The aim of the present study was to evaluate the lingual fracture patterns after sagittal split osteotomy (SSO) using short and long medial osteotomy cuts, via three-dimensional (3D) cone beam computed tomography (CBCT). Forty-six subjects participated in this prospective study. Two types of medial osteotomy line were made: for type I, the medial osteotomy line was finished just before the lingula; for type II, the medial osteotomy line was extended 3–4 mm beyond the lingula. Three fracture patterns were observed after SSOs. There were no significant differences in the medial fracture patterns between the two medial osteotomy types ( P = 0.16). The buccolingual thickness of the ramus was lower in fractures with pattern 3 (bad split) than in the two other fracture patterns. The length of the medial osteotomy line – short or long – did not alter the prevalence of a bad split. The bone thickness of the ramus may affect the type of fracture pattern on the medial side of the ramus.
The sagittal split osteotomy (SSO) of the mandible is a versatile and reliable operation used to change the mandibular position. Surgeons make an osteotomy at various sites and of various lengths on the medial and lateral aspects of the mandible. Since its introduction, many modifications of the original technique have been described in an attempt to decrease the risk of bad splits, to avoid non-union, and to prevent trauma to the inferior alveolar nerve. Originally, two osteotomy models were suggested for the lingual side of the ramus in SSO, namely a long osteotomy line that extends beyond the lingula and finishes at the posterior border or near it, and a short osteotomy line that ends approximately at the lingula.
The identification of the various patterns of medial fracture may be important to prevent bad splits and nerve injuries. Splits of the mandibular ramus are mainly evaluated using conventional radiographic techniques, such as orthopantomography (OPG). However, this method does not allow precise observation of the split lines. Patterns of lingual fracture after SSO have been studied using conventional three-dimensional (3D) computed tomography (CT) scans. Cone beam CT (CBCT) produces two-dimensional (2D) images that may be reconstructed into 3D. CBCT has a lower radiation dose compared with a conventional CT scan. Choosing an appropriate medial osteotomy model to prevent a bad split is desirable.
The aim of the present study was to evaluate the lingual fracture patterns and occurrence of bad splits after SSO via CBCT.
Materials and methods
This prospective study was performed between 1 September 2010 and 31 October 2012; approval was obtained from the medical ethics committee of the study institution. Subjects eligible for inclusion had a skeletal deformity (class II or III) and required an SSO to correct it. Subjects were excluded from enrolment in the study if they had experienced a previous trauma or fracture in the mandible, had undergone a previous osteotomy, had an asymmetry, or had an impacted mandibular third molar tooth. Subjects were informed of the study protocol and signed a consent form for the use of CBCT scans taken 1–2 days after surgery; all surgeries were performed by a maxillofacial surgeon (the second author).
CBCT scans were done using a NewTom VGi CBCT unit (ImageWorks, Verona, Italy). A standardized NewTom protocol was used, comprising an extended field of view (FOV) (15 × 15 cm) with 0.3-mm slice thickness and 26.9 s acquisition time. The raw images were exported using NewTom software (NNT viewer) into DICOM multi-files. Image processing software was used for measurement analysis (Mimics Innovation Suite, version 15; Materialise, Leuven, Belgium).
The process was started by importing CBCT images of patients in DICOM format into the software, followed by segmentation of the region of interest (ROI) ( Fig. 1 ). The ROI selected in the segmentation process was converted into a 3D model. The 3D measurement tool of the software was used to measure the following items: (1) the line from the osteotomy to the inferior border of the mandible in the medial ramus, (2) the anterior border to the posterior border of the ramus at the level of the lingula, (3) buccolingual thickness of the ramus, (4) lingula to the line of segmentation, and (5) length of the medial osteotomy cut ( Fig. 1 ).
An incision was made over the anterior portion of the vertical ramus, extending to the mesial aspect of the first molar. Sub-periosteal dissection was carried down to the inferior border of the mandible, where a lateral channel retractor was placed. A long bur was used to make a horizontal bone cut through the medial cortex of the ramus, just above the lingula. One of two types of medial osteotomy line was made randomly: type I where the medial osteotomy line was finished just before the lingula, or type II where the medial osteotomy line was extended 3–4 mm beyond the lingula. The vertical cut was made through the buccal cortex, distal to the second molar or further anteriorly. The two osteotomies were then connected with a 701 fissure bur. A spreader and a narrow osteotome were used to lift the lateral cortex and the osteotome was used to step along the connecting cut to ensure that the split stayed close to the lateral cortex.
The statistical analyses were performed using IBM SPSS Statistics for Windows, version 19.0 (IBM Corp., Armonk, NY, USA). The χ 2 test was used to compare the types of fracture, and analysis of variance (ANOVA) was used to assess the bone thickness of the ramus at the lingula. A post-hoc test was used to determine differences when ANOVA showed significant differences.
Forty-six subjects (15 males and 31 females) underwent bilateral SSO. Their mean age was 23.37 ± 4.61 years. The mean length of the lingual osteotomy line was 19.49 ± 2.37 mm. The mean vertical distance from the lingual osteotomy line to the lingula was 5.04 ± 3.31 mm. Ninety-two SSOs were studied in 46 subjects (23 subjects underwent a short medial osteotomy and 23 subjects underwent a long medial osteotomy).
Three patterns of lingual fracture were observed: (1) pattern 1, comprising a fracture line beginning from the end of the medial osteotomy, extending obliquely through the mandibular foramen, and finishing at the mandibular angle; (2) pattern 2, comprising a fracture line beginning from the end of the medial osteotomy, extending obliquely through the mandibular foramen, and finishing at the inferior border; and (3) pattern 3, a fracture line extending randomly in various directions as multiple lines and indicating a bad split of the proximal segment. Figs 2–7 demonstrate the three patterns in the two types of medial osteotomy line.
The mean buccolingual bone thickness was 5.71 ± 0.74 mm in pattern 1 fractures, 5.80 ± 0.65 mm in pattern 2, and 4.88 ± 0.85 mm in pattern 3. Analysis of the data demonstrated a significant difference in buccolingual bone thickness among the fracture patterns ( P < 0.05) ( Table 1 ). Assessment of the data using the post-hoc test showed a substantial difference in buccolingual bone thickness in pattern 3 than in the other two fracture patterns ( Table 2 ).
|Variable factors||Fracture pattern||ANOVA test|
|Pattern 1||Pattern 2||Pattern 3|
|Buccolingual bone thickness (mm)||5.71 ± 0.74||5.80 ± 0.65||4.88 ± 0.85||P < 0.05|
|Anterior–posterior width of the ramus (mm)||30.08 ± 3.41||30.10 ± 3.25||30.09 ± 3.38||P > 0.05|
|Vertical distance of the medial cut to the inferior border (mm)||33.84 ± 5.6||32.95 ± 4.8||32.87 ± 5.1||P > 0.05|
|Vertical distance of the medial cut to the lingula (mm)||3.8 ± 4.64||3.85 ± 3.97||3.78 ± 4.2||P > 0.05|
|Patterns||Buccolingual thickness, mm||95% CI
|Pattern 1||Pattern 2||5.80 ± 0.65||0.12|
|Pattern 3||4.88 ± 0.85||0.02|
|Pattern 2||Pattern 1||5.71 ± 0.74||0.15|
|Pattern 3||4.88 ± 0.85||0.01|
|Pattern 3||Pattern 1||5.71 ± 0.74||0.02|
|Pattern 2||5.80 ± 0.65||0.01|