- Discussion:
- bone provides and environment that significantly alters interaction
between bacterial and host defenses in favor of the invader;
- bone contains numerous microscopic channels, the walls of which are
impermeable to such host defenses as the large molecules of
immunoglobulins;
- bacteria, once gaining a foothold in these minute channels, may be
isolated from both the cellular and chemical elements of host
defenses and free to proliferate unimpeded;
- microcirculation of bone within these channels is highly vulnerable to
bacterial toxins, which may produce local thrombosis of these small
vessels leading to the death of bone in the vicinity of infection,
which further impedes access by host defenses;
- bone
resorption, bone formation, and necrosis are tissue responses
seen in varying degrees in all bone infections;
- tortuous course of
nutrient vessels in bone causes
bacteria to be trapped in the metaphysis;
- metaphysis is located between the epiphysis and the diaphysis;
- epiphyseal plate prevents infection from entering the joint space in
older children but not in neonates;
- joint infection secondary to osteomyelitis may occur in shoulder and
hip as a result of synovial membrane inserting distally to epiphysis,
allowing bacteria to spread directly from metaphysis to the joint space;
- Pathology:
- in classical hematogenous osteomyelitis of childhood, organisms in
bloodstream gain access to bone by way of the
nutrient artery and
pass through branches of this vessel to the metaphysis;
- terminal vessels enter small, bone-lined channels on metaphyseal side
of the
epiphyseal plate;
- these channels have a blind end, and each terminates in the last
hypertrophic cartilage cell of epiphyseal plate;
- on reaching the end of the tunnel, capillary inside this channel must
turn 180 deg and exit through the same passage by which it entered;
- it is in this area of sluggish circulation, in tunnels whose walls are
essentially impermeable, that bacteria lodge and hematogenous
osteomyelitis begins;
- once the process starts, thrombosis of small terminal vessel and
exudate seal the entrance to channel;
- in depths of this bone tunnel the organisms begin to proliferate,
unimpeded by the host defenses that are unable to reach them;
- after the infection is established in one channel, it extends into
metaphysis and involves other channels;
- hordes of PMNs are mobilized in metaphysis, and cellular destruction
occurs at a rapid rate;
- in adolescents, infection usually begins in metaphysis because of
tendency for sluggish & turbulent blood flow in capillary-sinusoid
loops adjacent to growth plate & absent or nonfunctioning phagocytes;
- bacteria that localize to this area proliferate and easily obstruct
blood flow, resulting in infarction and creation of environment
conducive to further bacterial proliferation;
- pus in the metaphysial area expands under pressure and travels up the
marrow cavity or passes out of the cortex through the many small
metaphyseal openings;
- in certain areas such as the hip, where epiphyseal plate is situated
w/in joint capsule, early joint involvement by infection is common;
- at knee, the joint is usually spared, and infections beginning in the
distal femoral or proximal tibial metaphysis usually spread along
shaft of the bone moving away from the joint;
- stripping of the periosteum from the shaft by pus stimulates intense
osteoblastic response;
- new immature bone is formed as response to periosteal stripping,
and, in severe cases, the entire shaft may be encased in a
sheath of new bone referred to as an involucrum;
- where a major portion of the shaft has been deprived of blood supply,
resulting sequestrum lies within the involucrum;
- openings in the involucrum, called cloacae, may permit escape of
pus from bone may be fulminant, w/ overwhelming sepsis & death;
- septicemia can cause bacterial seeding to other bones, leading to
multiple sites of osteomyelitis;
- if disease is treated early w/ appropriate antibiotics, septicemia
is controlled, and there may be only limited bone damage;
