Osteomyelitis is defined as bone inflammation,
usually caused by pyogenic infection.
It is often a difficult-to-treat condition,
associated with considerable morbidity and important health care costs.
Osteomyelitis can occur in every age,
although it has a two-peak incidence,
one infantile,
in children before 5 years of age and,
another, in adults older than 50 years of age.
Its incidence is increasing in developing countries,
since the incidence of predisposal conditions such as diabetes mellitus is also raising.
Staphylococcus aureus is the most common causative agent,
both in children and adults.
Other agents are common in specific groups,
such as Salmonella spp.,
frequent in patients with sickle-cell disease,
and Pseudomonas spp.,
in endovenous-drug users.
Osteomyelitis can be typically classified according to its duration as acute,
subacute or chronic,
although there is no consensus on the duration criteria to define each phase.
Chronic osteomyelitis is more common in adults than in children.
Pathogenic agents may reach the bone by three main routes.
Hematogenous dissemination is most common in children and affects most frequently the metaphysis of long bones,
the most susceptible part of the bone,
due to its high vascularization.
Hematogenous spread is less common in adults and may typically affect vertebral bodies,
but also long bones,
pelvis and clavicles.
Osteomyelitis may as well be caused by contiguous spread of infection from adjacent tissues,
frequently in diabetic patients,
and by direct inoculation,
due to previous trauma or surgery.
After bacterial proliferation,
bone undergo important inflammatory reactions that lead to local edema and increased medullary pressure.
This raised pressure induce inffection spreading to subperiosteal bone,
periosteum and lastly to contiguous soft tissues.
In chronic stages of the disease,
there can be a separated necrotic bone fragment,
called sequestrum.
If this sequestrum is present,
its detection is crucial because it can perpetuate the infection and it needs surgical removal.
As infections progress,
the pus needs to exit the medullary cavity,
and the cloaca is formed,
which is a cortical defect that drains the pus to adjacent tissues.
New bone is formed and deposited,
covering the necrotic bone,
as an involucrum.
Imaging exams play a crucial role on the diagnosis of osteomyelitis because its associated symptoms are varied and non-specific and,
additionally,
some patients may have normal serum inflammatory markers.
However,
the diagnosis can be sometimes challenging,
as imaging features vary according to the duration of the inflammatory process and many conditions may mimic osteomyelitis.