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AS, General aspects
AS, Sacroiliitis - radiography
AS, Sacroiliitis - MRI
AS, Spine - radiography
AS, Spine - MRI
AS, Spinal fractures
AS, Other joints
Other forms of SpA
Psoriatic arthritis
Reactive arthritis
Enteropathic arthritis
Undifferentiated SpA
PAO / SAPHO / CNO
Juvenile spondyloarthritis
General aspects

Psoriatic arthritis is a chronic inflammatory rheumatic disease affecting patients with the dermal disease psoriasis.

Up to 20-25% of patients with psoriasis have psoriatic arthritis (PsA) with the highest frequencies occurring in patients with moderate to severe psoriasis, doi:10.1016/J.JAAD.2018.06.027. Involvement of the axial skeleton (sacroiliac joints and spine) is a relatively frequent manifestation, mostly together with peripheral arthritis, enthesitis and/or dactylitis.

Some features typical for axial involvement in PsA are compared with axial SpA without psoriasis somewhat different such as lower prevalence of inflammatory back pain and HLA-B27, and the occurrence of isolated involvement of the spine without concomitant SIJ changes in up to 30% of the patients with axial PsA, doi: 10.1136/ANNRHEUMDIS-2016-209853. The morphology of the syndesmophytes is often different from ypical syndesmophytes in AS, being asymmetrical, non-marginal and more voluminous/”chunky” doi: 10.31138/MJR.33.1.142. However, a clear distinction between axial PsA and axSpA without psoriasis is not always possible due to a natural overlap between these conditions.

According to the ASAS criteria, patients with axial PsA can be classified as patients with axSpA if fulfilling the ASAS criteria, presenting chronic back pain with onset prior to the age of 45 years plus presence of sacroiliitis by MRI or radiography and one additional SpA feature that can be psoriasis, or alternatively having HLA-B27 plus 2 additional SpA features. However, PsA patients with axial involvement may have characteristics that do not allow an axSpA classification such as late onset of back pain, involvement of the spine without SIJ changes and lack of HLA-B27, doi:10.1136/ARD.2008.104018.

There are no widely accepted criteria for axial involvement in PsA. Therefore, clinical and imaging manifestations indicative of axial PsA are currently the subject of a large international study including imaging encompassing radiography and MRI of the SIJ and spine, doi:10.1177/1759720X211057975.

Because symptoms and signs of axial involvement often occur at older ages than AS, the imaging features can be mixed with degenerative and/or load-related changes.
 
Sacroiliac joints - radiography

The changes may be asymmetrical and more limited than seen in AS, but there can be manifest structural changes with concomitant subchondral edema and fat deposition as in AS. However, the changes can also be minimal and not detectable by radiography and by MRI confined to edematous changes in erosion or limited areas of subchondral edema without a subsequent development of manifest homogenous subchondral fat metaplasia.

Radiograph showing manifest right-sided sacroiliitis in the form of erosion and joint space widening (arrow) with concomitant subchondral sclerosis, especially in the iliac bone.
 
Spine - radiography

Radiographic spinal changes are ofteny characterized by more voluminous paravertebral ossification than seen in AS being located to the paravertebral ligaments, mainly the anterior longitudinal ligament. The ossifications are usually named non-marginal syndesmophytes or para-syndesmophytes.
 

Lateral radiograph of the cervical spine and frontal radiograph of the lumbar spine in a patient with psoriatic arthritis. There are manifest characteristic paravertebral ossifications (para-syndesmophytes - arrows). For comparison the slim syndesmophytes characteristic of AS are shown beneath, marked with open arrows.
 
Sacroiliac joints and spine - MRI

By MRI active spinal changes may present as edematous changes corresponding to the paravertebral ossifications and their attachment in the vertebral bone, often with additional osseous edema anteriorly in the vertebral bodies beneath the anterior longitudinal as shown in a typical PsA patient beneath complaining of low back pain and primarily referred to radiographic examination.

Anterior and lateral radiograph of the lumbar spine with enlargement of the L1-L4 region to the right show fluffy new bone formation anteriorly attached below the vertebral plate at the attachment site for the anterior longitudinal ligament, most pronounced at L4 (arrows). The anterior radiograph showed signs of right-sided sacroiliitis (open arrow).

Supplementary MRI, sagittal T1 image at the right side of the spine and two sagittal STIR images, shows osseous edema at the attachment of the longitudinal ligament at the Th11-L2 region (arrows) with concomitant edema anteriorly in the vertebral bodies beneath the ligament (open arrows). The new bone formation was difficult to detect, but could be seen at the Th11-L2 region marked with arrows on the T1 image.

Concomitant MRI of the SIJ shows manifest active right-sided sacroiliitis with additional erosion of the iliac joint facet.
 

The axial changes in PsA can occasionally present as predominant osseous inflammation, which especially occur in patients with pustular psoriasis. The inflammation can result in osseous sclerosis and may (as shown below) be accompanied sacroiliitis as well as osseous inflammation in the anterior chest wall making a link to CNO lesions.

MRI of the spine in a young patient with pustular psoriasis and back pain in addition to recurrent pain in the sterno-costo-clavicular region, sagittal T1 and STIR image showing rather diffuse edema in the 5th lumbar vertebra (arrow) with a signal void area superiorly (open arrow) consistent with osseous sclerosis. There is also BME posteriorly in the 5th thoracic vertebra.

Concomitant MRI of the SIJ, semi-coronal T1 and semi-axial STIR, shows manifest active left-sided sacroiliitis with BME in the sacrum (arrow) with concomitant pronounced subchondral fat deposition (open arrow) and subchondral sclerosis in the iliac bone.

CT of the anterior chest wall, frontal and sagittal reconstruction, showed erosion of the manubriosternal joint with surrounding pronounced sclerosis (arrows) in addition to sclerosis of the left clavicular bone (open arrow) with an irregular joint facet, but no changes at the sternal side of the sternoclavicular joint.
 
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