Failure analysis of a locking compression plate with asymmetric holes and polyaxial screws

Ning-Ze Zhang; Bo-Lun Liu; Yi-Chao Luan; Min Zhang; Cheng-Kung Cheng

doi:10.1016/j.jmbbm.2022.105645

Failure analysis of a locking compression plate with asymmetric holes and polyaxial screws

J Mech Behav Biomed Mater. 2023 Feb:138:105645. doi: 10.1016/j.jmbbm.2022.105645. Epub 2022 Dec 29.

Authors

Ning-Ze Zhang¹, Bo-Lun Liu¹, Yi-Chao Luan¹, Min Zhang², Cheng-Kung Cheng³

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, 100083, China.
² 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, 100083, China. Electronic address: m.zhang@buaa.edu.cn.
³ 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, 100083, China; School of Biomedical Engineering, Shanghai Jiao Tong University, Shanghai, 200030, China. Electronic address: ckcheng2020@sjtu.edu.cn.

PMID: 36603526
DOI: 10.1016/j.jmbbm.2022.105645

Abstract

Locking compression plates (LCP) with asymmetrical holes and polyaxial screws are effective for treating mid-femoral fractures, but are prone to failure in cases of bone nonunion. To understand the failure mechanism of the LCP, this study assessed the material composition, microhardness, metallography, fractography and biomechanical performance of a retrieved LCP used for treating a bone fracture of AO type 32-A1. For the biomechanical assessment, a finite element surgical model implanted with the intact fixation-plate system was constructed to understand the stresses and structural stiffness on the construct. Also, to avoid positioning screws around the bone fracture, different working lengths of the plate (the distance between the two innermost screws) and screw inclinations (±5°, ±10° and ±15°) were investigated. The fracture site of the retrieved LCP was divided into a narrow side and broad side due to the asymmetrical distribution of holes on the plate. The results indicated that the chemical composition and microhardness of the LCP complied with ASTM standards. The fatigue failure was found to originate on the narrow side of the hole, while the broad side showed overloading characteristics of crack growth. When the screws were inserted away from the region of the bone fracture by increasing the working length, the stress of the fixation-plate system decreased. Regardless of the screw insertion angle, the maximum stress on the LCP always appeared on the narrow side, and there was little change in the structural stiffness. However, angling the screws at -10° resulted in the most even stress distribution on the fixation-plate system. In conclusion, the LCP assessed in this study failed by fatigue fracture due to bone nonunion and stress concentration. The narrow side of the LCP was vulnerable to failure and needs to be strengthened. When treating an AO type 32-A1 fracture using an LCP with asymmetrical holes and polyaxial screws, inserting the screws at -10° may reduce the risk of implant failure and positing screws around the fractured area of the bone should be avoided.

Keywords: Asymmetric holes; Failure analysis; Finite element model; Locking compression plate; Polyaxial screws.

MeSH terms

Biomechanical Phenomena
Bone Plates
Bone Screws
Equipment Failure
Femoral Fractures* / surgery
Fracture Fixation, Internal* / methods
Humans