Imaging findings

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Conventional radiography

Plain radiographs are the first step in the imaging assessment of osteomyelitis because they may suggest the correct diagnosis, exclude other pathology, or provide clues for other pathologic conditions. It takes from 10 to 21 days for an osseous lesion to become visible on conventional radiographs, because a 30% to 50% reduction of bone density must occur before radiographic change is apparent. It is the insensitivity of this technique in the early diagnosis of bone infection that has prompted the use of alternative methods, such as scintigraphy and MR imaging, for the prompt recognition of osteomyelitis [22].

Radiographic evidence of significant osseous destruction in hematoge-nous pyogenic osteomyelitis is delayed for a period of days to weeks. Initial and subtle radiographic changes in the soft tissues may appear within 3 days of bacterial contamination of bone; however, focal deep soft tissue swelling in the metaphyseal region of infants and children may be the first important radiographic sign. Such swelling, which is temporally related to the vascular changes and edema of the early osteomyelitic process, results in displacement of the lucent tissue planes from the underlying bone. In the neonate, this displacement is difficult to detect because of the lack of subcutaneous fat and the presence of poorly defined fascial planes. A few days after the appearance of the initial soft tissue changes, muscle swelling and obliteration of the soft tissue planes can be observed. The deep muscles and soft tissues are affected first, followed later by involvement of the more superficial muscles and subcutaneous tissue [22].

In pyogenic infection, radiographically evident bone destruction and periostitis can be delayed for 1 to 2 weeks after intraosseous lodgment of the organisms. At all early stages, the degree of bony involvement that is visible on the radiograph is considerably less than is evident on pathologic examination. Eventually, large destructive lesions become evident on the radiograph (Fig. 5). In the child, these lesions appear as enlarging, poorly defined lucent shadows of the metaphysis surrounded by varying amounts of eburnation; the lucent lesions can extend to the growth plate and, on rare occasions, may violate it (Fig. 6). in addition, destruction progresses horizontally, reaching the cortex, and periostitis follows.

in the infant, the epiphyses are unossified or only partially ossified, so that radiographic recognition of epiphyseal destruction can be extremely difficult. Metaphyseal lucent lesions, periostitis, and a joint effusion are helpful radiographic clues. in the adult, soft tissue alterations are more difficult to detect on radiographic examination. Epiphyseal, metaphyseal, and diaphyseal osseous destruction create radiolucent areas of varying size, which are associated with mild periostitis. Cortical resorption can be identified as endosteal scalloping, intracortical lucent regions or tunneling, and poorly defined subperiosteal bony defects or gaps.

Single or multiple radiolucent abscesses can be evident during subacute or chronic stages of osteomyelitis. These abscesses now are defined as circumscribed lesions showing predilection for (but not confinement to) the ends of tubular bones; they are found characteristically in subacute pyogenic osteomyelitis, usually of Staphylococcal origin. Brodie's abscesses (Fig. 7) are especially common in children, more typically boys. in this age group, they appear in the metaphysis, particularly that of the distal or proximal portions of the tibia. Abscesses vary from less than 1 cm to more than

Metaphysis Elbow

Fig. 5. Variola (Smallpox) osteomyelitis and septic arthritis. AP (A) and oblique (B) views of the elbow. The presence of symmetric changes, epiphyseal extension and destruction, predilection for the elbow, and extensive diaphyseal osteoperiostitis of tubular bones (arrow) suggest the diagnosis of osteomyelitis variolosa.

Fig. 5. Variola (Smallpox) osteomyelitis and septic arthritis. AP (A) and oblique (B) views of the elbow. The presence of symmetric changes, epiphyseal extension and destruction, predilection for the elbow, and extensive diaphyseal osteoperiostitis of tubular bones (arrow) suggest the diagnosis of osteomyelitis variolosa.

Fig. 6. In this child who had pain and swelling of the ankle, initial radiographs at the time of clinical presentation do not reveal osseous destruction. Four weeks later, a lytic metaphyseal focus in the distal tibia readily is apparent. It extends to the growth cartilage. The presence of multilayered type of periostitis is also seen. (Courtesy of Sergio Fernandez Tapia, MD, Tampico, Mexico.)

4 cm in diameter. The wall of the abscess is lined by inflammatory granulation tissue that is surrounded by spongy bone eburnation. The fluid in the abscess may be purulent or mucoid; bacteriologic examination of the fluid may or may not reveal the infecting organisms. Closed needle biopsy or aspiration guided by imaging techniques can be useful in establishing

Fig. 7. Brodie's abscess. Anteroposterior radiograph of the tibia. Observe the elongated radio-lucent lesion with surrounding sclerosis not extending to the closing growth plate. (Courtesy of Sergio Fernandez Tapia, MD, Tampico, Mexico.)

a bacteriologic diagnosis of osteomyelitis. Material should be obtained for both histologic diagnosis and appropriate tissue culture (Fig. 8). Radiographs outline radiolucency with adjacent sclerosis. This lucent region commonly is located in the metaphysis, where it may connect with the growth plate by a tortuous channel. A circular or elliptic radiolucent lesion without calcification that is smaller or larger than 2 cm is characteristic of a cortical abscess, a circular lucent area with or without calcification smaller than 2 cm is typical of an osteoid osteoma, and a linear lucent shadow without calcification is characteristic of a stress fracture [22]. in any skeletal location, CT or MR imaging can be used to better assess the extent of the abscess and any signs of its reactivation. Radiographic detection of this channel is important; identification of a metaphyseal defect connected to the growth plate by such a tract ensures the diagnosis of osteomyelitis. When an abscess is located in the cortex, its radiographic appearance, consisting of a lucent lesion with surrounding sclerosis and periostitis, simulates that of an osteoid osteoma or a stress fracture.

The periosteal reaction (PR) of bone deserves a special mentioning. it is observed in a wide variety of benign, malignant, and systemic conditions, and in periods of normal growth and in response to injury. PR can be classified broadly as lamellated (linear-single or multiple), solid, spiculated, Codman's triangle, and expanded shell, based on radiographic appearances. The differential diagnosis of multiple layered PR (onion-skin image) includes osteomyelitis, Ewing's sarcoma, osteosarcoma, hypertrophic osteo-arthropathy, and Langerhans cell histiocytosis [33]. When an underlying process persists, the matrix between multiple lamellations or between a single layer and the cortex eventually ossifies, giving rise to a continuous, solid

Bone Biopsy Osteomyelitis
Fig. 8. Distal tibial bone biopsy and aspiration. AP (A) and lateral (B) views. Closed needle biopsy and aspiration accomplished with fluoroscopic guidance confirm the presence of S aureus osteomyelitis.

layer of periosteal new bone. Solid PR is associated with osteomyelitis, LCH, chondroblastoma, and healing fractures. Codman's triangle typically is associated with osteosarcoma and osteomyelitis. This cuff of reactive bone at the extreme ends of a lesion represents periosteal elevation by pus, hemorrhage, or the leading edge of a neoplastic lesion [33].

In the axial skeleton, hematogenous spread of infection frequently leads to a focus in the anterior subchondral regions of the vertebral body adjacent to the intervertebral disc. Extension to the ventral surface of the vertebra can be associated with infection of the adjacent longitudinal ligaments, but more typically discal perforation soon ensues. At this stage, radiographs may be entirely normal. Soon (1 to 3 weeks), however, a decrease in height of the intervertebral disc is accompanied by loss of normal definition of the subchondral bone plate and enlarging destructive foci within the neighboring vertebral body. The combination of rapid loss of intervertebral disc height and adjacent lysis of bone is most suggestive of an infectious process. With further spread of infection, progressive destruction of the vertebral body and the intervertebral disc becomes evident, and the process soon contaminates the adjacent vertebra. Such involvement of two contiguous vertebral bodies almost uniformly is associated with transdiscal infection and rarely is the result of multicentric involvement.

After a variable period (10 to 12 weeks), regenerative changes appear in the bone with sclerosis or eburnation. The osteosclerotic response is variable in severity and has been used in the past as a helpful sign in differentiating pyogenic from tuberculous infection. Although such sclerosis is indeed common in pyogenic (nontuberculous) spondylitis, it also may be evident in tuberculosis, particularly in black patients. Furthermore, some people who have pyogenic spinal infection do not reveal significant eburnation, particularly when symptoms and signs have not been of long duration, so that using the presence or absence of bony sclerosis as a foolproof way of differentiating tuberculous and nontuberculous spondylitis can lead to an erroneous diagnosis. More helpful in this differentiation is a combination of findings that strongly indicates tuberculous spondylitis (Fig. 9), including the presence of a slowly progressive vertebral process with preservation of intervertebral discs, subligamentous spread of infection with erosion of anterior vertebral margins, large and calcified soft tissue abscesses, and the absence of severe bony eburnation.

A radiographic-pathologic correlation of osteomyelitis is summarized in Table 1.

The sensitivity for plain film radiography has been reported to range from 43% to 75%, and the specificity from 75% to 83% [24,34-37]. Radiographs, when positive, are helpful, but negative radiographic findings are unreliable to exclude the diagnosis of osteomyelitis in patients who have violated bone. In these situations, radiographic findings are nonspecific, being diagnostic in as few as 3% to 5% of culture-positive cases [25,38].

Picture Mri And Lupus
Fig. 9. Tuberculous spondylitis: Vertebra plana and kyphosis. In this patient, a wafer-like remnant of the infected vertebral body can be seen (arrow). Abnormal kyphosis is present. The adjacent vertebrae appear normal.

Sinography

Opacification of a sinus tract can produce important information that influences the choice of therapy. In this technique, a small flexible catheter is placed within a cutaneous opening. Retrograde injection of contrast material defines the course and extent of the sinus tract and its possible communications with neighboring structures (Fig. 10). Sinography may be combined with CT for better delineation of the sinus tracts.

Conventional tomography

The major role of this technique in osteomyelitis is the detection of sequestra in cases of chronic osteomyelitis, as these pieces of necrotic bone can be obscured by the surrounding osseous abnormalities on conventional radiography. Because the presence of pieces of sequestered bone suggests activity of the infectious process, their detection is important to the infectious diseases and orthopedic specialists to guide the therapeutic options (Fig. 11).

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