Biomechanical analysis of titanium plate systems in mandibular condyle fractures . A systematized literature review

PURPOSE: To conduct a systematized review of the literature about the main methodologies used to evaluate the biomechanical fixation systems with titanium plates in fractures of the mandibular condyle. METHODS: A systematized review of literature was performed in the electronic databases PubMed, EMBASE, LILACS and MEDLINE without restriction of the publication date. The eligibility criteria were laboratory studies involving mandibular condyle fractures, studies using titanium plates, biomechanical studies, in vitro and computational studies involving the finite element method (FEM). RESULTS: Eleven articles that met the eligibility criteria were selected, including seven articles involving in vitro studies and four studies with biomechanical analysis by using FEM. CONCLUSION: Although few articles have used the finite element method, the results of in vitro studies were similar to those found in computational studies, regarding to the stable use of two titanium miniplates.


Introduction
Craniomaxillofacial trauma represents severe health hazards in a significant proportion of patients throughout the world, varying in type, severity and etiology according to the studied population 1 .Such injuries may occur alone or in association with other relevant damage in the brain, spine and other parts of the skeleton 2 .In this context, jaw is one of the facial bones most commonly fractured, representing between 17.5 and 52% of cases in the craniomaxillofacial region 3 .Anatomically, the main mandibular regions affected by fractures that have been reported, in decreasing order of occurrence, are angle, condyle, parasymphysis, body, symphysis, ascendant ramus and coronoid process 4 .
Mandibular condyle fractures represent 25-35% of all mandibular fractures 5 , being considered one of the most controversial fractures concerning to diagnosis and treatment 6 .
Although choosing the best treatment still remains under discussion 7 , the indication for open reduction and fixation of the fractured segments with the use of titanium plates and screws has been reported by several authors [8][9][10][11][12][13] .
Different osteosynthesis techniques have been used against these fractures aiming the achievement of satisfactory results 14 .Studies have been conducted using biomechanical analysis, computational or not, with the purpose of selecting an appropriate system that provides maximum stability and minimum trauma during their insertion 14 .In this context, the objective of the present study was to perform a systematic review of the literature about the main methodologies used in the biomechanical evaluation of fixation systems using titanium plates in mandibular condyle fractures.

Methods
The following electronic databases were used without restriction concerning to date of publication: PubMed, EMBASE, LILACS and MEDLINE.The search strategy used terms or combinations exemplified in Table 1 with restriction to English, Spanish and Portuguese languages.After this action, two reviewers (MFB and FWGC) independently evaluated the titles and abstracts of the selected articles (Table 1).The adopted inclusion criteria were: studies involving mandibular condyle fractures, studies using titanium plates, biomechanical, in vitro and computational studies involving the finite element method (FEM).Studies in animals, research involving other anatomical sites, studies using non biomechanical methods, analysis of biodegradable materials or osteosynthesis systems that were not titanium plates, case reports, literature reviews and articles written in language not compatible with the search strategy adopted in this work were excluded.Data were organized into two groups (G1: non computational biomechanical studies, G2: computational biomechanical studies) being analyzed and properly interpreted.

Results
Between 1999 and 2010, 11 articles that met the eligibility criteria listed above were selected, including seven articles involving in vitro studies and four studies involving biomechanical analysis using FEM (Table 2).[25] Austria Computational study

Elastic compression tests
Meyer et al. [26]  France In vitro study (fresh human mandibules)

Elastic compression tests and photoelastic stress
Tominaga et al. [19]  Japan In vitro study (synthetic human mandibules)

Elastic compression tests
Lauer et al.

Finite element method
Meyer et al. [16]  France In vitro study (fresh human mandibules)

Elastic compression tests and photoelastic stress
Seemann et al. [24]  Austria Computational study

G1: non computational biomechanical studies
Between the seven in vitro selected studies, three used jaws obtained from human cadavers and four studied polyurethane replicas of the jaw.The three articles that used fresh human mandibles compared different schemes for fixing condylar fractures, including fixation with a single miniplate in the posterior edge, dynamic compression miniplate, single reconstruction plate, schemes using two miniplates and single rectangular/trapezoidal plates.A study applied just one load simulating a condylar movement upward, forward and medially 15 .The other two studies applied loads simulating eight jaw movements according to Meyer et al. 16 .The studies evaluated the resistance level of the fixation by the displacement degree of the condylar fragment and/ or analysis of photoelastic stress.All fixation schemes using two plates or rectangular/trapezoidal plates were superior to fixation with a single plate concerning to the displacement degree and counterclockwise rotation of the condylar segment.In addition, photoelastic analysis showed a reproduction more close to the normality of tension/compression areas when using two plates or rectangular/trapezoidal plates.The four studies that assessed the resistance of the fixation systems in polyurethane mandible replicas also used various schemes of plates/screws and evaluation methods.The fixation methods varied from two plates positioned on the anterior and posterior edge of the condyle, two overlapping plates, a single lag screw, a single dynamic compression plate, one against two plates with different lengths of screws and one plate from the lock system.Two studies applied forces directed to condyle from medial to lateral and from anterior to posterior.One study added force from lateral to medial of the condyle.Another study used the method described by Ziccardi et al. 15 with load simulating a condylar movement upward, forward and medially.All works that used two plates apart showed that this fixation scheme is superior to others.One result showed that two plates overlapping one another was not superior to the model using two plates apart.When only one plate is used, the dynamic compression system is superior to the non-compressive and to the lock system.Additionally, when clinically it is not possible to insert two plates, a biomechanical study showed that longer and bicortical screws can provide increased resistance to fractured segments.

G2: computational biomechanical studies
Between the four selected studies, two used only one technique testing one 1 union element (one plate), and the other two studies compared two protocols (one versus two union elements).Three studies used union elements simulating plates with four holes, while one study used plates with six holes.
Regarding the geometry of the computationally simulated plate, two articles tested plates with new designs (trapezoidal), while the others reproduced conventional plates with straight format.
All works applied simulations using muscle loads, although only two articles have explained them.In one study, mandible was subjected to six muscle forces, while in other study, four muscle forces were applied.The distribution of stress lines was prevalent around the screw area (three studies).Research involving the test of only one union element considered it suitable for regular use, while the studies comparing different protocols showed greater stability when affixing two plates.

Discussion
The accuracy about the fractured bone reduction and the fixation stability have been considered as important prerequisites for the function restoration in situations of injury to the mandibular condyle 7 .It is recognized that the temporomandibular joint is subject to loads from various muscle groups that are active during the masticatory efforts 17 .Thus, the fixation methods should have sufficient resistance to oppose these forces and these methods can not interfere with the condyle position after reduction.Several types of laboratory methods have been described in the literature to assess the efficacy of fixation schemes in relation to functional loads experienced by the mandibular condyle 16,18,21 .There is still no consensus about the optimal fixation method for condylar process fractures 19 .The research mainly use jaws from fresh human cadavers 16,26,27 , polyurethane replica of mandibles [19][20][21][22] and, recently, finite element methods 18,[23][24][25] .

Non computational biomechanical studies
Biomechanical studies in the treatment of condylar fractures help in the knowledge about how the various osteosynthesis systems behave in the face of forces and loads experienced by the temporomandibular joint (TMJ).Due to the unique feature of TMJ regarding to the distribution of tension and compression zones after masticatory efforts, their fixing principles differ from the other regions of the jaw.During chewing efforts in the first molars and incisors regions, compression zones are generated in the region of the condyle posterior border and tension areas next to the ramus superior border (mandibular notch) 26 .
Considering the principles of functionally stable fixation of Champy, osteosynthesis should be performed in tension areas to counteract the trend of opening the fracture line.This explains the poor primary stability of most schemes that used only one plate on the ramus posterior border.Even stronger or two superimposed plates (lock, dynamic compression or reconstruction) can fail or break when used alone 27 .The main reason for this is that even they have been more robust plates, they are not positioned in the correct anatomical site to support the physiological forces.
The simulation methods of the muscle forces must also be taken into account in the critical analysis of the biomechanical studies results.Two reviewed articles simulated only the loads experienced in the joint when occurs the maximum bite force 15,19,27 .These two articles found that two plates presented greater resistance to permanent deformation than the methods of a single plate (reconstruction, dynamic compression, miniplate) (p<0.05).Additionally, the authors found that the use of a dynamic compression miniplate provides good stability to the condylar segment between all single plate schemes.Three studies simulated lateral and medial loads to condyle [20][21][22] .In these studies, almost all single fixing schemes suffered small deformations when subjected to loads similar to masticatory forces.Haug et al. 22 proved that the dynamic compression plates were more favorable to counter the latero-lateral moves of the mandibular condyle.Asprino et al. 20 suggest the use of two plates, but when it is not possible due to the segment size or surgical difficulty, the single plate should be fixed with longer screws (8mm).Gealh et al. 21proved that two overlapping plates do not increase the resistance of the condyle to the lateral loads.
Only two articles studied the use of synthetic materials specifically designed for use in condylar fractures 16,26 .These plates (four and nine holes) designed in a trapezoidal shape showed better results than the use of a single plate or lag screws when forces simulating the various masticatory muscles were generated in the fixed condyle.The authors advocate the use of these specific fixing schemes saying that these plates meet the principles of the functionally stable fixation in condylar region and are able to resist the physiological forces.

Computational biomechanical studies
The computational biomechanical studies employ a conventional method for obtaining virtual prototypes, based on a real jaw surface, which is reproduced by the finite element method, taking into consideration the anisotropic typical properties of the jawbone 18,[23][24][25] .
In order to obtain an experimental model using finite elements is necessary to define initially the geometry of the structure to be analyzed.Lauer et al. 23 and Parascandolo et al. 18 reproduced plates on an artificial model of the human mandible, while Wagner et al. 25 18 .However, the first authors applied proportionately more muscular forces, especially the temporal muscle (42.4%).
Studies that used the finite element method have also attempted to determine an effective and stable osteosynthesis protocol for condylar fractures.However, only two studies conducted comparative tests between the use of a single plate and the use of two appositional plates at the fracture site 18,25 .
Both studies found that the protocol involving two plates was the most appropriate, indicating it as a method of choice.However, Lauer et al. 23 and Seemann et al. 24 , when using the technique of a single plate, observed satisfactory results in terms of stability.
Probably this fact had happened because they tested new plates with favorable formats.These authors performed clinical studies with these plates and no fracture cases were found.

Conclusions
Although few studies have used the finite element method, the results of in vitro studies are similar in some ways, to those found in computational studies, regarding the use of two stable titanium miniplates in such fractures.In addition, future clinical trials, applying the methodology with finite elements or not, are necessary to better indicate the most appropriate technique for osteosynthesis in cases of mandibular condyle fractures.
Biomechanical analysis of titanium plate systems in mandibular condyle fractures.A systematized literature review

TABLE 1 -
Search strategies for electronic databases.

TABLE 2 -
Description of selected studies.

Table 3
details the non computational biomechanical studies.

TABLE 4 -
Characteristics of methods used in computational studies.