Abstract
This study was a comparative evaluation of the bending resistance of metallic and resorbable plates and screws in a mandibular body fracture model. Forty polyurethane synthetic hemimandibles were used; a vertical linear cut was made between the second and first premolars. These 40 hemimandibles were divided into four groups of 10 and were fixed with titanium plates and screws or resorbable plates and screws, with monocortical screws in the upper sector and bicortical screws in the lower sector. Bending resistance tests were done on a universal testing machine with a linear displacement speed of 1 mm/min, a cell load of 500 N, and a load cell on the lower central incisor or on the lower second premolar. Results were analyzed using the Student’s t -test, with the significance level set at 5%. No statistically significant differences were observed between the groups studied, either in the analysis of the osteosynthesis materials or related to the load-bearing points. The variables of displacement and peak load did not present any significant differences. In this in vitro model of a mandibular body fracture, the mechanical behaviour of a resorbable osteosynthesis was similar to that of a titanium osteosynthesis.
Surgical treatment of a mandibular body fracture is done with maxillomandibular fixation or surgically with plates and screws. The latter has advantages, such as the possibility of primary bone repair, the rapid re-establishment of function, and the elimination of postoperative intermaxillary fixation.
Surgical treatment has been the choice in many cases, with good results. One of the options for osteosynthesis in the area of tension and compression has been employed with great success and has demonstrated great versatility when different types of systems are used with monocortical or bicortical screws, depending on the area of approach.
Different technologies, including resorbable internal fixation systems, have been used in different cases of maxillofacial surgery with good results. In terms of bone repair, the observations in animal models treated with resorbable materials have been positive. Suuronen treated rabbit models of condylar fracture using resorbable and metallic screws, demonstrating adequate bone repair with the different treatments applied. Suuronen et al., in a sheep model of mandibular body fractures treated with metallic and resorbable systems, concluded that there was adequate bone repair in both groups. Quereshy et al. also treated models of mandibular fractures in dogs using resorbable and metallic systems, concluding that the bone repair was similar in the two groups when the X-ray and histological studies of the animals were analyzed.
Resorbable materials have been applied in patients for more than 20 years. Bergsma et al. used resorbable systems in zygomatic bone fractures, where after 3 years only four subjects presented episodes of increased facial volume, opting for removal of the elements installed; the authors concluded that the chemical and biological characteristics of the materials could be the cause of this complication. Ylikontiola et al. treated patients with mandibular fracture by means of resorbable systems, confirming the efficacy of these systems in this type of clinical situation. A more critical stance was taken by Wittwer et al., who showed that in 24.5% of zygomatic bone fractures it was necessary to stabilize the fractures with metallic plates since the resorbable plates were not able to stabilize the fractures.
The mechanical stability of resorbable plates have also been considered in some areas of maxillofacial surgery, where mandibular body osteotomies or the simulation of a mandibular body fracture have not been sufficiently studied. Mandibular body fractures suggest complex mechanical conditions whenever there is a high demand for the attached muscles, as well as a high demand for the postoperative functional stress.
The aim of this investigation was to compare the bending resistance of resorbable and metallic systems in a mandibular body fracture model.
Materials and methods
Forty rigid polyurethane replica hemimandibles with teeth, with a density of 200 g/l, were used (Nacional, Jaú, SP, Brazil). A linear cut was made in each hemimandible starting between the second premolar and the first premolar of each sample, advancing perpendicularly to the surface and axially to the root of the first premolar; the cut was made with a Stryker reciprocating saw blade.
Analysis group
The osteosynthesis systems used in this study were 4-hole straight plates with titanium alloy screws (Ti–6Al–4V/Grade V; Engimplan, Rio Claro SP, Brazil) and 4-hole straight plates with screws made from an amorphous poly l -co- dl lactic acid 70:30 copolymer developed by the Department of Materials Engineering at the State University of Campinas, Brazil. In order to determine the width of the lower sector of the mandible, a measurement was taken with a digital Vernier calliper, determining a width of 11 mm in this sector; it was decided that 12-mm screws should be used in the basilar region of the mandible, as exemplified by the use of bicortical fixation.
The analysis groups are described in Table 1 . In order to establish the exact position of the metallic and resorbable plates and screws, surgical guides made of chemically activated colourless acrylic resin were used (Dental Vipi Ltda, Pirassununga, SP, Brazil). These were generated from the model hemimandible in both metallic and resorbable fixations ( Fig. 1 ). Each hemimandible was fixed as per the manufacturer’s instructions, using screws in the metallic system and screws with a self-drilling tap in the resorbable system. The plates did not need manipulation or adjustment because the installation area was flat, with the screws being installed perpendicularly to the surface of the analysis substrate.
