|Year : 2022 | Volume
| Issue : 3 | Page : 443-448
Biomechanical evaluation of different fixation techniques following modified sagittal split ramus osteotomy using buccal step: An in vitro study
Pratik Dhananjay Warade, Rajesh Kshirsagar, Prasamita Mishra, Pranave Prasad, Amruta Sardeshmukh
Department of Oral and Maxillofacial Surgery, Bharati Vidyapeeth Dental College and Hospital, Pune, Maharashtra, India
|Date of Submission||12-Feb-2021|
|Date of Acceptance||21-Oct-2021|
|Date of Web Publication||10-Dec-2022|
Dr. Pratik Dhananjay Warade
Department of Oral and Maxillofacial Surgery Bharati Vidyapeeth Dental College and Hospital, Pune, Maharashtra
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Objective: This study was to evaluate the efficacy of three different designs of rigid fixation using polyurethane hemimandible replicas following modified sagittal split ramus osteotomy with buccal step.
Materials and Methods: Forty-eight polyurethane hemimandible replicas have been used. Following the osteotomy and fixation, each specimen has been mounted on a Universal Testing unit using a prefabricated customized jig and has been tested to a range of vertical forces starting from 0 N on the incisal edge of lateral incisor and lateral forces of on lateral surface of mandibular first molar. Progressive load has been applied until a deflection of 10 mm was observed. SPSS version 20 has been used for analyzing the data. The biomechanical stability of different osteosynthesis has been compared using a one-way ANOVA with post hoc Bonferroni test. P ≤ 0.05 will be considered statistically significant.
Results: After advancement of 7 mm, comparison of vertical load and lateral load among different miniplate systems showed that load was maximum in bicortical screws in inverted L pattern followed by T-shaped miniplate system and was least in single six-hole straight plate (0.001) at P ≤ 0.05.
Conclusion: Our study concludes that bicortical screws in inverted “L” pattern are more resistant to lateral load as well as on the application of vertical load when compared to other fixation techniques and statistical difference was evident.
Keywords: Buccal step, compressive force, modified osteotomy, orthognathic, T-shaped miniplate, tensile strength
|How to cite this article:|
Warade PD, Kshirsagar R, Mishra P, Prasad P, Sardeshmukh A. Biomechanical evaluation of different fixation techniques following modified sagittal split ramus osteotomy using buccal step: An in vitro study. Natl J Maxillofac Surg 2022;13:443-8
|How to cite this URL:|
Warade PD, Kshirsagar R, Mishra P, Prasad P, Sardeshmukh A. Biomechanical evaluation of different fixation techniques following modified sagittal split ramus osteotomy using buccal step: An in vitro study. Natl J Maxillofac Surg [serial online] 2022 [cited 2023 Jan 27];13:443-8. Available from: https://www.njms.in/text.asp?2022/13/3/443/363068
| Inroduction|| |
The bilateral sagittal split osteotomy (BSSO) is recognized as a standard modality for treatment of most mandibular deficiencies. The first osteotomy by Blair in 1897 of the whole mandibular body for prognathism has given a boon for the new era. Various methods of internal fixation have been used to allow mandibular movements and an early return to function after BSSO. Spiessl first described the use of rigid fixation for the sagittal split osteotomy using transcutaneously placed bicortical compression lag screws, two above and one below the mandibular canal. Later, Steinhäuser placed lag screws one above and two below the mandibular canal. Both methods yielded primary bone healing.
However, some unavoidable situations related to methods of rigid fixation such as displacement of the condyle, irreversible nerve injuries, difficulty in cases of reintervention, and occlusal changes should be studied further.
The effectiveness of internal fixation in maxillofacial trauma and orthognathic and reconstructive surgery is well-documented and clinically accepted. To better understand the mechanics of semi-rigid fixation and to develop improved fixation devices, experimental investigations are often conducted in an in vitro environment. Bovine bone, synthetic polyurethane mandibles, and red oak are the materials of choice to mimic the behavior of cadaveric bone.
When evaluating the functional stability of the fixation by simulating the vectors and load intensity that they will be subjected to, the results may be extrapolated to the clinical conditions. This will require a model simulating the human mandible anatomy. For the present study, a polyurethane hemimandible replica was employed as a substitute for the human mandible during the biomechanical testing.
We intend to perform modified sagittal split ramus osteotomy (MSSRO) with a buccal step laid adjacent to the mandibular second molar area. The design of the buccal step, which was originally presented by Gallo et al. and reported by Wolford et al., creates a horizontal bone ledge superior to the condylar segment that gives a more controlled condylar segment, preventing its alternation and therefore decreasing mechanical adherence. This step [Figure 1] is particularly advantageous for patients with retrognathism because the area of bone contact is increased. The design of the step creates two sites for fixation on the mandibular body.
The most common method of internal fixation following BSSO includes a single straight miniplate or bicortical screws placed in inverted “L” shaped formation. A newer design of a single “T” shaped plate has been introduced by Michelet et al. in 1973.
The MSSRO incorporating a buccal step is a unique modification for major mandibular advancements. It is imperative to provide stable fixation during the healing period to prevent malocclusion and relapse. This study on polyurethane mandibles will provide an insight into the suitability of various fixation techniques employed. This study was designed to evaluate the efficacy of three different designs of rigid fixation using hemimandible polyurethane replicas using MSSRO.
| Materials and Methods|| |
The study has been conducted on 48 polyurethane hemimandibles following approval of the Institutional review board vide letter number BVDU/DCH/1358/2018-19. As this is an in vitro study, where no humans or animals have been subjected to the study, therefore refuting grounds for approval from the institutional ethical committee. The replica matches the average norms of the height and thickness of a normal mandible bone. They have been successfully tested in related studies and shown to be good simulators of the human bone, eliminating many of the variables associated with human cadaveric mandibles and bone from animal resources. The 48 hemimandibles were divided into 3 Groups of 16 each to study different fixation options for MSSRO with buccal step and further divided into two Groups of eight each for vertical and lateral loading, respectively. After the osteotomy, advancement of 7 mm was done. Fixation modalities for the three Groups included a single straight six-hole miniplate (Group A) [Figure 2], bicortical screws placed in inverted L shaped [Figure 3] (Group B), and a single T-shaped miniplate (Group C) [Figure 4], respectively.
After fixation with any one of the three fixation options, each specimen was been mounted on a Universal Testing unit using a prefabricated customized jig. Each specimen was either tested to a range of vertical forces on the incisal edge of lateral incisor or lateral forces of on lateral surface of mandibular first molar. The machine used was the Universal Testing Machine (STS 248, Star testing systems, India) which records the results on the computer, has a 1000 kg load cell (maximum load capacity 10,000 kg), and is sensitive to changes of at least 0.1 mm and 3 mm/min speed in a transverse direction.
Sixteen hemimandibles in each Group were further divided into two equal sets of eight specimens each and exposed to vertical and lateral displacement forces using a Universal Testing Machine [Figure 5] till 10 mm deflection was achieved. Each fixed specimen was mounted on Universal Testing unit using a prefabricated customized jig, and was tested to a range of forces of 10–120 N. Deflection in our study was termed as inferior deviation/displacement of the fixed mandible from the fixed original position [Figure 6] and [Figure 7]. Progressive inferior load was applied till the deflection up to 10 mm was recorded. It was further decided that the force application would be discontinued when 10 mm deflection was recorded.
|Figure 6: Yellow line represents the level of point of application of load, red line represents the deflection at 10 mm after applying the vertical load|
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|Figure 7: (a) Straight six hole miniplate subjected to vertical force on incisal edge of lateral incisor and lateral force on lateral surface of mandibular first molar. (b) Bicortical screw in inverted L pattern subjected to vertical force on incisal edge of lateral incisor and lateral force on lateral surface of mandibular first molar. (c) Bicortical screw in inverted L pattern subjected to vertical force on incisal edge of lateral incisor and lateral force on lateral surface of mandibular first molar|
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| Results|| |
Comparison of vertical load among different fixation systems [Table 1] showed that load required for 10 mm deflection was maximum in bicortical screws in inverted L pattern (17.982 N) for 10 mm of deflection followed by T-shaped miniplate system and was least in single six-hole straight plate.
|Table 1: Comparison of vertical load (n) among all the miniplate systems|
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Comparison of lateral load among different miniplate systems [Table 2] showed that maximum load was resisted by bicortical screws in inverted L pattern for (22.909 N) for 10 mm of deflection followed by T-shaped miniplate system and was least in single six-holed straight miniplate. This difference in lateral load among the three Groups was significant.
| Discussion|| |
Different biomechanical studies have been conducted for evaluating different fixation techniques following mandibular orthognathic surgery. It is well known that the biomechanical function of fixation systems clinically depends on the interaction between plate, screws, and bone; therefore, it is proposed that an appropriate in vitro testing model should investigate fixation systems acting as a unit rather than testing the plate strength alone.
Polyurethane mandibles are widely used in biomechanical studies because of their similarities to the human mandible. Furthermore, the anatomy of human mandible can be replicated in polyurethane mandible. They are easy to obtain, economical, and enable reproducible measurements with biomechanical testing units. In addition, repetition of any tests can be performed without escalation of costs and difficulty in procurement of the sample. In research using biomechanical tests, the main concern is the proper simulation of the action of masticatory muscles during mandibular movement., Polyurethane mandibles were chosen for this study to standardize the samples and to avoid problems that occur with cadaver or animal mandibles. In addition, there are no ethical issues as animals are not sacrificed for study purposes. Major mandibular advancement is known to be an unsteady procedure due to the lack of bone contact between mandibular segments, prevalent muscular insertion in the submandibular region, and extreme stretching of those muscles during counterclockwise rotation.
Several clinical studies have reported that a single plate is sufficient to provide stability in mandibular advancement., However, using a single miniplate on an osteotomy site confers a 1-point fixation scheme, this point had been shown to become the area where stresses from occlusion or surrounding musculatures gather; therefore, failure of the fixation is not a remote possibility, particularly with large advancements.,, A report made by Ellis and Esmail in 2009 introduced three cases of fixation failure after a sagittal split advancement fixed using a standard single straight miniplate. For each case, they noticed a malocclusion formed within a 3-month period, and plate deformation was confirmed radiographically. They attributed the failure to the muscle forces that were exerted on the fixation site after advancement. We stabilized all specimens in “2 point fixation” system at the proximal segment of osteotomized mandible and distal segment was allowed for free movements. It simulated the articulation of mandible at the proximal end. In case of “3 point fixation”, hemimandibles are rigidly stabilized at proximal as well as distal segment and load is applied at predesigned position, usually the body of the mandible. Here, stabilization and application of load is achieved at three different points.
Most commonly vertical forces are directed to the central fossa of the mandibular first molar or the incisal edge of the central and lateral mandibular incisors, and lateral forces are applied on the lateral surface of the first mandibular molar or the interdental area between the first and second mandibular molars, replacing the flexional and torsional forces, respectively, found during mastication.,
In this study, MSSRO with buccal step has been selected. The design of the buccal step creates a horizontal bone ledge superior to the condylar segment that gives more control over the condylar segment, preventing its rotation and therefore reducing mechanical resistance. This step is predominantly advantageous for patients with retrognathism because the vicinity of bone contact is increased. From a clinical aspect, it is highly recommended to direct the horizontal cut of the buccal step parallel to the selected movement of the distal segment as simulated on paper. This is primarily helpful in maintaining condylar segment alignment and ultimate fitting at the buccal step, particularly when attempting advancements accompanying clockwise rotations, for example, in Class II deep bite cases or mild counterclockwise rotations (i.e., mild open bite deformity).
Bohluli et al. in 2010 state that fixation should be stable in healing period and should resist displacement during masticatory forces. Adequate fixation can provide sufficient resistance to the forces that cause micromovements across the osteotomy site. Rigid fixation provides skeletal stability, fast bony healing, early recovery of mandibular function, and easier airway maintenance. Rigid fixation methods using bicortical screws and miniplates with monocortical screws are widely used in SSRO.
On comparison of results of all the three fixation techniques, it was observed that the fixation technique with bicortical screws in inverted L pattern is the most resistant when subjected to vertical forces. V. A. Pereira Filho conducted a study in which hemimandibles were subjected to vertical compressive loads, by a mechanical testing unit and the result concluded that bicortical screws presented the greatest level of stability and fixation strength followed by conventional miniplates. Despite the great ability of bicortical screws to sustain the masticatory forces as compared with miniplates, the clinical data have not verified any tendency to omit the use of miniplates. This is because, clinically, this sort of attachment has had excellent results. Although it has smaller rigidity, several studies have demonstrated that during the 1st postoperative weeks, a significant reduction in the masticatory forces occurs;, thus, miniplates are able to provide enough stabilization during the initial stages of bone repair.
Van Sickels et al. (2005) reported fixation levels for SSRO of up to 140 N using three bicortical screws and cylinder plates for fixing mandibular models after advancement with loads applied on the anterior teeth.
On comparison of results of all the three fixation techniques, it was observed that the fixation technique with bicortical screws in inverted “L” pattern is the most resistant when subjected to lateral forces. Bernardo Ferreira Brasileiro conducted a study in which the result of the study concluded that the rigid internal fixation technique for sagittal split ramus osteotomies based on three bicortical screws within the inverted L pattern was foremost stable during a laboratory environment. Moreover, the results recommended that installation of a bicortical positional screw in the retromolar region may optimize the resistance of the semi rigid fixation. According to an in vitro study by Foley et al. and an experiment on cadavers by Andary et al., the greatest rigidity was obtained by placing two-position screws above and one below the mandibular canal.
Anucul et al. performed an in vitro study using bovine ribs to compare the strength of monocortical plates with bicortical position screws. An osteotomy was created to replicate a sagittal split and a 5 mm bone breach was produced. Four study groups were created: screw nongap, screw with gap, plate nongap, and plate with gap. Parameters of strength were analyzed by elastic deformation, stiffness ratio, permanent deformation, and breaking load.
The results showed that monocortical plate fixation in bovine ribs provides less rigidity and is more susceptible to deformation than is bicortical position screw fixation.
The advantage of using a T miniplate with MSSRO and buccal step is that the osteotomy is believed to prevent condylar segment rotation. In addition, the surgical movement and osteosynthesis within the region of mandibular body intraorally confers a 2-point fixation effect with a single construct, providing stability for osteotomy site sufficiently over the levels to permit consumption of usual food items during the early postoperative period. Peterson et al. conducted a study on miniplates with different designs, comparing various fixation systems using bicortical screws in an L-shape pattern, conventional straight plates, conventional double-bar plates, and conventional double-Y sagittal plates. The authors classified the loading protocol into two groups (incisor and molar) and used a cantilever model. The results showed better resistance performance for the bicortical screws, and the plates with the different designs were superior to the traditional straight plate. Nevertheless, the technique is more sensitive than the conventional split regarding the proposal of the osteotomy. The potential of having unfavorable splits of the buccal plates remains and requires additional attention. The drawbacks of conventional miniplates, such as screw loosening, need of plate adaptation to the bone, stability problems, and alteration in occlusion, are eliminated or decreased with the introduction of the locking plate/screw system.,
| Conclusion|| |
In our study, we conclude that when using bicortical screws in inverted “L” pattern, it showed maximum resistance at 10 mm of deflection when subjected to lateral as well as vertical forces when compared to single six-hole miniplate and “T” shaped miniplate.
The work presented in this paper was performed in the Department of Oral and Maxillofacial Surgery, Bharati Vidyapeeth Dental College, Pune, India.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2]