Applied Radiology

Diagnosis

Chronic osteomyelitis from Staphylococcus aureus

Findings

Radiography of the left femur revealed lucency with fuzzy sclerotic margin of Brodie's abscess and bony remodeling (Figure 1). MR imaging showed an ovoid well-circumscribed area of low T1 signal intensity with peripheral ring enhancement and surrounding bone marrow edema (Figures 2 through 3). The short tau inversion recovery (STIR) images showed high signal intensity within the marrow, which is characteristic of osteomyelitis (Figure 4).

Incision and trephination were performed; pathologic examination after orthopedic intramedullary placement is shown in Figure 5. A tobramycinimpregnated cemented implant was subsequently placed (Figure 6), and the patient was treated with 6 weeks of intravenous antibiotics. The cemented implant was removed after 8 weeks.

Discussion

Osteomyelitis presents in 3 stages with ill-defined transitions: Acute, subacute, and chronic.¹ Transition from acute to subacute or chronic indicates that therapeutic measures have been inadequate or inappropriate or that the organisms are especially resistant to accepted modes of therapy. Four routes of infection predominate: Hematogenous spread, contiguous spread (usually cutaneous, sinus, or dental in origin), direct implantation (usually from trauma), or postoperative infection.¹ Hematogenous spread is usually caused by Staphylococcus aureus. Delay in the treatment of osteomyelitis significantly decreases the cure rate and increases the rate of complications and morbidity.² The differential diagnostic considerations for osteolytic foci in the epiphysis include giant-cell tumor, clear-cell chondrosarcoma, osteonecrosis, fibrous dysplasia, intraosseus ganglion, and subchondral cyst.¹

The radiographic findings may be normal for the first 10 to 21 days after the onset of infection because 30% to 50% of bone density must be lost before a lucency can be appreciated.² Detection of chronic osteomyelitis is also difficult because a sequestrum is visible in only 9% of cases. Progressive changes on serial radiographs have a sensitivity of 14% and specificity of 70%.²

Radiographs are typically obtained first, but the detection of marrow abnormality on MRI is a more sensitive indicator of osteomyelitis than are the lytic changes seen on plain radiography.³ Osteomyelitis is visible as a low signal intensity on T1-weighted images and high signal intensity on T2weighted, STIR, or fat-saturated images.² The STIR pulse sequence is considered highly sensitive for abnormalities with a negative predictive value approaching 100% for acute osteomyelitis. The STIR images have a lower spatial resolution and cannot clearly illustrate the differentiation of abscess from soft-tissue edema.²

Acute and chronic osteomyelitis can be differentiated in several ways. MR imaging findings that suggest the presence of active chronic osteomyelitis include sequestra, cloacae, abscesses, and subperiosteal fluid collection. MRI findings favoring acute infection include a wide zone of transition and poor definition between normal and diseased marrow. Findings favoring chronic infection include a relatively sharp interface between normal and diseased marrow.²

Brodie's abscess is a form of subacute to chronic osteomyelitis, classically defined as a sharply delineated focus of infection lined by granulation tissue and frequently surrounded by eburnated bone.⁴This case revealed a rounded, well-circumscribed area of low T1 signal intensity with gadolinium administration characteristic of chronic osteomyelitis.² It has also been reported that the more profound the T2 signal intensity in the bone marrow, the more likely the abnormality is osteomyelitis.²

CONCLUSION

The authors have presented a case of chronic osteomyelitis with characteristic radiographic, MR, and pathologic features. MR imaging can be very helpful in the workup of patients suspected of having this diagnosis.

1. Resnick D. Diagnosis of Bone and Joint Disorders.Vol 3. 4th ed. Philadelphia, PA: W.B. Saunders; 2002:2378-2379, 2418-2419.
2. Tehranzadeh J, Wong E, Wang F, Sadighpour M. Imaging of osteomyelitis in the mature skeleton. Radiol Clin North Am.2001;39:223-250.
3. Jurriaans E, Singh NP, Finlay K, Friedman L. Imaging chronic recurrent multifocal osteomyelitis. Radiol Clin North Am.2001;39:305-327.
4. Resnick D. Bone and Joint Imaging.2nd ed. Philadelphia, PA: W.B. Saunders; 1996:650.

Products used

• Omniscan (GE Healthcare, Princeton, NJ)
• 1.5T MR scanner (Philips Medical Systems, Bothell, WA)
• Plain film equipment (GE Healthcare, Waukesha, WI)