Radiology Quiz

Radiology quiz

aWinter 2003

An 8-year-old boy had acute left elbow pain following trauma. Anteroposterior (AP) and lateral radiographs of the injured left elbow (Figures A and B) were obtained, as well as comparison views of the noninjured right elbow (Figures C and D).

Figure A Injured left elbow, AP view.

Figure B Injured left elbow, lateral view.

Figure C Noninjured right elbow, AP view

Figure D Noninjured right elbow, lateral view.

What is the diagnosis?

Findings: The key to the diagnosis is an abnormal round piece of bone that projects between the proximal end of the ulna and the medial aspect of the distal humeral metaphysis on the frontal view (Figure E). On the lateral view, this piece of bone projects over the posterior aspect of elbow joint space (Figure F). The noninjured right elbow does not show a bone fragment in this region, but rather shows a similar size ossicle nestled into a notch on the medial aspect of the distal humeral metaphysis on the frontal view (Figure G). The lateral view of the noninjured elbow is blurred, but demonstrates that this ossicle is relatively posterior (Figures H). This is the anatomic position of the medial epicondyle.

Figure E Injured left elbow, AP view. Arrow indicates abnormally positioned ossicle.

Figure F Injured left elbow, lateral view. Arrow indicates abnormally positioned ossicle.

Figure G Noninjured right elbow, AP view. Arrow indicates normal medial epicondyle.

Figure H Noninjured right elbow, lateral view. Arrow indicates normal position of the medial epicondyle, although it is not ideally seen in this case because the image is blurred.

Interpretation: The ossicle described above is the medial epicondyle. An avulsion fracture has occurred, which has displaced the medial epicondyle from its normal position along the medial and posterior aspect of the distal humeral metaphysis. In addition to being displaced, the medial epicondyle has also become positioned within the elbow joint space, indicating that it is entrapped.

Answer: The diagnosis is medial epicondyle avulsion with entrapment into the elbow joint.

Discussion: The medial epicondyle is the protuberance from the posteromedial aspect of the distal humeral metaphysis, from which the flexor and pronator muscles and medial collateral ligaments arise. During childhood, the medial epicondyle exists as an apophysis that will later fuse with the metaphysis. Apophyses differ from epiphyses in that they do not contribute to the longitudinal growth of the bone. Also the force between an apophysis and the metaphysis is traction rather than the compression force between an epiphysis and the metaphysis. The medial epicondyle ossification center is usually seen by ages 4-6, and it is the last to fuse with the metaphysis, generally by age 15, earlier in females than males.

Medial epicondyle fractures are usually caused by excessive traction force, either acute or chronic muscular forces, or by acute valgus stress experienced in an injury. Muscular forces are most often associated with throwing. “Little league elbow,” for example, is caused by excessively hard throwing, which may result in increased avulsion stress on the medial epicondyle, causing avulsion fracture, as well as increased compressive force on the lateral side of the elbow. This compression force can lead to Panner’s disease, characterized by fragmentation and sclerosis of the capitellum, suggesting avascular necrosis.

More frequently, medial epicondyle fractures are caused by a fall on an outstretched arm with the elbow locked in full extension. With the elbow fully extended and with normal elbow valgus, an axial load leads to acute valgus stress, which either distracts the medial side of the elbow or compresses the lateral side of the elbow. In adults, this usually leads to a compression injury on the lateral side of the elbow, causing radial head and neck fractures. In children, acute avulsion of the medial epicondyle is more frequent. As this injury occurs, the elbow joint is opened in valgus with the proximal ulna distracted from the medial condyle. Simultaneously, the avulsed medial epicondyle is pulled down by the muscles and ligaments. During this time, the medial epicondyle can enter the opened medial elbow joint space. When the valgus stress is released and the elbow returns to more normal alignment, the medial epicondyle will become entrapped in the elbow joint, as demonstrated in this case.

Medial epicondyle avulsion fractures are also caused by posterolateral elbow dislocation with acute posterior distraction by the ulnar collateral ligament. The medial epicondyle may also be injured by a direct blow, but this is much less common than the other mechanisms.

Radiological diagnosis of medial epicondyle injuries is highly dependent on knowledge of normal elbow anatomy and the timing with which the elbow ossification centers appear. In evaluating this case, one might consider that the ossicle present in the medial joint space is an ossification center of the trochlea (which forms the medial condyle). However, the medial epicondyle becomes ossified prior to the trochlea. Hence, if one does not see the medial epicondyle, it would be illogical to assume that the ossicle in the joint region would be the trochlea.

In evaluating elbow trauma radiographs in children, it is always important to account for those structures that should be ossified. If the child is mature enough that the medial epicondyle should be present, look carefully for it. If it is not seen in the normal location, start looking elsewhere, including within the elbow joint space. A diagnosis of an entrapped medial epicondyle is important and should not be missed, since surgical correction is needed.

FOR FURTHER READING

1. Rogers LF. The Radiology of Skeletal Trauma. 2nd ed. New York, NY: Churchill Livingstone; 1992:772-777.

2. Chambers HG, Wilkins KE. Apophyseal injuries of the distal humerus. In: Rockwood CA, et al. Fractures in Children. Vol 3. 4th ed. Philadelphia, Pa: Lippincott-Raven; 1996:801-819.

3. Shore RM, Grayhack JJ. Elbow Trauma—Pediatric. Online publication. 2002. Available at: http://www.emedicine.com/radio/topic868.htm. Accessed November 29, 2002.

TOP