Imaging description

As opposed to the relative ease of recognizing classic anterior subglenoid dislocation of the shoulder, the findings of posterior shoulder dislocation on the anterior-posterior (AP) view of the shoulder are subtle and require a high degree of suspicion to detect [1–3]. Nearly one-quarter of posterior shoulder dislocations are missed on initial radiographic assessment [4]. The normal anatomic appearance of the shoulder is reviewed in Figure 83.1. Signs to look for on the AP view include the “lightbulb” appearance of the humeral head, due to fixed internal rotation of the humerus (Figure 83.2), and a “vacant” glenoid fossa due to lateral displacement of the humeral head, creating the “rim sign” (Figure 83.3). The “rim sign” can be due to hemarthrosis or septic arthritis. There may be absence of normal half-moon overlap between the humeral head and glenoid. The “trough sign” is caused by impaction of the humeral head on the glenoid, and reflects the parallel lines created by the medial humeral head cortex and the reverse Hill–Sachs fracture fragment (Figure 83.4). More recently, the “Mouzopoulos” sign on AP radiographs has been described [2]. In posterior shoulder dislocation, projection of the greater and lesser tuberosities on the AP view of the internally rotated humeral head creates a capital “M” appearance (Figure 83.5). A false-positive Mouzopoulos sign may be seen when there is marked internal rotation of the humerus in the absence of dislocation [2].

Imaging description

In the setting of knee pain following trauma, an effusion raises concern for an internal joint derangement. Close inspection of the radiographs may reveal subtle clues to the presence of acute or chronic anterior cruciate ligament (ACL) tear, such as a deep lateral condylar notch [1]. To assess the lateral condylar notch sign, draw a line tangential to the lower articular surface of the lateral femoral condyle. Measure the depth of the notch perpendicular to this line [2]. If the sulcus measures greater than 1.5 mm in depth, an ACL injury should be suspected (Figure 87.1) [3]. Additionally abnormal angularity of the notch should also raise concern for ACL injury. A sulcus shallower than 1.5 mm does not assure integrity of the ACL (Figure 87.2) [3, 4].

Pathologically, the deep lateral femoral sulcus reflects the presence of a transchondral fracture, presumed to result from impaction on the lateral or posterolateral tibia during the twisting injury that simultaneously tore the ACL. If the impaction injury results in only a bone bruise or isolated chondral injury, it will be occult on radiograph but not on MRI.

Imaging description

In addition to the standard anterior-posterior (AP) and lateral radiographs of the knee, medial and lateral oblique views, or tangential views, should be obtained as part of the standard radiographic assessment of the injured knee. While the lateral radiograph is sensitive for knee effusion and therefore has been suggested as a screening tool for intra-articular pathology [1], additional views are often needed to identify the fracture. Lateral and medial oblique views at 45 degrees were advocated by Daffner and Tabas to remove superimposition of the patella over the distal femur and to better show the medial and lateral tibial plateaus [2]. The combination of tangential, AP, and lateral radiographs of the knee has been reported to be more sensitive, at 85%, for acute fracture detection than AP and lateral radiographs alone (79% sensitive) [3]. Multidetector CT still plays a role in fracture characterization and preoperative planning.

Any cortical defect or unexplained sclerotic or lucent line should be viewed with suspicion, even if only seen on one projection. CT or MR should be used in confirming or excluding a fracture when radiographs are indeterminate or clinical suspicion is high despite negative radiographs.

Imaging description

There are several sesamoid bones that we expect to see in most, if not all, patients, as well as other variants that may also be detected. The largest and best known sesamoid bone is the patella, which is part of the extensor mechanism of the knee (Figures 85.1–85.4).

In the foot, there are typically two first hallux sesamoid bones within the two heads of the flexor hallucis brevis tendon, forming part of the first metatarsophalangeal joint capsule along the plantar surface of the first metatarsal head (Figures 85.5–85.9) [1]. The tibial sesamoid is medial and the fibular sesamoid lies laterally. Additionally, the os peroneum may be seen along the lateral aspect of the midfoot within the distal peroneus longus tendon (Figures 85.10–85.12).

Each hand typically has five sesamoid bones: two at the first metacarpophalangeal (MCP) joint, one each at the second and fifth MCP joints, and one at the first interphalangeal joint (Figures 85.13 and 85.14).

Importance

Sesamoids are accessory ossific structures that are contained within a tendon or joint capsule and reduce friction during flexion and extension as they slide over adjacent structures. In distinction to accessory ossicles, sesamoids form from their own ossification center. Like accessory ossicles, the importance of sesamoids is recognizing their normal and variant appearances that maybe mimic or hide acute pathology.

Imaging description

By ultrasound, gallbladder wall edema is present when the gallbladder mural thickness is 3 mm or more (Figure 55.1). The wall may also appear striated, with alternating hypoechoic and hyperechoic layers (Figure 55.2). These imaging features are non-specific in isolation. Additional sonographic signs of acute cholecystitis include the presence of mobile stones or sludge, a non-mobile stone in the gallbladder neck, gallbladder luminal distension, pericholecystic fluid, and extrahepatic and intrahepatic biliary dilation. A sonographic Murphy’s sign (SMS) is positive when there is maximal tenderness over the sonographically localized gallbladder, and negative if the pain is diffuse or localized to a site distant from the gallbladder [1]. If both gallstones and a positive SMS are present with gallbladder wall thickening, the positive predictive value is greater than 90%. Gallbladder wall thickening found in the absence of stones or a SMS has a reported negative predictive value for acute cholecystitis of approximately 95% [1].

Importance

Gallbladder wall edema can be found in patients with both biliary and non-biliary causes of right upper quadrant pain. Additional sonographic features that favor a non-biliary source of the gallbladder wall thickening include sonographic signs of cirrhosis, a hypoechoic liver, and decompressed gallbladder lumen.

Imaging description

Fat planes are often present on radiographs but may be displaced or obliterated by soft tissue swelling and hemorrhage after acute trauma. Several of these fat pads have been described [1], but by far the most useful are the intracapsular fat pads of the elbow and suprapatellar fat pads of the knee.

In the absence of an effusion, the posterior fat pad of the elbow usually rests within the olecranon fossa, hidden from view by the humeral condyles on the lateral radiograph of the elbow in 90 degrees of flexion (Figure 82.1). If the elbow is extended, the posterior fat pad may be seen even in the absence of an effusion [2, 3]. There are also two normal anterior fat pads, which lie along the anterior aspect of the distal humerus; one in the coronoid fossa, and the second in the radial fossa. The anterior humeral fat pads are normally visible in adults without fractures. On the lateral elbow radiograph, they are superimposed and appear flat or triangular in shape (Figure 82.2).

Any process that distends the joint capsule of the elbow will elevate the fat pads. Identification of the posterior fat pad on a technically adequate radiograph virtually assures the presence of effusion [4]. Displacement of the anterior fat pad creates a sail shape, though this may be difficult to see if the paired fat pads are no longer superimposed [5]. Following trauma, the most likely reason for an effusion is an intracapsular fracture (Figures 82.3–82.5), most commonly a radial head fracture in adults (Figure 82.3) [5], and a supracondylar fracture in children [6]. However, 6–29% of children with an isolated posterior fat pad sign will not have an intracapsular fracture on follow-up imaging and clinical assessment [6].

Imaging description

Open mouth odontoid radiographs or cervical spine CT demonstrate asymmetric position of the dens between the lateral masses of C1 [1]. The C1 lateral mass contralateral to the direction of head rotation is subluxed anteriorly relative to the C2 lateral mass, while the ipsilateral C1 lateral mass might be posteriorly displaced. This asymmetry can be normal during head rotation, and is very frequently encountered on imaging. If, however, the asymmetric relationship persists when the head is then turned to the contralateral side, atlantoaxial rotatory fixation (AARF) is diagnosed.

Importance

At our institution, far more trauma patients are transferred from outside institutions for further evaluation for AARF than are ultimately diagnosed with it. Differentiation between AARF and acute torticollis, however, is important because torticollis responds to analgesics and conservative management, while AARF requires reduction. Furthermore, reduction has been more successful when performed early in the course of the condition.

Imaging description

When multiple structures overlap or abut on a radiograph, an optical illusion known as the “Mach effect” may simulate light and dark lines. This enhances edges and in some cases makes the structures easier to differentiate. However, the Mach effect may also simulate a fracture. The Mach effect is a normal phenomenon that results from the physiologic process called “lateral inhibition” in the radiologist’s retina [1]. As shown on Figure 79.1, each gray rectangle appears shaded, light at the top and darker at the bottom. Now use a sheet of paper to cover all but one of the gray rectangles – the illusion of a gradient of gray within the remaining rectangle disappears, revealing that each rectangle is, in fact, homogeneous in color.

When the Mach effect is being considered, careful inspection is necessary to avoid misdiagnosis. Additional views may remove the superimposition of structures creating the effect. In other cases, a CT may be necessary to exclude underlying pathology.

Imaging description

Misregistration is a CT artifact caused by both gross patient or physiologic motion during helical or axial acquisition [1, 2]. Motion artifacts can cause a variety of appearances on CT, including shading, streaking and double contours [1]. The appearance and severity of CT motion artifacts varies depending on the magnitude, speed, and direction of patient movement, as well as on the speed of the CT scanner itself. Slight patient motion, such as from cardiac motion, peristalsis, or tremor, can cause misregistration during image reconstruction, and is detected as bands and streaks on the axial image at the level of motion [1]. This motion is most problematic for evaluation of soft tissues and rarely causes significant diagnostic dilemmas in the skeleton. Gross patient motion, which occurs in intoxicated patients who cannot lie still, or can be due to respiratory motion, can cause step-off between axial images that may be confused for fracture or dislocation (Figures 78.1–78.3). In this case, the step-off will involve not only the cortex but also overlying soft tissue planes, such as the posterior wall of the pharynx in the neck or the skin overlying the sternum (Figure 78.4). Blurring may also be seen in the adjacent soft tissues, confirming the presence of motion (Figure 78.5).

Imaging description

At birth, the primary ossification centers of the ilium, pubis, and ischium converge on the triradiate cartilage at the hip (Figure 97.1). On a poorly positioned radiograph (Figure 97.2), the ischium may appear to be displaced medially relative to the ilium, which should not be mistaken for a fracture through the triradiate cartilage. The triradiate cartilage gradually thins (Figure 97.3), and the roof of the acetabulum may appear irregular in children 7–12 years of age. Particularly when viewed on axial CT images, this should not be mistaken for comminuted acetabular fracture (Figure 97.4). The bony acetabulum fuses around 11 to 14 years of age, achieving its adult appearance slightly earlier in girls than boys [1].

Around puberty, three secondary ossification centers develop around the acetabulum, including the os acetabuli (epiphysis of os pubis along the anterior wall of the acetabulum), epiphysis of the ilium (forms the superior wall of the acetabulum), and a small epiphysis of the ischium (Figures 97.5 and 97.6) [2]. These contribute to the depth of the acetabulum but may be confused with avulsion injuries (Figure 97.7). The os acetabuli may persist into adulthood as a separate, well-corticated ossicle (Figure 97.8).

The body and alae of the sacrum develop from several separate primary ossification centers (Figure 97.1), which typically fuse between one and seven years of age [3]. Cartilage bordering the articular surfaces of the sacroiliac joints in young children makes them appear wider on radiographs than would be normal for an adult (Figure 97.1). Small triangular secondary ossification centers appear around puberty along the anterior sacroiliac joint spaces at the levels of S1 and S3 (Figure 97.6) [4]. These begin to fuse to the lateral os sacrum around 18 years of age.

Imaging description

A radiograph obtained for acute trauma or musculoskeletal extremity pain reveals an extraosseous structure. Close inspection reveals that the bone is round and well corticated on all sides, as opposed to the appearance expected for an acutely avulsed fragment of bone. Perhaps correlation with physical exam reveals no focal tenderness at the site of the accessory ossicle, effectively excluding acute fracture. Alternatively, if the physical exam is unreliable or if clinical suspicion persists, a CT might show the complete cortication and lack of osseous donor site. An MRI may be obtained if there is concern that the accessory ossicle is itself the source of ongoing subacute or chronic pain, in which case bone marrow edema would be revealed on both sides of the synchrondrosis between the accessory ossicle and parent bone.

Importance

The number of described accessory ossicles is very large. Their clinical significance lies in the fact that they can simulate acute avulsions radiographically and they can be symptomatic themselves.

Imaging description

Inflammatory changes in the fat generally provide reliable evidence of an underlying acute intra-abdominal process. Disproportionate fat stranding in the pericolic region with mild colonic wall thickening suggests a pericolonic inflammatory process, including diverticulitis, appendicitis, epiploic appendagitis, and omental infarction, as opposed to colonic wall thickening centered on the colon, typical of infection, ischemia, and inflammatory bowel disease [1]. Occasionally, the underlying pathologic process is in the fat itself, involving the omentum, mesentery, or epiploic appendages.

Appendices epiploicae arise in two rows from the serosal surface of the colon, from the cecum to the rectosigmoid junction (Figure 52.1). Pedunculated and with tenuous blood supply, they are prone to torsion, infarction, and subsequent inflammation.

Imaging of epiploic appendagitis reveals an ovoid or lobular lesion of fat density less than 5 cm in size (usually 1–4 cm) adjacent to the anterior colonic wall (Figures 52.2 and 52.3) [2]. There is typically a well-defined hyperattenuating rim and surrounding inflammatory change. As with other pericolic processes, wall thickening of adjacent colon is mild compared with the degree of fat stranding.

Epiploic appendagitis can occur anywhere along the colon, but is most common adjacent to the sigmoid [3]. The “central dot sign” has been described as a specific sign for epiploic appendagitis and reflects thrombosed or obstructed central vessels within the torsed appendage.

Imaging description

In the setting of blunt trauma to the chest, injuries such as rib fractures, pneumothorax, hemothorax, and pulmonary contusions are relatively common and easily diagnosed by CT if not by radiograph. Fractures of the sternum and scapula and sternoclavicular joint dislocation are less common but reflect higher-energy trauma. Furthermore, they can be easily overlooked, even on CT.

Fractures of the sternum occur in up to 10% of polytrauma cases [1]. Substernal mediastinal hemorrhage should prompt close examination of the sternum, though hemorrhage is not universally present (Figure 84.1). The horizontal sternal fracture may be occult on axial CT images, and dedicated reformations in sagittal and coronal planes relative to the sternum may be necessary for diagnosis (Figure 84.2). An important caveat is that motion artifact may create artifactual step-off in the sternum, but close inspection for matching step-off in the overlying skin line usually helps differentiation. Because sternal fractures usually result from direct frontal injury, additional thoracic, cardiac, and spinal injuries should be excluded (Figure 84.3) [2].

Sternoclavicular dislocation is a rare injury. Anterior dislocations are more common than posterior and are usually clinically obvious. Posterior dislocations, conversely, are often clinically and radiographically occult. Furthermore, the posteriorly dislocated clavicle may be associated with vascular, nerve, or tracheal injuries, necessitating further evaluation. For greater detail, see Case 36.