CBCT assessment of bone thickness in maxillary and mandibular teeth: an anatomic study

Abstract The site of the sinus tract depends on the rate of resistance against abscess exudate drainage, bone morphology, and distance from the root apex to the outer cortical bone. Objective To assess apical bone thickness in buccal and palatal/lingual aspects of maxillary and mandibular teeth, using a high-resolution cone-beam computed tomography (CBCT) system. Methodology In total, 422 CBCT examinations were included in the study, resulting in a sample of 1400 teeth. The scans were acquired by PreXion 3D, with a high-resolution protocol. The bone thickness was taken as the distance between the center of the apical foramen and the buccal and lingual/palatal cortical bone. The quantitative variables were expressed as mean values±standard deviation. The independent samples were analyzed using the t-test or the Mann-Whitney test (p<0.05). Results The lowest mean value of bone thickness was observed in the buccal cortical bone of the upper canines (1.49 mm±0.86) and in the upper central incisors (1.59 mm±0.67). In premolar teeth, the lowest values were found in the buccal cortical bone of upper first premolars (1.13 mm±0.68). In the posterior teeth, the lowest values were found in the buccal cortical bone of upper first molars (1.98 mm±1.33). In the lower second molar region, the buccal cortical bone (8.36 mm±1.84) was thicker than the lingual cortical bone (2.95 mm±1.16) (p<0.05). Conclusions The lowest mean values of bone thickness are in the buccal cortical bone of the maxillary teeth. In the mandible, bone thickness is thinner in the buccal bone around the anterior and premolar teeth, and in the lingual aspect of mandibular molars. All these anatomic characteristics could make the occurrence of the sinus tract more susceptible in these specific regions of the maxillary and mandibular alveolar bone.


Introduction
Periapical inflammation is a frequent consequence of a chronic infection of endodontic origin. One of the most common inflammatory periapical lesions is the abscess, 1 which may present a chronic course due to persistence of an endodontic infection, resulting in the formation of a sinus tract. 2 The sinus tract is a pathologic means of abscess drainage along the path of least resistance through bone and soft tissue, ultimately gaining access to intraoral or extraoral surfaces. 2,3 The site of the sinus tract depends on the rate of resistance against abscess exudate drainage, bone morphology and distance between the root apex and the outer cortical bone 4 . Therefore, the study of bone thickness in maxillary and mandibular teeth could be a manner to understand the possible drainage routes of a periapical abscess, as well as the epidemiology of the odontogenic sinus tract.
The study of bone thickness in maxillary and mandibular dentition has been gaining attention in Implantology, 5,6 Periodontology, 7 and Oral Surgery. 8 The investigation of bone anatomy is important in many branches of Dentistry, influencing surgical planning, 6,9 dental implant rehabilitation outcome, 10 and selection of the best positioning for skeletal anchorage, which improves orthodontic mechanics. 11 However, information regarding bone thickness in the apical region of maxillary and mandibular teeth is scarce, though very important for endodontic purposes, specially for the surgical planning in paraendodontic surgery.

Cone-beam computed tomography (CBCT) is an
imaging technique that enables the anatomic study of dental and maxillofacial bone structures in crosssectional high-resolution images in vivo. 12,13 This imaging technique also enables linear measurements of dental and bone structures to be performed with accuracy and reliability. [14][15][16][17] However, the accuracy of reformatted CBCT images is affected by technical parameters that could depend on the CBCT system, such as nominal resolution, image quality, voxel size, kV, mA, number of basis images, field of view (FOV), and the algorithm of the software used in the acquisition and reconstruction of dimensional measurements. 12,18,19 Advanced CBCT systems with high spatial resolution, submillimeter voxel sizes, small FOV, and a smaller focal spot, are considered more accurate in regard to linear measurements. 18 Although some CBCT studies have been conducted to analyze bone thickness in maxillary and mandibular teeth, the information regarding bone thickness in the apical region have been under-represented.
Therefore, the aim of this study was to assess apical bone thickness in the buccal and palatal/lingual aspects of maxillary and mandibular teeth, using a high-resolution CBCT system.

Image analysis
The map-reading dynamic feature of the CBCT was applied as described previously, 20  The bone thickness was measured using the PreXion 3D Viewer software (TeraRecon Inc., Foster City, CA, USA). The standard reference for the location of the apical foramen was the main root canal. Axial navigation was used for each root individually. In the upper molars, axial navigation began in the mesiobuccal root (MB), followed by analysis of the distobuccal (DB) and palatal roots (PR). In the lower molars, the navigation started in the mesial root (M), followed by analysis of the distal root (D). In the presence of fused roots, the axial navigation analyzed the two roots concomitantly. Two observers, specialists in dental radiology with more than 10 years of experience, analyzed all the images. When differences were found, a consensus was reached by discussion of each case between the two examiners.

Statistical analysis
The mean and standard deviation of the quantitative variables were obtained. Data normality was assessed by the Kolmogorov-Smirnov test. The variance of the groups was assessed by the Levene's Test. Comparison analysis of independent samples was assessed by the t-test for independent samples -used for data with normal distribution and for groups with statistically homogeneous variances -or by the Mann-Whitney canines, n=100; first premolars, n=100; second premolars, n=100; first molars, n=100; second molars, n=100. Mandibular teeth: central incisors, n=100; lateral incisors, n=100; canines, n=100; first premolars, n=100; second premolars, n=100; first molars, n=100; and second molars, n=100.

Discussion
Bone thickness could influence the drainage routes of the odontogenic periapical abscess, and consequent formation of the sinus tract. 21 Therefore, the study of maxillary and mandibular apical bone thickness could be an important aid to understand the formation of sinus tract, and to consolidate the data published about its epidemiology and diagnosis in endodontics. In this respect, our study aimed to assess the apical buccal and palatal/lingual bone thickness in maxillary and mandibular teeth, using a high-resolution CBCT unit.
In this study, the lowest mean values of apical bone thickness were found in the buccal cortical bone of the maxillary teeth, especially in the anterior canines, central incisors, first premolars, and first molars.
These results corroborate those of epidemiological studies, which have found a higher prevalence of the odontogenic sinus tract in the maxilla, 2,4 particularly in the buccal aspect of upper incisors, upper premolars, and molars. 2 The thin cortical bone found in the buccal aspect of maxillary teeth could contribute to a higher prevalence of the sinus tract in these locations, for the distance between the tooth apices and the external cortical surface in these regions is usually short, and the sinus tract typically follows a path of least resistance through the alveolar bone. 3 In fact, the palatal alveolar bone in the apical region appears to be thicker than the buccal bone, as observed in this investigation, and is generally more compact, 3 thus explaining why it is rare to have a palatal sinus tract. 2,4,22 This study found that the palatal root of the upper second premolars is closer to the buccal cortical bone than the palatal cortical bone itself. This may explain why the sinus tract in maxillary teeth is often detected in the buccal alveolar bone.
Regarding the mandible, the bone thickness was thinner in the buccal bone around the anterior and premolar teeth. These findings corroborate published epidemiological data that indicate a prevalence of the sinus tract in the buccal aspect of the mandible. 2,4,22 Curiously, the occurrence of a lingual sinus tract is typically observed in mandibular molars. 22 Our findings

Bone thickness acts as an important factor
influencing the development of the sinus tract in bone, 25 associated with dental caries and trauma incidence.
These adverse factors may influence the prevalence of periapical abscess, and consequential prevalence of odontogenic sinus tract in specific dental groups. 4,26 This finding is based on the premise that the most common initiating factors of a periapical abscess have low incidence in teeth where the sinus tract is very uncommon, e.g., maxillary and mandibular canines.
According to Slutzky-Goldberg, et al. 22  of the alveolar bone. We also believe that sex and age may be factors that influence the thickness of the alveolar bone. However, in our sample there was a predominance of women, with mean age above 35 years, hindering the verification of these differences.
In this study, maxillary and mandibular bone thickness were analyzed by a high-resolution CBCT system, selected due to its ability to represent bony structures in a highly accurate way. 30 In most clinical applications, CBCT is considered an accurate imaging J Appl Oral Sci. 2020;28:e20190148 8/9 examination providing reliable information with respect to linear measurements. 14,31 It is recognized that some technical parameters of the CBCT, such as spatial resolution, voxel size, FOV, focal point, number of basis images and the reconstruction algorithm, can influence in the dimensional measurements obtained by this imaging examination. 32 In this study, CBCT system images were used with high spatial resolution, submillimeter isotropic voxel (0.100 mm), small FOV (60x56 mm), small focal spot (0.3 mm) and 1024 basis images, with the objective of reducing the influence of these parameters on the linear measurements of bone thickness. It should be highlighted that the reconstruction of images and the linear measurements were performed in native CBCT system software, respecting the reconstruction algorithms determined by the manufacturer. The combination of all these technical parameters produced a more accurate CBCT image in regard to linear measurements. 12 Consequently, this high-resolution CBCT system could be considered reliable in defining alveolar bone thickness.

Conclusions
The lowest mean values of apical bone thickness were found in the buccal cortical bone of the maxillary teeth, especially in anterior canines, central incisors, first premolars and first molars. In the mandible, the bone thickness in the buccal bone is thinner around the anterior and premolar teeth, and in the lingual aspect of the apical region of mandibular first and second molars. All these anatomic characteristics could increase the occurrence of the sinus tract in these specific regions of maxillary and mandibular alveolar bone.