The AO classification system for tibial plateau fractures: An independent inter and intraobserver agreement study

Purpose: Classifying tibial plateau fractures is paramount in determining treatment regimens and systemizing decision making. The original AO classification described by Muller in 1996 and the Schatzker classification of 1970 are the most cited classifications for tibial plateau fractures, demonstrating substantial to almost perfect agreement. The main problem with these classifications schemes is that they lack the detail required to convey the variety of fracture patterns encountered. In 2018, the AO foundation published a new classification system for proximal tibia fractures, highlighting a more complete and detailed number of categories and subcategories. We sought to independently determine inter and intraobserver agreement of the AO classification system, compared to the previous systems described by Muller and Schatzker. Methods: One hundred seven consecutive tibial plateau fractures were screened, and a representative data set of 69 was created. Six independent evaluators (three knee surgeons, three senior orthopedic residents) classified the fractures using the original AO, the Schatzker and the new AO classifications. After six weeks, the 69 cases were randomized and reclassified by all evaluators. The Kappa coefficient (k) was calculated for inter- and intraobserver correlation and is expressed with 95% confidence intervals. Results: interobserver agreement was moderate for all three classifications. k = 0.464 (0.383-0.560) for the original AO; k = 0.404 (0.337-0.489) for Schatzker; and k = 0.457 (0.371-0.545) for the base categories of the new AO classification. The inclusion of subcategories and letter modifiers to the new classification worsened agreement to k = 0.358 (0.302-0.423) and k = 0.174 (0.134-0.222), respectively. There were no significant differences between knee surgeons and residents for the new classification. Intra-observer correlation was also moderate for each of the scores: k = 0.630 (0.578-0.682) for the original AO; k = 0.623 (0.569-0.674) for Schatzker; and k = 0.621 (0.566-0.678) for the new AO base categories; without differences between knee surgeons or residents. Conclusions: This study demonstrated an adequate inter and intra-observer agreement for the new AO tibial plateau fractures classification system for its base categories, but not at the subcategory or letter modifier levels.