Retained foreign bodies are a common and frequently complicated presentation in both the emergency department as well as outpatient medical offices. While retained foreign bodies are usually superficial, they can be missed even when a well-trained clinician performs a thorough history and physical. Therefore, retained foreign bodies are one of the leading causes of malpractice lawsuits in the emergency department. If not found, they can lead to inflammation, delayed healing, and damage to surrounding tissues. Organic objects, such as thorns, splinters, or spines can cause severe inflammation, hypersensitivity reactions, and even infections. These infections are very resistant to antibiotic treatment and require surgical removal the foreign body for definitive treatment.
Most foreign bodies can be located during an extensive wound-probing, physical exam. Imaging studies should be ordered in cases where a foreign body is suspected, but not identified. This becomes especially true if multiple pieces have already been removed or when wound exploration is not possible. Imaging modalities available for detecting foreign bodies include plain film radiographs, computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound. Each modality has its strengths and weaknesses that vary depending on the material the foreign body is made of as well as the object’s location.
When it comes to foreign body detection, plain film radiography is the initial imaging modality of choice due to its ability to detect most foreign bodies quickly and cheaply with relatively low radiation exposure. Objects denser than soft tissue will absorb more rays, and therefore, appear in greater contrast to the surrounding areas. Due to increased density objects such as metal, glass and gravel are considered radiopaque, and multi-view, x-ray imaging is highly sensitive and specific when looking for these objects in soft tissues. However, gravel and graphite in or near bone may be difficult to identify due to the objects having a similar density as the bone. Radiography is also less effective for identifying radiolucent objects such as wood, plastic, and organics (such as thorns or spines) in soft tissue due to matching densities. One case review found physicians missed 75 out of 200 foreign bodies, of which 58 were radiolucent. The object’s size also limits radiographic accuracy, and glass, in particular, becomes difficult to locate when under 1 mm. If clinical suspicion remains high despite negative, plain film radiography, other modalities should be considered.
Imaging is effective at detecting most foreign bodies as well as aiding in their removal by clearly localizing the object of interest within the tissue. It works similar to radiography, but it has an improved ability to differentiate tissue densities, allowing for better visualization of inflammation, abscesses, and granulomas that are frequently secondary clues to a retained foreign body. It comes with the added benefit of being able to provide a more accurate three-dimensional localization of the foreign body. One study found a sensitivity of 68% and specificity of 98% for the detection of all types of foreign bodies using CT imaging. CT studies also come at an increased cost and radiation dose to the patient.
Magnetic resonance imaging, also known as MRI, is the most sensitive study for retained organic material, and like CT imaging, it provides information regarding the three-dimensional location of the foreign body as well as the surrounding structures such as ligaments, tendons, vessels, and nerves. One study found the sensitivity and specificity to be 58% and 100% for various types of foreign bodies inserted into the forefoot. It is however ineffective at imaging gravel or metallic foreign bodies due to ferromagnetic streak artifacts in addition to the inherent danger of movement or dislodgement of magnetic materials as they are attracted to the strong static magnetic field. It also comes at a significant monetary and time cost, making it frequently ineffective for imaging foreign bodies.
Ultrasound is extremely useful in detecting and localizing foreign bodies, especially in conjunction with plain radiography. A 2015 meta-analysis found it to be equally as effective for the detection of superficial, radiopaque objects as x-rays. However, ultrasound became particularly useful in the detection of radiolucent objects. One of the studies in the meta-analysis found a sensitivity of 87% and a specificity of 97% for wooden objects of at least 2.5 mm. Ultrasonography does require prior training, an understanding of anatomy and clinical time. Objects may be mistaken for anatomic structures such as tendons, vessels, or bursa, especially in hands, feet or joints. Objects deeper than 2 cm will also be more difficult to as imaging deeper into tissue comes with the cost of decreased resolution.
Foreign bodies will appear hyperechoic with variable shadowing and reverberation. After the foreign body has been found, clues to the type of material can be gathered by its acoustic details. When imaging gravel or wood, the foreign body will appear hyperechoic with a strong and distinct shadow. Metallic objects display a "comet tail" or distal, regular, parallel lines secondary to acoustic reverberation. Glass can present with variable acoustic shadows, including both comet tails or diffuse beam scattering.
Since no modality is perfect at identifying foreign bodies, a combination of high clinical suspicion, a thorough physical exam, and appropriate selection of the correct imaging modality will provide the best outcomes. A suggested algorithm begins with correctly identifying high-risk patients with a sound history and physical. Most foreign bodies are secondary to trauma or accidental injury, although special consideration must be given to psychiatric patients who may purposefully insert foreign bodies into soft tissue. All wounds harbor the potential for foreign bodies, and if the clinician or the patient has a reasonable level of suspicion, the next step should be to obtain plain film radiographs with views in at least two anatomical planes. If the exam is negative and only radiopaque objects (gravel, glass, or metal) are suspected, a provider may stop here. However, if radiolucent objects such as thorns, wood or plastic are suspected, an ultrasound examination of the area should be performed. If the foreign body is still not located, the clinician may choose to move on to CT or MRI depending on the level of suspicion as well as the type of foreign body.