Assessment of Lingual Stability in Mandible Fracture: Monocortical Versus Bicortical Fixation Using FEM Analysis

Abstract

Aim of the study

The present study compares lingual stability of monocortical miniplate fixation with that of bicortical miniplate fixation in parasymphysis fracture using FEM analysis.

Materials and methods

Using multislice CT scanner, 3D FEM of patient’s mandible was created. Fracture was simulated at parasymphysis region and fixed with 2-mm titanium miniplates and screws of length 8, 10, 12 mm, respectively. Loading force of 120 N applied at molar region and 62.8 N at incisor region. These three models were imported into ANSYS Workbench FEM software.

Result

There is no significant difference in results between bicortical fixation and monocortical fixation.

Conclusion

It was concluded that use of monocortical fixation provides sufficient lingual stability. This suggests monocortical fixation system is as reliable as bicortical fixation.

Introduction

An important aspect of oral and maxillofacial surgery practice is the manipulation of fractured facial skeleton; the treatment of such fractures has evolved essentially from minimal intervention with closed reduction to invasive surgical intervention in the form of open reduction and internal fixation (ORIF).

Currently, several methods are considered acceptable in treatment of fracture of mandible. These methods include closed reduction, open reduction and fixation using transosseous wires and miniplates. Plating system includes monocortical and bicortical system. Osteosynthesis with miniplates in maxillofacial region was first advocated by Michelet. Thereafter, Champy et al. demonstrated monocortical, juxta alveolar, subapical osteosynthesis without compression and intermaxillary fixation.

The present study compares the lingual stability of monocortical miniplate fixation with that of bicortical miniplate fixation in parasymphysis fracture using finite element model (FEM) analysis. FEM analysis is a computational technique originally developed by engineers to model the mechanical behavior of structures such as buildings, aircraft and engine parts. Several studies have demonstrated the accuracy of FEM analysis in describing biomechanical behavior of bone as well as evaluation of different plating techniques in the management of facial fracture [1–7].

Materials and Methods

The present study is performed to measure and compare the gap between the fracture segments on lingual side, which is reduced and fixed with monocortical miniplate and other with almost bicortical miniplates in parasymphysis region. For this purpose, a three-dimensional (3D) FEM of the mandible was developed.

3D Mandible FEM Generation

Data collected using multislice spiral CT scanner which is made by Philips were converted to the DICOM specification file. The procedures for image processing and preparation of the analytical model are shown in Fig. 1. The scanning condition is 120 kV, 400 mA, 1500 mS, and the horizontal distinguish is 1024 × 1024. The scan planes are parallel lines of Frankfort Horizontal Plane. The scan thickness is 1.5 mm [8, 9]. These DICOM files were delivered to the Slicer version 4 software. Then threshold 405 of CT gray value was selected to reverse the model of mandible. The gray scale above 1676 was extracted using the gray value scale selection. The additional noise was eliminated with the method that is the same to extract the mandible. The high-quality 3D model of the mandible and lower denture was obtained. The integrity of the model was verified and exported in Standard Tessellation Language (STL) format. The STL data model was optimized in Geomagic to get a close match with the mandible model to be analyzed. XYZ coordinate system was assigned to the model such that the origin is located on the X–Y plane at a point midway between the left and right condylar processes; the X direction is mediolateral, the Y direction is superoinferior, and the Z direction is anteroposterior. In present FE modeling, the limited role of the teeth in mechanical response of the mandible is ignored and removed to simplify the modeling [1, 10].

Fig. 1.

FEM of mandible

Plate and Screw Model

Two-millimeter-thick titanium straight miniplates with four holes were scanned, and the image files were imported into AutoCAD and were used to sketch miniplates in two dimensions using a combination of lines and curves. These 2D geometrical CAD data were imported into Unigraphics CAD (computer-aided design) software package where it was converted to 3D data. Also adaptation between miniplate and screws was done using Unigraphics CAD software. Material properties were assigned for titanium as 1.1 E5 MPa for Young modulus and 0.34 for Poisson ratio (strain limit − 0.2%). The titanium screws of 8, 10 and 12 mm length were also modeled, and the same material properties of titanium were used [7].

Fixation of Fracture Site with Miniplates

Complete fracture on right side of parasymphysis of mandible was selected for this investigation. Interfragmentary bone contact between fractured bone segments was simulated. The fracture line was rough without deep serrations. Titanium miniplates of 2 mm thickness are adapted to the underlying fractured bone to prevent alterations in the alignment of segments and changes in the occlusal relationship using Unigraphics CAD software (Fig. 2). Three separate models were generated, and fracture line is fixed with screws of 8, 10 and 12 mm, respectively. Model A is fixed with two titanium miniplates of 2 mm thickness of four holes using 8-mm screws. One plate is fixed at superior border, 4 mm from alveolar crest, and another plate is fixed at lower border of mandible. Model B and Model C are fixed with same two titanium miniplates of 2 mm thickness of four holes using 10-mm screws and 12-mm screws, respectively. It was found that 12-mm screw pierced both the buccal and lingual cortexes (Fig. 3). So Model C is bicortical fixation.

Fig. 2.

FEM of mandible with plate and screws

Fig. 3.

Model C with bicortical fixation using 12-mm screws

Muscle force vectors were assigned based on publishing work by Van Eijden et al. [11]. A substantial simplification of boundary conditions is assumed, and the transitional degree of freedom of condyle is set to zero [10, 12]. Maximum bite force has been demonstrated to be between 300 and 400 N for the average non-injured man. Bite forces are reduced during fracture healing period. Maximum bite force values are different in the molar, premolar, canine and incisor regions, with highest bite forces in the molar region and lowest forces in the incisor region. After 6 weeks of mandible fracture repair, bite force would be 57 N at incisor and 119 N at molar region [13]. In the present study, bite force applied at three different locations: ipsilateral fracture site, contralateral fracture site, incisor region. 120 N applied at molar region and 62.8 N applied at incisor region (Fig. 4).

Fig. 4.

Load application

Finite Element Analysis of the Model

These three models were imported into ANSYS Workbench FEM software [7, 14]. ANSYS workbench has interactive 3D FEM generation for modeling a multiconnected mandible structure. FEM is broken down into approximately one lakh finite elements which are connected by nodes. The ten-node tetrahedron is chosen as the basic element type due to its rigorous adaptability to structures with geometric complexities. The material properties were assigned to the FEM. Interaction of the pieces is considered by an FEM analysis using ANSYS software. The contact problem is of a nonlinear type, in which the system stiffness matrix is modified so that the contribution of the separate pieces is taken into account according to the state of contact. ANSYS has three different types of contact analyses: node-to-node contact, node-to-surface contact and surface-to-surface contact [10]. In a contact analysis, the regions of possible contact during the deformation of the model need to be identified. Later, contact regions are defined via target and contact elements, which will then track the kinematics of the deformation process. Contact elements were constrained against penetration into the target surface at their integration points [14]. When factors such as boundary conditions and loading stress are applied to the model, the deformations and stresses of these simple elements can be calculated at each node. Due to their mutual interlinking, displacement and deformation of overall structure can be calculated using FEM software. Thus, FEM software calculates the stress across the miniplate, bending and torsion moments in fracture site resulting from the bite force and the muscle and joint reaction forces that act on the proximal and distal fragments. Movement of the proximal segment with respect to distal one in the lingual aspect and the displacement gap between fracture segments while loading at bite points are calculated across X (mediolateral), Y (superoinferior) and Z (anteroposterior) directions [7]. Thus, this 3D model was successfully used to evaluate the stability between mandible fragments treated with different designs of miniplates under various loading conditions, which proved the viability and effectiveness of the proposed method (Fig. 5).

Fig. 5.

FEM analyses

Results

The vectorial summation of 3D translational displacements of the nodes on the fractured section gives the separation or gap distance, i.e., movement in X, Y, Z directions. Stability is required for appropriate osteogenesis and bone healing. These three finite element model observations showed that upper limit of relative gap distance on lingual side of mandible fracture site under bite force, 120 N on right and left molar region and 62.8 N on incisor region did not exceed the physiologic limit of 150 µm (0.15 mm) in accordance with findings of Perren [15]. It was found that all bite points on the fractured side resulted in negative bending moments. Here, zone of tension appeared on lower border. The bite points on the non-fractured side resulted in positive bending moments, gap appeared on upper border, and zone of compression appeared on lower border. And there is no significant difference in results between bicortical fixation and monocortical fixation. This suggests monocortical fixation system is as reliable as bicortical fixation. The lingual gap between fracture segments is measured and charted as shown in Table 1 (Fig. 6).

Table 1.

Lingual gap between fracture segments fixed with 12-, 10-, 8-mm screws

Load application	12 mm	10 mm	8 mm
Incisor (62.8 N)	21.66 µm	21.67 µm	21.72 µm
Lt. molar 120 N	25.955 µm	25.946 µm	25.938 µm
Rt. molar 120 N	12.891 µm	12.871 µm	12.852 µm

Fig. 6.

Representation of lingual gap between fracture segments fixed with 12 mm, 10 mm, 8 mm screws

Discussion

Although the use of compression plating for the treatment of mandible fracture was initiated by Luhr, popularized by Spiessl [16], rigid internal fixation is not necessary for primary bone healing which led Michelet et al. [17] to develop a system of craniofacial fixation using small functionally adaptable plates, miniplates. Champy et al. [18] later refined the plates and positioning of fixation. Miniplate osteosynthesis provides stable fixation without need of interfragmentary compression or MMF. The miniplates achieve this goal by neutralizing undesirable tensile forces while retaining favorable compression forces during function. The smaller size of miniplates offers several advantages; not only they are less palpable than compression plates, but they also entail small incisions and less soft tissue dissection. It could be placed near to tooth roots also. When the decision is made to treat mandible fracture with ORIF, discussion may arise whether to use monocortical fixation or bicortical fixation. Bicortical fixation holds both cortexes in direct contact and fixed with absolute stability, so that it will heal without formation of external callus. However, clinically may damage inferior alveolar nerve by hitting directly or squeezing the nerve due to compression of segments. Excessive stress around fixative appliances can cause gradual resorption of the surrounding bone and loosening of screws [19].

Monocortical fixation can provide sufficient resistance to displacing forces that encourage micro-movements across the fracture site. At the same time appropriate immobilization of the fracture site ensures an uncomplicated healing process, which guaranteed favorable long-term results. The use of bone plates and monocortical screw system permits a stable semirigid fixation that may eliminate the necessity for maxillomandibular immobilization. Monocortical screws and plate are much simpler and highly functional in use. Another main advantage of semirigid fixation is that the plates and screws do not disturb the underlying cortical bone perfusion. Biomechanical functions of monocortical fixation system clinically depend on interaction between three components, plate–screw–bone. A bone–plate–screw system requires precise adaptation of plate to the underlying bone. Without such intimate contact, the tightening of the screw draws the bone segment toward plate and results in alteration in the position of segments and the occlusal relationship. The smaller size of miniplates reduces the degree of stress shielding seen after rigid fixation [20]. However, miniplates also have limitations and potential complications. Their small size reduces rigidity and may lead to torsion or movement of fracture segments under loading [21]. Because of potential reduction in stability after miniplates fixation, a reduction in function is sometimes recommended for a time. This controversy encourages us to do this investigation. In the planning phase of fracture management, the determination of the best position, orientation, selection of plate and material type are important.

According to Vedrana Braut, the width of mandible at parasymphysis region, at 4 mm below alveolar crest, is 5–8 mm and near inferior border is 9–13 mm [22] as shown in Table 2.

Table 2.

Width of mandible at premolar and molar region

	Premolar region	Molar region
BW2	5.7 mm	8.6 mm
	(5–8 mm)	(8–13 mm)
BW3	9 mm	10 mm
	(9–13 mm)	(10–15 mm)

BW2: 4 mm below alveolar crest, BW3: at superior border of inferior alveolar canal

Finite element analysis is a numerical approach that addresses the complexity of the modeling by deriving an approximation to the solution. This is achieved through simplifying a complex shape which has infinite degree of freedom into a number of simpler interconnected shapes/elements in which the displacement and stress field in the elements are approximated by simpler functions. Nowadays, CBCT technology has evolved that allows 3D visualization of the oral and maxillofacial complex from any plane. This digital imaging system is capable of producing 2D and 3D images, as well as ortho-radial views of the jaws, to permit more accurate assessment of bone. Such cone beam system allows the physician to acquire 3D volume data in one rotation at reasonably low levels of radiation dosage. Medical CT differs from CBCT as it uses fan shaped beam and captures portions or slices of anatomy as the source and receptor move along the long axis of the section of anatomy being examined. The effective dose equivalent measured using CBCT is between 30 and 400 μSv, as compared to 2100 μSv from a conventional medical CT scan of the maxilla and mandible. The accuracy of a solution is the function of many variables including the accuracy of geometric replication, the number and complexity of the elements used in the model, material properties and the simulated loads [23]. In relation to material properties, orthotropic properties were chosen for cortical bone to enable the creation of a model that represented reality as closely possible [24]. However, most studies used isotropic properties with different values for cortical and medullary bone [25]. According to Andersen et al. [26], results are affected to a minimal degree with a distinction maintained between cortical and medullary bone allowing the simplification of the model to a single type of alveolar bone. The model used in our study closely simulates the clinical picture because the vertical plates are bent to fit the bony surfaces and each individual model screw attaches to the bony surfaces in a manner designed to model the mechanical connection of screws to bone. This 3D finite element model evaluated the complex stress field under posterior and anterior occlusal load condition on same side and opposite side of fixation screws and plates. This analysis allows for a more realistic representation of the stress distribution in the fixative material and the adjacent bone tissue.

The bite forces are the loading forces on the mandible during daily function. Maximum bite force has been demonstrated to be between 300 and 400 N for average non-injured person. Bite forces are reduced during healing period [27]. Maximum bite force values are different in molar, premolar, canine and incisor region with highest bite force in molar region and lowest forces in the incisor region [28]. Up to 4 weeks after mandibular fracture repair, masticatory force ranges from 57 N in the incisor region to 119 N in the molar region, with a mean of 100 N [13]. These forces may vary depending on the patient and the composition of the opposing dental arch (natural teeth/prosthesis). Bite force applied close to fracture site results in negative bending moments which give a zone of compression in alveolar region and zone of tension in lower border. Bite forces applied on opposite side result in positive bending moments with a compression zone at lower border and tension zone in alveolar region [29].

The highest recorded von Mises stress values were 274 MPa in the molar region and 163 MPa in the incisor region for forces of 62.8 N in the incisor region and 120 N in the molar region. So here VMS values were less than 600 MPa, i.e., plate rupture limit and yield strength limit (yield strength limit = 450 MPa) [30]. The loads transmitted through the plates should not exceed the limit of strength of material. Ideally plate fixation should result in undisturbed primary fracture healing. Such healing is only possible if displacement between fracture surfaces is within the range of 100–150 µm [15, 26].

Conclusion

Therefore, accurate placement of screws should be the goal of treatment in maxillofacial fracture management. Furthermore, an ideal internal fixation method should obtain a maximum rigidity in between segments, while it constitutes minimum stress on surrounding tissue for healing property. It was concluded that use of monocortical fixation provides sufficient lingual stability. FEM proved to be a suitable investigation for simulating complex mechanical stress situations in the maxillofacial region.

Conflict of interest

Dr. Udupikrishna Joshi and Dr. Manju Kurakar declare that we have no conflict of interest in this study.

Informed Consent

This article does not contain any studies with human participants or animals performed by any of the authors. For this type of study, formal consent is not required.