What Is the Normal Ulnar Bow in Adult Patients?

Background Rotation of the forearm is a result of the complex interaction among the radius, ulna, and interosseous membrane. Although the radius is recognized as curved, the ulna is generally thought of as a straight bone. To better describe normal anatomy, which may lead to more successful anatomic fixation of forearm fractures, we aimed to apply a method of measuring the normal ulnar bow and determine the mean ulnar bow in adults. Questions/purposes (1) To what degree is the ulna bowed in the coronal and sagittal planes in normal adult forearms? (2) To what degree is the radius bowed in the coronal plane in normal adult forearms? Methods Radiographs of the forearms of adults taken during a 1-year period were initially obtained retrospectively. These radiographs were performed for various reasons, including forearm pain and routine radiographic follow-up. Radiographs were excluded if evidence of a fracture or post-fracture fixation was found, if a patient had missing AP or lateral images, or if a suboptimal technique was used. The coronal and sagittal bow of the ulna was measured with a method adapted from previous studies that assessed radial bow using AP and lateral radiographs, respectively. Similar measurements were made in the coronal plane for the radius. All measurements were performed independently by the four authors. There was excellent interobserver reliability for ulnar bow in the coronal and sagittal planes (interclass correlation coefficient = 0.96 and 0.97, respectively) and for radial bow in the coronal plane (interclass correlation coefficient = 0.90). Results The mean maximal coronal ulnar bow was 7 +/- 2 mm and was located at 75% of the ulnar length, measured proximally to distally. The location of coronal bow was consistently distal to the radial bow location. The mean maximal sagittal ulnar bow was 6 +/- 3 mm and was located at 39% of the ulnar length. The mean maximal coronal bow of the radius was 14 +/- 2.0 mm and was 59% of the total length of the radius from proximal to distal. Conclusions The ulna is not a straight bone, as is commonly thought, but rather has a bow in both the coronal and sagittal planes.