Imaging in osteomyelitis and septic arthritis

Abstract

The diagnosis of osteomyelitis and septic arthritis depends on clinical presentation, appropriate cultures, laboratory tests, and imaging studies. During the initial evaluation of a patient suspected of having these musculoskeletal infections, radiography, ultrasound, magnetic resonance imaging (MRI) or computed tomography (CT), and radionuclide scans selectively are ordered to assist in the diagnosis, assess the extent of involvement, and guide the site selection for the bone biopsy. Conventional radiography should be performed on all patients. Radiographic changes in early osteomyelitis often are difficult to interpret and lag at least 2 weeks behind the evolution of infection. In patients with septic arthritis, radiographic images often are not revealing in the first few days of infection. However, initial radiographic images may be used to determine associated conditions such as osteoarthritis or simultaneous osteomyelitis, or to exclude neoplasm or injury. Therefore, after the infectious or inflammatory process has been detected and localized with this technique, further examinations by CT, MRI, biopsy, and cultures may be necessary to delineate the extent and etiology of the process. The use of ultrasound in the diagnosis of osteomyelitis is limited to the detection of soft tissue abnormalities around the bone. Ultrasound is a very powerful tool to detect early fluid effusions and to guide initial joint aspiration and drainage procedures. Ultrasound is the method of choice for treatment of patients with acute septic arthritis. CT provides images that display high spatial resolution and explicit cortical bony detail in patients with osteomyelitis. However, CT scans have limited use during the early stages of septic arthritis. MRI is superior to CT in localizing marrow extension and soft tissue changes. In patients with vertebral osteomyelitis, MRI should be used. MRI has the highest accuracy of all imaging techniques. The spatial resolution of MRI makes it more useful than CT or scintigraphy in the diagnosis of septic arthritis. Imaging with radiopharmaceuticals provides information regarding the pathophysiologic and pathobiochemical processes of inflammation, whereas radiography provides high-resolution morphologic information about the pathologic process. Technetium-99m methylene diphosphonate (99mTc MDP) usually is positive in biopsy-confirmed cases of hematogenous osteomyelitis. However, conditions that result in bone injury and repair cause constant bone turnover and focal uptake of 99mTc in bone. Using a three-phase 99mTc MDP followed by gallium-67 imaging increases specificity. Indium-111-labeled white blood cell (In-111 WBC) scintigraphy is very sensitive (except in some cases of chronic osteomyelitis), specific, and the method of choice for diagnosing and localizing distal appendicular skeletal osteomyelitis.