Abstract
This study constituted a comparative assessment of the mechanical resistance of square and rectangular 2.0-mm system three-dimensional miniplates as compared to the standard configuration using two straight miniplates. 90 polyurethane replica mandibles were used for the mechanical trials. Groups 1, 2, and 3 simulated complete symphyseal fractures characterized by linear separation of the central incisors; groups 4, 5, and 6 simulated parasymphyseal fractures with an oblique configuration. Groups 1 and 4 represented the standard method with two straight miniplates set parallel to one another. Square miniplates were used in groups 2 and 5, and rectangular miniplates in groups 3 and 6. A universal testing machine set to a velocity of 10 mm/min and delivering a vertical linear load to the first left molar was used to test each group. Maximum load values and load values with pre-established dislocation of 5 mm were obtained and submitted to statistical analysis using a calculated reliability interval of 95%. The mechanical performances of the devices were similar, except in the case of rectangular plates used in the parasymphyseal fractures. The innovative fixation methods used showed significantly better results in the case of symphyseal fractures.
In recent years, different methods have been proposed for the surgical treatment of mandibular fractures. Rigid internal fixation (RIF) is used to achieve a stable anatomical reduction, thereby reducing the risk of postoperative displacement of fractured bone fragments, avoiding the need for maxillomandibular fixation and favouring an early return to normal functioning.
The mandible is subject to forces generated by the chewing muscles transmitted via the teeth and the temporomandibular joints. During treatment, tensions and deformations occur according to the distribution of the external forces and the properties and geometry of the material being used. It is well known that the mandible is normally subjected to tensioning forces on its superior border and compression forces on its inferior border. However, that is only true for fractures in the body and angle of the mandible; in the case of fractures in the symphyseal and parasymphyseal regions, the opposite situation prevails and a single form of biomechanical behaviour can be expected for this latter region as a whole.
Irrespective of the fixation method used, stability is a key factor for the successful treatment of the symphyseal fracture. This can be evaluated by mechanical tests that simulate the forces that the mandibular fractures will be subjected to, making it possible to determine the resistance of the fixation material and the behaviour of the fractured region.
The symphysis is one of the most common mandibular fracture sites, with reports of prevalence varying from 9% to 57%; it is only surpassed by fractures of the condyle or of the angle. Each mandibular region has its own peculiarities, including variations in the forces exerted by the chewing muscles, zones of fragility, and the possible presence of mechanical forces acting in different directions. These factors determine the extent of a region’s susceptibility to trauma and its propensity for a favourable response to treatment.
Recent work done by Madsen et al. and by Oliveira and Passeri has involved the comparative biomechanical assessment of different fixation techniques applied to symphyseal and parasymphyseal mandibular fractures, but they did not make use of three-dimensional (3D) fixations.
Farmand , who was the first to undertake a biomechanical investigation of 3D plates in 1996, studied the performance of a plate in the shape of the four sides of a square open in the middle. In his view, the device, which was fixed by screws, would foster stability in three dimensions, and its biomechanical characteristics were comparable to those of conventional miniplates. The open-centred square configuration would be the smallest possible one for a 3D plate component. In that study, the clinical results and investigations showed that 3D plates provided good stability during osteosynthesis associated with more complicated cases of mandibular fracture. The author also stated that the 1-mm profile of the connecting arms of the device made its adaptation to the bone without distortions easier, and that the untrammelled areas between the connecting arms ensured a good blood supply to the bone.
In spite of the small number of in vitro studies using 3D plates in fractures in the anterior region of the mandible, there are some authors who have reported the efficacy of the method. In two clinical studies, Jain et al. demonstrated the effectiveness of 3D miniplates for fixation in the treatment of mandibular fractures in that region and analysed their advantages and disadvantages over a 2-month follow-up period. In the configuration used in their work, the material used proved to be less costly and readily adaptable, as well as reducing the operation time and providing greater stability.
While there have been some investigations using 3D plates as the fixation method, there is no scientific evidence concerning their use in the symphysis/parasymphysis region. Thus the present work was undertaken to perform a laboratory evaluation of the resistance and performance of square and rectangular 2.0-mm system 3D miniplates used to stabilize fractures in the anterior region of the mandible, as compared to the performance of standard pattern plates, namely two straight plates also of the 2.0-mm (screw diameter) system.
Materials and methods
For the purposes of this study, two rigid polyurethane mandible models with complete sets of teeth were prepared (Nacional Ossos, Jaú, São Paulo, Brazil) and two different ‘fractures’ in the form of cuts were made in them using a metal disc at low speed rotation. The ‘osteotomies’ simulated symphyseal and parasymphyseal fractures. The simulated symphyseal fracture consisted of a linear cut in the centre of the mandible, from between the median incisors down to the basal part of the mandible. The parasymphyseal fracture was represented by an oblique cut originating between the median incisors and going down in a slightly posterior direction to the base of the mandible on the right side. The two models were sent to the model company, which then produced 90 standardized replicas, 45 for each type of ‘fracture’.
Four hundred and eighty titanium–aluminium–vanadium alloy (Ti–6Al–V) screws (PROMM, Indústria de Materiais Cirúrgicos, Porto Alegre, Rio Grande do Sul, Brazil) were used, of which 240 were 6 mm long and 240 were 12 mm long, all compatible with the 2.0-mm system. 120 miniplates were used as follows: 60 straight four-hole miniplates that are the standard pattern for the 2.0-mm system, 30 square four-hole miniplates, and 30 rectangular four-hole miniplates. The square and rectangular miniplates were designed by the authors and made to order by the suppliers (PROMM, Indústria de Materiais Cirúrgicos).
Sample preparation
The rigid polyurethane mandibles were divided into six groups of 15 mandibles each for mechanical trials in accordance with a statistical design obtained using a programme for sample size determination (Diman 1.0); the confidence interval established was 95%.
The mandibular fractures in the replicas of groups 1 and 4 were stabilized using two straight four-hole miniplates of the 2-mm system on each. These were fixed in parallel, one in the superior position and the other in an inferior position, with care taken so that the superior plates were always lower than the level of the dental root apices. These two groups were considered to be the standard pattern or control groups. Fixation in groups 2 and 5 used square miniplates, and in groups 3 and 6, rectangular miniplates were used. Each square and rectangular miniplate was stabilized with four screws ( Figs. 1–3 ). In all groups, the screws that were nearest to the dental apices were 6 mm long, while those inserted near the inferior border of the mandible were 12 mm long.
