Capability of auxetic femoral stems to reduce stress shielding after total hip arthroplasty

Bolun Liu; Huizhi Wang; Min Zhang; Junwei Li; Ningze Zhang; Yichao Luan; Chaohua Fang; Cheng-Kung Cheng

doi:10.1016/j.jot.2022.11.001

Capability of auxetic femoral stems to reduce stress shielding after total hip arthroplasty

J Orthop Translat. 2022 Nov 23:38:220-228. doi: 10.1016/j.jot.2022.11.001. eCollection 2023 Jan.

Authors

Bolun Liu¹, Huizhi Wang², Min Zhang¹, Junwei Li¹, Ningze Zhang¹, Yichao Luan¹, Chaohua Fang^{3

2}, Cheng-Kung Cheng^{1

2}

Affiliations

¹ Key Laboratory of Biomechanics and Mechanobiology, Ministry of Education, Beijing Advanced Innovation Center for Biomedical Engineering, School of Biological Science and Medical Engineering, Beihang University, Beijing, China.
² School of Biomedical Engineering & Med-X Research Institute, Shanghai Jiao Tong University, Engineering Research Center of Digital Medicine, Ministry of Education, Shanghai, China.
³ Department of Joint Surgery, The 6th Hospital of Ningbo, No.1059 Zhongshan Road, Yinzhou District, Ningbo, 315000, Zhejiang, China.

Abstract

Background: Stress shielding (SS) is considered the main mechanical cause of femoral stem loosening after total hip arthroplasty (THA). This study introduces an auxetic lattice femoral stem structure with negative Poisson's ratio that can expand laterally, with the intent of transferring more load to surrounding bone and thereby reducing SS. This study aims to evaluate how the geometry profile of different femoral stems with auxetic structures affects the level of SS. Different re-entrant angles for the auxetic unit cells were also evaluated.

Methods: This study assessed three commercial femoral stem designs (Mayo, CLS and Fitmore) and three re-entrant angles for the auxetic structures (60°, 70° and 80°). Nine auxetic femoral stems (three M-type, three C-type and three F-type) and three solid femoral stems (control group) were designed. All femoral stems were implanted into a finite element model of the human femur to compare levels of SS between the auxetic stems and their traditional solid counterparts.

Results: The results showed that incorporating an auxetic structure into the stem design caused less SS of the surrounding bone than the control models. The M-type stems had the lowest level of SS, followed by the C-type and F-type stems. A re-entrant angle of 70° for the M-type stem, 80° for the C-type stem and 60° for the F-type stem were the designs most capable of reducing SS.

Conclusions: This study found that femoral stems with an auxetic lattice structure caused less SS after THA than comparable solid femoral stems. A femoral stem based on the M-type geometry profile is recommended when designing auxetic femoral stems to minimize SS of surrounding bone.

The translational potential of this article: The novel solution provided in this study may serve to increase the survival rate of femoral stems by reducing SS after THA.

Keywords: Auxetic structure; Femoral stem; Negative Poisson's ratio; Stress shielding; Total hip arthroplasty.