Initial metaphyseal abscess or direct innoculation
Resolves with appropriate treatment
Contained by host defences & persists as subacute or chronic localised infection
Spreads to involve adjacent structures
Medullary canal fills with pus & pressure forces infection through Haversian canals to the periosteum to form subperiosteal abscess
The hole in the cortex is the cloaca
Periosteum lifts & may form new bone resulting in an involucrum
Vascular obstruction with thrombosis may result from many factors
Pressure
Leucocytic enzymes & acidic pH
Periosteal stripping
Results in segment of dead bone called the sequestrum
In the adult the periosteum is adherent to the cortex so pus tends to break through to form multiple soft tissue abscesses
overlying skin is affected in chronic conditions
Indurated, puckered & adherent to bone
Sinus often connects the skin with bony lesion
histological picture is one of chronic inflammatory cell infiltrate around areas of acellular bone or microscopic sequestra