Role of imaging in spondyloarthropathies
The group of spondyloarthropathies includes ankylosing spondylitis, psoriatic, reactive, and enteropathic arthritides. Ankylosing spondylitis (Ankylosing spondylitis) typically debuts in adolescents and young adults (HLA-B27 positive) presenting with inflammatory back pain. A distinctive feature of the pathology is sacroiliitis—inflammation of the sacroiliac joints, which is symmetric and bilateral in the majority of cases.
Various imaging modalities are used to evaluate the joint apparatus:
- MRI has the highest sensitivity. It allows the detection of the earliest marker of the disease: subchondral bone marrow edema. In diagnostics, it is important to differentiate inflammatory edema from areas of normal bone vascularization.
- Conventional radiography and CT demonstrate established structural changes: erosions, subchondral sclerosis, and in late stages, complete bony ankylosis (fusion of articular surfaces).
Spinal involvement and syndesmophyte formation
The pathological process in the axial skeleton often develops in parallel with sacroiliitis or even precedes it. It is based on enthesitis, an inflammatory reaction at the sites of ligament attachment. In the intervertebral disc segment, involvement begins with the outer fibers of the annulus fibrosus (Sharpey's fibers).
On lateral radiographs, early changes appear as sclerosis of the vertebral body corners, known as Romanus lesions (Romanus lesions). On MRI, this sign corresponds to localized bone marrow edema, which is detected before radiographic sclerosis appears. Subsequently, the anterior longitudinal ligament ossifies, giving the vertebrae a characteristic "squared" shape.
In later stages, thin vertical syndesmophytes form, bridging across the discs. Total fusion of the vertebral bodies and sacroiliac joints creates the radiographic picture of a "bamboo spine" (bamboo spine). Meanwhile, the zygapophyseal (facet) joints may fuse before the vertebral bodies ankylose.
Traumatic injuries of the pelvic ring
Imaging methods are critical for assessing the mechanism of pelvic trauma. Several injury patterns are distinguished:
- Anteroposterior compression (type 3): Occurs due to anterior-to-posterior force application. Characterized by total disruption of the ligamentous apparatus, including the pubic symphysis and sacroiliac ligaments (ligg. sacroiliaca anteriora and posteriora). Radiographs and CT reliably visualize marked pathological widening of the sacroiliac joint.
- Lateral compression (type 1): Manifests as an impacted fracture of the inferior pubic ramus combined with an oblique fracture of the superior ramus.
- Lateral compression (type 3): Severe combined injury resulting from a "side-swipe" or rollover mechanism. Characterized by severe internal rotation of the affected hemipelvis and severe disruption of the posterior ring structures (sacral fractures, joint disruptions with fragment displacement).
Musculoskeletal complications
Long-standing ankylosing spondylitis leads to severe biomechanical complications:
- Spinal fractures. Despite massive syndesmophyte formation, bone density decreases (osteopenia develops). The rigid, fused spine cannot absorb loads and is therefore prone to unstable transverse fractures (often at the C6-C7 level).
- Andersson lesion (Andersson lesion). In a fused spine, movement is preserved only in a few free segments. Overload leads to inflammatory injury in these segments, which manifests radiographically as sclerosis and irregular contours. This sign can be mistaken for spondylodiscitis.
The pathology also affects peripheral joints (in 30% of patients). In the hip joint, bone proliferation around the entheses flattens the femoral head, mimicking a cam deformity. Diffuse joint space narrowing is observed, while overall bone density remains preserved.
Romanus shines at the corners (vertebral corner sclerosis); Andersson erodes between blocks (inflammation of mobile segments).
Class and exam questions
What is the advantage of MRI over radiography when sacroiliitis is suspected?
MRI can visualize subchondral bone marrow edema, the earliest sign of the inflammatory process. Radiography and CT show only late changes, such as erosions and ankylosis.
How to differentiate syndesmophytes in ankylosing spondylitis from osteophytes?
Syndesmophytes in this pathology grow vertically and appear as thin bony outgrowths at the sites of Romanus lesions. This distinguishes them from coarser proliferations in other arthritides and from osteophytes.
Why are patients with a "bamboo spine" at high risk for fractures?
Due to total fusion of the vertebral bodies and facet joints, the spine becomes rigid. Combined with developing osteopenia, this causes any trauma to result in highly unstable transverse fractures.
What is the radiographic marker of type 3 anteroposterior pelvic compression?
The main sign is marked widening of the sacroiliac joint space, indicating complete rupture of both the anterior and posterior portions of the pelvic ring ligaments.
What else to study on this topic
- Mechanisms of axial skeleton enthesitis formation
- Differential diagnosis of vertebral corner edema in degenerative processes
- Imaging semiotics of Andersson lesions versus infectious spondylodiscitis
- Assessment of shoulder and hip joint involvement in spondyloarthropathies
- Complete classification of lateral compression pelvic injuries
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