|ESSR 2019 / P-0092||
|Sacroiliac joint. How to differentiate inflammatory and non inflammatory conditions|
Imaging findings OR Procedure Details
Anatomy and Biomechanics
The main portion of the SIJ joint is surrounded by a complex capsule and lined with cartilage (diarthrosis). Its shape is auricular, and ‘opens’ posteriorly. The sacrum and ilia have an extracapsular, dorsally located articulation (synarthrosis), which is augmented by vast ligaments that provides internal stability.
If the sacroiliac joint is craniocaudally divided into thirds, it is widely admited that all of the inferior one-third is a true synovial-cartilaginous joint. The posterior part of the middle one-third is syndesmotic while the anterior part is synovial-cartilaginous. The posterior part of the superior one-third is syndesmotic along with the most superior aspect of the anterior part Fig. 2 .
The sacral cartilage is overall about 3 times thicker than the iliac cartilage. Cartilage thickness varies across the joint, with the sacral cartilage being much thicker (6 mm) anteriorly than posteriorly (1 mm), making it more prone tobe affected by any pathological change
Furthermore, the pelvis is anteriorly tilted inward and must be taken into account when interpreting the usually acquired axial or coronal oblique planes.
Although SI joint is firmly fixed, the ligament structures allows a rotational movement of up to 10 degrees and translational up to 6 mm Fig. 3
SIJ is prone to mechanical changes due to trauma or microtrauma secondary to having either too much laxity or excessive compressional stiffness in the joint mainly in the weight bearing area, that is the anterior segment of the upper and middle thirds. Fig. 4 Fig. 5
Mechanical overload may represent an early stage of osteoarthritis (OA), especially in young active individuals and athletes, and may manifest as in other areas of the body as bone marrow edema (BME), mimicking the Spa pattern
When only a mild or equivocal BME is found, the distribution patterns and signal characteristics can be used to differentiate inflammatory from non-inflammatory conditions.
Image features to analyze Fig. 6
-Spa is more common in the iliac side.
-Affected bone marrow areas are typically located periarticularly (subchondral bone marrow).
2.- Edema appareance
- Inflammatory lesions are usually at least 1 cm long or deep.
- Signal intensity is usually greater in inflammatory lesions compared to degenerative lesions.
- In the absence of additional imaging features of SpA in the SIJs or spine, two tiny lesions < 1 cm in diameter are not sufficient for the diagnosis of SpA.
-Erosions are considered the most disease-specific measurable imaging findings in SI MRI of patients with Spa. Although sparse erosions may be present in the anterior part of the degenerative SIJ, and small erosions have been identified in up to 20%of controls, subchondral erosions remain the most helpful MRI feature of sacroiliitis. Fig. 10
-Capsulitis, enthesitis, and synovitis are less commonly encountered and are supportive, though not diagnostic, features of SpA. Fig. 11
-Backfill phenomenon is reparative fat metaplasia characterized by high T1-signal alongside the sacroiliac joints, and this may be more specific to SpA than initially considered.“Backfill” may be an intermediate step between erosion and ankylosis. Fig. 12
-Sclerosis attributable to SpA should extend at least 5 mm from the SI joint space since small areas of periarticular sclerosis can be observed in healthy individuals (physiological sclerosis).
-Spine findings characteristic of Spa can add specifity to the diagnosis although it is now considered that imaging of the spine in addition to the sacroiliac joints significantly increases the length of the MR examination but does not necessarily add to specificity and sensitivity
Infective sacroiliitis Fig. 13
- BME in infective sacroiliitis tends to be more intense and there is more intra-articular fluid
- Second, inflammation in infective sacroiliitis spreads to involve the peri-articular soft tissues, particularly the iliacus and gluteal muscles
- Peri-articular fluid collection or abscess is practically pathognomonic of an infective sacroiliitis.
Osteoarthritis Fig. 8
-anterior marginal osteophytosis, joint space narrowing, joint surface irregularity with minimal subchondral sclerosis and subchondral cysts
-BME tends to be milder in degree than in inflammatory sacroiliitis and tends to be confined to the immediate subchondral areas
-Subchondral fatty change is a common feature of osteoarthritis that can mimic inflammatory ‘backfill’.
Diffuse idiopathic skeletal hyperostosis (DISH) Fig. 14
-Para-articular bony ankylosis that should not be mistaken for the true intra-articular bony ankylosis seen in ankylosing spondylitis
-There are no subchondral erosions or sclerosis in DISH and the synovial joints are spared.
Sacral insufficiency fracture Fig. 15
-The demographic profile of patients who develop sacral insufficiency fractures is clearly very different to that of most patients with inflammatory sacroiliitis.
-The fracture line is commonly aligned vertically through the sacral ala and horizontally through the sacral body forming an H-shape which has been termed the “Honda sign”.
Osteitis condensans ilii Fig. 16
-It is caused by bone deposition at stress areas alongside the sacroiliac joint.
-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.