Background: The "ulnar bow sign" is a novel marker of plastic ulnar deformity with radial head dislocation. This sign is best assessed on true lateral radiographs. However, such radiographs are rarely obtained in routine clinical situations. Misdiagnosis can result from using suboptimum radiographs. The present study examined the clinical radiographs of normal forearms in children to identify factors that affect the assessment of ulnar bowing.
Methods: We retrospectively analyzed the lateral forearm radiographs of 175 normal children ranging in age from 2 to 12 years. Radiographs were classified according to humeral position. The size and direction of the maximum ulnar bow were evaluated. The effect of humeral position, age, sex, and arm side on ulnar bow assessment were analyzed.
Results: Of the 175 radiographs, 27 showed a concave dorsal ulnar border (15.4%), 90 showed a straight dorsal ulnar border (51.4%), and 58 showed a convex dorsal ulnar border (33.2%). Only 22 (12.6%) radiographs were found to be true lateral radiographs; the remainder showed evidence of humeral rotation and/or tilting. Humeral tilting was found to affect the assessment of ulnar bowing (P<0.05), whereas the other measured factors did not.
Conclusions: The present study found that humeral tilting at the time of forearm radiography affected the assessment of ulnar bowing. Therefore, physicians should be cautious when assessing ulnar bow in the clinical setting.
Level of evidence: Level IV.