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. 2021 Mar 24;16:212. doi: 10.1186/s13018-021-02328-2

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© The Author(s) 2021

Open AccessThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

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Fig. 2 — A coordinate system was established based on the femur. a The fovea of the medial epicondyle (point M) and the highest point of the lateral epicondyle (point L) were selected to form the TEA, which was defined as the x-axis, with the midpoint of the TEA as the origin (point O); two virtual balls—with the origin as their centres and with 0.7 and 0.8 times of the TEA length as their radius respectively—were crossed with the femur to obtain two section surfaces, whose centroids were linked to determine the femoral shaft axis. b The y-axis was defined as the line passing through the origin and perpendicular to the TEA and the femoral shaft axis meanwhile. c The z-axis was perpendicular to the x-axis and y-axis through the origin. d Three planes were determined by the x-, y- and z-axes (red: level plane; yellow: coronal plane; blue: midsagittal plane)