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
     this point?

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 
intra-articular fracture.
     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