Biomechanical evaluation of different osteosynthesis methods after mandibular sagittal split osteotomy in major advancements

A New Approach to Virtual Occlusion in Orthognathic Surgery Planning Using Mixed Reality-A Technical Note and Review of the Literature

Orthognathic surgery plays a vital role in correcting various skeletal discrepancies of the maxillofacial region. Achieving optimal occlusion is a fundamental aspect of orthognathic surgery planning, as it directly influences postoperative outcomes and patient satisfaction. Traditional methods for setting final occlusion involve the use of dental casts which are time-consuming, prone to errors and cannot be easily shared among collaborating specialties. In recent years, advancements in digital technology have introduced innovative approaches, such as virtual occlusion, which may offer enhanced accuracy and efficiency in orthognathic surgery planning. Furthermore, the emergence of mixed reality devices and their 3D visualization capabilities have brought about novel benefits in the medical field, particularly in computer-assisted planning. This paper presents for the first time a prototype tool for setting virtual occlusion during orthognathic surgery planning using mixed reality technology. A complete walkthrough of the workflow is presented including an explanation of the implicit advantages of this novel tool. The new approach to defining virtual occlusion is set into context with other published methods of virtual occlusion setting, discussing advantages and limitations as well as concepts of surgical occlusion for orthognathic surgery.

Biomechanical Evaluation of Seven Fixation Methods for Sagittal Split Ramus Osteotomy with Four Advancement Levels by Finite Element Analysis

Background

Mandibular sagittal split ramus osteotomy (SSRO) is a routine surgery to correct mandibular deformities, such as mandibular retrusion, protrusion, deficiency, and asymmetry. However, nonunion/malunion of the fragments and relapse caused by fixation failure after SSRO are major concerns. Rigid fixation to maintain postosteotomy segmental stabilization is critical for success. Additionally, understanding the biomechanical characteristics of different fixation methods in SSRO with large advancements is extremely important for clinical guidance. Therefore, the aim of the present study was to evaluate the biomechanical characteristics of different SSRO methods by finite element analysis.

Methods

SSRO finite element models with 5-, 10-, 15-, and 20-mm advancements were developed. Seven fixation methods, namely, two types of bicortical screws, single miniplate, dual miniplates, grid plate, dual L-shaped plates, and hybrid fixation, were positioned into the SSRO models. Molar and incisal biomechanical loads were applied to all models to simulate bite forces. We then investigated the immediate postoperative stability from four aspects, namely, the stability of the distal osteotomy segment, osteotomy regional stability, stress distribution on the mandible, and implant stress performance.

Results

The stability of the distal osteotomy segment and osteotomy region decreased when the advancement increased. All seven fixation methods displayed favorable biomechanical stability under minor advancement (5 mm). With large advancements, bicortical screws, dual miniplates, and grid plates provided better stability. The von Mises stress was concentrated around the screws close to the osteotomy region for the proximal segment for all fixation methods, and the von Mises stress on implants increased with larger advancements. With small advancements, five fixation methods endured tolerable maximum stresses of <880 MPa. A single miniplate and dual L-shaped plates generally suffered high stresses using larger advancements. The biomechanical characteristics were similar under molar and incisal loads.

Conclusions

The current study investigated the biomechanical properties of seven fixation devices after SSRO under molar and incisal loads. Generally, bicortical screws, grid plates, and dual miniplates provided better biomechanical stability using finite element analysis.

Does the type of sagittal split ramus osteotomy influence fixation strength? Evaluation of the mechanical behavior of two types of fixation used in three types of sagittal split ramus osteotomy

PURPOSE

This study compared the mechanical behavior of two fixation techniques used in three sections representing the sagittal split ramus osteotomy (SSRO) in polyurethane replicas that were divided into groups, according to type of section, and sub-groups according to type of fixation, simulating 11-mm advancement and 6º clockwise mandibular rotation.

METHODS

Loads were applied in two regions, aiming at progressive application and consequent strength value, measured in kilogram-force in displacements of 1, 3, 5, and 7 mm, from the load application tip. Shapiro-Wilk test was performed, followed by two-way analysis of variance (ANOVA-2 way), and Bonferroni's multiple comparison.

RESULTS

The results showed no statistically significant difference in the type of section and type of fixation used when load was applied to the inter-incisor region. However, when load was applied to the first molar region, statistically significant difference was observed in 1-mm displacement, in which section described by Epker with two modifications showed greater strength, regardless of type of fixation used (p = 0.007).

CONCLUSION

In the application of load in the inter-incisor region, there was no statistical difference between the type of osteotomy and the type of fixation used. When applying loads to molars, there was a difference for the type of osteotomy, where the Epker osteotomy with 2 modifications presented greater resistance, regardless of the type of fixation used.

Comparison of bicortical, miniplate and hybrid fixation techniques in mandibular advancement and counterclockwise rotation: A finite element analysis study

OBJECTIVE

This study aims to evaluate biomechanical stability and stress distribution of five different fixation types with finite element analysis using 10-mm advancement with or without counterclockwise rotation of the mandible.

MATERIALS AND METHODS

After sagittal split osteotomy, 10-mm advancement was performed in the first group and 10-mm advancement and 10-degree counterclockwise rotation were performed in the second group. One miniplate (M-1), two-miniplate (M-2), one miniplate and a bicortical screw (H), l-shaped bicortical screw (B-1), and inverted l-shaped bicortical screw (B-2) systems were placed. Totally, 120 N force was applied to the models at a 45-degree angle from the lower edge of the symphysis.

RESULTS

The highest values on fixation were seen with miniplate, while the mean values were obtained with bicortical screw system. The highest values on bone were achieved using bicortical screws. One miniplate (M-1) showed both the highest and mean displacement. The highest values in counterclockwise-rotated models increased in all parameters, compared to non-rotated models.

CONCLUSION

In cases in which passive alignment between segments and adequate bone contact are ensured, inverted l-shaped bicortical screw, two-miniplate, or hybrid systems are recommended.

Stability of fixation methods in large mandibular advancements after sagittal split ramus osteotomy: an in vitro biomechanical study

Sagittal split ramus osteotomy (SSRO) with large mandibular advancements is a common surgical procedure and could be indicated for patients with sleep apnoea. As a large variety of fixation methods is used for the stabilisation of SSRO, a biomechanical test model was used to analyse which fixation technique was most stable. For this in vitro study, 80 polyurethane hemimandibles with a prefabricated SSRO were used as substrates. Loads in Newtons were recorded at displacements of the mandibular incisive edge at 1mm, 3mm and 5mm. The samples were divided into two groups: mandibular advancements of 10mm and 15mm. In both groups, four fixation techniques were used: (A) one four-hole miniplate; (B) two four-hole miniplates; (C) one four-hole miniplate plus one bicortical screw; and (D) three bicortical screws in an inverted-L arrangement. In group 1, three bicortical screws resulted in the best stability, and in group 2, two miniplates resulted in the best stability. The use of two miniplates did not show significant differences between both groups. Other fixation methods showed more stability with 10mm advancements. This study therefore suggests that in SSRO with advancements exceeding 10mm, the use of two miniplates is the optimal means of providing rigid fixation.

Which type of method shows the best mechanical behavior for internal fixation of bilateral sagittal split osteotomy in major advancements with clockwise rotation? Comparison of four methods

PURPOSE

The aim of the present study was to evaluate the four methods for bilateral sagittal osteotomy fixation.

METHODS

In this study, 56 replicas of whole mandibles made of rigid polyurethane were used. After simulation of major advancement (11 mm) with clockwise rotation of the mandible (6^o) in relation to the occlusal plane, the bone segments were fixed with plates and screws of the 2.0-mm system on both the right and left sides: group I, double "H" plate; group II, two mini-plates; group III, "hybrid technique"; and group IV, three bicortical screws in the "inverted L" pattern. The mandibles were submitted to load on the central incisors and right first molar.

RESULTS

The mean value of group I was higher than those of groups IV and II in the displacement of 1 mm (F = 4.705; p = 0.010) with load on the incisor. The mean value of group III was higher than those of groups I and II in the displacement of 1 mm (F = 5.166; p = 0.007) and 3 mm (F = 5.166; p = 0.007). The mean value of group IV was higher than that of group II (F = 3.142; p = 0.044) with load on the molar.

CONCLUSION

Therefore, after the analyses, the hybrid technique was the one that showed the best results.

Fixation methods in sagittal split ramus osteotomy: a systematic review on in vitro biomechanical assessments

The aim of this systematic review was to assess the stability of rigid internal fixation (RIF) techniques in sagittal split ramus osteotomy (SSRO) based on in vitro biomechanical assessments, with particular interest in large mandibular advancements. In general, RIF methods can be divided into three groups: bicortical screws, miniplates, and a combination of the two. An electronic search of the PubMed, CINAHL, and Embase databases was performed, and studies published between January 2003 and March 2018 were screened for inclusion. Comparative studies with an in vitro experimental design, using biomechanical assessments to measure the stability of RIF methods in SSRO, were included. Of 104 unique studies identified in the initial search, 24 were included. Twenty-two of these 24 studies analyzed an advancement of the mandible of 7mm or less. The use of a single four-hole or six-hole miniplate was less stable than the use of bicortical screws, hybrid techniques, double miniplates, or grid plates. Two studies analyzed advancements of 10mm, for which two miniplates placed in parallel and a grid plate showed most stability. Although there was agreement between studies with regard to results, more biomechanical studies are required to quantify the stability of fixation methods in larger mandibular advancements.