Design of a biodegradable plate for femoral shaft fracture fixation

https://doi.org/10.1016/j.medengphy.2020.05.010Get rights and content

Highlights

  • Design of a biodegradable implant plate for femoral shaft fracture.

  • Optimization of dimensions and analysis of implant plate based on computational structural analysis for adequate support and safe design.

  • Striking a balance between the life-span of biodegradable implant plate and its time-dependent degradation across the healing of bone.

  • Understanding the significance of implant plate thickness and the role played by it in ensuring desired balance between mechanical strength, biodegradation rate and plate degradation time.

Abstract

Biodegradable materials have been generating increasing bit of interest in biomedical applications and associated research. The evolution of implants made up of such materials (Mg-alloys, etc.) has the potential to be a game changer in fracture surgeries. These implants are essentially made up of a plate and a number of screws. In orthopaedic applications, they offer the biggest advantage of complete degradation after successfully supporting the fractured bone for the desired period. They may provide some nutrients that accelerate the healing process while simultaneously ensuring adequate mechanical stability. This article essentially focuses on design of a biodegradable implant plate for femoral shaft fracture, taking into consideration the dimensional accuracy of the plate, uniform biodegradation rate and adequate mechanical stability of the plate across the entire process span. The design of a biodegradable implant plate and associated specified screws that support the plate, fitted over two segments for fixation of femoral shaft fracture, has been made on the basis of femur's standard dimensions, optimized plate dimensions and uniform biodegradation rate. A confirmation regarding the safe design of the implant plate is obtained through computational structural analysis. The implant plate design turns out to be safe at specific optimized dimensions for a human being weighing 80 Kg, at corresponding loading and boundary conditions. For average monthly degradation of the plate across a period of six months, the factor of safety comes out to be more than unity. The implant plate eventually goes through complete degradation 3–6 months after the completion of the healing process and this is where the plate thickness plays a significant role.

Introduction

The implants used in fracture surgeries consist of plates, screws, nails, etc. In the past implants made up of non-biodegradable materials were quite common. The interaction of the physiological environment inside the human body with such implants could give rise to some harmful effects, especially after remaining in the body for a long time [1]. An external foreign body of this nature needs to be removed and/or replaced after a certain time during/after healing of the fractured bone. It can be considered safe for human beings only for a short period and doesn't degrade significantly after completion of the healing process. A secondary surgery after the completion of the healing process is therefore necessitated for its removal or replacement, and this can be rather inconvenient and painful for patients. Some examples of commonly used non-biodegradable materials in implant applications are Titanium (Ti)-alloy, Stainless Steel, Cobalt-Chromium alloy, etc. [2].

Of late, there have been significant bit of attempts to use implants which go through near-complete degradation within the body under the existing physiological environment. Such medical devices, called biodegradable implants, consist mainly of plates, screws and nails. They are increasingly used in orthopaedic applications, can be manufactured by using bio-inert or bio-active materials, and can be formed by naturally transforming compounds, e.g. Mg-alloy, etc. [3].

The usage of implants made up of biodegradable materials has all the promise to eliminate the secondary surgeries and can go a long way in providing an effective femoral bone fixation solution without accompanying harmful complications. During the course of healing, they start degrading and almost completely dissolve within the desired duration after the completion of the healing process. Some of the dissolved quantity offers a collateral benefit of promoting healing whereas the remaining exits the body simultaneously [4]. Biodegradable implants made up of such materials must provide desired mechanical stability and resist the effect of compression, shear, etc., to support the fractured bone during healing process. Whereas the biodegradable implant plate needs to have sufficient strength at every point throughout the process of healing, it needs to degrade completely over this period [5]. The plate thickness and degradation rate consequently become truly important parameters as there are two conflicting requirements that need to be balanced over the period of healing. Commonly used biodegradable materials like - Mg - alloy, Zinc - alloy, etc., serve the purpose reasonably well [6,7].

Designing a biodegradable plate for implant applications in accordance with the degradation behaviour is a challenging task [8]. Locking compression plates made up of non-degradable materials have been conventionally used in orthopaedic applications due to some advantages like satisfactory locking between the screws and plate that ensures limited contact in downward position along the contact surfaces [9].

The von Mises yield criterion (also known as the maximum distortion energy criterion) suggests that yielding of a ductile material begins when the second deviatoric stress invariant reaches a critical value [10]. To reach the yielding state, the material property distributions can be assumed to exhibit linear elastic behaviour characterized by the presence of an elastic region, or a nonlinear elastic-plastic behaviour exhibiting the corresponding regions.

Finite element method (FEM) is a numerical method used extensively for solving plethora of problems in engineering and science. It is widely used in finding solutions to problems arising in structural analysis, heat transfer, fluid flow, mass transport, etc. Determining analytical solutions to these problems generally requires finding solutions of partial differential equations and nonlinear equations that arise out of the mathematical representations for boundary value problems. The method approximates the unknown function over the domain to solve the problem by subdividing a large system into a number of small elements [10,11].

Discretization error is a key concept for the study of mesh convergence study to determine the most economical mesh refinement. Local re-meshing in surgical simulations leads to improved convergence with the most economical mesh at each time-step. Finite element analysis convergence also defines the relationship between the number of elements and the analysis accuracy [12]. The current work utilizes a linear approach for design and analysis.

Many types of fractures in the femoral shaft have already been observed and studied. Most bone fracture analyses have focused on the longest and strongest bone (i.e. femur bone) because if any plate and the corresponding screws can bear the load and stress under the impact load prevailing in femoral shaft fracture then it may be expected, with a reasonable bit of accuracy, to bear relatively less harsh loading conditions for different fractures on different bones [13]. Most commonly observed fracture types in femoral shaft are shown below in Fig. 1.

This work has focused on the design of implant plate (and associated supporting screws) made up of biodegradable materials (Mg-alloys), with the objective of optimizing the dimensions in accordance with the mechanical behavior, continuous and uniform average degradation rate throughout the life span while taking an average healing time duration into consideration.

Section snippets

Selection of material

The selection of biodegradable material for a biomedical application is a critical factor because its density, mechanical properties and corrosion (biodegradation) rates significantly influence the design and behaviour of the implant plate under consideration. Some of the important characteristics based on mechanical and corrosion behaviour have been listed in Table 1. These properties serve as important parameters for designing a biodegradable implant (the plate and screws, in particular), as

Meshing

To analyze the optimal results in the structure, the implant plate and screw have been divided into many nodes and elements. Regular geometries, medium smoothening meshed shape and adaptive sizes have been taken. Each parameter is identified in many elements with flexible stiffness behaviour to identify deformation/stress accurately in every element. Adequate meshing details are shown in Table 2 that indicates how accurately the elements and respective nodes have been divided in each part of

Design selection

Selection of optimal design is based on a factor of safety that can be calculated by considering yield strength of materials used for computational structural analysis with a static structure model. The selection of individual dimensions has been carried out by picking those dimension values that correspond to the minimum of the maximum stress values obtained. The factor of safety will be higher than one. The combination of a high biodegradation rate and low plate thickness may result in

Conclusion

The current work has its focus on designing a biodegradable implant plate for femoral shaft fracture. An analysis has been carried out with Mg - alloy with the primary objective of optimizing the dimensions of plate and screw. This is based on von Mises theory and a factor of safety (>1) has been determined. All dimensions have been properly verified under the criterion utilizing computational structural analysis for ensuring safe design. Overall analysis of final dimensions of implant plate

Declaration of Competing Interest

The authors declare that the current research work does not involve any conflict of interest in any manner whatsoever.

Funding

None

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