eMedicine Specialties > Orthopedic Surgery > Knee

Subacute Osteomyelitis (Brodie Abscess): Workup

Author: Khalid Khoshhal, MBBS, FRCS Ed, ABOS, Vice Dean, College of Medicine; Assistant Professor, Pediatric Orthopedic Surgery, Department of Orthopedics, Taibah University, Saudi Arabia
Coauthor(s): Robert Mervyn Letts, MD, FRCS(C), FACS, Former Chief, Department of Surgery, Division of Pediatric Orthopedics, Children's Hospital of Eastern Ontario, University of Ottawa; Consultant Pediatric Orthopedic Surgeon, Sheikh Khalifa Medical City, UAE
Contributor Information and Disclosures

Updated: Jul 18, 2008

Workup

Laboratory Studies

The laboratory workup of subacute osteomyelitis includes the following:

• The white blood cell (WBC) count is usually within the reference ranges or occasionally slightly elevated, with a normal differential.
• The erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) measurements are usually mildly elevated, but they may be within the reference ranges in 30-50% of patients.
• Blood culture results are usually negative.

Imaging Studies

• Radiologic findings
  • The various radiologic techniques involved in the diagnosis of subacute osteomyelitis are important and complementary, rather than competitive. Radiologic osseous changes are often 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 (see Images 4-5).
  • 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 (see Images 6-7), or the lesion may affect the epiphysis alone, although the articular cartilage itself is unaffected (see Image 1). Soft-tissue swelling overlying the lesion earlier in the course of the disease might be seen. A central bone density is occasionally 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 a snakelike fashion (the "serpentine sign" described by Letts.)¹¹ (See Images 9-10).
  • In diaphyseal lesions, periosteal reaction may occur with a single layer or it may be laminated with or without bony destruction (see Image 2).
  • 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 (see Image 8). The principal feature that helps to distinguish subacute osteomyelitis 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 is usually much smaller than in tuberculosis infections.
• Bone scanning
  • Findings on technetium (Tc) bone scans are often positive (see Images 11-12), but false-negative results or, less likely, false-positive results are also possible. In addition, bone scan findings are nonspecific, simply demonstrating an increased vascularity or metabolic activity within the bone on the delayed image. Close proximity of the focus of infection to the growth plate may render interpretation of bone scans difficult. The sensitivity and specificity of bone scanning have not been studied in subacute osteomyelitis, but they are better than 90% in cases of osteomyelitis of nonviolated bone when a 3-phase bone scan is performed.
  • Because subacute osteomyelitis has such characteristic features on normal radiographic examination, bone scanning is seldom indicated unless the diagnosis is unclear and a bone scan is performed as part of a tumor workup. Also, bone scanning might be of help in delineating the rarely occurring multifocal subacute osteomyelitis.
  • Gallium scans and scans with Indium 111 (¹¹¹ In)–labeled WBCs (WBC scan) have been used in conjunction with the Tc bone scan in the localization of infection, but they also remain nonspecific. Fractures and infarctions can lead to false-positive results with a WBC scan. In addition, these scans are more expensive, take longer to complete, and entail more radiation exposure (high absorbed radiation to the spleen and lymphocytes limit the injected dose in WBC scans) than Tc scans.
  • Insufficient data exist regarding the specificity of the newer scintigraphic agents, Tc-99m (^99m Tc) hexamethylpropyleneamine oxime (HMPAO)–labeled leukocytes, and nonspecific polyclonal¹¹¹ In-labeled immunoglobulin G (IgG). Although Roddie et al reported the use of^99m Tc HMPAO–WBCs in 20 patients with suspected osteomyelitis in general, with a reported sensitivity of 100% and specificity of 93%,²⁸ the use of polyclonal human IgG is not approved in the US despite its use in some European countries. The advantages of polyclonal human IgG include the fact that it has a simple preparation procedure compared with that of¹¹¹ In-labeled WBCs, eliminates the need for phlebotomy and laborious labeling methods, and reduces the patient radiation dose.
  • Infecton (Draximage Inc, Kirkland, Quebec, Canada) is another radiopharmaceutical, which is based on ciprofloxacin that is labeled with^99m Tc. The sensitivity is reduced for microorganisms with membranes impermeable to ciprofloxacin, but this method allows better differentiation between infection and sterile inflammation. Infecton is not available in the US but it is used in some hospitals in Europe.
  • In spinal infections, single photon emission (SPE) may reveal abnormalities not seen on the planar images. Degenerative disc disease is a cause of false-positive bone scan results. Gallium specificity is greater than 90% and is almost equivalent to magnetic resonance imaging (MRI) for spinal infections. WBC scanning, however, is not sensitive to vertebral osteomyelitis (40%).
• Computed tomography (CT) scanning
  • Broaching of the physis may not always be apparent on plain radiographs. It is more readily demonstrated by tomography or by CT.
  • CT scanning is valuable in detecting lesions in difficult anatomic locations that could not be seen with plain radiographs, as in the pelvis and sacrum (see Images 13-14).
  • CT scanning is also valuable in differentiating subacute osteomyelitis from osteoid osteoma. In osteoid osteoma, the nidus is central to the lesion, round, smooth, and well defined. In subacute osteomyelitis, a sequestrum, which is usually irregular and eccentric with respect to the radiolucent cavity, may occasionally be present.
• MRI
  • MRI is the most sensitive investigation in the evaluation of bone marrow pathology.
  • Signal intensity is decreased on T1-weighted images of the lesion (see Image 15), whereas signal intensity is increased on T2-weighted images (see Image 16), with a rim of decreased intensity due to sclerotic bone.
  • A gadolinium-enhanced image depicts a well-circumscribed nonenhancing area with slight rim enhancement (see Images 17-18.)
  • 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 to have 75% sensitivity, 99% specificity, and 99% accuracy²⁹ ; 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 MRIs (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 a 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 may be due to the high protein content of the granulation tissue. A similar appearance has been described in the wall of brain abscesses.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], and gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy.

NSF/NFD has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.

Diagnostic Procedures

• Fine needle aspirations (FNAs) of the subacute osteomyelitis abscess cavity do not usually allow isolation of the organism. Open drainage culture findings are positive in 50-75% of patients. Whether the culture-negative abscesses are truly negative or whether they are caused by fastidious organisms remains to be investigated. K kingae, for example, is a relatively new pathogen that has appeared to replace the predominance H influenzae in children younger than 3 years and is known to have a tenuous nature that can make it difficult to isolate on cultures.¹⁸ For this reason, K kingae or other similar organisms may be the causative organisms associated with some cases of so called culture-negative osteomyelitis.

Histologic Findings

In subacute osteomyelitis, the surrounding bone is usually sclerotic, but it is of variable thickness, most often thin rather than dense and thick. For most lesions, granulation tissue lines the abscess cavities, and the presence of fat, fibroblastic response (commonly, a fibrin layer separates bone from granulation tissue), remnant of necrotic bone, and new bone formation is seen.
 
Inflammatory infiltration in the form of acute and chronic cells consisting of polymorphonuclear lymphocytes (PMLs), lymphocytes, and plasma cells are seen (see Images 19-21). Pus under pressure is rarely encountered. The fluid content of the cavity may be purulent, oily, or even mucoid.³⁰  In diaphyseal lesions at operation, thickened periosteum with a thickened hard cortex without pus is usually encountered. The histologic appearance is usually that of subperiosteal new bone formation with inflammatory cells (plasma cells, fibroblasts, and PMLs) between the trabeculae of the medulla.

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