- Imaging findings of osteomyelitis
-Conventional radiography
Conventional radiography should be the first imaging exam requested since it may show some osteomyelitis features and can also exclude other diagnosis such as fractures or bone tumors.
However,
it has low sensitivity and radiologic findings only appear one to three weeks after the beginning of the infection.
Initially,
the imaging features are subtle.
Then radiography may show a wide-range of findings,
such as soft-tissue edema,
osteopenia,
impaired trabecular arrangement,
lytic lesions,
endosteal scalloping,
periosteal reaction and new bone apposition.
In sub-acute stage,
an intraosseous abscess called Brodie Abscess may be seen mainly the in extremity of tubular bones,
typically tibia and femur (Fig. 1).
It appears as a well-defined lytic lesion,
with a sclerotic halo,
ranging from 1 to 4 cm.
Fig. 1: Brodie abscesses in three different patients appearing as a lytic lesion with a sclerotic rim. A- AP knee x-ray shows an abscess on the proximal tibial metaphysis. B- Left ankle x-ray shows an abscess on the distal tibial metaphysis. C- AP knee x-ray shows two abscesses, one in distal femur and the other on the proximal extremity of the tibia.
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
In chronic phases,
radiographs may show sequestrum, appearing as sclerotic calcified bone within lucent bone.
Osteoid osteoma,
eosinophilic granuloma and lymphoma are some conditions that may mimic the sequestrum.
Radiographs may also show the involucrum,
ill-defined lytic lesions and cortical destruction (Fig. 2).
Fig. 2: Osteomyelitis. Right AP foot x-ray (magnified on the right) shows ill-defined lytic lesions and cortical destruction in the distal phalange of the hallux.
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
-Computed tomography (CT)
On CT,
osteomyelitis findings are similar to those shown by radiography (Fig. 3),
but they can be detected on an earlier phase.
Fig. 3: Brodie abscesses. Right knee CT coronal (A) and axial (B, C) images, from the same patient as Figure 1-C, show two lytic lesions in a child presenting with fever and pain, representing Brodie abscesses: one in the distal femoral metaphysis (A, B) and the second on the proximal metaphysis of the tibia (A, C).
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
CT is better than MRI to identify sequestrum,
cloaca (Fig. 4),
involucrum and intra-osseous gas,
a rare sign of osteomyelitis.
Fig. 4: Ankle CT sagittal (A) and axial (B) images show a lytic lesion on the calcaneum, containing a bone fragment inside, consistent with a Brodie abscess with a sequestrum (circles). On the axial image (B), a cortical defect representing the cloaca (arrow) is seen. MRI was also performed to exclude osteoid osteoma.
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
CT allows guidance for intervention radiology procedures,
although it uses ionizing radiation.
Not to forget that it is inferior to MRI in the study of soft-tissues,
as it does not show bone marrow edema,
an important finding in the earlier stages of osteomyelitis.
Its accuracy may be limited in the presence of metallic prosthesis causing beam-hardening artifacts.
-Ultrasound (US)
US has an important role in the pediatric population,
as it is easily accessed,
gives real time information,
does not use ionizing radiation and is a relatively low-cost technique.
Like CT,
it also allows guidance for intervention radiology procedures.
US may show elevated periosteum by hypoechoic purulent material (periosteal abscesses) (Fig. 5),
adjacent soft tissue involvement with collections (Fig. 6) or fistulas,
cortical erosions,
concomitant septic arthritis,
soft tissue edema and foreign bodies.
Fig. 5: Gray-scale US images show a subperiosteal abscess, seen as irregularity of the tibial periosteum with hyperechoic material extending towards soft-tissue, in a child with osteomyelitis.
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
Fig. 6: Chest wall gray-scale US images in transverse (A) and longitudinal (B) plans show a fluid collection (C) representing a soft-tissue abscess in a child with rib (b) osteomyelitis. Periosteum irregularity can also be seen on both images.
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
-Magnetic resonance imaging (MRI)
MRI is the gold standard in the diagnosis of osteomyelitis,
due to its high sensitivity,
specificity and its good spatial resolution.
Imaging findings are best seen on T1-weighted and fluid sensitive sequences.
Whenever an abscess or sinus tract is suspected to be present,
T1-weighted sequences with fat saturation after gadolinium injection should also be acquired.
The key feature of osteomyelitis is bone marrow edema,
which is seen as a low signal on T1-weighted and high signal on T2-weighted and short tau inversion recovery (STIR) images.
It is the first imaging sign and appears as early as in 24 to 48 hours.
(Fig. 7)
Fig. 7: Acute osteomyelitis. Left hip x-ray (A) of a boy presenting with left hip pain and fever. No clearly lesions are seen. MR images show abnormal marrow signal on the left acetabulum, seen as low-signal on Fat-saturated (FS) T1 (B and D) and T1 (C). After gadolinium injection (D), the bone marrow enhances.
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
Abscesses are hypointense on T1-weighted images and hyperintense on fluid sensitive sequences.
After the administration of gadolinium,
the granulation tissue enhances but the central cavity does not,
representing the “penumbra sign” (Fig. 8).
Fig. 8: Penumbra sign. Sagittal MRI FS T1-weighted image with contrast show a round hypointense area with peripheral enhancement (circle), representing an intra-osseous abscess. Anteriorly, a cutaneous ulcer is seen as subcutaneous tissue thinning. Also, in the most distal part of the tibia, bone avascular necrosis is seen (arrow).
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
Sequestrum is seen on MRI as an area of low signal,
surrounded by granulation tissue,
which has low to intermediate signal on T1-weighted and high signal on fluid sensitive sequences.
After contrast,
granulation tissue enhances,
but central dead bone don’t (Fig. 9 and Fig. 10).
Sinus tracts and fistulas may appear as a fluid-linear structure draining to skin surface.
Fig. 9: Sequestrum. Axial MRI images in a patient with osteomyelitis of the right tibia show a round area (inside circles) of low-signal on T2-weighted (A) and FS T1-weighted sequences (B), that exhibits peripheral enhancement on the FS T1 with contrast image (C), representing a sequestrum with outlying enhancing granulation tissue.
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
Fig. 10: AP leg x-ray (A) shows an ill-defined lytic lesion (arrow) on the proximal metaphysis of the tibia. MRI fat-sat T1-weighted images with gadolinium on coronal (B) and sagittal (C) planes show an abscess (arrows) on the proximal metaphysis of the tibia with peripheral enhancement – penumbra sign. In the most distal part of the tibia, an area of bone avascular necrosis is also seen (circles). Axial MRI FS PD with contrast image (D) show a small cortical defect representing the cloaca (*). The patient was submitted to abscess drainage as seen on the intraoperative photograph (E).
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
MRI is also important to determinate the extent of the inflammatory process to contiguous tissues (Fig. 11),
to detect acute inflammatory activity in chronic osteomyelitis and to help on planning surgical treatment.
Fig. 11: Sagittal fluid-sensitive sequence MRI image shows a sub-periosteal abscess (circle) in a patient with acute osteomyelitis and associated soft-tissue involvement. Two days after, the patient developed a compartment syndrome and fasciotomy was performed, as seen on the intra-operative photograph (B). Five days after this surgery, MRI show muscles edema due to fasciitis and a defect on the subcutaneous tissue related to the surgical procedure performed (arrow). An intramuscular abscess is also seen (*).
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
On the other hand,
MRI has the disadvantage of requiring sedation or general anaesthesia in children.
Several conditions with different etiologies may mimic osteomyelitis.
-Stress injuries
Stress fractures may be a diagnostic challenge.
They occur more frequently in weight-bearing bones of the lower limb and present with insidious pain that gets more frequent over time.
The first radiographs may be normal.
MRI plays a major role on the diagnosis of these fractures,
showing a fracture line surrounded by abnormal bone marrow signal (Fig. 12).
Like in osteomyelitis,
periosteal reaction may be seen,
but no soft tissue affection is expected to be present.
Fig. 12: Stress fracture in a adolescent presenting with night pain. Technetium 99m bone scan (A) show increased uptake in the right medial malleolus. Coronal MRI FS PD image (B) show abnormal marrow signal on the distal tibia and a small hypointense cortical interruption representing the fracture line (arrow). Sagittal MRI images show surrounding bone marrow edema, seen as low signal on T1-weighted (C) and high signal on STIR images (D).
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
-Primary bone neoplasms
Some primary bone tumors may mimic osteomyelitis,
such as multiple myeloma,
Ewing sarcoma and osteosarcoma.
To differentiate them,
the age of the patient must be considered and also his clinical history,
as tumors tend to have a slower growth when compared to osteomyelitis.
Multiple myeloma is the most prevalent primary bone tumor in adults.
Its imaging features range from multiple “punched-out” lytic lesions involving predominantly the axial skeletal,
to diffuse osteopenia or a single expansive lesion (plasmocytoma).
MRI has a good sensitivity to detect bone marrow infiltration.
Osteosarcoma is the most common bone tumor in childhood.
It may be classified as primary or secondary.
Primary osteosarcomas are more common in males between the ages of 10 to 20 years.
They usually affect the metaphysis of long bones,
most frequently around the knee.
On radiographs,
a bone-destroying lesion is seen,
usually associated with aggressive periosteal reaction and with a soft-tissue mass.
On MRI,
a mass with heterogeneous signal is seen,
which reflects the mixed tumoral matrix composed by both mineralized and non-mineralized parts.
Ewing sarcoma is the second most prevalent bone tumor in children,
after osteosarcoma.
It also has a male-predominance and is more common in adolescents and young adults. Radiographs typically show a poorly-defined permeative lytic lesion located on the metaphysis/diaphysis of long bones that may have associated prominent soft-tissue mass.
On MRI,
the tumoral lesion has low signal on T1 and high signal on T2-weighted sequences (Fig. 13).
The abnormal marrow signal tends to be well-defined,
unlike what is seen in osteomyelitis.
Fig. 13: Ewing sarcoma. AP x-ray of the right elbow (A) shows an expansive lesion, predominantly lytic, on the proximal extremity of the humerus. MRI coronal STIR image (B) shows abnormal marrow signal depicted by hyperintensity on the meta-diaphysis of the humerus.
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
-Bone metastasis
Many tumors can invade bone,
but skeletal metastasis are more frequently found in association with lung,
renal cell,
breast and prostate cancers.
Bone metastasis are usually present in well irrigated parts of the skeleton such as skull,
vertebral bodies,
proximal femur and humerus and pelvis.
They can have a lytic (Fig. 14),
sclerotic or mixed appearance.
Periosteal reaction may be seen,
generally less exuberant than what is seen in primary tumors.
Fig. 14: Bone metastasis in a patient with breast cancer. Right hip x-ray shows a permeative pattern on the right femoral neck. Coronal CT scan shows lytic lesions on this location. Technetium 99m bone scan (C) shows increased uptake in the proximal femur and in D11 vertebral body, consistent with metastatic disease.
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
-Charcot Joint/Neuropathic arthropathy
Charcot Joint is a degenerative disorder that affects patients with loss of sensation,
most commonly caused by diabetes mellitus neuropathy.
This arthropathy leads to the destruction of weight-bearing joints and has two main forms: atrophic and hypertrophic.
It usually affects multiple bones with a juxta-articular distribution,
mainly in the midfoot (Fig. 15),
whereas osteomyelitis is more common in a single weight-bearing bone,
in the forefoot or hindfoot.
MRI is an important tool to differentiate Charcot from osteomyelitis,
but also to identify a superimposed osteomyelitis on a Charcot foot.
Charcot is the most likely diagnosis if,
beside abnormal bone marrow signal,
there are no other associated features such as soft-tissue inflammatory changes,
cutaneous ulcers or sinus tracts.
Fig. 15: Charcot joint in a diabetic patient. Oblique foot x-ray (A) shows deformity of the mid-foot with associated subchondral sclerosis. Sagittal MRI STIR (B) and T1-weighted (C) images show diffuse mid-foot bone marrow edema with some extension to the calcaneum. No skin ulcer was seen.
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
-Langerhans cell histiocytosis (LCH)
LCH is a rare systemic disease caused by abnormal proliferation of Langerhans cells.
Patients may present with a wide-range of symptoms,
which vary according to the body part that is involved.
The musculoskeletal system is the most affected one,
mainly in young males.
Imaging exams may show a wide range of findings,
such as single or multiple “punched-out” lytic lesions (Fig. 16),
vertebra plana or floating tooth.
In long bones,
periosteal reaction and endosteal scalloping may be seen,
like in osteomyelitis.
However,
in LCH,
diaphysis are more frequently affected,
rather than metaphysis.
Fig. 16: Bone involvement in Langerhans cell histiocytosis. Skull x-ray (A) and axial CT image (D) show an isolated right parietal lytic lesion (arrows).
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
- Chronic recurrent multifocal osteomyelitis (CRMO)
CRMO is an idiopathic condition that affects most frequently female children or adolescents.
It presents with chronic,
multifocal and typically symmetric pain,
with a relapsing and remitting pattern.
Lesions are most commonly located on lower extremities,
committing metaphysis of long bones,
with possible associated physis involvement.
Medial clavicles and spine may also be affected.
Unlike osteomyelitis,
in CRMO inflammatory markers are usually low.
Radiographs initially show lytic lesions with a sclerotic halo.
Over time,
these lesions can get progressively sclerotic.
MRI is more sensitive to detect CRMO,
showing abnormal marrow signal,
periostitis and soft-tissue edema (Fig. 17).
Abscesses,
fistulas or sequestrum are not present on CRMO,
distinguishing it from osteomyelitis.
Fig. 17: CRMO in a patient presenting with left hip pain. Pelvis x-ray (A) show mild sclerosis on the proximal femur metaphysis, but also on the right iliac bone. MRI coronal FS-T1 with contrast image (B) shows abnormal bone marrow signal on both locations, associated with periosteal reaction and adjacent soft-tissue inflammatory changes.
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal
Promptly treatment of osteomyelitis is important to prevent bone necrosis,
thus decreasing complications and morbidity rates.
The mainstay of acute osteomyelitis treatment is optimal long-term antibiotic therapy.
Surgery is most likely to be necessary in adults,
rather than in children with acute osteomyelitis.
Surgery may be necessary in some situations,
for example: if a sequestrum is present,
in patients with infected orthopedic devices or if there’s no response to antibiotics.
In some cases a multi-step approach is preferred,
including multiple procedures such as sequestrum removal,
dead space management with bone grafting or tissue flaps and local therapy with antibiotic-impregnated material.
Especially in diabetic patients,
minor or major amputations are sometimes necessary due to bad outcomes with antibiotic therapy only.
Imaging findings of some surgical procedures are seen on Fig. 18.
Fig. 18: Imaging findings of some surgical procedures. A- Right foot AP x-ray of a diabetic patient submitted to transmetatarsal amputation of the first toe. B- Side knee x-ray shows a lateral cortical window (arrow) on the femur, made for bone abscess drainage. Also, signs of knee osteoarthritis may be seen.
References: Department of Radiology, Centro Hospitalar de São João/Faculdade de Medicina da Universidade do Porto, Portugal