The impact of mal-angulated femoral rotational osteotomies on mechanical leg axis: a computer simulation model

Jud et al. BMC Musculoskeletal Disorders https://doi.org/10.1186/s12891-020-3075-1 (2020) 21:50 RESEARCH ARTICLE Open Access The impact of mal-angulated femoral rotational osteotomies on mechanical leg axis: a computer simulation model Lukas Jud* , Lazaros Vlachopoulos, Thomas V. Häller, Sandro F. Fucentese, Stefan Rahm and Patrick O. Zingg Abstract Background: Subtrochanteric or supracondylar femoral rotational osteotomies are established surgical treatments for femoral rotational deformities. Unintended change of the mechanical leg axis is an identified problem. Different attempts exist to plan a correct osteotomy plane, but implementation of the preoperative planning into the surgical situation can be challenging. Goal of this study was to identify the critical threshold of mal-angulation of the osteotomy plane and of femoral rotation that leads to a relevant deviation of the postoperative mechanical leg axis using a computer simulation approach. Methods: Three-dimensional (3D) surface models of the lower extremity of two patients (Model 1: 42° femoral antetorsion; Model 2: 6° femoral retrotorsion) were generated from computed tomography data. First, baseline subtrochanteric and supracondylar rotational osteotomies, perpendicular to the femoral mechanical axis were simulated. Afterwards, mal-angulated osteotomies in sagittal and frontal plane followed by different degrees of rotation were simulated and frontal mechanical axis was analyzed. Results: 400 mal-angulated osteotomies have been simulated. Mal-angulation of ±30° with 30° rotation showed maximum deviation from preoperative mechanical axis in subtrochanteric osteotomies (4.0° ± 0.4°) and in supracondylar osteotomies (12.4° ± 0.8°). Minimal mal-angulation of 15° in sagittal plane in subtrochanteric osteotomies and mal-angulation of 10° in sagittal plane in supracondylar osteotomies altered the mechanical axis by > 2°. Mal-angulation in sagittal plane showed higher deviations of the mechanical axis (up to 12.4° ± 0.8°), than in frontal plane mal-angulation (up to 4.0° ± 1.9°). Conclusion: A femoral rotational osteotomy, perpendicular to the femoral mechanical axis, has no considerable influence on the mechanical leg axis. However, mal-angulation of femoral rotational osteotomies showed relevant changes of the mechanical leg axis. In supracondylar respectively subtrochanteric procedures, mal-angulation of only 10° in combination with already 15° of femoral rotation respectively mal-angulation of 15° in combination with 30° of femoral rotation, can lead to a relevant postoperative mechanical leg axis deviation of more than 2°, wherefore these patients probably would benefit from the use of navigation aids. Keywords: Subtrochanteric osteotomy, Supracondylar osteotomy, Rotational osteotomy, Mechanical leg axis * Correspondence: Department of Orthopedics, Balgrist University Hospital, University of Zurich, Forchstrasse 340, 8008 Zürich, Switzerland © The Author(s). 2020 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. 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. Jud et al. BMC Musculoskeletal Disorders (2020) 21:50 Background Femoral rotational deformities with excessive antetorsion or retrotorsion are frequently seen in patients with femoroacetabular impingement [1, 2], hip dysplasia [3, 4] or patellofemoral instability [5, 6]. Established treatment options are free-hand subtrochanteric or supracondylar femoral rotational osteotomies [7–9], bearing the risk of unintended changes in mechanical leg axis [10, 11]. Furthermore, a computer model study by Nelitz M et al. [12] showed a tendency to varus angulation in proximal- and a tendency to valgus angulation in distal femoral external-rotational osteotomies. In their study, the osteotomy plane was defined perpendicular to the femoral anatomical axis, probably the most common intraoperative landmark for orientation of the osteotomy plane. However, other authors propose to perform the osteotomy perpendicular to the femoral mechanical axis [13], possibly with less influence on the postoperative mechanical leg axis. There are different other attempts for preoperative planning of the correct osteotomy plane in femoral rotational osteotomies [14–16]. Nonetheless, implementation of the preoperative planning into the surgical situation can be challenging, wherefore some deviation from the planning is likely in most cases. A possible remedy could be the use of patient specific instruments (PSI) [15, 17]. However, PSI are not yet routinely used in such surgical procedures and they are not always available. Moreover, the additional costs of PSI need to be considered. Probably their use should, however, be considered in risk-prone patients, such as cases with the need of higher degrees of femoral rotation. So far no study exists that investigates the change of the mechanical leg axis in case of a femoral rotational osteotomy perpendicular to the femoral mechanical axis, and that assess the effect of an unintentionally mal-angulated osteotomy plane. Therefore, threedimensional (3D) patient models with excessive femoral antetorsion and retrotorsion were used to simulate subtrochanteric and supracondylar rotational osteotomies with different angulated osteotomy planes and different degrees of rotation. As an intended correction of the mechanical leg axis in high tibial osteotomy shows accuracy of about 2° [18], a postoperative mechanical leg axis deviation of more than 2° was defined as a relevant mechanical axis deviation. Using this computer simulation approach, it was the goal of this study to investigate a femoral rotational osteotomy perpendicular to the femoral mechanical axis and to identify the critical threshold of mal-angulation and femoral rotation that leads to a relevant deviation in postoperative mechanical leg axis of more than 2°, respectively to identify surgical procedures that are more risk-prone for relevant postoperative mechanical leg axis deviation and Page 2 of 8 therefore would benefit from the use of navigation aids (e.g. PSI). Methods 3D surface models of the lower extremity of the right side of a patient with femoral antetorsion (42 degrees of antetorsion, Model 1) and of a patient with femoral retrotorsion (6 degrees of retrotorsion, Model 2) were generated from computed tomography (CT) data. Besides the rotational deformity, both used patient models had a normal femoral anatomy with a femoral antecurvatum angle of 8° in Model 1 and 14° in Model 2 and a me (...truncated)