Tuberculous osteomyelitis
Overview
- Definition: Osteomyelitis caused by Mycobacterium tuberculosis.
- Pathophysiology:
- Usually haematogenous dissemination from a primary pulmonary focus (which may be active or latent).
- Can also spread directly from adjacent lymph nodes or joints.
- Epidemiology:
- Thai context: Extremely common. Must be in the differential for any chronic lytic bone lesion or monoarthritis.
- Risk factors: HIV/AIDS, immunocompromise, elderly, endemic regions.
- The "Great mimicker": Can resemble cellular tumours, metastases, or fungal infections due to its indolent course.
Anatomical distribution
- Spine (50%): "Pott's disease" (thoracolumbar junction).
- Appendicular skeleton (30%):
- Large weight-bearing joints: Hip, knee.
- Metaphyses of long bones (can mimic Brodie's abscess).
- Small bones: Hands/feet (Tuberculous dactylitis / Spina ventosa).
Imaging features
Appendicular / metaphyseal TB
- "Cystic" tuberculosis:
- Well-defined lytic lesions in the metaphysis.
- Minimal reactive sclerosis (unlike pyogenic Brodie's abscess).
- Minimal periosteal reaction (indolent nature).
- Can cross the physis in children (transphyseal spread).
Spinal TB (Pott's disease)
- Pattern: Spondylodiscitis targeting the anterior vertebral body.
- Spread: Subligamentous spread under the anterior longitudinal ligament (ALL)
non-contiguous "skip lesions". - Disc: Intervertebral disc destruction occurs later than in pyogenic infection (or is relatively preserved).
- Deformity: Anterior wedging leads to severe kyphosis (Gibbus deformity).
- Soft tissue: Disproportionately large paravertebral abscesses ("cold abscesses") with thin, smooth walls.
MRI features
- Marrow: Oedema typically extends beyond the margins of bone destruction.
- Abscess:
- Thin, smooth wall enhancement (pyogenic abscesses tend to have thick, irregular, shaggy walls).
- Internal debris/calcification (calcified psoas abscess is pathognomonic for old TB).
- "Kissing sequestra": In joints (e.g., knee), focal sequestra on opposing articular surfaces.
Differential diagnosis: Pyogenic vs Tuberculous
| Feature | Pyogenic osteomyelitis | Tuberculous osteomyelitis |
|---|---|---|
| Course | Acute/Subacute | Chronic/Indolent |
| Sclerosis | Prominent (thick rim) | Minimal / Absent |
| Sequestrum | Common, often large | Rare (or "sand-like" microsequestra) |
| Periosteal reaction | Thick, irregular | Minimal / None |
| Joint space | Rapid destruction | Preserved until late |
| Abscess wall | Thick, irregular, shaggy | Thin, smooth |
| Soft tissue | Proportional to bone injury | Disproportionately large |
High-yield exam pearls
The Thai Board rule
In the Thai exams, if you see a lytic lesion that looks like a tumour (e.g., GCT or metastasis) but the clinical history is "chronic pain" or "mild swelling" without severe systemic signs:
TB is the top differential.
- Keyword to look for: "Cold abscess" (soft tissue mass without acute inflammation/erythema).
Transphyseal spread
Unlike pyogenic osteomyelitis, which is often halted by the physis in children (due to avascular cartilage), TB can cross the growth plate.
- If a lesion involves both the metaphysis and epiphysis in a child, think TB (or rarer entities like infantile osteomyelitis).
