Conventional radiographs are notoriously insensitive for detecting stress injuries in bone. The sensitivity of initial radiographs, obtained at the time of presentation, has been reported to be as low as 15% and will become positive over time in only 50% of patients [17,18].
The earliest radiographic finding in a stress reaction involving cortical bone is the ''gray cortex'' sign . This is an area of subtle, ill-defined intracortical lucency, related to the osteoclastic tunneling that occurs in an area of stress damage. This finding is extremely difficult to identify, and most stress reactions are detected somewhat later when periosteal or endosteal new bone formation is evident on radiographs. If the offending activity is continued, the microfractures and osteoclastic tunnels within the area of stress reaction will coalesce into a lucent intracortical fracture line or even a displaced fracture.
Within trabecular bone, the earliest radiographic finding is usually that of a subtle linear or curvilinear focus of sclerosis, related to the microcallus that forms at the sites of trabecular fractures. This finding also is difficult to detect in the early stages, and a high index of suspicion is often needed to do so.
CT is also insensitive in detecting stress injuries in their early stages but can be helpful for identifying a fracture line in an area of probable stress reaction demonstrated on a radionuclide or MRI study .
Radionuclide bone scanning is extremely sensitive for detecting osseous stress injuries, even in their early stages . Because the radiopharmaceutical is taken up at any area of active bone turnover, the scan will become positive at the stage of accelerated bone remodeling, before the development of clinical symptoms. Studies have demonstrated that as many as 46% of abnormal foci on bone scans are asymptomatic .
MRI is at least as sensitive as radionuclide scanning for detecting these injuries. The earliest MRI findings include marrow edema or periosteal fluid at the site of a stress reaction . In the later stages of the stress injury continuum, a discrete fracture line will be seen. However, the findings must be correlated with clinical information, because a recent study revealed that MRI findings suggesting stress reactions were evident in as many as 43% of asymptomatic runners
MRI exhibits certain advantages over radionuclide scanning: shorter scan time, no ionizing radiation, improved specificity (given the better spatial resolution of MRI), and the capability simultaneously to image bone and soft tissues. One disadvantage is its current inability to screen the entire skeleton, as is possible with radionuclide scanning.
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