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Basic and Clinical Research

Volumetric Evaluation of Safe Zone for Bone Harvesting From Symphysis Region by Using Cone Beam Computed Tomography

Altug, Hasan Ayberk; Coskun, Abdullah Tugrul DDS; Kamburoglu, Kvanc; Zerener, Tamer PhD, DDS; Gulen, Orhan PhD, DDS; Sencimen, Metin; Ozkan, Aydn PhD, DDS

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doi: 10.1097/ID.0000000000000450
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Abstract

Successful implant therapy requires adequate alveolar bone level to obtain both function and aesthetics.1 Endosseous implant placement is relatively difficult and complex in cases where there is inadequate horizontal and vertical bone.2 Insufficient bone volume is considered to be a major limitation factor during dental implantation procedures.3,4 Bone volume is often reduced as a consequence of dental trauma, tooth loss, infectious disease, and advanced periodontal disease.4 Various techniques have been developed to restore inadequate horizontal and vertical bone level, such as onlay bone grafts, distraction osteogenesis, alveolar ridge preservation, bone splitting, and guided bone regeneration.4 Although, several graft materials are used with these techniques, autogenous bone grafting remains the most predictable and accurate technique for bone augmentation procedures.1 Osteogenic potential of autogenous bone grafting is one of the major considerations in promoting it as the gold standard.2,4 Autologous bone grafts can be harvested from intraoral or extraoral sites. The symphysis menti, mandibular ramus, and maxillary tuberosity are among intraoral sites, whereas commonly used extraoral sites are the iliac crest, rib, tibial plateau, and calvaria.2,4,5 The selection of the donor site depends on the level of the bone defect.2 When there is limited jaw opening or temporomandibular joint dysfunction, obtaining grafts from the mandibular symphysis area can be a better alternative to mandibular ramus region; besides, the symphysis graft contains larger cancellous component than that of the mandibular ramus.1,5 To avoid possible risks and render successful surgical outcome, clinicians need to assess the accessibility of the preferred bone graft and the assessment of donor site.5 Cone beam computed tomography (CBCT) is a useful technique for 3-dimensional (3D) assessment of donor site anatomy.1 CBCT systems operate by focusing a cone-shaped beam on a 2-dimensional (2D) detector that performs one pass or less around the patient's head to produce a series of 2D images. A cone beam reconstruction algorithm is then applied to this image data set, allowing the operator to extract planar and curved reconstructions of varying thicknesses in any orientation and to generate accurate 3D images of bone and soft-tissue surfaces that can be read on a personal computer monitor. In clinical practice, CBCT possesses a number of advantages over medical computed tomography (CT), such as lower effective radiation doses, lower costs, fewer space requirements, easier image acquisition, higher image accuracy, and interactive display modes such as multiplanar reconstruction that are applicable to maxillofacial imaging.6 CBCT helps to plan the augmentation procedure, determine the size of the block required, and assess the feasibility of the donor site in the mandibular symphysis. A previous study evaluated the amount of harvestable bone graft in the mandibular symphysis using medical CT.4 The aim of the present study was to retrospectively make a volumetric evaluation of symphysis sites of patients by the use of CBCT.

Methods

After receiving approval from the local ethical committee of the Gulhane Military Medical Academy in December 12, 2014 (approval 47), in compliance with the Helsinki Declaration of 2000, this retrospective study evaluated the dentate symphysis region of mandibles in CBCT scans taken for different reasons which included preimplant surgery, preprosthetic evaluation, and radiographic examination in 90 patients (45 women and 45 men). The mean age of the patients was 46 years (women: 38 years, age range 22–45 years; men: 33 years, age range 20–46 years). For each patient, sex and age were recorded and images of each individual were labeled without disclosure of their names and personal data. 3D data were obtained using a CBCT device Kodak 9000 3D (Eastman Kodak Co., Rochester, NY) CMOS sensor with optical fiber, offering a single 5 × 3.8-cm field of view with voxel sizes ranging from 0.076 to 0.250 mm operated at 70 kVp, 10 mA, and an exposure time of 10.8 seconds (Fig. 1).

Fig. 1
Fig. 1:
The sagittal, coronal, and axial sections of the mandible in CBCT scans.

The CBCT images with artefacts and pathologies in the mandible, edentulous area in the symphysis region and outside the age range were excluded. CBCT images of the mandibles were cropped from the symphysis site. The cropping was made in consideration to the safety zone for harvestable symphysis bone grafts. The safety zone's borders were 5 mm below from the teeth apexes, 4 mm superior to the inferior border of the mandible, 5 mm anterior to the mental foramen, and 4 mm anterior to the lingual cortex of the mandible (Fig. 2). Reconstructed CBCT images were then transferred as digital imaging and communications in medicine files and imported into a volumetric rendering software 3D DOCTOR (Able Software Corp., Lexington, MA) capable of measurements of vector-based segmentation technology for volumetric measurements. The 3D DOCTOR software allows for anatomical segmentation on consecutive axial slices of 0.4 mm thickness, thereby enabling symphysis visualization at every level from the teeth apexes to lingual cortex of the mandible (Fig. 3, A and B). Using a computer mouse, a turquoise border was manually drawn on each slice to delineate the symphysis borders, and the total volume (in cubic millimeters) of the symphysis region was then automatically calculated by the software (Fig. 4). This was performed twice by 1 observer, and the average was taken. Statistical analysis was conducted by t test.

Fig. 2
Fig. 2:
Cropped symphyseal bone graft boundaries on the sagittal, coronal, and axial CBCT slices.
Fig. 3
Fig. 3:
A and B, 3D image of the symphyseal corticocancellous bone graft in Kodak 9000 (A), and the volume measured on the 3D DOCTOR software program (B).
Fig. 4
Fig. 4:
The cortical and cancellous bones are rendered as a 3D image.

Results

The results are summarized in Table 1. The lower bone volume was 1179.94 mm3 in group IV. The higher bone volume was 3943.63 mm3 in group I. The total average bone volume is 2616.45 mm3. Significant differences in the average bone volume were found between group I and group IV (P < 0.001). Higher bone volume was obtained in the male groups (2903.01 mm3) than in the female groups (2329.89 mm3). The highest average bone volume of the symphysis area was in group I (3166.19 mm3).

Table 1
Table 1:
Number, Age, and Sex Distribution of Average Volumetric Measurements of the Symphysis Bone Grafts

Discussion

Autogenous bone grafts are commonly used for reconstruction of bone defects in the maxillofacial region due to their osteoinductive and osteoconductive potential.5 Mandibular symphysis has been considered as the best common donor site to provide adequate bone for reconstruction of bone defects.3 The bone graft obtained from the mandibular symphysis has been used for ridge augmentation, sinus augmentation, and the reconstruction of alveolar clefts.7 The most likely complications which may occur when harvesting the mandibular symphysis are injuries of the inferior alveolar neurovascular bundle, tooth roots, and mental foramen.7 To render a successful operation and to avoid surgical complications, clinicians need to know the morphology and volume of the bone grafts. CBCT is a good candidate for the 3D assessment of high-contrast structures in the oral region with a significantly reduced radiation dose.8

Previous studies suggested using the mandible with 5-mm safety margins, caudal to the expected position of the mandibular dentition, anterior to the position of the mental foramen, and cephalad to the inferior border of the mandible, as the borders of symphysis region.4,7 Similarly, we used the mentioned borders for the quantitative evaluation of the mandibular symphysis region.

Yavuz et al5 were the first to evaluate the volume of the mandibular symphysis bone grafts by using medical CT. They calculated an average bone volume of 3491.08 mm3. Möhlhenrich et al4 also used medical CT scans and calculated an average bone volume of 4026.25 mm3. We used CBCT and found similar results as shown in Table 1. CBCT has replaced medical CT for most dental diagnostic tasks. Similarly, we used CBCT for the quantitative evaluation of the mandibular symphysis region in our study.

Moshfeghi et al9 reported that the symphyseal height and depth were also found to be greater in females.

The volume of the mandibular symphysis shows increase with growth and development. Krarup et al10 showed that the symphysis menti increased in anteroposterior width, primarily as a consequence of bone apposition. In a similar study, Kingsmill et al11 reported that age could be a potential factor affecting osseous density and cortical thickness. Bone density and thickness increased with age and dentate status.10 In our study, similar results were shown in Table 1. A different study showed that patients older than 33 years had less mandibular bone density and volume.12 Bone density and volume had decreased with age, and the distances between the anatomical markers may vary.

The thicknesses of the CT slices are very important for evaluating the bone volume. Quirynen et al13 have suggested that thinner CT slices increase the accuracy of the measurements. Therefore, in our study, axial slices of 0.5 mm thickness were used. Also, by using CBCT instead of medical CT, we had the advantage of making reconstructions with small and isotropic voxels that resulted in higher quality images.

A previous study used Mimics software for measuring the volume of harvestable symphysis bone graft on the 3D CT scans.5 Another study used CAD software program (AutoCAD).14 We used a vector-based, 3D DOCTOR software program to calculate the bone volume of symphysis grafts from CBCT scans. According to company information, 3D Doctor uses a unique vector-based technology for better 3D mesh model creation and easy editing. In addition, surface model uses smaller number of triangles while maintaining all details for high-quality rapid prototyping applications.

The mandibular symphysis region has often been used as a donor site in maxillofacial surgery and oral implantology. Using the mandibular symphysis as a donor site proposes ease of access, good bone quality, and a corticocancellous block graft morphology.15 We suggested that the use of 3D CBCT in combination with a software program is a dependable means of measuring the volume of the symphysis bone graft. We find that the use of CBCT in combination with the 3D DOCTOR software program is a dependable means of measuring the volume of the symphysis bone graft.

Conclusions

Mandibular symphysis quantitative dimensions were found to vary according to patient age and sex. A detailed CBCT image analysis of the symphysis morphology should be included as part of preoperative symphysis graft operation. Moreover, third party software was found to generate 3D models that provide useful information about the shape, size, and volume of the graft procedures before implant placement.

Disclosure

The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.

Approval

This study was approved by the local ethical committee of the Gulhane Military Medical Academy in February 12, 2014.

References

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Keywords:

symphysis graft; mandible; volumetric analysis

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