Write a short note on Sequestrum & Involucrum.
Definitions
In osteomyelitis, a sequestrum is a fragment of dead, avascular bone that has become separated from the surrounding living bone. It arises because pus strips the periosteum and thromboses the vessels, depriving a segment of cortex of its blood supply. An involucrum is the sheath of new, living bone formed by the stripped-but-viable periosteum around the sequestrum.
The dead sequestrum lies within a new-bone involucrum, which is perforated by a cloaca.
WHY They Matter
The sequestrum is the key to chronicity: being avascular, it is inaccessible to antibiotics and to host defences, so it harbours bacteria and perpetuates the infection until it is surgically removed. A cloaca is an opening in the involucrum through which pus (and sometimes small sequestra) discharges, connecting to the skin via a sinus. On radiographs a sequestrum appears as a dense, sclerotic fragment (it cannot decalcify because it has no blood supply) lying within a lucent cavity.
CLINICAL PEARL
A radiodense fragment surrounded by lucency is a sequestrum; the surrounding sheath of new bone is the involucrum. Removing the sequestrum (sequestrectomy) is essential to cure chronic osteomyelitis — antibiotics alone will not eradicate it.
DANGER / REMEMBER
Because the sequestrum is avascular, it stays dense on X-ray while the surrounding living bone becomes osteoporotic — this contrast is a useful diagnostic clue.
KEY POINT
Key points TO remember
- Sequestrum = dead avascular bone; involucrum = new living bone around it.
- Cloaca = opening in the involucrum; sinus = track to the skin.
- Sequestrum harbours infection and causes chronicity — remove it surgically.
- On X-ray the sequestrum is dense within a lucent cavity.
EXAM TIP
Sources: Maheshwari's Essential Orthopaedics; Apley & Solomon's System of Orthopaedics and Trauma; AO Principles of Fracture Management.
All questions in Bone & Joint Infection
- Q1Define Acute Haematogenous Osteomyelitis. Describe its clinical features, investigations, management and complications. ★
- Q2Define Chronic Osteomyelitis. Describe its clinical features, investigations, management and complications. ★
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- SN1Write a short note on Sequestrum & Involucrum. ★
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