Brodie's abscess
Overview
- Definition: A form of subacute or chronic osteomyelitis where the infection is contained within a wall of sclerotic bone and granulation tissue.
- Pathophysiology:
- Usually follows haematogenous spread.
- The host immune system is strong enough to contain the infection but fails to eradicate it completely.
- Microbiology:
- Staphylococcus aureus (> 50% of cases).
- Culture negative (20–25% of cases).
- Thai context: Always consider Mycobacterium tuberculosis (TB) and Salmonella (in sickle cell/thalassaemia traits).
- Clinical presentation:
- Insidious onset of local pain (often worse at night, mimicking osteoid osteoma).
- Systemic signs (fever, elevated WBC) are frequently absent.
Anatomical distribution
- Location: Metaphysis (classic).
- Bones:
- Tibia (distal or proximal) is the most common site.
- Distal femur.
- Fibula.
- Position: Central or eccentric.
Imaging features
Plain radiograph
- Appearance: Geographic lytic lesion.
- Margins: Well-defined with a thick sclerotic rim (indicates chronicity).
- Shape: Often oval, with the long axis parallel to the bone.
- Pathognomonic sign: A serpiginous tract (tortuous lucent channel) extending from the abscess towards the growth plate (representing the path of vascular invasion).
- Periosteal reaction: Thick, organised, and solid (benign type).
CT
- Identifies the sequestrum (dead bone fragment within the cavity) much better than X-ray.
- Confirms the breach of cortex (cloaca) if a sinus tract is forming.
MRI (The problem solver)
- T1: Central low signal (pus) with an intermediate-to-high signal rim.
- T2/STIR: Target sign or Double line sign.
- Centre: Hyperintense (pus/fluid).
- Inner rim: Hyperintense (granulation tissue).
- Outer rim: Hypointense (sclerotic bone).
- Post-contrast: Intense ring enhancement of the granulation tissue lining.
- The "Penumbra sign" (Key differentiator):
- A discrete rim of high T1 signal (without contrast) surrounding the abscess cavity.
- Represents vascular granulation tissue.
- Significance: Highly specific for infection; rarely seen in tumours (99% specificity).
Differential diagnosis
| Lesion | Key distinguishing features |
|---|---|
| Osteoid osteoma | Central nidus is calcified/ossified (targetoid) rather than fluid; usually smaller (< 1.5 cm); intense reactive sclerosis obscuring the lesion; dramatic relief with aspirin. |
| Eosinophilic granuloma | "Beveled edge"; no extensive sclerotic rim in acute phase; often multiple; "floating tooth" if in jaw. |
| Chondromyxoid fibroma | Eccentric; lobulated; no serpiginous tract; T2 markedly hyperintense (myxoid) without the layered "target" look of an abscess. |
| Ewing sarcoma | Permeative/moth-eaten; aggressive periosteal reaction (onion-skin); soft tissue mass; no sclerotic rim. |
| Tuberculous osteomyelitis | Can look identical but often has less reactive sclerosis ("cold abscess") and more extensive soft tissue involvement relative to bone destruction. |
High-yield exam pearls
The "penumbra sign" physics
On unenhanced T1-weighted images, the granulation tissue lining a Brodie's abscess appears slightly hyperintense relative to the central pus and the surrounding muscle.
- Why? High protein content and paramagnetic effects in the granulation tissue.
- Utility: Helps distinguish Brodie's from a cystic tumour (which usually has a dark wall on T1).
The Thai Board context
In Thailand, a "Brodie's abscess" appearance is frequently TB.
- If the patient has no systemic symptoms and a "cold" presentation, or if the sclerotic rim is thin/absent despite a lytic cavity, suggest acid-fast bacilli (AFB) stain/PCR in the biopsy recommendation.
The night pain trap
Both Brodie's abscess and osteoid osteoma cause night pain.
- Osteoid osteoma: Pain is prostaglandin-mediated
relieved by NSAIDs. - Brodie's: Pain is pressure-mediated (throbbing)
variable relief with NSAIDs.
