Andersson lesion
Overview
- Definition: A destructive, inflammatory, or post-traumatic lesion involving the vertebral body-disc complex in a patient with a rigid, ankylosed spine (typically Ankylosing spondylitis).
- Pathophysiology: Represents a pseudoarthrosis (false joint) forming at a single mobile segment within a fused spine.
- Mechanism A (Traumatic): A stress fracture occurs (often missed)
persistent motion at the fracture site non-union and mechanical destruction. - Mechanism B (Inflammatory): Localised severe inflammation at the discovertebral junction (less common theory).
- Mechanism A (Traumatic): A stress fracture occurs (often missed)
- Epidemiology: Occurs in ~5–10% of patients with established ankylosing spondylitis.
Anatomical distribution
- Location:
- Thoracolumbar junction (T10–L2): Most common (site of maximal mechanical stress).
- Cervicothoracic junction.
- Usually involves a single level (or rarely two adjacent levels).
Imaging features
Plain radiograph
- Appearance: Looks deceptively like infection.
- Disc space: Irregular widening or narrowing.
- Endplates: Extensive erosion and destruction.
- Sclerosis: Significant reactive sclerosis ("shiny corners" become distinct sclerotic blocks) surrounding the destruction.
- Alignment: Often mild kyphosis or subluxation at the level.
CT (The problem solver)
- Key finding: Vacuum phenomenon (gas) within the disc space.
- Significance: Gas indicates motion (negative pressure). It is a strong predictor of pseudoarthrosis and argues against acute infection (which is fluid-filled).
- Posterior elements: Look for the fracture line extending through the fused facet joints (indicating a 3-column injury).
MRI
- T1: Hypointense signal in the disc and adjacent marrow (sclerosis/oedema).
- T2/STIR:
- Hyperintense signal at the disc space and endplates (marrow oedema).
- Can look identical to infective spondylodiscitis.
- Signal extent: Oedema often extends into the posterior elements (pedicles/lamina) due to the stress fracture, which is less common in pure discitis.
- Enhancement: Enhances avidly (granulation tissue), further mimicking infection.
Differential diagnosis: The "infection" battle
| Feature | Andersson lesion (Pseudoarthrosis) | Spondylodiscitis(Infection/TB) |
|---|---|---|
| Context | Rigid/ankylosed spine (Ankylosing spondylitis/DISH) | Any patient (often immunocompromised) |
| Vacuum phenomenon | Common (The "clinching" sign) | Rare (fluid/pus fills the space) |
| Posterior involvement | Fracture line visible through fused facets | Rare (unless advanced TB) |
| Soft tissue mass | Minimal / None (mild paraspinal oedema) | Paravertebral abscess / epidural collection |
| Fracture line | Often "through-and-through" the spine | Absent |
Management principles
- Goal: Stop the motion to allow healing.
- Conservative: External immobilisation (bracing) – often fails due to the long lever arm.
- Surgical: Posterior fixation (instrumentation) to stabilise the spine and promote fusion.
- Exam context: If diagnosed, the treatment is surgical stabilisation, not antibiotics.
High-yield exam pearls
The "vacuum" rule
If you see a destructive disc lesion in an AS patient:
Look for gas (vacuum phenomenon) on CT.
- Gas = Andersson lesion (Mechanical).
- Fluid/Pus = Infection (Biological).
The Thai Board trap
In Thailand, TB spine is very common. An AS patient can get TB.
- Do not diagnose Andersson lesion solely on X-ray.
- If there is soft tissue abscess formation on MRI, it is TB/Infection until proven otherwise, even if they have AS. Andersson lesions generally do not form drainable abscesses.
