Design and Analysis of Customized Fixation Plate for Femoral Shaft

Abstract

Background

In the recent years, several techniques have been used to treat femur diaphyseal fracture. Among all the traditional fixation techniques, unstable fixation remains the biggest challenge for orthopedists. Researchers have recommended new approaches to deal with diaphyseal femur fracture. However, solely few had been successful in getting some better results. In the present work, a methodology comprising of design and finite-element analysis of a counter fit customized fixation plate has been suggested to provide a stable fixation.

Materials and Methods

In the present work, reverse engineering (RE) approach has been invoked to create a 3D model of a fresh fractured femur diaphysis bone using the computed tomography (CT) scan data available in digital imaging and communications in medicine (DICOM) format. To provide stable fixation, a counter fit customized fixation plate at medial side has been designed and simulated under static physiological loading conditions for three different biocompatible materials, viz., titanium alloy (Ti6Al4V), stainless steel (SS-316L), and cobalt–chromium–molybdenum alloy (Co–Cr–Mo).

Results

Static stress distribution and deformation analysis of the clinical setup have been performed for the aforementioned materials. It has been observed that the stresses and deformation developed in all the materials are quite low. It implies that customized fixation plates will provide stable fixation resulting in improved fracture union.

Conclusion

The proposed work will assist the medical practitioners regarding the design and analysis of customized implants. This will reduce the post surgical failures and residual pain due to non-union fractured region.

Introduction

Motor vehicle accidents and falling from height are types of accidents which result in high-energy trauma in femur bone [1]. Most favored therapy of femur diaphyseal fracture (FDF) is intramedullary (IM) nailing [1–5]. Its success rate is high, but everything is not perfect. There are some possibilities of complications with this treatment, such as malunion, non-union, leg length discrepancy, infection, and other potential complications [6–12].

FDF non-union varies from 0.9 to 7.5% after most important nail therapy as research revealed [1, 13–16]. There are some common outcomes of non-union like diabetes, obesity, unstable fixation, and infection. In the present work, unstable fixation has been accomplished. For imparting steady fixation, anatomic discount should be achieved and fixation should be able to face up to the high shear forces throughout the fracture with motion, weight-bearing, and muscle tone. Higher shear forces develop stress shielding throughout the fixation screw and intake bone. It also increases the risk of unstable fixation due to loosening of the screws [17].

In the present study, authors have proposed a methodology consisting of a framework of plan and evaluation of a custom-made counter match implant for resisting shear force and stress shielding with a constant compression pressure over fractured region [18, 19]. Customized counter fit implant has been designed with the help of reverse engineering (RE) technique. RE is a method of creating a replica of a present object by way of extracting the dimensional elements of the authentic part [20]. There are a number of levels in RE such as: scanning of body dimensions of an object, processing of scanned data, utilizing these records to create a 3D mannequin of the object, and finally, this model is physically fabricated using additive manufacturing approach [21].

Researchers have developed several physical dimensional scanning strategies. These techniques are widely categorized as contact and non-contact type. In the field of clinical applications, typically non-contact kind of imaging strategies is employed such as: computed tomography (CT), magnetic resonance imaging (MRI), ultrasonography (USG), etc. [22, 23]. CT scan is generally performed for hard tissues like bone [24]. These CT scan data are available in DICOM file format. Many researchers have invoked RE approach for creating personalized counter in shape implants, prosthetics, and medical models for different uses [25–28].

Limitations of the usage of typical orthopedic implant is that it requires alterations (bending and shaping) for better counter fit on the fractured/damaged region of the bone relying on the anatomy of a precise affected person [28]. Aforementioned implant modifications can be completed only after incision and observing the fractured region, thereby increasing surgical procedure time as well as threat of contamination [29]. Moreover, tailoring and alteration in the structure of implants affect its mechanical strength and eventually resulting in unstable fixation [28, 30]. Due to this, non-union fracture occurs and thus leading to post-operative failures and residual pain borne by the patient [17]. These limitations have inspired the authors to design a patient-specific counter fit implants.

Proposed work is to simulate the conduct of bone and customized implant system considering physiological loading prerequisites. Moreover, it is a non-invasive method for analysis of stress distribution and deformation. Analysis has been performed for three different biocompatible materials.

Methodology

In the existing work, a fresh diaphyseal fractured femur CT scan data along medial facet have been collected. Moreover, counter fit personalized fixation plate has been designed and simulated under static physiological loading prerequisites for three exclusive biocompatible materials. The following steps had been performed:

Initially, a CT scan datum of fractured femur bone in DICOM file format is obtained. These data are invoked to create a 3D geometric model using RE approach in Materialise Mimics software, which is stored in the form of standard tessellation language (STL) format. Then, 3D CAD Model is prepared using the STL model. Moreover, to achieve stable fixation, location of the fixation plate has been decided on the basis of principles of biomechanics of fracture and consultations with the orthopedic surgeons. Thereafter, fracture is reduced digitally and a free form counter fit implant is designed pertaining to the morphology of the fractured region. Then, assembled 3D CAD model of the reduced fracture bone alongside with the implant and screws is organized which is mentioned here as clinical setup. Finally, biomechanical analysis has been performed on the clinical setup. Various stages involved in the existing research are summarized and presented in Fig. 1.

Fig. 1.

Methodology to prepare clinical setup and FEA model

Finite-Element Model Generation

Initially, data saved in DICOM file format comprise of a number of images taken with reference to three perpendicular planes viz. top view (axial), side view (sagittal), and front view (coronal).

The DICOM file of the femur bone has been imported into medical image processing software Materialise Mimics software for converting the stacks of 2D images into a 3D model. To develop 3D CAD model for analysis, the 3D model created saved in STL file format which is universally accepted and almost compatible file format for 3D printing. This STL file contains some noise and errors like inverted normal, gaps, etc., which needs to be identified and accordingly rectified before further proceedings. These recognized errors have been rectified and in addition re-meshing of STL model has been executed using MeshLab software. MeshLab is superior mesh processing software with features of automatic as well as manual filtering, cleaning, editing, rendering, and conversion of irregular meshed region. The STL file obtained from Materialise Mimics software has been loaded into MeshLab. Here, it has been checked for errors such as: duplicate facets, unreferenced vertices, null faces, small isolated pieces, and non-manifold faces. These errors have been removed and small holes had been filled up automatically.

After removal of errors in MeshLab software, STL model has been imported into SolidWorks 2016 software. In this software, 3D CAD model of the fractured femur bone has been created the use of the STL model.

This 3D CAD model has been studied and in addition in consultation with an orthopedic physician customized implant has been modeled hence in SolidWorks 2016 software. This implant has been designed keeping in idea that it will maintain the fractured bone intact and also provide steady support under static physiological loading conditions. Moreover, vicinity of drilled hole in the implant for tightening the screws plays very important role. The location of holes should be neither very near nor very far from the fracture line. However, traditional implants have pre countered holes and consequently positioning of implant along with the screws posed a variety of hindrances. Moreover, traditional implants are available in standard sizes only. Therefore, it has to be tailor-made before fixing it on the bone. These limitations can be overcome by means of designing a customized implant two.

Further, implant screws has been designed in the identical software on the groundwork of bone and implant dimensions. In the existing work, five implant screws have been designed, for suitable fixation of the implant on the fractured bone.

Finally, clinical setup has been prepared using assembling the fractured bone along with the customized implant and screws in a suited way.

Biomechanical Evaluation Using Finite-Element Analysis (FEA)

Clinical setup has been imported into ANSYS workbench 14 software for biomechanical evaluation. In this analysis, stress variation and deformation of the fractured bone have been done considering Von Mises and Rankine’s (maximum principal stress) stress criteria. Moreover, aforementioned analysis has been done for implants of three different biocompatible materials. The detailed analysis has been presented in the ensuing text.

Mesh Preparation

Meshing is an operation to divide the region of analysis into small size elements for evaluation. In the present work, mesh model for the fractured femur bone has been created using model wizard in ANSYS Workbench 14. Element considered for meshing purpose was tetrahedral. The number of tetrahedral elements and nodes used for the femur model was 19,496 and 33,732, respectively.

Boundary Conditions

Boundary conditions considered for finite-element analysis of clinical setup are as follows:

1. Distal end of the femur has been fixed considering the human bone to be inflexible.

2. A fixed loading condition has been applied on the proximal end of the femur.

Loading Conditions

In the present work, static load analysis has been done. It has been assumed that a person is standing straight and weight of the person is 1000 N. This load is transmitted equally the through pelvic bone at the head of femur. Thus, it has been assumed that 500 N load is acting vertically downward on each femur neck of the person. Biomechanical evaluation of clinical setup has been performed for three biocompatible materials, viz., titanium alloy (Ti6Al4V), stainless steel (SS-316L), and cobalt–chromium–molybdenum alloy (Co–Cr–Mo). Material properties of respective materials are mentioned in Table 1.

Table 1.

Implant material properties [9, 10]

Materials (alloy)	Young modulus (GPa)	Poisson’s ratio	Yield strength (MPa)	Ultimate strength (MPa)
Ti6Al4V	110	0.33	825	1080
SS-316L	200	0.3	290	580
Co–Cr–Mo	100	0.3	450	720

Ti6Al4V titanium alloy, SS-316L stainless steel, Co–Cr–Mo cobalt–chromium–molybdenum alloy

Results and Discussion

Stress and deformation analysis of clinical setup has been done under static physiological loading conditions. Results of stresses and deformation have been plotted for three different materials and discussed in detail as follows.

Stress Analysis

Stress analysis of three biocompatible materials has been done considering Von Mises and Rankine’s (Maximum Principal Stress) stress criteria. It has been observed that average Von mises stresses developed over the whole region of clinical setup for titanium alloy (Ti6Al4V), stainless steel (SS-316L), and cobalt–chromium–molybdenum alloy (Co–Cr–Mo) was approximately 4.56, 4.85, and 5.48 MPa, respectively, as shown in Fig. 2. However, on analyzing the stresses developed in the implant and screws, it has been noted that stress is raised up to 18.62, 19.40, and 27.40 MPa for titanium alloy, stainless steel, and cobalt–chromium–molybdenum alloy. This stress was maximum in the screw thread of the nearest screw from the fractured region towards the distal side, as shown in Fig. 2, respectively, for titanium alloy (Ti6Al4V), stainless steel (SS-316L), and cobalt–chromium–molybdenum alloy (Co–Cr–Mo). This stress is higher in this region because of stress concentration. Moreover, this stress is also developed because of contact pressure between the bone and the screw threads.

Fig. 2.

Von Mises stress distribution in clinical setup of a Ti6Al4V, b SS 316L, and c Co–Cr–Mo. Ti6Al4V titanium alloy, SS-316L stainless steel, Co–Cr–Mo cobalt–chromium–molybdenum alloy

Similarly, stress distribution has been analyzed considering maximum principal stress criterion for titanium alloy, stainless steel, and cobalt–chromium–molybdenum alloy, as shown in Fig. 3, respectively. Overall stress distribution in the clinical setup was uniform 6.35, 4.62, and 1.1 MPa for titanium alloy, stainless steel, and cobalt–chromium–molybdenum alloy, respectively. However, stresses developed in the screw near the vicinity of fractured region were higher and observed to be 17.31, 18.30, and 20.74 MPa for titanium alloy, stainless steel, and cobalt–chromium–molybdenum alloy, respectively.

Fig. 3.

Maximum principal stress in clinical setup of a Ti6Al4V, b SS 316L, c Co–Cr–Mo. Ti6Al4V titanium alloy, SS-316L stainless steel, Co–Cr–Mo cobalt–chromium–molybdenum alloy

Deformation Analysis

Total deformation has been analyzed for three biocompatible material implants, viz., titanium alloy, stainless steel, and cobalt–chromium–molybdenum alloy. The variations of deformation on three clinical setups are presented in Fig. 4. From these figures, it is evident that the deformation is maximum near the proximal end, where load is applied. Moreover, the proximal region is a spongy bone and so deformation is higher as compared to the rest portion which is comparatively more rigid. However, the maximum deformation observed in titanium alloy, stainless steel, and cobalt–chromium–molybdenum alloy implant were 0.39 mm, 0.29 mm, and 0.17 mm, respectively. This value is in the considerable range.

Fig. 4.

Total deformation variation in clinical setup of a Ti6Al4V, b SS 316L, c Co–Cr–Mo. Ti6Al4V titanium alloy, SS-316L stainless steel, Co–Cr–Mo cobalt–chromium–molybdenum alloy

On analyzing the stresses developed in all the three clinical setups, it is inferred that both Von Mises and Principal stress were maximum on the screws near the fractured region towards the distal end. However, for the three materials, these stresses developed are much below their ultimate and yield strengths as given in Table 1. A comparative stress plot has been drawn to compare the relative Von Mises and principal stresses developed in these three implant materials, as shown in Fig. 5a, b. Similarly, deformation developed in these materials has been analyzed and a graph is plotted to compare the deformation in three clinical setups, as shown in Fig. 5c. It is also found that the deformation in all these three materials was within the acceptable range. Therefore, the customized design of the implant for this type of fracture in femur bone is quite apt.

Fig. 5.

a Von Mises stresses, b maximum principal stresses, and c deformation developed in three clinical setups

Conclusion

To create and analyze a clinical setup of a fractured femur bone considering CT image data and RE technique has been presented in this work. Furthermore, to provide better fitting and overcome the stress shielding on fractured region, a customized counter fit fixation plate has been created for the aforementioned clinical setup. Moreover, three biocompatible materials named as titanium alloy (Ti6Al4V), stainless steel (SS-316L), and cobalt–chromium–molybdenum alloy (Co–Cr–Mo) have been considered for analyses. Furthermore, static stress distribution and deformation analysis of the clinical setup have been performed for mentioned materials. After analysis, the following conclusions have been drawn:

• Stresses and stress shielding developed are found to be approximately 1/10 of its ultimate strength and deformation, and are quite considerable in all the aforesaid materials. It shows that mechanical properties of implant are satisfactory for stable fixation.

• Created clinical setup is helpful for pre-operative and intra-operative planning which reduces the surgery time.

Proposed work will serve as a guideline for the medical practitioners to design and analyze suitable implants for the respective fractured bone. Moreover, it will enable the surgeons to select the most suitable material for these implants.

Compliance with Ethical Standards

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical standard statement

This article does not contain any studies with human or animal subjects performed by the any of the authors.

Informed consent

For this type of study informed consent is not required.