Occult fracture

An occult fracture is a fracture that is not readily visible, generally in regard to projectional radiography ("X-ray"). Radiographically, occult and subtle fractures are a diagnostic challenge. They may be divided into 1) high energy trauma fracture, 2) fatigue fracture from cyclical and sustained mechanical stress, and 3) insufficiency fracture occurring in weakened bone (e.g., in osteoporosis and postradiotherapy). Independently of the cause, the initial radiographic examination can be negative either because the findings seem normal or are too subtle. Advanced imaging tools such as computed tomography, magnetic resonance imaging (MRI), and scintigraphy are highly valuable in the early detection of these fractures. Fractures represent up to 80% of the missed diagnoses in the emergency department. Failure to recognize the subtle signs of osseous injury is one of the reasons behind this major diagnostic challenge. While occult fractures present no radiographic findings, radiographically subtle fractures are easily overlooked on initial radiographs. In both cases, a negative radiographic diagnosis with prominent clinical suspicion of osseous injury will prompt advanced imaging examination such as CT scan, magnetic resonance imaging, ultrasound, and nuclear medicine to confirm or exclude the clinically suspected diagnosis. The burden entailed in missing these fractures includes prolonged pain with a loss of function, and disability. Early detection, on the other hand, enables more effective treatment, a shorter hospitalization period if necessary, and decreased medical costs in the long run. It will also prevent inherent complications such as nonunion, malunion, premature osteoarthritis, and avascular osteonecrosis (as in scaphoid fracture). Of the three types of occult fractures mentioned above, the latter two, fatigue fracture secondary to repetitive and unusual stress being applied to bone with normal elastic resistance, and insufficiency fracture resulting from normal or minimal stress on a bone with decreased elastic resistance are also described as "stress fractures". These fractures are often a challenging diagnostic problem in daily clinical practice. Radiologists should be aware of the different situations and mechanisms of these injuries as well as the subtle radiographic signs that can be encountered in each situation. The knowledge of normal images and the consideration of the clinical context are of great value in improving the detection of these fractures either on conventional radiographs or with more advanced imaging tools.

Imaging tools Thanks to rapid technological advancement, new and more efficient imaging hardware is constantly released for all imaging modalities including CT, MRI, nuclear medicine, and ultrasound.

Projectional radiography Radiography is the first step for detection of fractures. The detection of subtle signs of fracture requires a high standard for the acquisition technique and a thorough and systematic interpretation of radiographic images. Correct diagnosis primarily relies on the reader's experience. Awareness of normal anatomic features is crucial for the interpreter to be able to detect subtle signs of fracture. Fat pads should be carefully examined for convexity, which implies joint effusion (e.g., in the hip and elbow). However, the radiographic technique (positioning in particular) must be optimal for this evaluation to be valid. Osseous lines should be checked for integrity (e.g., acetabular rim in the hip). Trabecular angulation, impaction lines, and sclerotic bands also suggest fracture in osseous structures with a significant proportion of cancellous bone such as proximal femur. The general rule is to perform two orthogonal views, but more specific views should be added if there is any suspicion of fracture. Moreover, one should be aware of the commonly encountered lesions and their locations. In wrist trauma, for instance, the interpreter should pay close attention to the scaphoid and triquetrum, which are the two most commonly injured carpal bones. The mechanism of trauma may also be helpful to locate the potential fracture. A fall on an outstretched hand suggests scaphoid fracture. Although the classical presentation consists of a radiolucent line and cortical disruption, the radiographic signs will depend upon the time elapsed between the first clinical symptoms and the time of radiographic examination, the location of the fracture within the bone, and the ratio of cortical to cancellous bone. Particular attention should be paid when analysing the subchondral plate, which may be disrupted or deformed. In metaphyseal areas, delayed signs of fracture include a band of sclerosis perpendicular to the trabeculae, while diaphyseal fractures may present as periosteal thickening. Digital radiography known as tomosynthesis has been shown to be superior to conventional radiographs in the detection of occult fracture of the scaphoid. Tomosynthesis has the ability to demonstrate cortical, as well as moderately displaced trabecular fractures. Thus, the performance of tomosynthesis in detecting radiographically occult fractures is considered as comparable to CT.

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