Osteomyelitis is the inflammation/infection of the bone and
its marrow. Osteomyelitis can be acute or chronic based on the duration. It is
said to be acute when it is identified within 7 to 14 days of onset and chronic
when the infection is present for atleast several months.
The source of infection can be from:
1. Haematogenous
spread from an ongoing infection (known as acute haematogenous osteomyelitis).
This is mainly a disease of children. It is most frequently encountered in the metaphysis of long bones in
children.
2. Abrasion in
surrounding structure of the bone and puncture wound that is neglected.
In acute
osteomyelitis in children, the metaphysis is commonly involved. Predilection
for this site has traditionally been attributed to the peculiar arrangement of
blood vessels in this area. The non-anastomosing terminal branches of the
nutrient artery twist back in hairpin loops (hence said to be cockscrew end
vessels) before entering the large network of sinusoidal veins. Also, these end
vessels of the nutrient artery empty into the much larger sinusoidal veins.
This arrangement causes a slow and turbulent flow of blood at this junction.
The relative vascular stasis and consequent lowered oxygen tension favours
bacterial colonisation.
This
infection leads to acute inflammatory reaction with vascular congestion,
exudation of fluid and infiltration by polymorphonuclear leucocytes. The
intra-osseous pressure rises causing intense pain, further obstruction to blood
and intravascular thrombosis. Pus forms within the bone and forces its way
along the Volkmann canals to surface where it produces a subperiosteal abscess.
This leads to swelling, further pain, tenderness and irritability. The
subperiosteal abscess is more evident in children because of the relatively
loose attachment of the periosteum than in adults.
The
subperiosteal abscess can spread along the shaft of the bone to re-enter at
another level or burst into the surrounding soft tissues. If the subperiosteal
abscess is not released either spontaneously or by decompression, it will
stimulate the formation of periosteal new bone known as involucrum. Also, the
rising intra-osseous pressure, vascular stasis, small vessel thrombosis and
periosteal stripping, increasingly compromise the blood supply leading to bone
death as the duration increases. The dead bone is called a sequestrum.
Furthermore, if the infection persists, pus may track through perforations
(known as cloaca) in the involucrum and track by draining sinuses to the skin
surface.
Acute
osteomyelitis in adults usually follows an injury, an operation or spread from
a contiguous focus of infection (e.g. neutropathic ulcer or an infected
diabetic foot.). True haematogenous is uncommon in adults.
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