**Relationship between Cortical Bone Thickness and Cancellous Bone Density at Dental Implant Sites in the Jawbone**

**Shiuan-Hui Wang <sup>1</sup> , Yen-Wen Shen 2,3, Lih-Jyh Fuh 2,3 , Shin-Lei Peng <sup>4</sup> , Ming-Tzu Tsai <sup>5</sup> , Heng-Li Huang 2,6 and Jui-Ting Hsu 2,6,\***


Received: 19 August 2020; Accepted: 15 September 2020; Published: 17 September 2020

**Abstract:** Dental implant surgery is a common treatment for missing teeth. Its survival rate is considerably affected by host bone quality and quantity, which is often assessed prior to surgery through dental cone-beam computed tomography (CBCT). Dental CBCT was used in this study to evaluate dental implant sites for (1) differences in and (2) correlations between cancellous bone density and cortical bone thickness among four regions of the jawbone. In total, 315 dental implant sites (39 in the anterior mandible, 42 in the anterior maxilla, 107 in the posterior mandible, and 127 in the posterior maxilla) were identified in dental CBCT images from 128 patients. All CBCT images were loaded into Mimics 15.0 to measure cancellous bone density (unit: grayscale value (GV) and cortical bone thickness (unit: mm)). Differences among the four regions of the jawbone were evaluated using one-way analysis of variance and Scheffe's posttest. Pearson coefficients for correlations between cancellous bone density and cortical bone thickness were also calculated for the four jawbone regions. The results revealed that the mean cancellous bone density was highest in the anterior mandible (722 ± 227 GV), followed by the anterior maxilla (542 ± 208 GV), posterior mandible (535 ± 206 GV), and posterior maxilla (388 ± 206 GV). Cortical bone thickness was highest in the posterior mandible (1.15 ± 0.42 mm), followed by the anterior mandible (1.01 ± 0.32 mm), anterior maxilla (0.89 ± 0.26 mm), and posterior maxilla (0.72 ± 0.19 mm). In the whole jawbone, a weak correlation (*r* = 0.133, *p* = 0.041) was detected between cancellous bone density and cortical bone thickness. Furthermore, except for the anterior maxilla (*r* = 0.306, *p* = 0.048), no correlation between the two bone parameters was observed (all *p* > 0.05). Cancellous bone density and cortical bone thickness varies by implant site in the four regions of the jawbone. The cortical and cancellous bone of a jawbone dental implant site should be evaluated individually before surgery.

**Keywords:** jawbone; cancellous bone density; cortical bone thickness; dental cone-beam computed tomography; dental implant site

#### **1. Introduction**

Dental implant surgery is a common treatment for missing teeth [1,2]. Its survival rate is closely related to osseointegration ability in patients. Specifically, dental implants with higher initial stability have more favorable osseointegration and are associated with a higher survival rate [3–7]; accordingly, examining the bone condition of a dental implant site in the jawbone is crucial. Generally, jawbone condition is assessed by measuring cancellous bone density and cortical bone thickness.

Numerous studies in dentistry have assessed jawbone condition by using computed tomography (CT) and dental cone-beam CT (CBCT), and have evaluated cancellous bone density prior to dental implant surgery on the basis of bone radiographic density. For CT and dental CBCT images, bone density at a dental implant site can be expressed as Hounsfield units (HU) and grayscale values (GVs) [8–12], respectively. Bone density is generally higher in the mandible and anterior region than it is in the maxilla and posterior regions [13,14]. Studies on bone density at different dental implant sites using CT and CBCT for assessment have reported the following ranking (in descending order) of bone density in different regions: anterior mandible, anterior maxilla, posterior mandible, and posterior mandible [9–12,15].

In addition to cancellous bone density, cortical bone thickness in the jawbone affects the initial stability of dental implants [4,16–20]. Miyamoto et al. [7] employed resonance frequency analyses to measure the initial stability of 225 dental implants; their results demonstrated that implant sites with thicker cortical bone had higher initial stability. Song et al. [21] reported that cortical bone thickness measured using dental CBCT at implant sites was highly correlated with the initial stability of dental implants. Hsu et al. [4] conducted artificial bone experiments and demonstrated that cortical bone thickness significantly influenced the initial stability of dental implants. Roze et al. [22] examined jawbone structures by using micro-CT and compared the results with the initial stability of dental implants. They discovered that dental implant stability depended greatly on cortical bone thickness. In brief, studies on cancellous bone density, cortical bone thickness, and the success rate of dental implant surgery have indicated that jawbone condition is highly correlated with the survival rate of dental implant surgery. In 2017, Ko et al. [23] analyzed the cortical bone thickness of 661 dental implant sites in 173 patients by using dental CBCT and reported the following rankings (in descending order) for cortical bone thickness in different regions: posterior mandible, anterior mandible, anterior maxilla, and posterior maxilla. In the same year, Gupta et al. [24] measured cortical bone thickness at dental implant sites by using dental CBCT and obtained a similar result.

Research has indicated that cancellous bone density and cortical bone thickness at dental implant sites differ among distinct jawbone regions; however, the posterior maxilla has the lowest cancellous bone density and cortical bone thickness. Researchers have examined cancellous bone density [9–12,15] and cortical bone thickness [23,24] by using CT and dental CBCT; however, few have analyzed the correlation between the two parameters. Therefore, dental CBCT was employed in this study to identify (1) differences in and (2) correlations between cancellous bone density and cortical bone thickness at dental implant sites in different jawbone regions.

#### **2. Materials and Methods**

#### *2.1. Dental CBCT Examinations of Patients and Implant Sites*

This study was conducted after receiving ethical approval from the Institutional Review Board of China Medical University Hospital (No. CMUH 108-REC2-083), approval date: 3 July 2019. Dental CBCT images were obtained from 128 patients (66 male patients, age: 54.14 ± 14.40 years (mean ± standard deviation); 62 female patients, age: 52.13 ± 13.73 years) who had received dental implants between August 2018 and March 2020. Dental CBCT was performed using a Promax 3D Max (Planmeca, Helsinki, Finland) with the following technical parameters: 96 kV, 12.5 mA, and a voxel resolution of 150 or 200 µm. In total, 315 dental implant sites (39 in the anterior mandible, 42 in the anterior maxilla, 107 in the posterior mandible, and 127 in the posterior maxilla) were identified in the dental CBCT images.
