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. CC-BY 3.0
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.
Computed Tomography Multidetector computed tomography (MDCT) is a highly valuable imaging tool for the diagnosis of occult fractures. CT has several advantages including short acquisition time (compared to MRI), the ability to acquire volumetric and isotropic image data sets, the opportunity to reconstruct multiplanar reformations in any arbitrary plane, and excellent spatial resolution. Moreover, the image quality for multiplanar reconstruction may be increased by reducing slice thickness and acquisition pitch. In general, bony structures are best demonstrated by using a small focal spot and using a "bone" algorithm. CT contributes much to the diagnosis of occult fractures by depicting subtle fracture lines, depressed, or distracted articular surfaces and by assessing bone loss It also detects late bony changes such as increased medullary density, endosteal sclerosis, sclerotic lines in trabecular bone, and periosteal thickening. Moreover, CT aids in excluding other differential diagnoses, especially in case of isolated bone marrow edema, by confirming the normal appearance of the remaining trabeculae and excluding space-occupying lesions such as malignancy and osteomyelitis.
Newest generation of CT, such as dedicated cone-beam CT (CBCT) system for musculoskeletal extremities, may be beneficial in various conditions, such as arthritis and occult fractures. Although dedicated CBCT for musculoskeletal extremities is still a matter of research, it has been shown to be of potential benefit as an adjunct for CT and MRI. It offers the possibility of volumetric imaging, which may be helpful in case of suspected occult fractures. It also provides higher spatial resolution and a potentially reduced dose compared to CT.
Magnetic resonance imaging The diagnostic performance of MRI in the detection of occult fractures has been shown to be comparable, or better than MDCT. Indeed, while the specificity of both CT and MRI for the diagnosis of fracture can be as high as 100%, the sensitivity was reported to be higher for MRI. The superiority of MRI over any other imaging modality including MDCT for the detection of occult hip fractures is now recognised. For instance, an occult intertrochanteric extension of a greater trochanter fracture can be most effectively appreciated on MRI Moreover, MRI is extremely helpful in detecting associated soft tissue abnormalities, especially ligamentous lesions. MRI is now considered as the standard in this context. However, because of its relative unavailability in emergency settings and high costs, MRI may only be performed in "high risk patients" with negative X-rays. For example, when a hip occult fracture is suspected, patients with reduced baseline mobility and pain on axial compression are considered at risk and, therefore, should be examined by MRI. MRI signs of occult fractures are evident several weeks before radiographic signs appear. In the hip, a limited and cost-effective MR protocol, with only T1 weighted () coronal images, may enable a reliable diagnosis or exclusion of occult fracture in very little time, for example, 7 minutes. Typically, a linear hypointensity is observed on T1 W images. MRI is also highly sensitive to marrow abnormalities surrounding the fracture line, which appear as hypointensity on T1 W images and hyperintensity on fluid-sensitive sequences. Such signal changes are thought to be a combination of bone marrow edema, intraosseous haemorrhage, and/or granulation tissue and help to identify even undisplaced fractures. However, in the absence of a history of trauma and linear hypointensities on T1 W images, isolated bone marrow edema may represent other pathologies such as osteoid osteoma and sclerosing osteomyelitis.
Although 1.5 T and 3 T MR is considered as the current gold standard for the detection of radiographically occult fractures, ultra-high field MR provide higher signal-to-noise ratio and, therefore, is expected to be superior to 1.5 T and 3 T. Ultra-high field MR seems to be promising in the diagnosis of a variety of musculoskeletal conditions including trauma, but it is not used in daily routine yet.
Nuclear medicine The most traditional method is bone scintigraphy. Although scintigraphy is highly sensitive for the detection of occult fracture, its lack of specificity limits its diagnostic utility. However, when MRI is not available, scintigraphy may be of value, especially in the absence of trauma history, for example, for detection of insufficiency and fatigue fractures. While radiography may show only late signs of bone reaction (such as periosteal thickening and band of sclerosis), scintigraphic examination allows for earlier detection of bony changes. Regarding Fluorine-18 2-deoxy-D-glucose (FDG) positron emission tomography (PET), it is critical to be aware that occult fractures may be responsible of marked metabolic uptake, and, thus, represent a potential false positive of metastatic disease. Integrated hybrid single-photon emission computerized tomography (SPECT)/CT combines the detection of abnormal bone metabolism with SPECT, to the precise anatomical detail provided by high resolution CT. For instance, SPECT/CT may be interesting in the detection of radiographic occult fractures of the wrist and other sport-related injuries.
Ultrasonography High frequency ultrasound has been shown to be of value, particularly in the pediatric population. In this case, and in an emergency setting, ultrasonography can be more accessible and less time-consuming than radiographs and has high specificity and sensitivity in the evaluation of suspected long bone fractures. The utility of ultrasonography has also been shown for adults with suspicion of wrist trauma or fatigue/stress fracture. Recently, it has been suggested that therapeutic ultrasound may be beneficial as a primary evaluation of bone stress injury; however, its benefit seems to be more evident in selected high-risk patients rather than general population.
High-energy trauma fractures