SCIENTIFIC ARTICLE Interobserver Reliability of Coronoid Fracture Classification: Two-Dimensional Versus Three-Dimensional Computed Tomography Anneluuk Lindenhovius, MSc, Paul Jack Karanicolas, MD, PhD, Mohit Bhandari, MD, Niek van Dijk, MD, PhD, David Ring, MD, PhD, Collaboration for Outcome Assessment in Surgical Trials* Purpose This study tests the hypothesis that 3-dimensional computed tomography (CT) reconstructions improve interobserver agreement on classification and treatment of coronoid fractures compared with 2-dimensional CT. Methods A total of 29 orthopedic surgeons evaluated 10 coronoid fractures on 2 occasions (first with radiographs and 2-dimensional CT and then with radiographs and 3-dimensional CT), separated by a minimum of 2 weeks. Surgeons classified fractures according to the classifications of Regan and Morrey and of O’Driscoll et al., identified specific character- istics, recommended the most appropriate treatment approach, and made treatment recom- mendations. The kappa multirater measure () was calculated to estimate agreement between observers. Results Regardless of the imaging modality used, there was fair to moderate agreement for most of the observations. Three-dimensional CT improved interobserver agreement in Regan and Morrey’s classsication ( 3-dimensional = 0.51 vs  2-dimensional = 0.40; p  .001) and O’Driscoll et al.’s classifications ( 3-dimensional = 0.48 vs  2-dimensional = 0.42; p = .009). There were trends toward better reliability for 3-dimensional reconstruction in recognition of coronoid tip fractures ( 3-dimensional = 0.19,  2-dimensional = 0.03; p = .268), comminution ( 3-dimensional = 0.41 vs  2-dimensional = 0.29; p = .133), and impacted fragments ( 3-dimensional = 0.39 vs  2-dimensional = 0.27; p = .094), and in surgeons’ opinions on the need for something other than screws or plate for surgical fixation ( 3-dimensional = 0.31 vs  2-dimensional = 0.15; p = .138). Interobserver agreement on treatment approach was better with 2-dimensional CT ( 3-dimensional = 0.27,  2-dimensional = 0.32; p = .015). Conclusions Three-dimensional CT reconstructions improve interobserver agreement with respect to fracture classification compared with 2-dimensional CT. (J Hand Surg 2009;34A: 1640 –1646. © 2009 Published by Elsevier Inc. on behalf of the American Society for Surgery of the Hand.) Type of study/level of evidence Diagnostic III. Key words Computed tomography, coronoid, fracture, interobserver, reliability. From the Orthopaedic Hand and Upper Extremity Service, Harvard Medical School, Massachusetts Gen- eral Hospital, Boston, MA; the Department of Surgery, University of Western Ontario, London, Ontario, Canada; Clarity Research, McMaster University, Hamilton, Ontario, Canada; and the Department of Or- thopaedic Surgery, Academic Medical Center, University of Amsterdam, The Netherlands. Received for publication March 12, 2009; accepted with revisions July 17, 2009. *Collaboration for Outcome Assessment in Surgical Trials surgeons are: Christopher Allan, Jeffrey An- glen, Terry Axelrod, Mark Baratz, Daphne Beingessner, Peter Brink, Charles Cassidy, Chad Coles, Joe Conflitti,BrettCrist,GregoryDellaRocca,NiekvanDijk,SanderDijkstra,L.H.G.J.Elmans,RogerFeibel,Luis Flores, Frede Frihagen, Taco Gosens, Carel Goslings, Jeffrey Greenberg, Elena Grosso, Neil Harness, Huub van der Heide, Kyle Jeray, David Kalainov, Albert van Kampen, Sumito Kawamura, Peter Kloen, Michael McKee, Sean Nork, Richard Page, Rodrigo Pesantez, Anil Peters, Rudolf Poolman, Michael Prayson, Mar- tin Richardson, John Seiler, Marc Swiontkowski, George Thomas, Tom Trumble, Arie van Vugt, Thomas Wright, Charalampos Zalavras, and Robert Zura. D.R. is a consultant for Wright Medical, Acumed, and Tornier. He received unrestricted research grants from Acumed and Tornier, and he receives royalties from Wright Medical and Skeletal Dynamics. Corresponding author: David Ring, MD, PhD, Orthopaedic Hand and Upper Extremity Service, Mas- sachusetts General Hospital, 55 Fruit Street, Boston, MA 02114; e-mail:dring@partners.org. 0363-5023/09/34A09-0008$36.00/0 doi:10.1016/j.jhsa.2009.07.009 1640  ©  Published by Elsevier, Inc. on behalf of the ASSH.