Radiographic Examination of the Elbow - The Hourglass Sign
Radiology Cases in Pediatric Emergency Medicine
Volume 1, Case 12
Alson S. Inaba, MD
Kapiolani Medical Center For Women And Children
University of Hawaii John A. Burns School of Medicine
A 5 year old girl presents to the ED with a painful
elbow after falling from the monkey bars at school. She
did not hit her head and there was no loss of
consciousness. She was brought to the ED by a
teacher because of left elbow pain and swelling.
Exam: Upon presentation to the ED, she prefers to
hold her left arm in the extended position with the
forearm pronated. She is able to wiggle all fingers and
her thumb without any difficulty, and her distal
sensation appears to be intact. She has full range of
motion about the wrist without any tenderness over the
scaphoid bone. The shoulder and clavicle also appear
to be nontender. The left elbow region reveals a mild
degree of swelling without any overlying lacerations or
abrasions. The child cries with attempted supination
and pronation of the forearm but there does not seem
to be much resistance during passive supination or
pronation. She cries a lot when you palpate near the
elbow joint and resists flexion of the elbow beyond 90
degrees. Because of the possibility of an elbow
fracture you order an AP and lateral view of the elbow.
View elbow radiographs.
a) Do you see any obvious fractures, and if so, where?
b) What are "fat pads", and what do they signify?
c) Is an anterior "fat pad" pathologic?
d) Is a posterior "fat pad" pathologic?
e) What is the radiologic significance of the anterior
humeral line and the radiocapitellar line?
f) Are these 2 views adequate to rule-out a fracture? If
not, then what additional view(s) would you obtain at
Discussion & Teaching Points:
a) A proper AP view of the elbow should be
performed with the forearm in supination and the elbow
in as much extension as tolerated. On the AP view
above, there are three ossification centers present in
their expected locations (refer to Case 11 - Elbow
Ossification Centers in a Child). There are no obvious
fractures or dislocations.
b) The anterior fat pad of the elbow normally lies
just over the coranoid fossa. This fat pad occasionally
is visualized as a thin radiolucent line just anterior to the
coranoid fossa (anterior border of the distal humerus)
seen on the lateral view in many normal radiographs of
the elbow. However, when the elbow joint capsule
becomes distended (eg., hemarthrosis secondary to a
fracture within the joint space), the anterior fat pad is
displaced further anteriorly and superiorly to form an
anterior "sail sign" or a more prominent lucency.
c) The posterior fat pad normally lies over the
olecranon fossa. Therefore the posterior fat pad is
never visualized on normal radiographs because the
olecranon fossa is much deeper (more concave) than
the coranoid fossa. Visualization of the posterior fat
pad (even as a thin radiolucent line on the lateral view)
signifies marked distention of the joint capsule (eg.,
hemarthrosis secondary to an intra-articular fracture)
and is therefore always pathologic. Thus remember,
although a thin radiolucent line anterior to the
coranoid fossa (anterior fat pad) could be normal, a thin
radiolucent line posterior to the olecranon fossa
indicates the presence of a traumatic joint effusion and
should always make one very suspicious of an
d) The anterior humeral line is a line that is drawn
along the anterior surface of the distal humeral on a
true lateral view of the distal humerus. Normally this
line should intersect the middle third of the capitellum.
If there is a supracondylar fracture with posterior
displacement of the distal segment, the anterior
humeral line will either intersect the anterior third of the
capitellum or pass completely anterior to the capitellum
(without intersecting the anterior surface of the
capitellum). An abnormal anterior humeral line may
also indicate the presence of a fracture through the
physis displacing the capitellum.
e) The radiocapitellar line is a line that is drawn
along the central axis of the radius on the lateral view.
Normally, this line should intersect the center of the
capitellum on the lateral view. If this line does not
transect the middle of the capitellum, this would
signify either a radial head dislocation and/or a fracture
through the radial neck region. The radial head should
point to the capitellum in all views.
f) On the lateral view of the elbow, there is a
hint of a faint anterior fat pad as well as a hint of a faint
posterior fat pad. These are not easy to appreciate.
You may have to adjust the brightness and contrast
controls on your monitor to see these findings (but you
may still not be able to see these findings). Both the
anterior humeral and radiocapitellar lines are within
normal limits; however, this lateral view of the elbow is
not a true lateral of the distal humeral region. On a true
lateral of the distal humerus, one should be able to
visualize an "hourglass" or "figure-of-eight"
configuration of the distal humerus. Refer to the next
radiograph, which is a true lateral of this patient's distal
View true lateral.
g) Notice that, on this view, an hour glass or
figure-of-eight configuration can be seen on the distal
humerus. Upon closer comparison of this true lateral
view with the other "lateral view" above [Click on
X-Ray], three findings become very obvious:
1) On this true lateral [Click on True], both anterior
and posterior fat pads are very obvious. In fact, both
of the fat pads have sail sign configurations. These
appear as faint soft tissue lucencies. The "sail sign"
refers to the triangular appearance of the fat pad as it is
pushed outward and upward out of the joint space. You
may have to adjust the brightness and contrast controls
on your monitor to see these findings.
2) One also notices that the anterior humeral line on
this view now intersects the anterior third of the
capitellum rather than intersecting the middle third.
Therefore, one must suspect a supracondylar fracture
with posterior displacement of the distal humeral
3) Upon closer inspection of this true lateral view, a
buckle-type fracture is observed along the posterior
aspect of the supracondylar region.
Simon RR, Koenigsknecht SJ: Emergency
Orthopedics: The Extremities (second edition).
Chapter 9 (Distal Humerus), Appleton & Lange, 1987.
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Web Page Author:
Loren Yamamoto, MD, MPH
Associate Professor of Pediatrics
University of Hawaii John A. Burns School of Medicine