This document summarizes pathologies of the sacroiliac (SI) joint that can be identified on MRI. It describes the anatomy of the SI joint and classifies diseases as spondyloarthropathies. Signs of active inflammation on MRI include bone marrow edema, synovitis, capsulitis, and enthesitis. Chronic changes include subchondral sclerosis, erosions, and ankylosis. Differential diagnoses discussed are infective sacroiliitis, osteoarthritis, stress reactions, insufficiency fractures, and osteitis condensans ilii. Images are provided to illustrate the radiological findings of various SI joint pathologies.

1. SI JOINT PATHOLOGIES

2. ANAT
T1 T2

4. 1 = Gluteus maximus
2 = Gluteus medius
3 = Gluteus minimus
4 = Iliacus
5 = Psoas
6 = Piriformis
7 = Common iliac artery
8 = Common iliac vein
9 = Inferior gluteal artery and vein
C = Coccyx
I = Ilium
L = L5 vertebral body
M = Sacral lateral mass
S = Sacrum with four sacral elements
and four neural foramina with
visualization of the nerve roots S1–S4

5. Diseases classified as
spondyloarthropathies
• 1.Ankylosing spondylitis
• 2.Arthritis associated with inflammatory
bowel diseases (ulcerative colitis – UC,
Crohn’s disease – CD)
• 3.Reactive arthritis
• 4.Undifferentiated spondyloarthropathies
• 5.Psoriatic arthritis
• 6.Juvenile spondyloarthropathies

8. Radiological classification of sacroilitis
according to New York criteria
• Grade 0:- No changes/sacroiliac joint normal
• Grade 1:-Suspected changes (blurry margins of
sacroiliac joint gap)
• Grade 2:-Minimal changes (single erosions and
periarticular sclerosis involving small area of iliac
or sacral bone)
• Grade 3:-Advanced changes (distinct periarticular
sclerosis, numerous erosions with widening of
articular gap, possible partial ankylosis)
• Grade 4:-Total ankylosis

9. Grade 1,2,3,4

10. Two types of inflammatory changes of sacroiliac
joint in MR examination according to ASAS
• Active inflammatory
lesions1. Bone
marrow edema
• 2. Capsulitis
• 3. Synovitis
• 4. Inflammation of
tendon, ligament
and capsule
attachments.
Chronic
inflammatory lesions
1. Subchondral
sclerosis
2. Erosions
3. Fatty bone marrow
transformation
Bony bridges,
ankylosis

11. MR examination of sacroiliac joints, images: T1- (A), T2 TIRM ;Turbo inversion recovery
magnitude (TIRM) (B), T1FSCE-weighted (C): bone marrow edema, more pronounced in
right sacroiliac joint, particularly in the iliac bone as well as erosions undergoing strong
enhancement following administration of contrast medium, contrast enhancement and
thickening of synovial membrane in the sacroiliac joint (synovitis), inflammation of right
sacroiliac joint capsule (capsulitis), subchondral sclerosis

12. Chronic inflammation in sacroiliac joints
in T1- (A) and T1FS-weighted (B) images:
fatty transformation of the sacral bone
and iliac bones, partial sacroiliac joint
ankylosis on right side.

13. Active inflammatory lesions in MR examination
Bone marrow edema (osteomyelitis, osteitis, BME)
It is visible as an area of increased signal intensity in T2-weighted images and STIR/TIRM
sequences and as an area of decreased signal intensity in T1-weighted images that become
enhanced on administration of contrast medium. Bone marrow edema is not specific
for sacroilitis, as it also occurs in other inflammatory, traumatic or neoplastic pathologies.
Subchondral, periarticular BME specific for sacroilitis is located in iliac or sacral bones. It may
surround erosions . It is thought that symmetric lesions are characteristic for ankylosing
spondylitis, while unilateral, asymmetric location of bone marrow edema point to other forms of
SpA – most commonly psoriatic arthritis . literature data indicate that at early stage of ankylosing
spondylitis bone marrow edema is asymmetrical and unilateral.
Synovitis and capsulitis
Synovitis and capsulitis are diagnosed based on the presence of increased synovial signal intensity
in T2-weighted and STIR/TIRM sequences. They are characteristic for inflammation of sacroiliac
joints provided that there is an adjacent area of bone edema . Presence of articular exudate is
often observed in the course of synovitis and capsulitis, which may be differentiated from
synovitis in T1FS-weighted images following intravenous administration of contrast .
Inflammation of tendon, ligament and capsule insertions (enthesitis)
Enthesitis is visible in the form of increased signal intensity of bony or tendinous (either
ligamentous or fibrous layer of the capsule) part of enthesis on STIR;short inversion time
inversion recovery/TIRM or T1FSCE images .

14. • CHRONIC INFLAMMATORY CHANGES IN MR EXAMINATION
Subchondral sclerosis of bones that constitute the sacroiliac joint characterized
by low signal in T1-weighted and STIR sequences as well as lack of contrast
enhancement. Area of sclerosis specific for SpA has blurry margins and, as
disease progresses, becomes broader as opposed to osteoarthritis, in which it is
clearly demarcated and narrower .
Erosions in the subchondral layer of sacral or iliac bones, which may be active, i.e.
filled with inflamed tissue (areas of low signal in T1-weighted sequences and
hyperintense in STIR and T1CE sequences), or inactive (hypointense in all
sequences).
Subchondral fatty bone marrow transformation with high signal in T1-weighted
images, which becomes completely suppressed in fat-saturation sequences and
does not become enhanced following administration of contrast medium.
Ankylosis, characterized by low signal in all sequences, is sometimes surrounded
by high signal in T1-weighted images corresponding to fatty bone marrow
transformation.

15. „Shiny corners” of anterior vertebral bodies in the lumbar region; inflammation of
intervertebral joints L1–L5 with signs of bone marrow edema in T2 TIRM image (A)
enhanced after administration of contrast medium in T2FSCE-weighted image (B).
Visualizing >3 shiny corners indicates SpA with 81% specificity, while in young patients
<40 years old, specificity of this symptom raises to 97%

16. Sacroiliitis on MRI. (A) Bone marrow oedema (arrows) on T2W FS oblique coronal
images and (B) corresponding enhancement (arrowheads) on T1W FS post-contrast
axial images. This degree of bone marrow oedema is sufficient to diagnose sacroiliitis.

17. Capsulitis can be visible (A) anteriorly (arrow) or (B) posteriorly
(arrowhead) and may extend to involve the adjacent bone. The presence of
capsulitis alone is not diagnostic of spondyloarthropathy.

18. Enthesitis is a common finding in active sacroiliitis on (A) T2W STIR and (B) post-
contrast T1W FS. Enthesitis is seen as ligamentous oedema, bony irregularity and
bone marrow oedema (arrow) deep to areas of ligamentous insertion. Both the sacral
and iliac sides can be involved. In this case, it occurs at the attachment of the
interosseous ligament. Enthesitis can also be seen at muscular attachments and at
the attachments of the extra-articular sacrospinous and sacrotuberous ligaments. The
presence of enthesitis alone is not diagnostic of spondyloarthropathy.

19. Synovitis (arrow) is best demonstrated on post-contrast images and should be
differentiated from peri-articular vasculature by ensuring that the enhancement
is seen on consecutive images

20. Peri-articular fatty deposition (arrows) can only be appreciated on MRI.
This finding is suggestive of chronic sacroiliitis, but is non-specific, being
also often seen in osteoarthritis and, less so, in osteitis condensans ilii.

21. Subchondral sclerosis (arrows) is a non-specific sign of
sacroiliitis. Sclerosis can be physiological, related to
osteitis condensans ilii, physical stress or
osteoarthritis.

22. Ankylosis or bony bridging (arrows) can be
readily appreciated on MRI. One ideally wants to
make the diagnosis of sacroiliitis well before
ankylosis becomes apparent.

23. INFECTIVE SACROILIITIS
Three features of infective sacroiliitis are particularly
helpful in differentiating infective from inflammatory
sacroiliitis.
First, bone marrow oedema in infective sacroiliitis
tends to be more intense and there is more intra-
articular fluid (Figure 17A).
Second, inflammation in infective sacroiliitis spreads to
involve the peri-articular soft tissues, particularly the
iliacus and gluteal muscles (Figure 17B).
Third, peri-articular fluid collection or abscess is
practically pathognomonic of an infective sacroiliitis

24. infective sacroiliitis. (A) Excessive sacroiliac joint fluid
(red arrow); (B) inflammation of iliacus muscles (white
arrow); and (C) abscess formation (arrowhead)
supports the diagnosis of infective sacroiliitis.

25. Osteoarthritis. (A) Inferior osteophytosis (arrow) on radiograph and (B)
anterior marginal osteophytosis (arrowhead) seen on MRI as features of
osteoarthritis or instability. (C) Joint irregularity with subchondral
sclerosis is due to osteoarthritis rather than sacroiliitis on radiograph

26. Stress reaction. Focal patchy
oedema (arrow) at the mid-sacral
area close to the left sacroiliac
joint in a 28-year-old male
athlete. No fracture line is
evident. The patient recovered
with rest.
Reactive stress-related soft tissue changes
around the sacroiliac joint. Twenty-seven-
year-old professional football player.
Radiograph was unremarkable. (A) On MRI,
there is severe thickening of the anterior
capsule (arrow) on the right side; (B) more
proximally, similar though less severe
capsular thickening (arrowhead) with
osteoarthritis is present on the left side.

27. insufficiency fracture. MRI has a very high positive and negative
predictive value for insufficiency fracture. It will demonstrate either (A)
bone oedema (arrow) ± or (B) a discrete fracture line (arrowhead).

28. OSTEITIS CONDENSANS ILII is caused by bone deposition at
stress areas alongside the sacroiliac joint. It is usually seen in
childbearing women and is most likely related to the abnormal
high stresses incurred during pregnancy. Radiography and MRI
typically reveal bilateral, symmetrical, sharply circumscribed,
triangular-shaped areas of subchondral sclerosis, without
erosions or joint space widening, at the anteroinferior aspect of
the iliac bone alongside the sacroiliac joint.
Bone marrow oedema and fatty infiltration are recognized
additional MR features of osteitis condensans ilii .
Bone marrow oedema in osteitis condensans ilii tends to appear
in a continuous pattern as an arcuate line beneath the anterior
subchondral bone sclerosis in contrast to inflammatory
sacroiliitis

29. Osteitis condensans ilii (arrows). Well-defined
triangular-shaped symmetrical subchondral sclerosis,
without erosions or joint space widening at the
anterior aspect of the iliac bone on (A) radiograph
and (B) MR