PATHOPHYSIOLOGY
The characteristic histopathological feature of SpA is enthesitis,
which is given by the pathological inflammation of the involved entheses,
which are the sites of insertion of tendons,
ligaments,
fascias and capsules to the bone; as well as the attachment of cartilage to the subchondral bone.
Three processes are observed at the entheses: inflammation,
bone erosion and syndesmophyte (spur) formation (Fig.1).
Each porcess is controlled by diferent mediators including CD4 and CD8 T lymphocytes,
macrophages, cytokines,
particularly tumor necrosis factor-α (TNF-α) and transforming growth factor-β (TGF-β) leading to fibrosis and ossification at sites of enthesitis.
Sacroiliac joint is rich in entheses and is subject to significant tensile forces,
not necessarily pathological so it is usually the first place of involvement. Same manifestations take place in spine with the particularity that there are numerous entheses that can be affected.
This process in spine occurs at the junction of the vertebrae and the anulus fibrosus of the intervertebral discs.
The outer fibers of the discs eventually undergo ossification to form syndesmophytes.
The condition progresses to the characteristic bamboo spine appearance.
The most commonly affected region is between the middle part of the dorsal spine (T7-T8) and the middle part of the lumbar (L2-L3).
This information is important when designing study protocols.
On the basis of this evidence,
Baraliakos et al suggest that at least the lower part of the thoracic spine and the lumbar spine should be included in MRI examinations in patients with active AS (6).
IMAGING TECHNIQUES
The first imaging method is still conventional radiography despite its known low sensitivity for early arthritis. Radiographic changes usually are not evident until symptoms have been present for 5-7years. Typical radiographic features are bone erosions,
new bone formation and ankylosis.
Osteoporosis is also a prominent feature.
Magnetic resonance imaging is useful in early detection of disease,
when radiographs are normal.
Furthermore,
MRI shows abnormalities in patients with clinical disease in whom laboratory markers such and CRP and ESR may be normal.
MRI technique is well established,
and sequences capable of detecting both components of the process are practiced: inflammatory activity and structural damage.
Stir-enhanced images are preferred for detecting inflammation activity,
since it is a fast,
sensitive and robust technique,
little influenced by artifacts or alterations of the magnetic field.
It usually affects subchondral bone rather than cartilage and synovial,
and includes edema,
cellular infiltration and hyperemia
Structural damage that occurs in response to chronic inflammatory involvement is better assessed in T1 sequences (Fig.5).
Protocols should include sagital sequences and be sure to expand enough FOV to include costotransverse joints since more than 40% of the inflammatory affectation at the dorsal level occurs in these elements.
More than 5 lesions are highly suggestive of Spa with a specificity of 98%
IMAGING FINDINGS
Distribution pattern includes inflammatory lesions of vertebral margins,
both anterior and posterior,
central inflammatory lesions and infallmatory lesions on lateral and posterior elements.
- The acute inflammatory lesion is a circumscribed triangular-shaped area of corner edema within the vertebral body demonstrated only in MRI corresponding to enthesitis (Fig.2).They appear in both the anterior and posterior margins and must appear in a number equal to or greater than 3 to be considered positive.
At an advanced stage this lesions lead to sclerosis (shiny corner sign) and erosion (Romanus lesion) that are well depicted in conventional radiographs (Fig.3).
Romanus lesion term should only be used in radiography.
This erosions give rise to a squared appearance of the vertebral body.
- The characteristic feature of SpA are syndesmophytes wich are vertically oriented osseous outgrowths along the vertebral body margins in the exact position of annulus fibrosus running parallel to spine (Fig.4).
When multiple levels are affected they give a typical appearance of bamboo spine.
- Erosions and destructions of end plates are common in later stages and they are mild and remain stable for months and years.
Intervertebral space is usually preserve (Fig.10).
- In the thoracic spine,
involvement of lateral and posterior elements is characteristic of SpA affecting the interapophyseal,
costovertebral and costotransverse joints (Fig.
6,7).
It is manifested by subchondral bone marrow and soft tissues edema.
Although posterior element lesions were less frequent than vertebral body enthesitis they are highly specific for SpA.
DIFERENTIAL DIAGNOSES
- Diffuse idiopathic skeletal hyperostosis (DISH) manifests thick flowing ossifications in contrast to the thin vertical syndesmophytes of AS.
Apophyseal joints are not ankylosed and sacroiliac joints are spared.
- In Spondylodiscitis end plate erosions have a rapid progression and the presence of a paraspinal or epidural collection should also orient the diagnosis toward an infectious process.
- Osteochondrosis / disc degeneration.
Modic changes can mimick Spa however the lack of corner lesions,
sacroiliac joint involvement or posterior element enthesitis distinguish it from SpA.