Computer Guided Generated Dual-Purpose Splint for Bilateral Sagittal Split Osteotomy

Abstract

Purpose

to evaluate the efficacy of dual-purpose computer-generated splint in guiding the proximal and the distal segment in bilateral sagittal split osteotomy.

Patients and Method

It was a prospective case series study directed on 8 class III patients indicating the need of maxillary advancement and mandibular set back by bilateral sagittal split osteotomy. A CAD/CAM splint is generated to guide the distal segment to the stable maxilla and at the same time a grooved extension to engage the proximal segment ensuring the condyle in its planned position during fixation. The primary outcome was measured by calculating the difference between the pre- and post-operative condylar segment position.

Results

The present study included five female patient and three male patient with mean age of 28.4 ± 5.1 years. The accuracy of the splint in positioning the mandibular proximal segment showed promising results ranging from 2.59 to 0.49.

Conclusion

The dual-purpose splint introduced in this study showed satisfied results in maintaining the pre-operative condylar position while securing the distal segment in the desired plan.

Introduction

Orthognathic surgery is based on the accurate identification of the original dentofacial skeleton position and the estimation of the required final installation [1, 2]. Skeletal stability and the exact position of the condyle after the rigid fixation of bilateral sagittal split osteotomy (BSSO) is considered of prime importance [3]. The mal positioning of the condylar segment during sagittal split fixation could impact the mandibular stability and the desired post-operative outcomes [4]. The alteration of condylar orientation could be the causative factor of several drawbacks including; malocclusion, TMJ dysfunction, inefficient mastication and finally relapse and condylar resorption [5, 6].

The investigation of the role of condylar positioning devices (CPDs) has been taken into consideration in the literature. The effect of muscle relaxant, the recumbent position of the patient, method of fixation and finally the amount and direction of mandibular movement are the main factors that support the clinical usage of CPDs to prevent skeletal instability and temporomandibular disorders [7]. However, CPDs have some drawbacks as; extended operating times, the stability of the intermaxillary fixation is mandatory during their use, the process depends on the experience of the surgeon during bone segment reposition, the rotation of the mandibular ramus and additional expenses [8].

Computer guided surgery has gained a foothold in orthognathic surgery where a 3 D illustration of the intended final position of the bony segments are presented [9]. CAD/CAM generated splints were assessed as being accurate, reproducible, not time consuming and cost effective [10–12]. The computer aided surgical simulation (CASS) allows accurate plan execution and transmission to the operating theater using surgical splint and pre-bent or customized plates [13, 14]. It permits precise positioning of the maxilla and mandible irrespective to the sequencing in cases of bimaxillary surgery [15, 16]. The three dimensional (3D) repositioning of the condylar segment after BSSO could be achieved using CASS, the condyle is secured to its original place at the glenoid fossa; anteroposteriorly, mediolaterally and vertically [17, 18]. The aim of this study is to present a dual-purpose splint with occlusal part to guide the mandibular distal segment to its final position and a posterior grooved extension that guarantee the position of the proximal segment.

Patients and Methods

Study Design, and Patient Selection

This study was approved by Cairo University Hospital institutional review board and followed Helsinki guidelines. It was a prospective case series conducted on 8 consecutive patients selected from Oral and Maxillofacial Surgery Department, Faculty of Dentistry, Cairo University. Patients were selected according to the following criteria: patients with class III malocclusion (horizontal mandibular excess) indicating the need of mandibular setback and maxillary advancement; with no signs of temporomandibular joint disorder; free from any systematic disease contraindicating the surgical procedures. All patients received preoperative orthodontic treatment, and were prepared for mandibular setback using bilateral sagittal split osteotomy.

Preoperative Preparation

Virtual Planning

Computed tomography (CT) was performed for the mandible and the midface using a multi-slice helical CT machine (I-CAT®PreciseTMfrom I-CAT®Technology, Hatfield, PA), with1 mm slice thickness image acquisition, 0.625 mm slice increment, 0.3 mm voxel size,17 X 23 cm extended field of view, 120 kV, 5 mA and 4 m exposure time. CT DICOM (digital Imaging and Communication in Medicine) files were imported to the planning software (Mimics 1.0, Materialise NV, Leuven, Belgium). Bony structures of the mandible and the midface were virtually segmented and separated using the software through a series of segmentation and simulation processes. Digital casts obtained from 3D scanning of plaster dental casts were imported and registered over the dental arches of the virtual 3D skull model to generate an artifact-free composite/dentition skull model.

The second step was virtual surgical planning of the planned bimaxillary orthognathic surgery. The virtual (simulation) surgery was performed for the mandible. An intermediate dual-purpose splint was constructed for mandible first movement guided by the planned final position of the mandible and the original position of the maxilla. The virtual splint has occlusal part that guide the mandibular distal segment to its final position and a posterior grooved extension that guarantee the position of the proximal segment (Fig. 1). Then, the simulation surgery was performed for the maxilla, and the final virtual splint was constructed.

Fig. 1.

a The intermediate dual-purpose splint constructed for mandible first movement guided by the planned final position of the mandible and the original position of the maxilla. b occlusal view of the splint and the grooved extension engaging the proximal and distal segment. c. occlusal view of the dual-purpose splint after prototyping

Rapid Prototyping and Preoperative Preparation

Stereolithographic (STL) files of the splints, the mandible, and the mid-face with the maxilla in their final positions were exported to multi-jet modelling printing machine (InVision Si2, 3D Systems e Rock Hill, SC), and fabricated using plastic material (VisiJet SR 200, 3D Systems e Rock Hill, SC). The mandibular and mid-face models were used for preoperative bending of the titanium plates (Fig. 2).

Fig. 2.

Titanium plates adapted at the stereolithographic model of the mandible, and mid-face in their final position

Surgical Procedures

Surgical procedures were performed under general anesthesia using nasotracheal intubation. Incision was done 5 mm away from the attached gingiva and extended along the external oblique ridge and the anterior border of the ascending ramus. Dissection was carried in subperiosteal plan to expose the lateral aspect of the ramus till the posterior border and the medial one above the mandibular foramen. The sequence of operative procedure was performed in an altered sequence starting with the mandible. BSSO was accomplished following the steps described by Trauner and Obwegeser (1957) and the late modification by DalPont (1961) and Epker (1977). After releasing the distal and proximal segment of the mandible, the former was guided to the desired position by the occlusal part of the dual splint and the later was guided to the original anatomic position of the condyle by the posterior grooved extension. Intermaxillary fixation was done while the splint securing the position of distal and proximal mandibular segments, and they were fixed in the new position by the pre bent plates, screws (Fig. 3). Then, Le fort I maxillary osteotomy was carried out and the maxilla was repositioned guided by the final splint and incisions were sutured.

Fig. 3.

a The intermediate dual splint guide ensuring the final position of the proximal and distal segments with intermaxillary fixation. b The proximal and distal segment fixed in final position by the pre-bend plate

Follow up and Outcomes

Postoperative CT was performed for all the patients to assess proper positioning of the mandible. The accuracy of the splint to position the mandibular proximal segments was considered as the difference between proximal segment position before and after the surgery. The following anatomical planes and points were located and reconstructed on the 3D reconstructed image (Fig. 4). Four anatomical points were located as follows: Condylon (CO), most prominent point of the condyle; Coronoid (Cr), most prominent point of the coronoid; Gonion (GO), mid-point of the gonial angle; lingula (Li), highest point of the lingual. Sagittal plane, Frankfort horizontal (FH) plane and Coronal plane were reconstructed on the 3D image. The distance was measured from each of the four points to the three planes in the postoperative and preoperative CT for each patient. The accuracy of the splint to position the mandibular proximal segments was calculated at each point as the difference between pre- and post-operative measures. To assess the accuracy of the splint in positioning the distal segment in the planned position, Sella-Nasion-B-point (SNB) and Pogonion-N-Perpendicular (Pog-N-Per) was measured in both the postoperative and planed image, and the difference between the two readings was calculated. The linear and angular radiographic analyses were performed by 2 assessors. Inter-observer agreement was assessed, and the final record was calculated as the average between the two readings. All patients were followed up for 6 months. Occlusion was evaluated clinically for all patients postoperatively and after 6 months. TMJ symptoms (such as clicking, pain, and range of motion) were assessed after 3 and 6 months postoperatively.

Fig. 4.

3D proximal segment assessment. a anatomical point distribution; Co, Cr, Li, Go. b FH plane passing through right porion and right and left orbital. c sagittal plane passing through nasion and perpendicular to FH. d coronal plane passing through nasion and perpendicular sagittal and FH planes.

Statistical Methods

Statistical analysis was performed using SPSS (Statistical package for the social sciences- IBM® SPSS® Statistics Version 20 for Windows, IBM Corp., Armonk, NY, USA). Data were represented as mean ± standard deviation. Data were explored for normality using Kolmogorov- Smirnov and Shapiro–Wilk tests. Spearman correlation coefficient was used to assess inter-observer agreement. The results were considered statistically significant if the p value was less than 0.05.

Results

This study was conducted on 8 patients (3 male, 5 female) with mean age of 28.4 ± 5.1 years. In all cases, the splint fitted the dentition; the surgical procedures were uneventful. Postoperative clinical examination showed stable occlusion achieved for all cases, with no significant TMJ symptoms. Radiographic assessment of proximal and distal segments showed promising results. The accuracy of the splint to position the mandibular proximal segments ranged from 0.32 to 1.75 mm at different points (Table 1). The accuracy of the splint to position the mandibular distal segments was 0.38 ± 1.12° for SNB, and − 0.51 ± 1.08° for Pog-N-Per. Correlation coefficient showed perfect inter-observer agreement (coefficient 0.91, P value < 0.05).

Table 1.

Accuracy of the splint in positioning the mandibular proximal segment (in mm)

	Condylon	Coronoid	Gonion	lingula
Anterior coronal plane	1.56 ± 0.79	1.75 ± 0.85	1.07 ± 1.13	1.04 ± 2.15
Sagittal plane	1.39 ± 1.37	− 0.98 ± 2.16	1.38 ± 2.52	1.21 ± 2.16
FH plane	− 0.47 ± 1.34	0.32 ± 2.23	− 1.02 ± 1.54	− 0.77 ± 1.57

Discussion

Rigid fixation of bilateral sagittal split osteotomy could be confronted with the risk of mal positioning of the proximal segment or displacement of the condyle from its normal anatomical position [19, 20]. Special concern should be given during surgery to prevent the distraction of the condyle from its seated position in the glenoid fossa [21–23]. The first reported positioning device was introduced by Leonard [24] in 1976. Since then, numerous methods have been used to secure the condyle in its ideal centric relation position ranging from manual methods to various positioning devices and plates [8, 25–29]. The need of (CPDs) to avoid skeletal instability and temporomandibular dysfunction have been questioned in the literature frequently. Ellis raised this question in 1994 and left a sense of doubts about their inevitability. However, the recent techniques of CPDs are taking priority over the conventional methods to enhance their accuracy. Lands [27] compared the intraoperative sonography by the traditional methods and proved the superiority of the dynamic positioning. The same idea was praised by Bettega et al. [18], who two years ahead demonstrated the clinical returns of computer assisted 3D condylar repositioning.

In the present study the dual-purpose splint (DPS) has been used as an intermediate splint and condylar positioning device. The DPS was applied to secure the dentate segment in the desired position and to ensure the position of the proximal segment to overcome the risk of condylar distraction from its original position.

Changes in condylar position after orthognathic surgery procedures are contributed to several factors [30, 31]. To accurately test the validity of the DPS different variables were unified among the present study. The sequence of bimaxillary orthognathic surgery was shifted to mandible first to limit the effect of errors that could occur during intermediate splint fabrication and maxillary movement. The direction of mandibular movement was set back to ensure that the proximal segment is the first contacting the V shape extension or the slot in the splint. In all cases, the fixation method was done by pre-bended 3D plate and monocortical screws to achieve maximum strength and maintaining malleability [32]. Evaluation of the proximal segment bodily movement and the spatial changes of the condylar position was accomplished in a detailed way. Points were distributed all over the ramus and identification of the amount of rotation, translation and deviation of the condyle was assessed, respectively, in relation to FH, coronal and sagittal planes.

The dual-purpose splint (DPS) used in this study showed non-significant changes of condylar position from its original position in sagittal, coronal and axial planes. Those findings were convenient with Lee et al. [33], who used computer assisted Simulation Surgery (CASS) to produce computer generated splint for the management of proximal segment. He evaluated the accuracy of the splint by the superimposition of the postoperative simulated surgical image with postoperative CT image. The results of the present study were also comparable to those introduced by Cortese et al. [34]; however, he evaluated the centric relation of the condyle within the glenoid fossa. In addition, the splint used by Cortese had both the tooth and bone born parts separated while the current splint was manipulated as single unit.

The CASS applied to generate the DPS eliminated the errors that could arise from the traditional maneuver and permitted accurate positioning of the distal segment in the pre-planned position. The DPS when compared to other CPDs is considered a simple and inexpensive method that brings the surgeon out of the dilemma of whether to use a CPD or not.

Within the limitations of this pilot study, the dual- purpose splint showed promising results regarding condylar positioning in mandibular set back, in addition to securing the distal segment in pre-planned position. However, the major limitation of this study is the relatively small sample size as we recommend conduction of further studies.

Funding

This study is not funded.

Declarations

Conflict of interest

The authors declare that there is no conflict of interest (Mamdouh Ahmed declares that there is no conflict of interest, Sherif Ali declares that there is no conflict of interest and Sara Soliman declares that there is no conflict of interest).

Ethical Approval

All procedures performed in this study were in accordance with the ethical standards of Cairo University research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.