Evaluation of two stable internal fixation techniques in mandibular ramus sagittal osteotomy: a three-dimensional finite element study

doi:10.21203/rs.3.rs-3871440/v1

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Evaluation of two stable internal fixation techniques in mandibular ramus sagittal osteotomy: a three-dimensional finite element study

https://doi.org/10.21203/rs.3.rs-3871440/v1

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Purpose

To evaluate and compare, using finite elements three-dimensional (3D) analysis, two techniques of stable internal fixation in 2 patterns of sagittal split ramus osteotomy (SSRO) and the repercussions of occlusal load distribution in the postoperative period.

Methods

A 3D finite element model of a hemimandible was created. The models were divided into 2 split patterns of SSRO advanced 5 mm forward, a split pattern (1) following the Epker osteotomy design and another split pattern (2) following Posnick osteotomy design, simulated by CAD (Computer Aided Engineering) Rhinoceros 3D®. Two internal fixation techniques were simulated by HyperMesh: A – one titanium miniplate of the Arnett system (1mm) for orthognathic surgery fixed with four monocortical screws; B – one titanium plate fixed with four monocortical screws and one bicortical screw. Resistance, stress and displacement were analyzed applying a 200N vertical load to the occlusal surface region of the mandibular first molar and a 100N to the incisal surface of the mandibular central incisor.

Results

The results were presented through the von Mises stress analysis. A difference in the displacement of the models fixed with only one miniplate and four monocortical screws was observed. The models with split pattern (2) presented smaller displacements when compared to the split pattern (1). For metallic components, the split pattern (2) exhibits lower strain values, although it has no significant difference.

Conclusion

The fixation technique B proved to be more rigid and lower stress values both in the bone and in the plate, as well as lower deformation and displacements were showed in comparison to fixation technique A for all mandibular movements and forces.

finite element analysis

orthognathic surgery

internal fixation device

mandibular osteotomy

Dentofacial deformities are defined as skeletal abnormalities that affect the bones of the face, mainly the maxilla and mandible [1]. The sagittal split ramus osteotomy (SSRO), initially described by Trauner and Obwegeser, in 1957, after several modifications, is the standard procedure in oral and maxillofacial surgery for the treatment of maxillomandibular discrepancies [2] and its stability is achieved through the use of titanium plates, screws and combinations of both, which is named stable internal fixation [3].

Most studies on the mechanical characteristics and differences between the techniques used to stabilize mandibular osteotomies were idealized in vitro models. However, the specific design, model geometry and the type of loading regimens chosen, frequently place a limit on these studies [4]. It is also hard to comprehend the subtle mechanical interactions between bone and fixation devices, resulting in a difficult task for improvements in surgical design [5, 6].

In view of this, finite element (FE) analysis is a promising alternative that can provide accurate information about biomechanical properties of bone and fixation techniques in different situations and loading regimens, filling gaps in other methodologies [7, 8]. FE analysis is based on the creation of computational models that are graphically represented by complex geometry meshes, permitting a 3-dimensional (3D) evaluation of the tensions and stress analysis generated in the fixation system, something that is not possible, for example, in photoelastic analysis [9–11].

In this study, we evaluated the mechanical characteristics and stress distribution on plates and screws of 2 stable internal fixation techniques through 3D mandible FE on simulated models with 2 designs of SSRO on clenching tasks in the postoperative period.

Mandible finite element modeling, plates and screws

The 3D mandibular model, miniplate, and screws were created from a bioengineering protocol [12](Renato Archer Information Technology Center, Campinas, SP, Brazil) using the Computer-Aided Engineering, through the software Rhinoceros 3D (version 7.0, Robert McNeel and Associates, Seattle, Washington, United States). The mandible was separated into two parts to generate a hemimandible due to asymmetry on both sides with the first molar only, to simplify modeling [13].

The model was geometrically formed by Rhinoceros 3D software and then transferred to the HyperMesh FE software (version 21 - Altair Engibeering GmbH, Michigan, United States), for contour definitions, biomechanical testing, properties, and loading regimens analysis (figure 01A).

Osteotomy designs

Four FE mesh of mandibular models were then simulated with 2 designs of SSRO advanced 5 mm forward [2, 9, 14, 15], which are commonly used in the clinical setting, divided into: Model 1, following Epker osteotomy design; and Model 2, following Posnick osteotomy design.

The proximal and distal bone segments were firmly fixed after cutting the bone, allowing displacement only in the masticatory force direction.

Internal fixation techniques

The SSRO described by Epker and the technique modified by Posnick were stabilized with 2 stable internal fixation techniques. A titanium miniplate (diamenter 1mm, length 29mm), from the Arnett system for orthognathic surgery (KLS Martin group), monocortical screws (diameter 2mm, length 5mm), and bicortical screws (2 mm in diameter, length 19mm) were selected. Technique A used 1 locking titanium miniplate with 4 monocortical screws; Technique B, a hybrid technique, used 1 titanium miniplate fixed with 4 monocortical screws and 1 bicortical screw, posterior to the last tooth and above the inferior alveolar nerve.

It was determined that each miniplate and screws were in perfect contact with the bone. The bone structure was differentiated between cortical and trabecular, in which the cortical structure has an average thickness of 2mm [13].

Materials properties

Young's modulus (E) was selected to analyze the elastic deformation capacity of materials, thus measuring their strength. The Poisson’s coefficient (ratio) (v) was used to represents the ratio between the transverse deformation value and the longitudinal deformation value.

The necessary properties for this analysis are described in table 1 [16-21]. These values were added to the simulation HyperMesh FE software.

Table 1. Mechanical characteristics of materials used in FE simulation [16-21].

Material	Young’s modulus (MPa)	Poisson’s coefficient
Cortical bone	13700	0.3
Trabecular bone	7930	0.3
Plate and screws	115000	0.34
Dentin	18600	0.31
Periodontal ligament	68.9	0.45

Loads and Restrictions

The vertical load magnitude in this study was set at 200N applied to the occlusal surface region (central fossa) of the mandibular first molar and to the incisal surface of the mandibular central incisor, with a magnitude of 100N. Both loads were applied vertically towards the bone from top to bottom [2, 15, 16]. The load coincided with previous studies on bite force in patients with dentofacial deformities [2, 15, 22, 23]. The models were evaluated according to the maximum principal stress, whose MPa (N/mm2) stress scale measured the general material effective stress [2].

The condyle region and the lateral surface of the ramus in the masseter region were fixed in all three directions (figure 1 B, C and D), since these regions do not rotate and do not translate in any of the axes and, therefore, it is said they do not have degrees of freedom [16].

The basic load condition was investigated in the focal points of masticatory forces: biting with occlusal contact at the region of molars and incisors.

It is worth mentioning the loading forces in the models are static. Analyzes of the stress distributions formed in the cortical and trabecular bones and also in the region of the fixation material were performed. The stability of the stress in a 3-dimensional state was evaluated according to the hypothesis of the stress scale of Von Mises (1913) [24]. The scale did not represent the yield strength of the screw, but it provided a clear visualization of stress concentrations [2,15].

Comparisons between the models were performed correlating displacement, strain, and stress. Insignificant displacement of the condyle was common on both models, however a markedly displacement was noted in models with no bicortical screw. When the models were fixed with bicortical screw, lower stress values in bone and metallic components were observed.

In technique A, there was a difference in the set displacement, especially of the cortical bone in model 1. However, the displacement was nine times greater than predicted in the model with bicortical screw (figure 2).

In figure 3, both models 1 and 2 presented displacement values decreased when fixed with technique B. Further, model 2 presented smaller displacements, when compared to model 1.

The distribution of strain in bone along the condyle and the cortical bone in model 1 presented more uniform but with higher values. In model 2, the same region showed greater concentration of strain, yet, with lower values. Model 2B presented strain three times lower than model 1B (figure 4).

The osteotomy design did not show significant difference in the analysis of the periodontal ligaments, although Epker osteotomy has slight advantages, with lower strain and stress values (figure 5).

For metallic components, model 2 showed lower strain values, although the difference is not significant. The same happens with the tension distribution. In terms of stress distribution, technique B showed reduction on strain values and lower stress values in bones and metallic components.

Among the techniques for bone repositioning in orthognathic surgery, the sagittal split of the mandibular ramus osteotomy, as well as its variations, are considered versatile because they allow multiple movements such as advance, set-back and rotate the distal. The fixation technique applied in these cases must be stable enough to minimize postoperative risks and complications, even in cases of few bones contact such as in large advances. Among the available techniques, we can mention the use of bicortical screws, which are rigid and present an increased risk of damage to the inferior and lingual alveolar nerve [25]; miniplates, less rigid, with higher removal rates and can cause irritation to the gingival mucosa even when properly placed [26]; and the hybrid technique, which combines the advantages of fixation with bicortical screws and miniplates with monocortical screws.

The FE analysis is based on the creation of a mathematical model that simulates the geometric conditions and mechanical properties of the objects to be tested. The configuration of the simulation models is essential and any changes in force locations or constraints can affect the results. The comparison of results with other studies on orthognathic surgery through FE analysis presents some difficulties, such as difference in model construction, numbers of mesh elements, and several simulation protocols. For example, there are studies that use a complete mandible while others use a hemimandible [2, 13, 15, 16]. The ratio depends a lot on computer resources, and in certain cases models require simplification to reduce computational demand. However, FE analysis uses bone properties and has been considered as a safe simulation in biomechanical studies, when compared to mechanical tests results on bone specimens. Thus, it is reliable in the clinical setting [16].

To compare the biomechanical properties of two fixation techniques (mini plate and hybrid technique) of SSRO for 5mm mandibular advancement, forces to the first molar and central incisor were applied. This study results corroborate with the findings of previous FE analysis studies [2, 6, 16], where the hybrid technique proved to be more rigid than mini plate use, generating lower cortical movement, deformation, and tension values in both the bone and the plate. However, in the present study, the 1.0 mm plate use was analyzed, which proved to be satisfactory in controlling and maintaining bone positioning in the face of tension and deformation forces.

The vertical load magnitude in this study was set at 200N on the occlusal surface region (central fossa) of the mandibular first molar and at 100N on the incisal surface of the central incisor. Both loads were applied vertically towards the bone, that is, from top to bottom. The bite forces used in the study were obtained from research that investigated postoperative bite forces in orthognathic surgery patients [8, 25]. This study did not include maximum normal bite forces, as fixation is most required during the 4 to 6 weeks bone healing period. Since during the postoperative period, bite forces in healthy adults may drop to 30%, 50% or less as a neuromuscular protection result, and gradually increase to normal after 3 months to 2 years [8].

The hybrid technique, with the use of 2.0 mm bicortical screws, registered lower stress and displacements values for all mandibular movements and forces, compared to the use of only one titanium miniplate of the Arnett system (1mm) for orthognathic surgery and four monocortical screws (diameter of 2.0 mm and length of 5 mm). Bicortical screws are more rigid than miniplates with monocortical screws, probably because they penetrate both the buccal and lingual cortical of the mandibular segments [9, 10, 27]. This study did not show any deformation of the miniplates, and areas of high stress represented in red color could be tensile or compressive forces. Minimizing manipulation of miniplates in these high-stress regions during the surgical procedure is recommended to avoid postoperative complications, such as plate fracture.

The amount of displacement is a fixation rigidity indication, and bicortical screws proved to be the most rigid fixation. Fixation using only miniplates resulted in a displacement nine times greater than predicted in the model with bicortical screw (Fig. 2), which is undesirable and compromise the procedure success, when the patient moves the mandible. However, some degree of micro movement in the bone actually increases callus formation during bone healing, and this may be an advantage of stable fixation [5].

The osteotomy designs in the present study were shown to influence the distribution pattern of displacement forces. While the model 1A presented a displacement of 13.07 mm, model 2A presented a displacement of 7.11 mm, that is, 1.8 times smaller. The situation becomes less pronounced in the hybrid technique, where the displacement in the Epker osteotomy was 1.40 mm and in the Posnick osteotomy, 1.04 mm. There were found no studies in the literature that made a comparison between these two osteotomies types, since most focus on the repercussion of the fixation material against occlusal forces. In regard, we suggest new studies be carried out specifically comparing these osteotomies types.

It was noted that, in the model 1, the distribution of bone tension is more uniform along the condyle and cortical bone, but with higher tension values. On the other hand, in model 2, the condylar region and the cortical bone presented a higher tension concentration, however, with lower tension values. The model 2B presented tensions three times lower than the model 1B. On the other hand, it was noted that the influence of the osteotomy was not so accentuated in the periodontal ligaments analysis, with punctual differences between the models.

Regarding to miniplates, they can be rigid or functionally stable and both have been clinically proven to be successful in the treatment of mandibular fractures and orthognathic surgery [13, 19]. In our study, a greater maximum displacement was observed only with the miniplates use. However, it is noteworthy that the bone movement performed was mandibular advancement and the close approximation of bone segments, in this technique, is necessary to increase fracture stability [17, 21].

Using the bicortical screw, the displacement values decrease substantially both in models 1 and 2. However, it was noticed that the model 2B presented smaller displacements values and maximum strain forces in the bone and in the fixation material, when compared to the model 1. However, the success of orthognathic surgery does not depend only on the fixation techniques, but on other factors, including the dentofacial deformity severity, surgical planning, surgeon's technique, and the patient's soft and hard tissues [28].

The hybrid technique proved to be more rigid and lower stress values, both in the bone and in the plate, and lower strain and displacements values for all mandibular movements and forces were showed. The osteotomies types showed to influence the distribution pattern of displacement forces. The Posnick osteotomy presented 1.8 times smaller, where the difference in displacement was minimal in the hybrid technique.

3D – 3-dimensional;

FE – finite elements;

SSRO – sagittal split ramus osteotomy;

Author contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by KMS, LMRM, PYN, GPJ, ALM. The first draft of the manuscript was written by KMS and MGML and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors

Ethical approval This article does not contain any studies on humans or animals performed by any of the authors.

Competing interests The authors declare that they have no competing interests.

Consent to participate For this type of study, formal consent is not required.

Consent to publish For this type of study, consent to publish is not required

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No competing interests reported.

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Evaluation of two stable internal fixation techniques in mandibular ramus sagittal osteotomy: a three-dimensional finite element study

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