Evaluating the validity of lightweight talar replacement designs: rational models and topologically optimized models

Kang et al. Biomaterials Research (2022) 26:10 https://doi.org/10.1186/s40824-022-00256-8 RESEARCH ARTICLE Open Access Evaluating the validity of lightweight talar replacement designs: rational models and topologically optimized models Yeokyung Kang1,2, Seongjin Kim1, Jungsung Kim1, Jin Woo Lee3 and Jong-Chul Park2* Abstract Background: Total talar replacement is normally stable and satisfactory. We studied a rational scaffold talus model for each size range created through topology optimization (TO) and comparatively evaluated a topologically optimized scaffold bone talus model using a finite element analysis (FEA). We hypothesized that the rational scaffold would be more effective for application to the actual model than the topologically optimized scaffold. Methods: Size specification for the rational model was performed via TO and inner scaffold simplification. The load condition for worst-case selection reflected the peak point according to the ground reaction force tendency, and the load directions “plantar 10°” (P10), “dorsi 5°” (D5), and “dorsi 10°” (D10) were applied to select worst-case scenarios among the P10, D5, and D10 positions (total nine ranges) of respective size specifications. FEA was performed on each representative specification-standard model, reflecting a load of 5340 N. Among the small bone models selected as the worst-case, an arbitrary size was selected, and the validity of the standard model was evaluated. The standard model was applied to the rational structure during validity evaluation, and the TO model reflecting the internal structure derived by the TO of the arbitrary model was implemented. Result: In worst-case selection, the highest peak von Mises stress (PVMS) was calculated from the minimum D5 model (532.11 MPa). Thereafter, FEA revealed peak von Mises stress levels of 218.01 MPa and 565.35 MPa in the rational and topologically optimized models, respectively, confirming that the rational model yielded lower peak von Mises stress. The weight of the minimum model was reduced from 1106 g to 965.4 g after weight reduction through rational scaffold application. Conclusion: The rational inner-scaffold-design method is safer than topologically optimized scaffold design, and three types of rational scaffold, according to each size range, confirmed that all sizes of the talus within the anatomical dimension could be covered, which was a valid result in the total talar replacement design. Accordingly, we conclude that an implant design meeting the clinical design requirements, including patient customization, weight reduction, and mechanical stability, should be possible by applying a rational inner scaffold without performing TO design. The scaffold model weight was lower than that of the solid model, and the safety was also verified through FEA. Keywords: Talar replacement, Topology optimization, Finite element analysis, Rational scaffold, Optimized scaffold * Correspondence: 2 Cellbiocontrol Laboratory, Department of Medical Engineering, Yonsei University College of Medicine, Seoul, Republic of Korea Full list of author information is available at the end of the article © 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://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. Kang et al. Biomaterials Research (2022) 26:10 Background The talus forms the ankle joint and is located superiorly in the foot. It is located inferior to the tibia and fibula, supporting both bones, and is responsible for transferring weight to the feet. Moreover, the talus is covered in cartilage along with the fibula and other ankle bones to control feet movement. Since 60% of the talus is covered by cartilage and surrounding bones, blood supply is poor, and avascular necrosis (AVN) occurs frequently [1, 2]; 75% of cases of AVN of the talus are caused by trauma, and 25% have nontraumatic etiologies, including polycythemia [3, 4]. Currently, for AVN of the talus, conservative treatment is preferred over surgical treatment, and surgical treatment, such as talectomy and talus fusion, is performed when pain is severe or walking is impossible. Talectomy is no longer recommended as it leads to poor functional results, shortening of the lower extremities, and marked postoperative destruction of the calcaneus. When the damage to the existing talus bone is severe, talus fusion is performed, removing the existing talus and implanting a bone that is fixed in the ankle joint. Because the postoperative ankle joint is immobile, the patient’s gait is not natural and the load on the adjacent joint is heavy; patients prefer not to undergo such treatment, as the surgical outcome is not much better than with talectomy [5]. Ankle joint surgery that can be replaced includes total talus arthroplasty and total ankle arthroplasty. However, total ankle arthroplasty has a disadvantage in that the talus, as well as the tibia, where it is in contact with the talus, must be excised and replaced with an artificial joint, even when the tibia is intact. In contrast, total talus arthroplasty replaces only the talus, maintaining the limb length [1]. Moreover, total ankle arthroplasty is contraindicated in AVN patients [6]; in such patients, only the talus should be replaced with an implant with proven safety. Moreover, as the need for implants rises because of an increase in demand related to talar idiopathic AVN and trauma [1], total talar arthroplasty is expected to show superior results compared to total ankle arthroplasty for quick pain relief. To perform such total talus arthroplasty, the customized talus implants are generally manufactured using the powder bed fusion three-dimensional (3D) printing method. The first advantage of a talus implant manufactured using 3D printing is that an anatomically fitting ankle can be reconstructed using the patient’s normal contralateral talus as a template for the design, reducing postoperative discomfort and increasing ankle function [7]. The second advantage is that, with topology opti (...truncated)