Subacute osteomyelitis is a distinct form of osteomyelitis, and Brodie abscess is one type of subacute osteomyelitis. Subacute osteomyelitis is difficult to diagnose because the characteristic signs and symptoms of the acute form of the disease are absent. It has an insidious onset, mild symptoms, and lacks a systemic reaction, and supportive laboratory data are inconsistent. Subacute osteomyelitis may mimic various benign and malignant conditions, resulting in delayed diagnosis and treatment. The most frequently made incorrect diagnosis is that of tumor.
Site of infection
Subacute osteomyelitis occurs in a much wider variety of bones than does the acute type, and it occurs at various sites within the affected bones. The lower limb is affected much more often than the upper limb. The tibia is affected relatively more often than is the femur. Subacute osteomyelitis may involve only the epiphysis, which is contrary to the belief that primary bone infection does not occur in the epiphysis.
The diaphysis is occasionally affected, although more often in adults than in children; the most common site affected is the metaphysis. Communication of the lesion between the metaphysis and the epiphysis is also common.
Other sites in which subacute osteomyelitis is frequently reported are metaphyseal-equivalent locations, like the pelvis, the vertebrae, the calcaneum, the clavicle, and the talus. When subacute osteomyelitis occurs in tarsal bones, it usually occurs in the subchondral part or on the border of the apophysis of the calcaneus. Subacute lesions of the spine occur more often in adults than in children. When subacute osteomyelitis occurs in the long bones of adults, the diaphysis is involved as often as is the metaphysis. The patella is rarely involved.
Classification
Ross and Cole (1985) categorized these lesions either as aggressive or as cavities in the area of the metaphysis and epiphysis. This categorization helps in the treatment plan, as aggressive lesions should be treated surgically for diagnosis. Gledhill classified subacute osteomyelitis according to radiologic appearance (1973), and this classification scheme has since been modified by Roberts et al (1982). The classification scheme is useful for reporting results of treatment according to site and is not a prognosis or treatment plan. The authors have modified the latter one as follows :
- Type I is a metaphyseal lesion.
- Type Ia is a central metaphyseal lesion seen as a punched out radiolucency, often suggestive of Langerhans cell histocytosis.
- Type Ib is a metaphyseal lesion eccentrically located with cortical erosion, which may give the appearance of osteogenic sarcoma.
- Type II is a diaphyseAl lesion.
- Type IIa is a localized cortical and periosteal reaction simulating osteoid osteoma.
- A type IIb lesion is a medullary abscess in the diaphysis without cortical destruction but with onionskin periosteal reaction resembling Ewing sarcoma.
- Type III is an epiphyseal lesion.
- Type IIIa is a primary epiphyseal osteomyelitis and appears as a concentric radiolucency. This type is seen usually in children younger than 4-5 years.
- Type IIIb is a subacute infection that crosses the epiphysis and involves both the epiphysis and metaphysis.
- A type IV lesion is a metaphyseal-equivalent lesion, which is defined as the portion of a flat or irregular bone that borders cartilage (apophyseal growth plates, articular cartilage, or fibrocartilage), like the vertebrae, the pelvis, and small bones such as tarsal bones and clavicle (Nixon, 1978).
- Type IVa involves the vertebral body with an erosive or destructive process.
- Type IVb involves flat bones of the pelvis and is mostly sclerotic with neither erosion nor destructive processes. Ezra et al mentioned this type in 1993 and 1997.
- Type IVc involves the small bones, like the tarsal bones and clavicle.
Imaging Studies:
Radiologic findings
- The various radiologic techniques involved are important and complementary, rather than competitive, in the diagnosis of subacute osteomyelitis. Radiologic osseous changes often are present, even in patients with a short history of symptoms (at least >2 wk to fit the diagnosis). Typically, a localized destructive lesion of bone is present, with surrounding sclerosis in the metaphysis.
- In some cases, a similar lesion with no surrounding sclerosis may be present. The lesion may cross the epiphyseal plate to affect the epiphysis as well , or the lesion may affect the epiphysis alone, though the articular cartilage itself is unaffected. Soft tissue swelling overlying the lesion earlier in the course of the disease might be seen. A central bone density occasionally is seen in the presence of a sequestrum, which makes it difficult to differentiate subacute osteomyelitis from osteoid osteoma on plain films. On occasion, the lesion appears to become tethered to the growth plate, and the cavity progressively elongates with growth extending from the epiphysis into the diaphysis in snakelike fashion (the serpentine sign described by Letts [1988].
- In diaphyseal lesions, periosteal reaction may occur with a single layer or laminated with or without bony destruction.
- In spinal lesions (which occur more often in adults than in children), radiographs may show signs of healing by the time the diagnosis is made . The principle feature that helps to distinguish it from tuberculosis is sclerosis of the vertebral body with a variable degree of destruction of bone and disc space, associated with relatively early new bone formation in the form of bony bridging between adjacent vertebral bodies. A paravertebral abscess may be present, but it usually is much smaller than in tuberculosis infections.
Magnetic resonance imaging
- MRI is the most sensitive investigation in the evaluation of bone marrow pathology.
- Signal intensity is decreased on T1-weighted images of the lesion, and signal intensity is increased on T2-weighted images, with a rim of decreased intensity due to sclerotic bone.
- A gadolinium-enhanced image depicts a well-circumscribed nonenhancing area with slight rim enhancement.
- A characteristic but not pathognomonic MRI finding that supports the diagnosis of subacute osteomyelitis and helps to exclude the presence of a tumor is the penumbra sign, which was reported by Grey et al (1998) to have 75% sensitivity, 99% specificity, and 99% accuracy; in their experience, the penumbra sign did not appear to occur with any great frequency in other osseous conditions.
- The penumbra sign is characteristically seen on T1-weighted MRI images (2- to 5-mm thickness) and is due to a thick layer of highly vascularized granulation tissue. (The presence of a layer of granulation tissue lining a cavity is important in the differentiation of an abscess from tumor.)
- It is a discrete peripheral zone of marginally higher signal intensity than the abscess cavity and surrounding marrow edema/sclerosis and of lower signal intensity than the fatty bone marrow. The hyperintensity is may be due to the high protein content of the granulation tissue. A similar appearance has been described in the wall of brain abscesses.