Materials and methods

The 2.0-mm titanium-based system group consisted of 42 straight 4-hole plates with 168 self-tapping screws 6.0 mm long (Neoortho, Curitiba, Brazil). The 2.0-mm poly- l – dl -lactic acid-based system group consisted of 42 straight 4-hole plates and 168 screws 6.0 mm long, developed at the Mechanical Engineering School at the State University of Campinas, Campinas, São Paulo, Brazil. The titanium-based system was sterilized in an autoclave, whereas the poly- l – dl -lactic acid-based system was sterilized using gamma radiation.

84 human dentate mandibular replicas made of rigid polyurethane resin (Nacional, Jaú, Brazil) were used as a substrate. The mandibular replicas were divided into two groups: a titanium-based and a poly- l – dl -lactic acid-based group. Each group was divided into subgroups according to the type of sectioning and site-of-load application ( Table 1 ). Each subgroup consisted of seven mandibular replicas.

The experimental models were uniformly sectioned at the left mandibular angle using a microreciprocating saw containing a blade 0.2 mm thick (Dentscler, Ribeirão Preto, Brazil). A first point (A) was marked in the alveolar process 5 mm posterior to the distal face of the second molar. From this point, a line was traced perpendicular to the mandibular base and a second point (B) was identified in a more inferior aspect of the mandibular base. Two further points were obtained 10 mm equidistant from point B, one anterior (C) and another posterior (D) ( Fig. 1 ). To obtain defects simulating fractures, both favourable and unfavourable to the treatment, the sectioning was completed from points A to C ( Fig. 2 (I) ) and from points A to D ( Fig. 2 (II)), respectively.

Point A is marked on the alveolar process 5 mm posterior to the distal face of second molar. From this point, a line perpendicular to the mandibular base was traced (dotted line) and point B was obtained. Points C and D are each 10 mm from point B.

(I) The trajectory of the sectioning made from points A to C, simulating a fracture favourable to treatment; (II) the trajectory of the sectioning made from points A to D, simulating a fracture unfavourable to treatment.

To standardize the two types of sections in each subgroup (favourable and unfavourable to treatment), two acrylic jigs were made and adapted to the lateral aspect of the polyurethane mandibles during sectioning.

The plates were adapted and stabilized passively at the same site in both groups. The adaptation of the plates from the titanium-based system to the external oblique ridge was made using the specific instruments in the manufacturer’s set. After using a drill 1.6 mm in diameter, the plates were stabilized by applying four self-tapping screws 6.0 mm long ( Figs 3 and 4 ).The adaptation of the plates from the poly- l – dl -lactic acid-based system was made following the instructions of the researcher and consisted of immersing the plate for 10 s in physiological saline solution (0.9%) heated to 55 °C (131 °F). The plate became malleable, allowing easy adaptation to the external oblique ridge using digital pressure. After using a drill 1.6 mm in diameter, a tap was applied to obtain the threads on the bone and the plates were stabilized with four screws 6.0 mm long. The researcher only manufactured screws with a length of 12.0 mm, thus necessitating the use of a carborundum disc mounted in a low-speed electric motor to obtain the desired length of 6.0 mm.

The location of the titanium-based system in the group in which the fracture favourable to treatment was simulated.

Location of the poly- l – dl -lactic acid-based system in the group in which the fracture favourable to treatment was simulated.

To standardize the position of the plate, acrylic jigs were made for each subgroup and adapted to the lateral aspect of the polyurethane mandibles during the fixation of the plate.

The resistance-to-load test was performed at the Integrated Laboratory for Research on the Biocompatibility of Materials of the School of Dentistry at Ribeirão Preto, University of São Paulo, using an EMIC DL200 Universal Material Testing Machine (EMIC, São José dos Pinhais, Brazil). To standardize the test, two steel devices were made and set up on the EMIC machine, one as a supporter to stabilize the mandible replicas and another as a tip to apply the vertical loads.The force was applied through the tip perpendicular to the occlusal plane at a rate of 2 mm per min at the following three points: the first molar on the plated side; the first molar on the contralateral side; and between the central incisors ( Table 1 ). The data from the load applied were obtained in kilogram-force at three times: 1 mm and 2 mm of tip displacement, and at the time at which the fixation failed. The tip displacement was obtained in mm at the time of failure of the fixation.

The data from the subgroups with the same type of section and site-of-load application were cross-checked. All data obtained underwent analysis of variance (ANOVA) and the Tukey’s test at a level of significance of 5% using the software Winks DAS – Version 6.0.1 (Texasoft Copyright 1991–2007).