CASE 8870 Published on 02.11.2010

Radial head dislocation with plastic bowing ‘fracture’ of the ulna

Section

Paediatric radiology

Case Type

Clinical Cases

Authors

Claes H, Smet M
Department of Radiology, University Hospitals Leuven, Herestraat 49, 3000 Leuven, Belgium

Patient

7 years, male

Categories
Area of Interest Extremities ; Imaging Technique Conventional radiography
Clinical History
A seven-year-old boy fell on his left arm while jumping the trampoline. 22 months earlier, he already broke his left distal radius and ulna.
Now he had severe pain and loss of function in the left elbow. Initial radiographs of the elbow (Fig. 1) were reported negative.
Imaging Findings
Seven days after the initial radiographs the boy presented again with persisting symptoms. New radiographs, including the entire forearm (Fig. 2), showed an anteriorly dislocated radial head associated with the ‘ulnar bow sign,' compatible with a plastic deformation of the ulna. A very subtle transverse sclerotic band in the distal radial diaphysis was the only remnant of the double forearm fracture 22 months before.
Discussion
Probably all so-called “isolated” radial head dislocations are associated with plastic deformation of the ulna, visible as the 'ulnar bow sign' on radiographs of the forearm. A straight line is drawn along the dorsal border of the ulna from olecranon to distal metaphysis, which normally intersects with the ulnar diaphysis. The maximum perpendicular distance of this line from the diaphysis is measured (not including the natural curve of the distal metaphysis). As a rule of thumb, an ulnar bow of more than 1 mm is considered abnormal. In the control group from reference [1], the mean value was 0.01 +/- 0.1 mm.

Awareness of this plastic Monteggia fracture is crucial, as treatment of the ulnar deformation is necessary to adequately reposition the radial head (Fig. 3). Vice versa, finding an ulnar bow sign can lead to diagnosis of a subtle radial head subluxation. Therefore, in the assessment of a forearm or elbow injury, a lateral radiograph of the entire forearm should be obtained.

In many cases of missed radial head dislocation, the quality of the radiographs is rather poor. To obtain an optimum true lateral view of the elbow joint, some rules have to be considered. The shoulder and elbow joints need to be at the same level with the elbow flexed 90°, while the wrist should be a little higher than the elbow with the thumb pointing upwards. Although more difficult in trauma settings, adequate positioning is essential to get the necessary diagnostic information.
Differential Diagnosis List
Radial head dislocation with plastic bowing ‘fracture’ of the ulna
Final Diagnosis
Radial head dislocation with plastic bowing ‘fracture’ of the ulna
Case information
URL: https://www.eurorad.org/case/8870
DOI: 10.1594/EURORAD/CASE.8870
ISSN: 1563-4086

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