Cloaca (osteomyelitis)

Last revised by Calum Worsley on 29 Apr 2023

A cloaca (pl. cloacae or cloacas) is a gap in the cortex of a bone affected by chronic osteomyelitis that allows the drainage of pus or other material from the bone into the adjacent tissues.

The use of the term cloaca should be reserved for cortical breaches that are the result of material draining from them, which should be distinguished from pathological fractures that occur as the result of force exerted on bone weakened by infection 1. Non-specific terms like "cortical breakthrough" may be used when there is doubt as to the etiology. If the tract extends to the skin surface, the portion extending beyond the involucrum to the skin surface is called a sinus tract 1.

Pus and other necrotic debris fills the medullary cavity of infected bone which weakens and then perforates through the cortex of the bone 2. The resulting opening then matures as pus flows through it out of the bone. Once the infection has resolved, there may be callus formation if the cloaca begins to heal.

A cloaca may appear as a lucent gap in the cortex of a bone with other signs of osteomyelitis, such as periosteal thickening or involucrum formation.

CT will more clearly identify a cloaca as a gap in the cortex, and can also identify associated features of chronic osteomyelitis such as the presence of a sequestrum. CT is more sensitive than MRI for the detection of cloacas 3.

On MRI, a cloaca will appear as a gap in the low signal cortex of bone, which may be easier to identify on T2 weighted or fluid-sensitive sequences by the high signal of pus or debris within the cloaca 1.

The resolution of a cloaca depends on the successful treatment of the causative infection. If this is successful, the cloaca may heal through callus formation, though some lucency may remain visible 1.

"Cloaca" is the Latin word for "sewer".

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