The purpose of this study was to compare, by mechanical in vitro testing, a 2.0-mm system made with poly- l – dl -lactide acid with an analogue titanium-based system. Mandible replicas were used as a substrate and uniformly sectioned on the left mandibular angle. The 4-hole plates were adapted and stabilized passively in the same site in both groups using four screws, 6.0 mm long. During the resistance-to-load test, the force was applied perpendicular to the occlusal plane at three different points: first molar at the plated side; first molar at the contralateral side; and between the central incisors. At 1 mm of displacement, no statistically significant difference was found. At 2 mm displacement, a statistically significant difference was observed when an unfavourable fracture was simulated and the load was applied in the contralateral first molar and when a favourable fracture was simulated and the load was applied between the central incisors. At the failure displacement, a statistically significant difference was observed only when the favourable fracture was simulated and the load was applied on the first molar at the plated side. In conclusion, despite more failure, the poly- l – dl -lactic acid-based system was effective.
Rigid internal fixation (RIF) using titanium-based plates and screws is widely used in oral and maxillofacial surgery in orthognathic, posttraumatic and reconstructive applications . The use of titanium-based plates and screws in the treatment of mandibular fractures is well-established, but several potential postoperative problems have been associated with these permanent devices, including decreased visibility or palpability, hardware loosening with resultant extrusion, increased sensitivity to cold, hardware migration, bone atrophy or osteopenia caused by stress shielding and corrosion, interference with imaging examinations (e.g. radiographs, computed tomography scan and magnetic resonance imaging) and radiation therapy, allergic and foreign body reactions, possible growth restriction of the craniofacial skeleton in paediatric patients, infection, and physiological or psychological discomfort .
In an effort to address these issues, a new class of materials, bioresorbable polymers, has been developed for use in RIF . The main advantage of this type of material is the gradual transfer of functional stress to the bone while the device is resorbed, thereby decreasing or eliminating the ‘stress shielding’ phenomenon and the need for an additional surgical procedure to remove the device . Some clinical and in vitro studies have shown that the mechanical properties of this class of material are adequate for use in mandibular osteotomies and fractures , others have found them to have inadequate resistance and strength .
The purpose of this study was to compare the 2.0-mm system made with poly- l – dl -lactic acid (70:30) to the analogous metallic titanium-based system using mechanical in vitro testing.
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.
|Group||Type of sectioning||Site-of-load application||Subgroup|
|Titanium-based||Unfavourable to treatment||First molar on the sectioned side||1|
|Contralateral first molar||2|
|Between the central incisors||3|
|Favourable to treatment||First molar on the sectioned side||4|
|Contralateral first molar||5|
|Between the central incisors||6|
|Poly- l – dl -lactic acid-based||Unfavourable to treatment||First molar on the sectioned side||7|
|Contralateral first molar||8|
|Between the central incisors||9|
|Favourable to treatment||First molar on the sectioned side||10|
|Contralateral first molar||11|
|Between the central incisors||12|
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.
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.
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).