Design of a biodegradable plate for femoral shaft fracture fixation
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
Ethical Approval
Not required
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