Three-dimensional analysis of incisive canals in human dentulous and edentulous maxillary bones
© Fukuda et al.; licensee Springer. 2015
Received: 29 January 2015
Accepted: 13 April 2015
Published: 1 May 2015
The purpose of this study was to reveal the structural properties that need to be considered in dental implant treatment by investigating differences between dentulous and edentulous maxillae in the three-dimensional (3D) microstructure of the incisive canals (ICs) and their surrounding bone.
A total of 40 maxillary bones comprising 20 dentulous maxillae and 20 edentulous maxillae were imaged by micro-CT for 3D observation and measurement of the IC and alveolar bone in the anterior region of the IC.
The Y-morphology canal was most frequently observed at 60% in dentulous maxilla and 55% in edentulous maxilla. There was a significant difference between dentulous and edentulous maxillae in IC diameter and volume, but no significant difference between the two in the major axis of the ICs.
The anatomic structure surrounding the IC has limited area for implant placement. Therefore, where esthetic and long-term maintenance requirements are taken into account, careful attention is needed when setting the placement position. Also, due to the resorption of bone, edentulous maxillae have a different IC morphology from dentulous maxillae, and therefore, a cautious approach is required.
KeywordsAnatomy Dental implant Incisive canal Maxilla Micro-computed tomography Three-dimensional structure
In recent years, with the rapid spread of dental implant treatment and a dramatic increase in accidents, the importance of precise 3D data on the human jaw is ever-increasing [1-3]. Bone resorption after tooth extraction is an unavoidable event, and the morphological features of the jawbone change greatly after tooth loss [4-7]. Moreover, the maxilla has a thinner cortical bone than the mandible and presents with a structure where cancellous bone predominates . Therefore, implants have a lower survival rate than the mandible, and the search for sites suitable for implant placement has become an urgent need . The position of the incisive canal (IC) is one of the most important factors for implant placement in the premaxillary region, not only in terms of survival rate but also from an esthetic point of view [10-17]. The incisive canals tend to increase in size with aging and after tooth extraction . If there is a possibility of perforating the incisive canals during implant placement, then a cortical bone/cancellous bone block graft can be adapted to the incisive fossa without removing the contents of the fossa. The contents are pushed posteriorly, thus allowing proper implant placement .
The ICs are located in the maxillary midline posterior to the maxillary central incisors, and through them descend the nasopalatine nerve and nasopalatine artery, so they are anatomically important structures [20,21]. Mardinger et al. classified IC morphology as observed from sagittal sections into four types: cylinder-shaped, funnel-shaped, hourglass-shaped, and banana-shaped . Song et al. confirmed one to four branches at the midpoint of the length of the IC and made classifications depending on the number of branches. They reported that the major type of canal shape for the IC is Y-morphology, with four other types - spindle-shaped, very large, cystic, and narrowed - existing as minor types . Tolga et al. ran 2D bone measurements on the IC and surrounding bone of the IC and observed the differences between dentulous and edentulous maxillary bone . Accumulating more finely detailed data directly connected to clinical practice necessitates 3D structure analysis of the bone surrounding the IC.
Kim et al. reported on the usefulness of micro-CT in analyzing the internal microstructure of bones . 3D analysis using micro-CT, which has an imaging resolution of 5 μm at the maximum, makes it possible to reveal very complex structural properties, including even the trabecular bone.
The purpose of the present study is therefore to reveal the structural properties that need to be considered in dental implant treatment by investigating the differences in the IC and their surrounding bone structures between dentulous and edentulous maxillary bones.
A total of 20 dentulous human maxillae and 20 edentulous maxillae samples were resected from dried adult skull bones obtained from the Department of Anatomy, Tokyo Dental College. Dentulous jaw samples with maxillary central incisors present on both sides and no midline diastema were selected to ensure they had no tooth loss.
The samples were imaged with a micro-CT system (HMX-225 Actis4; TESCO, Tokyo, Japan). Imaging conditions were as follows: matrix size 512 × 512, tube voltage 120 kV, and tube current 80 μA. The micro-CT imaging intensifier (I.I.) had a 1-in. CCD camera with a 4-in., 16-bit 1,024 × 1,024 scanning line. A total of 1,200 images of raw data were output with this camera. On the basis of this raw data, a back projection method was used to produce 2D slice data.
Region of interest
Measurement was performed by preparing three-dimensional constructed models and thereafter taking a sagittal section at the region with maximum IC diameter. The region of interest was the distance from the anterior nasal spine to the maxillary first premolar. Taken as a reference plane at the time of measurement, the virtual plane was set so as to be a plane connecting the points of the largest bulge of the left/right maxillary tuberosities and the point of the incisive foramen, parallel to the Frankfort plane.
3D construction software (VG Studio; Volume Graphics, Heidelberg, Germany) was used to observe the internal structure of the bone by 3D construction with volume rendering from the slice data. At the same time, image processing software (Mimics; Materialise, Leuven, Belgium) was used to separate the IC and other structures, followed by reconstruction with the IC shown in red.
Classification of IC morphologies and calculation of their incidences
Measurements of surrounding bone
PASW Statistics 18 for Windows (SPSS Inc., Tokyo, Japan) was used for statistical processing. Student’s t-test was used to test for comparison between dentulous and edentulous maxillary bones. The p value was set to <0.05.
Number of individuals and proportions based on classification of the principal item
Classification of the IC from the overall appearance
Dentulous + edentulous
Number of individuals and proportions based on classification of the secondary item
Classification of the IC from the sagittal cross section
Observation of bone structure
Descriptive analysis of the IC and alveolar bone
Comparative analysis of measurements items in dentulous maxilla and edentulous maxilla
Dentulous ( n = 20)
Edentulous ( n = 20)
Mean ± SD
Mean ± SD
11.75 ± 1.70 (mm)
10.84 ± 2.42
17.97 ± 3.16 (mm)
14.01 ± 3.50
2.84 ± 0.79 (mm)
3.56 ± 0.91
4.28 ± 1.49 (mm)
5.40 ± 1.44
5.80 ± 1.21 (mm)
4.06 ± 1.77
5.61 ± 1.13 (mm)
5.08 ± 1.87
8.45 ± 1.84 (mm)
8.42 ± 1.59
60.67 ± 36.25 (mm3)
87.05 ± 32.34
Dental implant surgery in the premaxillary region often has limited bone for implant placement, and as a result, there is the risk of a poor prognosis in some instances . In particular, the ICs are located in the anterior part of the hard palate, and the probability of adverse effects during implant placement in the premaxillary region is about 4% . Therefore, practitioners must be aware of the risks of accidents and avoid them by using precise, detailed research data on the IC.
Close to 60% of all ICs exhibit Y-morphology. However, according to the report by Michael et al., the frequency of a cylindrical morphology of IC was 50% or more . Otherwise, in terms of rare cases, Frederico et al. studied prospective implant patients using CBCT and reported that two ICs that are completely independent exist . In the present study, instances where two or more IC branches existed accounted for 7.5% of the whole sample, and this suggests the possibility that they vary depending on various factors such as ethnicity or environment.
Regarding IC morphology as observed from the sagittal plane, on the other hand, funnel-shaped predominated, at 55% in dentulous maxilla and 50% in edentulous maxilla, followed by cylinder-shaped at 45% in dentulous maxilla and 45% in edentulous maxilla. This result is largely consistent with Mardinger et al.’s findings, namely, the principal shapes of the IC were shown to be funnel-shaped and cylinder-shaped, whether it be for dentulous or for edentulous maxilla. With funnel-shaped, expansion of the incisive foramen is observed, so depending on the angle of implant placement, there is a risk of perforating the IC and damaging the nasopalatine nerves and arteries. Next, the IC diameter (c, d) and volume (f) showed significant differences between dentulous and edentulous maxilla. This suggests that bone resorption due to the loss of teeth also has an impact on the IC. Bhola et al. reported that in order to obtain initial stability of the implant, there are two requisites: an implant fixture measuring 10 mm or longer must be used, and at the time of placement, there needs to be at least 3 mm or more alveolar bone from the implant body tip . In the present study, where attention is limited to the alveolar bone height (b) in the anterior region of the IC, adequate bone for implant placement was present in 60% of edentulous maxilla. It is, however, desirable for there to be a thickness of 2 mm or more of hard tissue on the labial side (buccal side) of the implant, when the long-term prognosis is taken into account . The mean bone width of the IC anterior alveolar bone was 4.05 mm in edentulous maxilla at e1. Considering the placement of an implant with a diameter of 4 mm, which is now commonly used, there are still comparatively many cases with high risk.
Typically, in implant surgery, a platform is prepared at the top of the alveolar bone . However, there are reported cases where a slight gap created at the platform after placement became a space for bacteria to grow, resulting in about 1 to 2 mm of bone resorption due to inflammation of the peri-implant gingiva [33-35]. As gingival recession affects the esthetics, it seems possible that long-term prognosis could be affected by the fine cancellous bone and thin cortical bone observed in the present study.
The structure surrounding the IC, anatomically, has limited area for implant placement, and when esthetic and long-term maintenance requirements are taken into account, careful attention is needed in setting the placement position. Also, due to the resorption of bone, edentulous maxillae have a different IC morphology from dentulous maxillae, and therefore, a cautious approach is required.
This study demonstrated that the IC displays many morphological as well as dimensional variations. These findings may assist clinicians in understanding the morphology and planning treatment.
This study was supported by an Oral Health Science Center Grant (#5A10: Satoru Matsunaga) from Tokyo Dental College and by a Grant-in Aid for Scientific Research (no. 22791905: Satoru Matsunaga) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We thank all staff of the Department of Anatomy, Tokyo Dental College for their whole-hearted cooperation.
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