Case 12 - Radiographic Examination of the Elbow - The Hourglass Sign

Volume 1, Case 12
Alson S. Inaba, MD
Kapiolani Medical Center For Women And Children
University of Hawaiʻi 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. 

Questions:

1. Do you see any obvious fractures, and if so, where?
2. What are "fat pads", and what do they signify?
3. Is an anterior "fat pad" pathologic?
4. Is a posterior "fat pad" pathologic?
5. What is the radiologic significance of the anterior humeral line and the radiocapitellar line?
6. 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:

1. 1. 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.
   2. 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.
   3. 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.
   4. 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.
   5. 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.
   6. 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 humerus.
      
      View true lateral.
      
      
      
   7. 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 segment.
3. Upon closer inspection of this true lateral view, a buckle-type fracture is observed along the posterior aspect of the supracondylar region.

References:

Simon RR, Koenigsknecht SJ: Emergency Orthopedics: The Extremities (second edition). Chapter 9 (Distal Humerus), Appleton & Lange, 1987.