Sagittal femoral bowing contributes to distal femoral valgus angle deviation in malrotated preoperative radiographs

Kokubu et al. BMC Musculoskeletal Disorders https://doi.org/10.1186/s12891-022-05542-z (2022) 23:579 Open Access RESEARCH Sagittal femoral bowing contributes to distal femoral valgus angle deviation in malrotated preoperative radiographs Yasuhiko Kokubu1, Shinya Kawahara1*, Satoshi Hamai1,2, Yukio Akasaki1, Hidetoshi Tsushima1, Kenta Momii1,3 and Yasuharu Nakashima1 Abstract Background: The coronal whole-leg radiograph is generally used for preoperative planning in total knee arthroplasty. The distal femoral valgus angle (DFVA) is measured for distal femoral bone resection using an intramedullary guide rod. The effect of coronal and sagittal femoral shaft bowing on DFVA measurement in the presence of malrotation or knee flexion contracture has not been well reported. The objectives of this study were: (1) to investigate the effects of whole-leg malrotation and knee flexion contracture on the DFVA in detail, (2) to determine the additional effect of coronal or sagittal femoral shaft bowing. Methods: We studied 100 consecutive varus and 100 valgus knees that underwent total or unicompartmental knee arthroplasty. Preoperative CT scans were used to create digitally reconstructed radiography (DRR) images in neutral rotation (NR, parallel to the surgical epicondylar axis), and at 5° and 10° external rotation (ER) and internal rotation (IR). The images were also reconstructed at 10° femoral flexion. The DFVA was evaluated in each DRR image, and the angular variation due to lower limb malposition was investigated. Results: The DFVA increased as the DRR image shifted from IR to ER, and all angles increased further from extension to 10° flexion. The DFVA variation in each position was 1.3° on average. A larger variation than 2° was seen in 12% of all. Multivariate regression analysis showed that sagittal femoral shaft bowing was independently associated with a large variation of DFVA. Receiver operating characteristic analysis showed that more than 12° of sagittal bowing caused the variation. Conclusion: If femoral sagittal bowing is more than 12°, close attention should be paid to the lower limb position when taking whole-leg radiographs. Preoperative planning with whole-leg CT data should be considered. Keywords: Distal femoral valgus angle, Femoral shaft bowing, Malrotation, Total knee arthroplasty, Whole-leg radiography *Correspondence: 1 Department of Orthopaedic Surgery, Graduate School of Medical Sciences, Kyushu University, 3‑1‑1 Maidashi, Higashi‑ku, Fukuoka 812‑8582, Japan Full list of author information is available at the end of the article Background During total knee arthroplasty (TKA), surgeons have traditionally tried to place the femoral component perpendicular to its mechanical axis [1, 2]. In preoperative planning, the whole‐leg radiograph is used to measure the angle between the femoral mechanical axis and the anatomical axis of the distal femur in the coronal plane, to facilitate the use of the intramedullary guide rod. For © The Author(s) 2022. Open Access This 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://creativeco mmons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Kokubu et al. BMC Musculoskeletal Disorders (2022) 23:579 accuracy, the coronal whole-leg radiograph must be evaluated in neutral rotation (NR). However, radiographs of varus knees are generally taken in a slightly externally rotated (ER) position, while valgus knees are usually examined in a slightly internally rotated (IR) position. Flexion contracture is also common in severely deformed knees. However, few studies have described in detail the effects of lower limb malrotation and knee flexion on the distal femoral valgus angle (DFVA) for TKA preoperative planning. Morphological features of the femur, such as coronal and sagittal femoral shaft bowing, may increase the effect of whole-leg malrotation and knee flexion contracture on the measured DFVA. Femoral shaft bowing has been associated with Asian ethnicity, age, and the progression of knee osteoarthritis (OA) [3]. Coronal femoral shaft bowing greater than 5° has been described as a risk factor for postoperative malalignment [4]. Sagittal femoral shaft bowing has been shown to cause increased femoral component flexion in TKA [5]. However, it is not clear how coronal and sagittal femoral shaft bowing affects the measurement of the DFVA when there is malrotation. The objectives of this study were to use three-dimensional (3D) computer simulations, first, to investigate the effects of whole-leg malrotation and knee flexion contracture on the DFVA and, second, to determine the additional effect of coronal or sagittal femoral shaft bowing. Materials and methods Data acquisition Consecutive patients with varus or valgus deformity who underwent TKA or unicompartmental knee arthroplasties in our institution were included in the study. Patients with any history of osteotomy, fracture, or arthroplasty of the hip or knee joint were excluded. We recruited 100 varus and 100 valgus knees. The varus knees were recruited between April 2019 and June 2021, and the valgus knees were recruited between April 2012 and June 2021. All the patients were Japanese and provided informed consent before participation. The local Institutional Review Board approved the study (No.2020–204). Varus or valgus alignment is based on the hip–kneeankle (HKA) angle (the angle between the mechanical axes of the femur and tibia). The HKA angle was measured with anteroposterior whole-leg standing radiographs using Fuji-film OP-A software (Fujifilm, Co., Ltd, Tokyo, Japan). Preoperative transverse CT scans (Aquilion ONE; Canon Medical Systems Corporation, Tochigi, Japan) of the lower extremity (including hip and ankle joints) were taken in all patients at 1.25 mm intervals and 1.25 mm thickness with a field of view of 400 and 1.375 pitch. The Page 2 of 9 patients, supine on the scanning table, were instructed to naturally extend their affected knee without any feeling of internal or external rotation. The CT images were acquired as Digital Imagin (...truncated)