Original ArticleUnderstanding the Radiographic Anatomy of the Proximal Ulna and Avoiding Inadvertent Intraarticular Screw PlacementGithens, Thomas C. DO; Campbell, Sean T. MD; Salazar, Brett BS; Goodnough, L. Henry MD, PhD; DeBaun, Malcolm R. MD; Bishop, Julius A. MD; Gardner, Michael J. MDAuthor Information Department of Orthopaedic Surgery, Stanford University Hospital, Palo Alto, CA. Reprints: Thomas Githens, DO, 450 Broadway St, Pavilion C, 4th Floor Redwood City, CA 94063-6342 (e-mail: email@example.com). The authors report no conflict of interest. Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal's Web site (www.jorthotrauma.com). Accepted September 05, 2019 Journal of Orthopaedic Trauma: February 2020 - Volume 34 - Issue 2 - p 102-107 doi: 10.1097/BOT.0000000000001638 Buy SDC Metrics Abstract Objectives: To map the proximal ulnar articular margins and ensure safe extraarticular placement of implants. Methods: Ten fresh frozen adult elbow cadaver specimens were obtained. Radiopaque wire was applied to the articular margin of the articular facets and the central trochlear ridge of the proximal ulna. Fluoroscopic images were obtained demonstrating the articular facet margins. Radiographic measurements were performed and used to identify relative safe screw zones. Results: All specimens demonstrated marked extension of the ulnar and radial facets dorsal to the central trochlear ridge. The dorsal extent of the ulnar facets from the central trochlear ridge averaged 9.7 mm (range, 7.9–13 mm; SD, 1.5 mm) and 6.2 mm (range, 3.4–9.4 mm; SD, 1.9 mm), respectively. The average footprint of the posterior ulnar facet occupied 44% (±4.9%) of the total ulnar height from the dorsal cortex to the trochlear ridge. Conclusions: The articular margins of the anterior and posterior facets of the proximal ulna are challenging to identify radiographically. A surgical “at-risk zone” exists within 9.7 mm from the radiographic margin of the central trochlear ridge. Implants placed within this zone have the potential to violate the articular surface. Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.