Fracture diagnosis is based on both clinical and radiographic
findings. Three clinical predictors of a fracture are: 1) the mechanism
of injury, 2) the findings on physical examination, and 3) age-related
common injuries (Table 1). Likewise, there are three radiographic
findings of a fracture: 1) visualization of the fracture line; 2)
alterations in skeletal contour or alignment (most useful in children);
and 3) soft tissue changes. Supplementary views are occasionally
needed to visualize the fracture. These principles are illustrated
for the elbow in this and the following chapter.
The elbow is a prime example of the usefulness of soft tissue
signs in fracture diagnosis. Soft tissue changes can sometimes be
easier to see than the fracture itself (see Principles of Skeletal Radiology,
Table 3, in Section III: Skeletal Radiology—Upper Extremity). Soft tissue changes include swelling, joint
effusions, and distortion or obliteration of the fats planes between
Post-traumatic joint effusions can
serve as a clue to an intra-articular fracture. An effusion usually
appears as an area of fluid density adjacent to the joint. Examples
include ankle and knee effusions (although knee effusions are more
reliably detected by physical examination).
The elbow anatomy is unusual because there are two collections
of fatty tissue contained within the joint capsule (anterior and
posterior fat pads) (Figure 3A). When
the elbow is flexed to 90°, these fat pads lie nearly entirely within
the coronoid and olecranon fossae of the distal humerus. On a properly performed
lateral radiograph, the anterior fat pad may appear as a small lucent
area just anterior to the distal humerus. The posterior fat pad
is not normally visible (Figure 4A).
If the joint space is filled by blood (due to a fracture) the
fat pads are displaced outwards (Figure 3B). The posterior fat pad
becomes visible posterior to the olecranon fossa, and the anterior
fat pad is displaced anteriorly, forming a radiolucent triangle,
the “sail sign” (Figure
4B). The elbow fat pad signs are not do to fat entering the joint
from bone marrow through an intra-articular fracture, as is the
case with a lipohemarthrosis of the
knee (see Lower Extremity Patient 3, Figure 16 in Knee Fractures—Tibial Plateau Fractures).
Radiographic technique can obscure or mimic the fat pad signs.
If the radiograph is overpenetrated (too dark) with respect to soft
tissues, a bright light (or a lightened digital image) may be needed
to detect the fat pads (Figure 5). This is often the case because
extremity radiographs are generally exposed to show bone detail.
If the lateral view is incorrectly positioned with an oblique
orientation, the fat pad signs can be obscured. If the elbow is
extended rather than flexed, the posterior fat pad may be visible
because it is displaced from the olecranon fossa by the olecranon.
Finally, fat pad signs can also caused by nontraumatic elbow effusions
such as infectious and inflammatory arthritides.
The Fat Pad
Sign in Adults
In adults, the fat pad sign is most frequently associated with
a radial head fracture. Radial head
fractures are by far the most common fracture in adults, accounting
for 50% of cases. This fracture can be difficult to see
if it is nondisplaced. An oblique view or radiocapitellar view (lateral
view taken with an angled x-ray beam) can sometimes reveal the fracture
(Figure 6). However, there is little need to obtain this additional
view because management is not altered. An adult patient with a
fat pad sign and no visible fracture is treated as though a non-displaced
radial head fracture were present, i.e., simply using a forearm
sling with early mobilization exercises.
In another patient, a radial head fracture is only seen
on an oblique view.
A radial head fracture is usually caused by a fall onto an outstretched
hand causing the radial head to impact against the capitellum. Tenderness
over the radial head can be difficult to elicit because of the thickness
of the surrounding muscles. Pain and limited range of motion (flexion–extension and
supination–pronation) are reliable clinical signs of a
radial head fracture. In fact, these signs are useful criteria in
deciding to order elbow radiographs, although this has not been
studied in a large clinical trial (Hawksworth and Freeland 1991,
Brasher and Macias 2001).
This patient’s lateral elbow
radiograph show both an anterior fat pad sail sign and a small
posterior fat pad, meaning that an intra-articular fracture is likely
present (Figure 1B and 4B). A radial head fracture is the most common,
However, given this patient’s mechanism of injury—a
direct blow to the extensor surface of his elbow by a nightstick—and
the findings on physical examination (tenderness over the olecranon), a
radial head fracture is unlikely. This patient’s direct
blow mechanism of injury is reflected by the “nightstick” fracture
(ulnar shaft fracture) of his left forearm (Figure 2).
Olecranon fractures are the second
most common about the elbow in adults (20%). Olecranon
fractures are usually transverse or avulsion fractures and are easily
seen on the lateral view (Figure 7).
Another patient with a more common type of olecranon
fracture —an avulsion fracture. Olecranon fractures are
usually readily detected on the lateral elbow view (arrow).
An intra-articular elbow fracture that can be caused by a direct
blow to the elbow and which may be difficult to detect radiographically
is a longitudinally-oriented olecranon fracture.
In this patient, a supplementary olecranon
view was necessary to demonstrate the fracture (Figure 8).
In retrospect, close examination of the AP view reveals a very faint
longitudinal fracture line through the olecranon (Figure 9).
(A) Olecranon view reveals
the longitudinal fracture through the olecranon (arrow).
(B) Positioning of the axial olecranon
A subtle longitudinal fracture of the olecranon is visible
on the patient’s AP radiograph (arrow).
This patient was treated with a padded posterior splint and sling
for the olecranon fracture of his right elbow and a long arm cast
for the “nightstick fracture” of his left forearm.
Supplementary radiographic views are
indicated when a fracture is suspected based on physical examination
findings, but is not clearly seen on the standard radiographic views.
The Fat Pad
Sign in Children
In children, the fat pad sign is especially important because
it serves as a clue to potentially serious fractures—most
commonly a supracondylar fracture or
a lateral condylar fracture (Table
1). When a pad sign is seen in a child, a search should be made
for subtle signs of these fractures (Figure 10). If a fracture cannot
be found, an occult supracondylar fracture is presumed to be present
and the child should be immobilized in a splint or cast.
Supracondylar fracture in a 5-year-old child.
Anterior and posterior fat pad signs are present (arrows). However, the fracture itself
is subtle. There is a slight interruption of the anterior cortex
of the distal humerus in the supracondylar region (arrowhead).
Supracondylar condylar fractures can cause neurovascular injuries
(most commonly median nerve and brachial artery) and growth deformities.
Although these complications are usually associated with displaced
fractures that are readily apparent on radiography, all supracondylar
fractures require specific orthopedic treatment—a posterior
splint or cast if nondisplaced, and reduction and pin fixation if
displaced—and all patients need close clinical follow-up.
and Specificity of the Fat Pad Sign
Because treatment of children having an elbow fat pad sign requires
more prolonged immobilization than in adults, the specificity (positive
predictive value) of the fat pad sign for an occult fracture is
important to consider in children. In early reports, only a minority
of patients with fat pad signs and no demonstrable fracture actually
had fractures (6–29%). This calls into question
the need to immobilize all children with isolated fat pad signs.
However, a larger prospective study (Skaggs 1999) found that 76% of
45 children ultimately had fractures (53% supracondylar,
26% proximal ulnar, 12% lateral condylar, and
9% radial neck). Two studies using MRI in elbow trauma
also revealed a high incidence of occult fractures, although treatment
was not altered on the basis of the MRI findings (Griffith 2001,
The sensitivity of the fat pad sign (i.e., does the absence of
a fat pad sign exclude a fracture) has not been studied in a systematic
fashion. However, it is reasonable to assume that when no fracture
is visible, the absence of a fat pad sign makes a significant fracture
unlikely. Nonetheless, fractures can be present without a visible
fat pad sign. In two large series, fat pad signs were seen on only
41-66% of patients with radiographically visible fractures
(Kohn 1959, Corbett 1978). A fat pad sign does not occur if the
joint capsule is disrupted or if correct radiographic positioning cannot
be attained because of the injury (elbow flexed to 90° on the lateral
Patient 1 also has a left ulnar
shaft fracture (Figure 11). Because the ulna and radius are rigidly
bound to each other by the interosseus ligament, a fracture of one
of these bones may be accompanied by a fracture or dislocation of
the other. With a displaced ulnar shaft fracture, there may be dislocation of
the radial head at the elbow. This is called a Monteggia
lesion (Figure 12). Patients with forearm fractures must be
carefully examined at the elbow and wrist, and radiographs should
be obtained when there is pain or tenderness of either joint.
Patient 1 had an isolated nondisplaced fracture of the left ulnar
shaft that was caused by a direct blow to the ulnar aspect of the
forearm by a nightstick. The elbow and wrist are normal.
This is known as a nightstick fracture.
The fracture is managed by immobilization in a cylindrical cast.
If the force of impact had been greater and the ulnar fracture
was displaced, a concomitant dislocation of the radial head could
be present—a Monteggia lesion.
When is a nightstick injury not a nightstick fracture?
(A and B)
In another patient who was hit by a nightstick on the ulnar aspect
of his forearm, there is a moderately displaced fracture of the
A radial head dislocation should be suspected in such cases and
the patient should be examined for elbow pain and limited range
On first glance, the radial head appears to articulate with the
distal humeral articular surface. There is, however, significant
malalignment. The radial head is actu-ally near the trochlea, not
to the capitellum.
(C) The elbow radiograph reveals dislocation of the radial head-a Monteggia lesion
(D) The Monteggia lesion is mechanically unstable and requires
open reduction and internal fixation
In this patient, the ulnar shaft fracture is not displaced and
the elbow and wrist are normal. This is termed a nightstick fracture owing to the
usual mechanism of injury—a direct blow to the forearm
as the patient holds up his forearm to protect himself from an assault.
A night-stick fracture is usually managed with cast immobilization,
although if the fracture is displaced, reduction can be difficult
to maintain and internal fixation may be necessary. A Monteggia
fracture is unstable and always needs internal fixation. The mechanism
of injury of a Monteggia fracture is either a direct blow to the
ulnar aspect of the forearm or a fall on an outstretched arm.