Abstract
The sagittal split ramus osteotomy (SSRO) is a surgical technique used widely to treat many congenital and acquired mandibular discrepancies. Stabilization of the osteotomy site and the potential for skeletal relapse after the procedure are still major problems. The aim of this study was to compare the mechanical stability of six methods of rigid fixation in SSRO using a biomechanical test model. Sixty polyurethane replicas of human hemimandibles were divided into six groups. In group I, the osteotomies were fixed with two four-hole titanium miniplates; in group II, with one four-hole miniplate; in group III, with one four-hole miniplate + a bicortical screw; in group IV, with a grid miniplate; in group V, with a four-hole locking miniplate; and in group VI, with a six-hole miniplate. A linear load in the premolar region was applied to the hemimandibles. The resistance forces (N) needed to displace the distal segment by 1, 3, and 5 mm were recorded and the data transmitted from the load cell to a computer. One-way analysis of variance with Tukey’s post hoc test was performed to compare the means between groups. For the three displacement conditions, there was a strong tendency for the 2.0-mm plate + screw and the grid plate to have higher values.
The sagittal split ramus osteotomy (SSRO) is a surgical technique used widely and successfully to treat many congenital and acquired mandibular discrepancies. Stabilization of the osteotomy site and the potential for skeletal relapse after the procedure are still major problems. Many fixation techniques have been developed to stabilize the proximal and distal segments, which vary from the interosseous fixation of the bone segments with wires to rigid internal fixation. The most uncomfortable aspect for the patient has been the requirement for several weeks of intermaxillary fixation of the jaws.
The introduction of titanium miniplates has overcome the disadvantages of intermaxillary fixation, shortening or even eliminating its use, and in many cases it has also improved the long-term stability of treatment. Studies have also found that the use of bicortical screws in conjunction with miniplates and monocortical screws (hybrid technique) results in better biomechanical properties in comparison to the traditional fixation techniques with standard miniplates or screws. Further, the locking miniplate/screw system has recently been introduced, which has advantages such as less screw loosening, greater stability, less need for plate adaptation, and less alteration in the occlusal relationship.
Various studies have evaluated the biomechanical stability of the different fixation methods and materials such as transosseous wiring, lag and positional screws, bicortical screws, titanium miniplates, locking screws/plates, bioresorbable plates, L-shaped plates, and the hybrid technique.
The biomechanical characteristics of the different types of fixation materials and techniques can be measured and evaluated in vitro with two- or three-point biomechanical test models using fresh sheep mandibles or polyurethane hemimandible replicas and photoelastic tests or finite element models.
The aim of this study was to compare the mechanical stability of six methods of rigid fixation in SSRO using polyurethane hemimandible replicas in a biomechanical test model.
Materials and methods
This study involved 60 polyurethane replicas of human hemimandibles (code 1052) in which SSROs mimicking the Dal Pont modification of the sagittal osteotomy were performed by the manufacturer (Nacional Ossos, Jaú, SP, Brazil). The distal segments of all hemimandibles were advanced 5 mm, and the specimens were divided randomly into six groups of 10 hemimandibles each. All fixation materials employed were obtained from the same manufacturer (Tóride Industria e Comércio Ltda, Mogi Mirim, SP, Brazil). An acrylic splint was constructed for each group to standardize the position of each plate ( Fig. 1 ).
The simulated osteotomies were fixed with the following six methods, which are shown in Fig. 2 : Group 1 osteotomies were fixed with two four-hole titanium standard miniplates set parallel to each other, with monocortical 2.0-mm × 6.0-mm screws. Group 2 were fixed with one four-hole titanium standard miniplate with monocortical 2.0-mm × 6.0-mm screws. Group 3 were fixed with one four-hole titanium standard miniplate with monocortical 2.0-mm × 6.0-mm screws and one additional 2.0-mm × 10.0-mm bicortical screw positioned posteriorly to the miniplate. Group 4 were fixed with one eight-hole titanium grid miniplate with monocortical 2.0-mm × 6.0-mm screws. Group 5 were fixed with one four-hole locking miniplate with monocortical 2.0-mm × 6.0-mm locking screws. Group 6 were fixed with one six-hole titanium standard miniplate with monocortical 2.0-mm × 6.0-mm screws.
After fixation, the specimens were mounted on a testing machine (model DL2000; EMIC, São José dos Pinhais, PR, Brazil), which was based on a biomechanical cantilever-bending model that simulates the masticatory forces, and stabilized in the condylar and coronoid areas. An initial 5-N load was applied to standardize the test requirements; the machine was then reset. A linear load in the premolar region was applied to the hemimandibles at a displacement speed of 1 mm per min. The resistance forces needed to displace the distal segment 1 mm, 3 mm, and 5 mm were recorded in Newtons (N) and the data were transmitted from the load cell to a computer.
One-way analysis of variance (ANOVA) with Tukey’s post hoc test was performed to compare the means between groups.
Results
The data were distributed parametrically (Kolmogorov–Smirnov test). A summary of the results – including the means, standard deviations, and 95% confidence intervals for all of the groups and testing conditions – is presented in Table 1 and Fig. 3 .
| Amount of displacement | Two 2.0-mm four-hole miniplates | One 2.0-mm four-hole miniplate | One 2.0-mm four-hole miniplate + bicortical screw | Grid miniplate | Four-hole locking miniplate | Six-hole miniplate |
|---|---|---|---|---|---|---|
| 1 mm | 13.2 ± 3.9 (10.4–16.0) | 9.4 ± 3.2 (7.1–11.7) | 18.1 ± 5.3 (14.3–21.9) | 16.1 ± 4.9 (12.6–19.5) | 8.7 ± 2.7 (6.7–10.6) | 10.4 ± 3.2 (8.1–12.7) |
| 3 mm | 32.1 ± 8.4 (26.0–38.1) | 16.9 ± 3.0 (14.8–19.1) | 40.9 ± 8.2 (35.0–46.8) | 38.1 ± 7.2 (33.0–43.3) | 24.6 ± 4.1 (21.6–27.5) | 21.0 ± 3.3 (18.6–23.4) |
| 5 mm | 48.4 ± 8.6 (42.2–54.6) | 20.1 ± 2.3 (18.5–21.8) | 55.4 ± 9.6 (48.5–62.3) | 49.9 ± 8.8 (43.5–56.2) | 34.9 ± 5.8 (30.7–39.0) | 23.2 ± 3.7 (20.6–25.9) |
For 1 mm of displacement, the results obtained for one 2.0-mm plate differed significantly from those obtained for the 2.0-mm plate + screw and the grid plate ( P < 0.0001 and P < 0.01, respectively). The 2.0-mm plate + screw results also differed significantly from those for the locking plate and the six-hole plate (both P < 0.0001). Lastly, the results for the grid plate differed significantly from those for the locking plate ( P < 0.01) and six-hole plate ( P < 0.05).
For 3 mm of displacement, the outcome using two 2.0-mm plates differed significantly from those for one 2.0-mm plate ( P < 0.0001), one 2.0-mm plate + screw ( P < 0.05), and the six-hole plate ( P < 0.01). The 2.0-mm plate results also differed significantly from those for the 2.0-mm plate + screw and grid plate (both P < 0.0001). In addition, the outcome for the 2.0-mm plate + screw differed significantly from those for the locking plate and six-hole plate (both P < 0.0001), and the results for the grid plate differed significantly from those for the locking plate and six-hole plate (both P < 0.0001).
For 5 mm displacement, the results for two 2.0-mm plates differed significantly from those for one 2.0-mm plate ( P < 0.0001), the locking plate ( P < 0.01), and the six-hole plate ( P < 0.0001). The results for the 2.0-mm plate also differed significantly from those for the 2.0-mm plate + screw, grid plate, and locking plate (all P < 0.0001), and the outcome for the 2.0-mm plate + screw differed significantly from those for the locking plate and six-hole plate (both P < 0.0001). Finally, the results for the grid plate differed significantly from those for the locking plate and six-hole plate (both P < 0.0001), and the result for the locking plate differed significantly from that of the six-hole plate ( P < 0.01).
For all of the test conditions, 3 mm of displacement led to higher values than 1 mm of displacement ( P < 0.0001), and 5 mm of displacement led to higher values than both 1 mm and 3 mm of displacement ( P < 0.0001). In addition, for the three displacement conditions, there was a strong tendency for the 2.0-mm plate + screw and the grid plate to have the higher values.
Results
The data were distributed parametrically (Kolmogorov–Smirnov test). A summary of the results – including the means, standard deviations, and 95% confidence intervals for all of the groups and testing conditions – is presented in Table 1 and Fig. 3 .
