Comparative Study of Miniplate vs Reconstruction Plate in the Management of Bilateral Parasymphysis Mandible Fracture: FEM Analysis

Abstract

Aim and Objective

The present study compared the stability of fracture fragments in the management of bilateral parasymphysis mandible fracture with Miniplate fixation and Reconstruction plate fixation using finite element analysis.

Material and Method

3D FE Mandible model was created using CT scanner. Two bilateral parasymphysis mandible fracture models were created. Model 1 was fixed with Miniplates, and Model 2 was fixed with Reconstruction plate. Loading forces of 120 N at molar region and 62.5 N at incisor region were applied. These two models were imported to ANSY’S Workbench Software.

Result

Miniplate fixation model showed comparatively reduced gap between fragments than Reconstruction plate. But the gap values of both the models were within the physiologic limit of healing under this specific loading. Analytically Miniplates were superior to Reconstruction plate in the management of bilateral parasymphysis fracture.

Conclusion

Analytically Miniplates are superior to Reconstruction plate in the management of bilateral parasymphysis fracture. As the masticatory forces were reduced during fracture healing period, both fixations provide satisfactory healing. So both Miniplate and Reconstruction plate can be considered as fixation method for bilateral parasymphysis mandible fracture.

Introduction

Mandible fractures are the second most common fractures in Maxillofacial region. Treatment of mandibular fractures is advanced over the years due to improved understanding of biomechanical principles, advances in biomaterials, instrumentation and scientifically based research of treatment outcomes [1, 2]. The therapeutic goals in mandibular fracture management are to re-establish premorbid anatomy, to provide fracture stabilization and to restore the functionality with least morbidity. Mandibular parasymphysis is highly dynamic area, constantly subjected to occlusal force, fracture in this zone creates dilemma for the surgeon regarding management. The biomechanics of bilateral mandible fracture is more complex than unilateral fracture [3]. Special attention should be given to bilateral fracture. The intermediate fragment present between the fracture lines is susceptible to movement during mastication.

Fixation with plate and screws is now a standard treatment modality for fracture. Various types of plating system have been developed from time to time for fixing fracture. A simple osteosynthesis technique which would guarantee fracture healing without intermaxillary compression was the monocortical plate osteosynthesis of Michelet et al.[4]. It was modified and developed into a practicable clinical method of load sharing osteosynthesis by Champy et al.[5]. Their biomechanical studies resulted in the concept of an ideal line of osteosynthesis and led to consistent success in mandible fracture treatment. The semi-rigid system with monocortical plates and screws is currently used universally for fixation of fractures in Maxillofacial region. The miniplates system gives sufficient support and stability to bone fragments and allows precise anatomic reduction and easy to use. It prevents cortical necrosis which is sometimes seen under plate which is if compressed against bone. A longstanding problem in miniplates osteosynthesis is loosening of screws and difficulty in meticulous adaptation of plate to the contour of bone, errors in fixation leads to permanent malocclusion.

According to Lavertes and Luhr, fracture fixation using reconstruction plates provides mechanical stabilization, functional and morphological rehabilitation by load bearing osteosynthesis [6, 7]. Reconstruction plate avoids dislocation of mandibular fragments which would be a major obstacle to later restoration of mastication. Incidence of 2.8–9.8% fracture cases of reconstruction plates reported that mechanical weakness may be present in reconstruction system itself. Clinical experience and prevalent literature have shown that the commercially available titanium reconstruction plates currently used for mandibular defects are often subject to excessive stress that may lead to fatigue fractures. Also, these plates are straight or slightly contoured metal plates with preformed retention screw holes in only generic shapes and sizes. Conventional reconstruction plate requires precise adaptation of plate with underlying bone. During surgery, the surgeon may have to spend a considerable amount of time bending and shaping the plate to fit the contour of the patient’s bone. It may disrupt the cortical bone perfusion and can lead to screw loosening also.

The aim of the present study was to compare stability of fractured fragments in bilateral mandibular parasymphysis fracture under functional load when fixed with conventional miniplates and Reconstruction Plates using finite element analysis. The finite element model analysis is an accepted theoretical technique used in the solution of Engineering Problems. Several studies have demonstrated the accuracy of FEM analysis in describing biomechanical behavior of bone as well as evaluation of different fixation techniques in the management of facial fracture [8–10].

Material and Methods

The finite element method is an accepted theoretical technique used in the solution of engineering problems. In FEM analysis, a structure is modeled with discrete element mathematical representation by subdividing it into simple geometric shapes or elements whose apices meet to form nodes.

3D Finite Element Mandible Model

Multi-slice Spiral CT scanner which is made by Philips in Netherlands was used to scan the patient. 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.

2D pictures were procured and put out with Dicom files. These Dicom files were delivered to the Slicer Version4 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 valve 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 STL (Standard Tessellation Language) format [8, 11, 12]. 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 and the X direction is Medio lateral, the Y direction is superioinferior, and the Z direction is anteroposterior. Teeth do not add to the mechanical strength/stiffness of the mandible. In present finite element modeling, the limited role of the teeth in mechanical response of the mandible is ignored and removed to simplify the modeling [13]. Mandible has a linear elastic behavior, and its mechanical properties are described in terms of Young’s Modulus, Poisson ratio, Material density for both Cortical and Trabecular bone. Cortical bone was modeled as orthotropic, and cancellous bone was modeled as isotropic [14–16].

Simulation of Fracture Sites in Finite Element Mandible Model

Complete fracture on both side parasymphysis of mandible region was created for this investigation. Interfragmentary bone contact between fractured bone segments was simulated. The fracture line was rough without deep serrations. The properties of bone were assumed similar in every plane and do not exhibit any nonlinear stress strain characteristics or plasticity. The behavior is characterized by the 2 material constants (Young modulus and Poisson ratio). The average values in the literature were attained [14, 15].

Plate Configuration

Two bilaterally fractured mandible models at parasymphysis region were selected for this study.

Model 1

FEM mandible model with bilateral parasymphysis fracture is bridged with 2 mm thickness titanium miniplates. One 2-holed titanium miniplate is bridged on the superior side of fracture, 4mm from alveolar crest using 8 mm screws. Second 4-holed titanium miniplates are fixed at lower border of mandible using 10 mm screws. Miniplates are perfectly adapted to the underlying bone to prevent alterations in the alignment of the segments and changes in the occlusal relationship. Perfect adaptation between miniplates/screws and the bone, with no slippage at their interfaces, was considered for load sharing osteosynthesis, along with the fixed contact of the miniplate with the bone, to evaluate the stresses transferred to the miniplate along the bone (Fig 1).

Fig. 1.

Model 1 Miniplate fixation at bilateral parasymphysis fracture

Model 2

FEM mandible model with bilateral parasymphysis fracture is bridged single reconstruction plate of 2.5mm thickness made of titanium using bicortical screws. The volume elements of ANSYS were meshed and elements obtained. The solid element type 10-node tetrahedron and 20-node hexahedron was chosen to model the bone segments, with three translations in the nodal x, y and z directions per node (Fig 2).

Fig. 2.

Model 2 Reconstruction plate fixation at bilateral parasymphysis fracture

Boundary Conditions

FEM is a numerical method allowing modeling of structures that approximates reality. The spatial geometry model is broken down into a large number of finite elements which are interconnected by nodes. This technique is termed discretization. When factors such as clamping conditions and loading stress are known, the deformations and tensions of these simple elements can be calculated at each node [17]. On the basis of the fact that muscles can only transmit traction, and joints can only transmit pressure, the resultants of the vertical components of the muscles responsible for closing the jaw were assumed as loading tensions.

The horizontal muscle forces acting at the joint and thus counterbalancing each other can be neglected in the further analysis. In idealized terms, the behavior of the materials involved was characterized as isotropic, homogeneous and linearly elastic. Titanium was taken as material for the reconstruction plate, miniplate and the osteosynthesis screws. The parameters of the material used are shown in Table 1. The muscle and chewing forces were distributed to the nodes perpendicularly to their main action interfaces.

Table 1.

Mechanical properties of the materials

	Young’s modulus (N/mm2)	Poisson’s ration	Tensile strength (N/mm2)
Titanium plate	105,000	0.3	290–740
Titanium screw	105,000	0.3	290–740
Cortical bone	8700	0.3	85
Cancellous bone	100	0.3	1–13

Validity of analysis result depends on mimicry of model, boundary conditions, loading condition, and material modeling in accordance with the physical reality [18]. During the application of bite forces or loading condition, to prevent rigid body rotation and translation, a substantial simplification of the boundary conditions is assumed and the transitional degree of freedom of the condyles is set to zero [19]. Closing muscle force vectors (path of origin to insertion) were assigned based on published work by Van Eijden et al. [20]. The magnitude of the force in each muscle was assumed to be directly proportional to the muscle cross section as reported in the same study. The sum of all of the muscle forces was calculated to create a moment sufficient to balance the prescribed bite force about a pivot point at the condyle. The condyle was fixed in all three spatial directions to represent the reaction force at the temporomandibular joint [21].

Simulation of Loading Condition

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 [22]. In the present study, load is applied on three configurations, right molar region, left molar region and incisor aspect. A load of 120N is applied on molar region, and 62.5N is loaded on incisor region.

Contact Analysis

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. Two models were imported into ANSYS Workbench FEM software. ANSYS workbench has interactive 3D FEM generation for modeling a multiconnected mandible structure [23]. 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. The contact decision is made when the contact and target surfaces touch together. Contact can occur only when the outward normal direction of the target surface points to contact surface. ANSYS surface-to-surface contact elements use integration points as contact detection points. Contact elements were constrained against penetration into the target surface at their integration points.

Results

Performance of finite element analysis in terms of fracture segment stability on loading condition is defined in three terms: total deformation, gap between fracture segment and Sliding distance between fragments and results are shown in Table 2. When bilateral parasymphysis fracture is fixed with single reconstruction plate and on loading 120N bite force on right molar region, fracture fragments exhibit a gap of 23 µm especially on left side parasymphysis on upper side of fracture fragments (Fig 3). When bilateral parasymphysis fracture is fixed with conventional miniplates on both sides, under loading of 120N on right molar region the gap between fracture fragments was only 9.9µm. On loading condition of 120N on molar region, sliding distance between fracture segments is observed 25 µm and 17 µm in reconstruction plate and miniplate, respectively (Fig 4). When load was on symphysis region, there was no appreciable change in gap between fracture segments observed between Model 1 and Model 2. When loading is applied on symphysis, comparatively more sliding distance is noted on model 2, bridged with reconstruction plate.

Table 2.

Results in terms of Total deformation (TD), Sliding distance (SD), Gap between fragments (GAP) in Model 1 and Model 2 under loading condition

	RH-TD (µm)	RH-GAP (µm)	RH-SD (µm)	LH-TD (µm)	LH-GAP (µm)	LH-SD (µm)	SYM-TD (µm)	SYM-GAP (µm)	SYM-SD (µm)
Model 1	47	9.9	17	46	9	16	89	15	2.8
		rt(up)	rt(up)		rt(up)	rt(lw)		rt(lw)
		9.5			10	7		13
		lt(up)			lt(lw)	lt(up)		lt(lw)
Model 2	47	23	25	41	16	30	64	15	4
		lt(up)	rt(up)		rt(up)	lt(up)		rt(up)	rt(lw)
								15	4
								lt(up)	lt(lw)

rt:Right

lt:Left

up:Upper

lw:Lower

Model 1: Bilateral parasymphysis fractured FEM mandible model bridged with miniplates

Model 2: Bilateral parasymphysis fractured FEM mandible model bridged with Reconstruction plate

RH-TD: Total deformation when loading on right molar side. RH-GAP: Gap between fracture fragments when loading on right molar side. RH-SD: Sliding distance when loading on right molar side. LH-TD: Total deformation when loading on left molar side. LH-GAP: Gap between fracture fragments when loading on left molar side. LH-SD: Sliding distance when loading on left molar side. SYM-TD: Total deformation when loading on symphysis region. SYM-GAP: Gap between fracture fragments when loading symphysis side. SYM-SD:Sliding distance when loading on symphysis region

Fig. 3.

Model 2 showed Total deformation on right side molar loading

Fig. 4.

Model 1 showed gap between fracture fragments on Right side molar loading

The gap distance or separation of the fracture section indicates stability or rigidity of the fixed bone fragments of the mandible. But theoretically on the basis of fracture fragments displacement, conventional miniplates are superior to reconstruction plate in bilateral parasymphysis fracture.

These two finite element model observations showed that upper limit of relative gap distance between mandible fracture fragments with bite force, 120 N on right are not exceeded the physiologic limit of 150 µm (0.15 mm) in accordance with findings of Perren. [24]. 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.

Discussion

Bilateral mandible fracture is prevalent in 10.2–50% of all fractures involving tooth bearing and non-tooth bearing segments of the mandible. Special attention should be given to bilateral fracture because it changes the complexity of mandible biomechanics. The intermediate fragment present between fracture lines is susceptible to movement during mastication. As length of intermediate fragment increases torque-moment arm and results in twisting of segment. This behavior emphasizes the importance of rigid fixation at least on one side [1, 25].

Parasymphysis of mandible is highly dynamic area constantly subjected to occlusal force. Parasymphysis region locates in transition zone. Fracture in this zone creates dilemma for the surgeon regarding management due to its complex biomechanics and the presence of mental nerve. Fracture in parasymphysis extends obliquely and transverse through transitional zone to body region, is relatively common and accounts for approximately 20% of mandible fracture. The presence of deeper roots, neurovascular bundle and change in density and orientation of trabeculae made fixation of reconstruction plate challenging in parasymphysis region [26].

There are four main goals for management of mandibular fractures: Anatomical restitution, immobilization, prevention of infection and rehabilitation of function. Achieving these goals is essential for successful bone healing and correct postoperative function of the stomatognathic system. Different techniques for the treatment of mandibular fractures have evolved in the past decade. These techniques have ranged from closed reduction with maxillomandibular fixation (MMF) to open reduction with wire osteosynthesis, to open reduction with either rigid internal fixation or adaptive miniplate fixation. In 1976, Spiessal and colleagues established open reduction and internal fixation with principles of AO/ASIF [27]. Primary bone healing occurs when there is minimal strain and good anatomic reduction such as when rigid fixation is performed. Primary bone healing is predicated on the direct restoration of lamellar bone. The callus formed when good anatomic reduction is achieved is minimal compared with secondary bone healing [28, 29]. Luhr introduced the concept of compression plating, the principle of axial compression for better adaptation of the fracture ends combined with distinct advantage of increased stability and early function.[7] According to Arbeit gemeinschaft fu¨r Osteosynthesefragen (AO)/Association for the Study of Internal Fixation (ASIF) principles, the main aim of open reduction and rigid internal fixation in the management of mandibular fractures is to achieve undisturbed healing and immediate restoration of form and function without the adjunctive use of MMF [30].

According to Laverter and Luhr, besides mechanical stabilization, reconstruction plates helps to prevent fragment dislocation during mastication later due to scar. Reconstruction plate system in bilateral fracture established functional, mechanical and morphological rehabilitation. But conventional reconstruction plates can cause cortical bone necrosis under the plate which if compressed against the bone [6, 7]. Conventional plate system should be perfectly adapted to the underlying bone to avoid gaping of the fracture and associated instability. The locking plating system has been developed and popularized by AO/ASIF to obviate the main disadvantage of conventional plate system. This bone-plate system acts as an internal–external fixator, which results in better distribution of the load and prevents load concentration on a single screw, thus decreasing the risk of a screw’s loosening and stripping. AO 2.4 mm locking reconstruction plates offer the advantages resulting from buttress plates, which can support a full functional load by acting as load bearing devices and can counter and convert shear forces to compressive axial forces at the fracture site. Locking reconstruction plate allows more rapid and undisturbed bone healing and decreased risk of delayed union, nonunion, or infection. First, the absence of pressure under the plate prevents the cortical blood supply from being disrupted and allows periosteum growth under the plate. Second, stress shielding below the plate is eliminated, which prevents chronic inflammation and subsequent bone necrosis [31].

According to champy’s line of osteosynthesis, mandible act as a class lll lever. Fulcrum is at summit of condyle. When the patient bites, the elevator muscle force is applied distal to fulcrum and occlusion load is distal to muscle force. In the presence of parasymphysis-body fracture, masticatory forces create tension above mandible canal pulling the bony fragment apart and compression along the inferior border promoting bony contact. The most common cause for postoperative failure in mandibular surgery is either plate failure (fracture due to loads) or the loosening of affixing screws. The treatment objective of achieving unimpeded healing and immediate restoration of form and function without MMF in the management of mandibular fractures can only be achieved by the reestablishment of previous normal functional tension (alveolar border) and compression (basal portion) bone trajectories. This is the prime requirement for complete recovery from injury and assured return to the preinjury state of function and aesthetics. The Michelet–Champy technique utilizing conventional miniplates has been used extensively during the last many decades to treat mandibular fractures. The miniplates offer multiple advantages of ease of handling, possibility of intraoral application, decreased chances of potential nerve damage and eliminating the need for MMF. The disadvantages of the conventional miniplate system include the precise adaptation required between the plate/bone interface, the requirement of an additional second plate at the superior border to counteract the tension forces and the theoretical increased interferences in cortical blood supply [4, 5].

Stress and strain in a structure can be solved exactly with analytical means for simple geometric shapes with homogeneous material properties. More complex shapes become difficult to solve, and the finite element method provides an approximate solution to such problems by subdividing complex geometries into finite but high number of smaller simpler elements of geometry. Computational models can be categorized as static, dynamic and kinematic model. Static models investigate the deformation of mandible under loads and muscle forces. The material properties of elements must be defined like elastic modulus, poison ratio. Boundary conditions are important to prevent movement of the model when loaded so that it can be displaced as rigid body thus allowing finite element computations. The completed model is solved to obtain the nodal displacements and results in stress and strain. The external force and mechanical properties are used to calculate the nodal displacements. Once nodal displacements are known, the displacement field is interpolated from nodal values using standard interpolation polynomial function [11, 13].

Numerical analysis suggests that maximum stress occurs in cortical bone where stiffness is higher and fracture is generated. While bending or cutting osteofixation plates, their surface may be damaged, increasing the risk of corrosion, which happens in over 35% cases. According to Jasmine et al, during the plating of bone fractures, osteopenia can occur beneath the plate because of the stress-shielding effect, which happens as a result of the rigidity of the reconstruction plate. In other words, if after the bone fracture treatment is finished, the osteofixation plate is left in the patient’s body, it may partially take over the load causing a decrease in mineralization and weakening of the bone leading to subsequent fractures [32, 33]. 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 [22]

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. 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 moment 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.

It was observed in our study that, when a loading force of 120N is applied to one side molar region, fracture fragments exhibited 9.9 µm gap between fragments and 17 µm sliding distance in bilateral parasymphysis fracture fixed with conventional miniplates. When the bilateral parasymphysis fracture fixed with single reconstruction plate and 120 N is applied on one side molar region, it showed 23 µm gap between fragments and 25 µm sliding distance between fragments. On the basis of these findings, it is exhibited that 2 miniplate fixation is better than single reconstruction plate. But according to Perren, these two values are with in physiological limit of healing [24]. Both the models can heal satisfactory with the selected loading condition.

Conclusion

As parasymphysis of mandible locates in a transition zone and owns deeper roots, neurovascular bundle, mental foramen, change in density and orientation of bony trabeculae, gradual change in the arch form of mandible, anticipated blood supply, extend of incision required for the fixation should be taken into consideration during surgical planning phase of bilateral parasymphysis fracture management. In our study, bilateral fixation of parasymphysis fracture with two miniplates showed reduced gap between fragments and better stability than single reconstruction plate fixation. However, these two model values came under physiologic limit of healing. As masticatory forces were reduced during fracture healing period, both reconstruction plate and miniplates can provide sufficient stability.

Author Contributions

Dr. Manju Kurakar made substantial contributions to the conception or design of the work or the acquisition, analysis and interpretation of data. Dr. Udupikrishna Joshi revised it critically for important intellectual content and approved the version to be published.

Declarations

Conflict of interest

Both the authors did not receive any funding from any organization for the submitted work and have no financial interest. The authors declare that there is no conflict of interest.