient’s finger progressed over a period of 4 years – erosion is seen in both the middle phalanx (MP) and the distal phalanx (DP) of the finger
The middle phalanx is narrowed, like a pencil, and the distal phalanx is eroded centrally, like a cup, hence 'pencil-in-cup' deformity
Note: Similar erosion may occur in a septic arthritis but progresses more rapidly
