Chapter 22 Recognizing Fractures and Dislocations

Recognizing an Acute Fracture

Everyone, it seems, is fascinated by a broken bone or two. Fractures are a favorite among those learning radiology, perhaps because of how common they are. Fractures due to moderate trauma are much more common than those due to severe trauma or pathologic fractures.

A fracture is described as a disruption in the continuity of all or part of the cortex of a bone.

• If the cortex is broken through and through, the fracture is called complete.

• If only a part of the cortex is fractured, it is called incomplete. Incomplete fractures tend to occur in bones that are “softer” than normal such as those in children, or in adults with bone-softening diseases such as osteomalacia or Paget disease (see Chapter 21, Recognizing Abnormalities of Bone Density).

• Examples of incomplete fractures in children are the greenstick fracture, which involves only one part of, but not the entire, cortex, and the torus fracture (buckle fracture), which represents compression of the cortex (Fig. 22-1).

Figure 22-1 Greenstick and torus fractures.

Incomplete fractures are those that involve only a portion of the cortex. They tend to occur in bones that are “softer” than normal, such as those in children or in adults with bone-softening diseases, such as osteomalacia or Paget disease. There is a greenstick fracture of the distal radius (A), which involves only one part of (dotted white arrow), but not the entire, cortex (solid white arrow). B, In this torus fracture (buckle fracture) of the distal radius, buckling of the cortex is seen (solid black arrows).

Radiologic features of acute fractures (Box 22-1)

• Fracture lines, when viewed in the correct plane, tend to be “blacker” (more lucent) than other lines normally found in bones, such as nutrient canals (Fig. 22-2A).

• There may be an abrupt discontinuity of the cortex, sometimes associated with acute angulation of the normally smooth contour of bone (Fig. 22-2B).

• Fracture lines tend to be straighter in their course yet more acute in their angulation than any naturally occurring lines (such as epiphyseal plates) (Fig. 22-3).

• The edges of a fracture tend to be jagged and rough.

Figure 22-2 Nutrient canal versus fracture.

Fracture lines, when viewed in the correct orientation, tend to be “blacker” (more lucent) than other lines normally found in bones, such as nutrient canals. This is a nutrient canal (A) (solid white arrows) while a true fracture is seen in another patient in (B) (dotted black arrows). Notice how the nutrient canal has a sclerotic (whiter) margin, which is not the case with fracture lines. The edges of a fracture tend to be jagged and rough.

Figure 22-3 Fracture versus epiphyseal plate.

Fracture lines (solid black arrow) tend to be straighter in their course and more acute in their angulation than any naturally occurring lines, such as the epiphyseal plate in the proximal humerus (solid white arrows). Because the top of the metaphysis has irregular hills and valleys, the epiphyseal plate has an undulating course that will allow you to see it in tangent both on the anterior and posterior margins of the humeral head. This gives the mistaken appearance that there is more than one epiphyseal plate.

Pitfalls: sesamoids, accessory ossicles, and unhealed fractures (Table 22-1)

• Sesamoids are bones that form in a tendon as it passes over a joint. The patella is the largest and most famous sesamoid bone.

• Accessory ossicles are accessory epiphyseal or apophyseal ossification centers that do not fuse with the parent bone.

• Old, unhealed fracture fragments can sometimes mimic acute fractures (Fig. 22-4A).

• Unlike fractures, these small bones are corticated (i.e., there is a white line that completely surrounds the bony fragment) and their edges are usually smooth.

• In the case of sesamoids and accessory ossicles, they are usually bilaterally symmetrical so that a view of the opposite extremity will usually demonstrate the same bone in the same location. They also occur at anatomically predictable sites.

• There are almost always sesamoids present in the thumb, the posterolateral aspect of the knee (fabella), and the great toe (Fig. 22-4B).

• Accessory ossicles are most common in the foot (Fig. 22-4C).

TABLE 22-1 DIFFERENTIATING FRACTURES, OSSICLES, AND SESAMOIDS

Finding	Acute Fracture	Sesamoids and Accessory Ossicles*
Abrupt disruption of cortex	Yes	No
Bilaterally symmetrical	Almost never	Almost always
“Fracture line”	Unsharp, jagged	Smooth
Bony fragment has a cortex completely around it	No	Yes

* Old, unhealed fractures will not be bilaterally symmetrical.

Figure 22-4 Pitfalls in fracture diagnosis.

A, Old, unhealed fracture fragments (solid white arrow). B, Sesamoids are bones that form in a tendon as it passes over a joint (solid white arrows). C, Accessory ossicles are accessory epiphyseal or apophyseal ossification centers that do not fuse with the parent bone, (like this os trigonum, open white arrow). Unlike fractures, these small bones are corticated (i.e., there is a white line that completely surrounds the bony fragment) and their edges are usually smooth. Sesamoids and accessory ossicles are usually bilaterally symmetrical.

Recognizing Dislocations and Subluxations

In a dislocation, the bones that originally formed the two components of a joint are no longer in apposition to each other. Dislocations occur only at joints (Fig. 22-5A).

In a subluxation, the bones that originally formed the two components of a joint are in partial contact with each other. Subluxations also occur only at joints (Fig. 22-5B).

Some characteristics of dislocations of the shoulder and hip are shown in Table 22-2.

Figure 22-5 Dislocation and subluxation.

A, In a dislocation, the bones that originally formed the two components of a joint are no longer in apposition to each other (solid white arrows). The terminal phalanx is dislocated lateral compared to the middle phalanx. B, In a subluxation, the bones that originally formed the two components of a joint are in partial contact with each other. The humeral head (H) is subluxed inferiorly (solid white arrow) in the glenoid (G) because of large hematoma in the joint secondary to a fracture of the humeral neck (solid black arrow). The hematoma itself is not visible by conventional radiography.

TABLE 22-2 DISLOCATIONS OF THE SHOULDER AND HIP

Shoulder	Hip
Anterior, subcoracoid most common	Posterior and superior more common
Caused by a combination of abduction, external rotation, and extension	Frequently caused by knee striking dashboard transmitting force to hip
Associated with fractures of humeral head (Hill-Sachs lesion) and glenoid (Bankart lesion)	Associated with fractures of posterior rim of the acetabulum

Describing Fractures

There is a common lexicon used in describing fractures to facilitate a reproducible description and to assure reliable and accurate communication.

Fractures are usually described using four major parameters (Table 22-3):

• The number of fragments

• The direction of the fracture line

• The relationship of the fragments to each other

• By communication of the fracture with the outside atmosphere

TABLE 22-3 HOW FRACTURES ARE DESCRIBED

Parameter	Terms Used
Number of fracture fragments	Simple or comminuted
Direction of fracture line	Transverse, oblique (diagonal), spiral
Relationship of one fragment to another	Displacement, angulation, shortening, and rotation
Open to the atmosphere (outside)	Closed or open (compound)

How Fractures are Described: by the Number of Fracture Fragments

If the fracture produces two fragments, it is called a simple fracture.

If the fracture produces more than two fragments, it is called a comminuted fracture. Some comminuted fractures have special names.

• A segmental fracture is a comminuted fracture in which a portion of the shaft exists as an isolated fragment (Fig. 22-6A).

• A butterfly fragment is a comminuted fracture in which the central fragment has a triangular shape (Fig. 22-6B).

Figure 22-6 Segmental fracture and butterfly fractures.

These are two comminuted fractures. A, This is a segmental fracture in which a portion of the shaft exists as an isolated fragment. Notice how the fibula has a center segment (S) and two additional fragments, one on either side (solid white arrows). B, A butterfly fragment is a comminuted fracture in which the central fragment has a triangular shape (dotted white arrow).

How Fractures are Described: by the Direction of the Fracture Line (Table 22-4)

In a transverse fracture, the fracture line is perpendicular to the long axis of the bone. Transverse fractures are caused by a force directed perpendicular to shaft (Fig. 22-7A).

In a diagonal or oblique fracture, the fracture line is diagonal in orientation relative to the long axis of the bone. Diagonal or oblique fractures are caused by a force usually applied along the same direction as the long axis of the affected bone (Fig. 22-7B).

With a spiral fracture, a twisting force or torque produces a fracture like those that might be caused by planting the foot in a hole while running. Spiral fractures are usually unstable and often associated with soft tissue injuries such as tears in ligaments or tendons (Fig. 22-7C).

TABLE 22-4 DIRECTION OF FRACTURE LINE AND MECHANISM OF INJURY

Direction of Fracture Line	Mechanism
Transverse	Force applied perpendicular to long axis of bone; fracture occurs at point of impact
Diagonal (also known as oblique)	Force applied along the long axis of bone; fracture occurs somewhere along shaft
Spiral	Twisting or torque injury