An ill-defined, hypointense signal on the T1 Weighted images is seen to involve the marrow of the distal half of the left tibia. This signal appears heterogeneously hyperintense on the T2 Weighted, GRASS and STIR images. There is periosteal elevation in the distal half of the left tibia with soft tissue deep to the periosteum. This soft tissue is of intermediate signal on the T1 Weighted images and appears hyperintense on the T2 Weighted images and is noted around the distal half of the left tibia. The fat planes around the distal half of the left tibia also shows an ill-defined hyperintense signal on the T2 Weighted and STIR images. There is erosion of the cortex along the medial margin of the left tibia, in it’s distal third segment. A tract is noted in the subcutaneous fat along the antero-medial margin of the left tibia in it's mid segment. There is stranding of the subcutaneous fat along the antero-medial margin of the involved segment of the left tibia. The lesion is seen to involve the metaphysis and diaphysis of the distal left tibia. Focal break of the growth plate of the distal end of the left tibia is noted.
Osteomyelitis is the simultaneous infection of bone and marrow. Usually it is caused by bacteria, although fungi and parasites can cause an infection of the bone and the marrow. Osteitis is an infection of the bone cortex. Infective periostitis is infection of the periosteum with subperiosteal accumulation of bacteria and fluid, often causing infective osteitis and osteomyelitis, as well as cortical necrosis.
Osteomyelitis may be acute, subacute, or chronic. Acute osteomyelitis is characterized by an abrupt onset of symptoms and signs during the initial phase of the infection. If the infection cannot be eliminated in the early stages because of inappropriate treatment or bacterial resistance, the process becomes subacute or, with increasing duration, chronic.
A fragment of necrotic bone, separated by granulation tissue from living bone, is termed a sequestrum. Such fragments may persist and harbor viable bacteria, which may cause acute flare-ups or lead to chronic osteomyelitis. A layer of living bone can form about the necrotic bone and is termed an involucrum. It surrounds and eventually merges with the parent bone. Discharge of pus and sequestra from the bone marrow to the soft tissue occurs through an opening in the involucrum, the cloaca. A sinus tract or fistula is a channel from the bone to either the skin surface (sinus tract) or an internal organ (fistula). An active infection that is sharply delineated within the bone and lined by granulation tissue is termed a bone abscess or Brodie's abscess. Garré's sclerosing osteomyelitis is a sclerotic nonpurulent form of osteomyelitis that results in marked periosteal bone deposition without necrosis and pus and with only little granulation tissue.
Etiology:
Bones and joints can become infected in the following ways:
Hematogenous
Spread of Infection:
Acute osteomyelitis:
Acute osteomyelitis mainly affects children. Hematogenous infection of
bone occurs at sites of slow blood flow and necrotic tissue. Staphylococcus
aureus, streptococci, Escherichia coli, and Haemophilus influenzae are among the
most frequently implicated organisms. Gram-negative organisms are seen more
commonly in adults. Generally, in younger age groups, the long tubular bones are
involved, whereas in adults the axial skeleton and the pelvis are the preferred
sites.
The pathological features of osteomyelitis differ in the infant, child, and
adult. These differences are related in part to the particular vascular anatomy
of the tubular bones evident in each of these age groups. In the infant,
diaphyseal vessels can penetrate the growth plate and reach the epiphysis, where
they end in sinusoidal lakes. This situation provides a vascular connection
between the metaphysis and the epiphysis and explains the frequency of
epiphyseal infections in neonates. Epiphyseal infection then can damage the
growth cartilage, resulting in disturbance or arrest of growth and maturation or
spread to the joint. In a child older than about 1 year, the capillaries of the
metaphysis do not cross the growth plate but form large sinusoidal lakes in the
metaphysis, which join venous capillaries. The predilection for involvement of
the metaphyses and equivalent locations in hematogenous infections in this age
group has been attributed to slow and turbulent blood flow in these areas. In
the adult, with closure of the growth plate, vascular continuity between the
epiphysis and metaphysis is restored. Nevertheless, hematogenous osteomyelitis
of tubular bones in the adult is rare. More commonly the spine, pelvis, and
small irregular bones are affected. If hematogenous infection occurs in a
tubular bone, it is likely to be seen in the subchondral region, and infection
may spread into the adjacent joint
Subacute and Chronic Hematogenous
Osteomyelitis:
Brodie's abscess:
A Brodie's abscess is a circumscribed lesion that may be present during
subacute and chronic stages of hematogenous osteomyelitis. These abscesses occur
more often in children than in adults, and they have a predilection for the ends
of tubular bones, especially the metaphysis and less commonly the epiphysis .
Preferred sites are the distal and proximal tibial metaphyses. Occasionally,
such abscesses are diaphyseal in location or are found in irregular or flat
bones, including vertebral bodies. On radiography, a lucent defect of 1 to 4 cm
in diameter is seen in the metaphyseal region with adjacent bone sclerosis. The
cavity is filled with mucus or pus, and the wall is lined with inflammatory
granulation tissue. Occasionally, a channel extends from the abscess to the
growth plate, which confirms the diagnosis of osteomyelitis. A Brodie's abscess
in the diaphysis may be central, subcortical, or cortical. Cortical location may
lead to confusion with an osteoid osteoma or a stress fracture. Brodie's
abscesses appear as well-circumscribed areas of low signal intensity on T1W
images and of high signal intensity on T2W images. These abscesses may show a
rim of low signal intensity on all sequences, resulting from bone sclerosis.
Surrounding bone marrow edema may be seen on T2W and STIR images. A double-line
sign on T2W images may be produced by a lining of granulation tissue on the
inner wall of the abscess. Delineation of Brodie's abscess may be facilitated by
intravenous administration of gadolinium contrast agent, which causes peripheral
rim enhancement.
Chronic osteomyelitis:
Chronic osteomyelitis may develop when acute osteomyelitis is treated
inadequately or when the patient has decreased immunity as a result of diabetes
mellitus, AIDS, or chemotherapy, although it is predominantly a complication of
open fracture. In cases of chronic osteomyelitis, CT can demonstrate involucra,
sequestra, cloacae, and sinus tracts or soft tissue abscesses. On MRI chronic
osteomyelitis may exhibit well-defined soft tissue abnormalities, thickened
cortex with or without sequestra and clear demarcation between affected and
nonaffected bone marrow. In chronic osteomyelitis, especially after trauma, a
well-described rim of low signal intensity on all sequences may represent
reactive fibrous tissue or bone formation.
Differentiation between active and inactive chronic osteomyelitis remains
problematic, however, and may be extremely difficult or impossible with
radiography alone. Although some investigators have indicated that increased
marrow signal intensity on T1W images reflects fatty replacement at sites of
healed osteomyelitis, others have been unable to distinguish between inactive
and active osteomyelitis. The combination of sequestration, periosteal elevation
with subperiosteal fluid and reactive bone formation is more consistent with
active infection. CT and MRI can confirm the presence of sequestra. MRI in
chronic osteomyelitis reveals areas of inhomogeneous signal intensity within the
medullary cavity. Active areas of inflammation exhibit low signal intensity on
T1W images and high signal intensity on T2W and STIR images, as well as
gadolinium-enhanced sequences; these findings relate to the presence of
granulation tissue as opposed to pus collections. However, diagnostic
difficulties may arise in cases after surgical intervention, since postoperative
and postinfectious scar tissue may show increased signal intensity on T2W
images. In these cases, an intensely increased signal intensity on T2W images
occurring over 9 months after the most recent surgical intervention is more
consistent with an infection. Nevertheless, these findings remain unreliable for
definitive diagnosis in an individual patient.
In chronic and subacute osteomyelitis, a low–signal intensity rim on T1W, T2W and STIR images has been described, which corresponds to an area of fibrous tissue or reactive bone. This “rim sign” has only rarely been seen in cases of acute osteomyelitis. Chronic fibrosis exhibits low signal intensity on both T1W and T2W images. The cortex of the infected bone is thickened and sequestra and sinus tracts can be demonstrated. Sinus tracts appear as linear or curvilinear areas of high signal intensity on T2W images, which involve both bone and soft tissue and may exhibit contrast enhancement.
Sequestration:
Occurring more commonly in tubular than in flat bones, sequestration can
be a feature of chronic hematogenous osteomyelitis. The necrotic bone
fragments have increased density on conventional radiographs, because of a
lack of blood supply. Sequestra consist of cortical or cancellous bone and are
usually surrounded by granulation tissue or exudate (cloacae). Eventually,
sequestra may be discharged through sinus tracts exiting on the skin surface.
In addition to conventional tomography and CT, MRI can be used successfully to
demonstrate bony sequestra. On MRI, sequestra appear as areas of low to
intermediate signal intensity on T1W and T2W MR images and do not exhibit
gadolinium enhancement.
Sclerosing osteomyelitis of
Garré:
In subacute and chronic osteomyelitis, periosteal bone formation can be
extensive and the cancellous bone also may be sclerotic. These features
suggest a healing response and are characterized by increased radiodensity and
irregularity of the bony contour with occasional cystic changes. The mandible
is the most common site for such alterations, although they may also occur in
the diaphyses of tubular bones. Osteoid osteoma, fibrous dysplasia, and
Ewing's sarcoma are differential diagnostic considerations.
MRI Findings:
MRI has proved to be extremely sensitive in the early detection of osteomyelitis
and has a higher sensitivity than CT in the detection of infective periostitis.
In the course of the infection, bone marrow is replaced by fluid and
inflammatory cells, which have longer T1 relaxation times than the unaffected
marrow fat. Infected areas therefore can be seen as regions of reduced signal
intensity on T1W sequences and as regions of increased signal intensity on T2W
and STIR sequences. Other abnormalities occurring in acute and chronic
osteomyelitis that can be observed with MRI include erosions and perforations of
the cortex, periosteal bone formation, and soft tissue involvement. In chronic
osteomyelitis abscesses, bone sequestration and sinus tracts may be found
Despite it's high sensitivity, MRI lacks specificity in the diagnosis of
osteomyelitis. Many disorders of the musculoskeletal system can produce similar
changes in signal intensity. These conditions include surgical alterations,
malignant bone tumors, bone infarction, metabolic disorders, fractures, and
sterile intraosseous fluid collections. Unless typical morphological changes are
present, as in fractures and infarcts, differentiation among these processes
with MRI alone may be difficult or impossible.
Pathophysiology
and Magnetic Resonance Imaging Correlation:
Acute inflammation of bone is characterized by vascular engorgement,
edema, cellular infiltration, and abscess formation. Increasing intramedullary
pressure in association with osteomyelitis leads to spread of the infectious
process into the cortical bone, with intracortical extension provided by the
Haversian and Volkmann's channels. The subperiosteal space subsequently becomes
infected. Because of the loose attachment of the periosteum to the bone in
infants and children, elevation of the periosteum is prominent. In the adult,
owing to the firm attachment of the periosteum to the bone, lifting of the
periosteum by the infectious process is less pronounced. The elevated periosteum
lays down bone in the form of an involucrum, especially in infants and children,
which can completely surround the bone. In infants and children, extensive
cortical necrosis and sequestration can occur, related to thrombosis of
metaphyseal vessels; in adults, however, this feature is unusual, and
pathological fractures are more frequently seen. Penetration of the infection
through the periosteum can lead to abscess formation in the adjacent soft
tissues. Sinus tracts are more common in adults than in infants and children.
Healing of osteomyelitis is characterized by thickening of the cortex. The
marrow cavity is replaced by granulation tissue and later by fibrous tissue.
Abscesses are transformed into cystic cavities, and fibrous tissue is ultimately
replaced by cellular or fatty marrow.
On T1W images, low to intermediate signal intensity of the bone marrow is
characteristic of both edema and pus. On T2W and STIR sequences, increased water
content and accumulation of inflammatory cells produce high signal intensity.
With infection of the subperiosteal space, cortical destruction may be detected
on MRI, and the periosteum may be lifted by fluid and abscesses. In some cases
of acute osteomyelitis, subperiosteal pus is detected in the absence of cortical
disruption or signal intensity alterations in the cortex. Subperiosteal
abscesses or fluid collections are low signal intensity on T1W and high signal
intensity on T2W and STIR sequences. Soft tissue edema around infected bone
leads to increased signal intensity on T2W and STIR images in the fatty tissues
and fascial planes, along with decreased signal intensity on T1W sequences. Soft
tissue edema and infection of the fasciae, muscles, and subcutaneous tissues
cannot be differentiated on T1W, T2W and fat-suppressed sequences. Intravenous
administration of gadolinium contrast agent, however, may demonstrate
enhancement of signal intensity in cases of cellulitis. Soft tissue abscesses
can be easily detected by a mass effect and distortion of soft tissue planes.
After intravenous gadolinium contrast agent administration, a typical rim
enhancement is seen around abscesses.
Contrast
Agents:
Generally, enhancement of signal intensity is seen in areas of
infection involving both soft tissues and bones, because of the presence of
vascularized inflammatory tissue. Enhancement of vascularized inflammatory
tissue and either nonenhancement or ring enhancement (of abscess collections has
been reported. Whereas the nonvascularized necrotic areas in an abscess do not
show enhancement, the rim or capsule of the abscess, corresponding to the
inflammatory cellular zone, enhances after gadolinium administration.
Gadolinium-enhanced MR imaging has also been reported to be helpful in the detection of active focuses in chronic osteomyelitis and in planning sites for percutaneous biopsy. Sequestra can be demonstrated as areas of low to intermediate signal intensity on T1W and T2W MR images that do not show gadolinium enhancement. Administration of a gadolinium contrast agent may be helpful for the delineation of sequestra, which do not enhance, and for differentiation of sequestra from vascularized granulation tissue. Furthermore, use of gadolinium contrast may be advantageous in defining intraosseous and soft tissue fluid collections, as well as sinus tracts, and facilitates planning of surgery or interventional procedures.
In septic and rheumatoid arthritis or bursitis, inflamed synovium in a joint or bursa enhances after intravenous gadolinium administration. Gadolinium enhancement is not helpful in the differentiation between osteomyelitis and bone infarction.