In males higher values of thickness and density than females were found, at different levels (2 mm, 4 mm, 6 mm, 8 mm) from the alveolar crest, with a statistically significant difference (P≤0.05).
In adults, the thickness of both jaws was greater than in adolescents with a statistically significant difference at 4, 6, 8 mm from the alveolar crest (P≤0.05). The density values did not show any difference according to the age of patients; only at 8 mm cortical oral bone level, the adolescents recorded higher density values with a statistically significant difference (P≤0.05).
The lower jaw was found both thicker and higher in density than the upper jaw with a statistically significant difference (P≤0.05).
Regarding the anterior and the posterior regions of the jaws, thickness values were higher in posterior region than in anterior with a statistically significant difference at 2, 4, 6 and 8 mm form the alveolar crest in the buccal cortical bone side (P≤0.05). Concerning density values the results were ambiguous with statistically significant higher density values, in the posterior region, only in the buccal cortical bone side at 4 and 6 mm from the alveolar crest (P≤0.05).
The paired comparison between buccal and oral side showed a thicker cortical bone in the oral side of both jaws with a statistically significant difference at 4 and 6 mm from the alveolar crest (P≤0.05).
The density of mandibular lingual cortical bone was found significantly higher at 2, 4, 6 and 8 mm whereas in maxillary palatal cortical bone the density was found higher at 4, 6 and 8 mm (P≤0.05).
The results of this study showed that there are differences in alveolar cortical bone thickness and density between, males and females, adolescents and adults, upper and lower arch, anterior and posterior area of the jaws, between buccal and oral side and from crest to base of alveolar crest.
Discussion
Several studies have proposed a variety of methods for assessing bone density, but in recent years, the use of a CT scan has been common for preoperative quantitative and qualitative assessment of implant sites, and the Hounsfield Unit (HU) is routinely used to determine the bone density objectively (
19
,
20
).
Even more recently, due to the need for less expensive image acquisition protocols or for scanners with lower radiation dose, cone beam CT (CBCT) has been widely employed for oral and maxillofacial imaging, as it seems to provide good spatial resolution, gray density range, and contrast, as well as a good pixel/noise ratio (
20
).
With CBCT, the dimensional accuracy is also comparable with CT, but unlike CT, the gray density values of the CBCT images (voxel value [VV]) are not absolute. In fact, CT could be calibrated using as a reference the density values of the air (-1,000 HU) and pure water (0 HU); otherwise, CBCT cannot be calibrated, and the values, which are based on the difference of gray scale, are already preset by the manufacturer (
21
) .
In a recent study (
20
) the possibility of correlating the gray density values recorded by CT and CBCT was demonstrated; in fact, a correlation between VV of CBCT and HU values of multislice CT was observed. More specifically the conversion ratio between the two gray values was determined and defined equal to 0.7; thus, to convert the CBCT gray values into CT, it is necessary to multiply CBCT values by 0.7.
This conversion ratio, moreover, is approximate and may vary based on the CBCT used.
On the basis of these results, in the present study only CT scans were evaluated.
In this study a statistically significant difference between males and females in alveolar cortical bone thickness and density in both jaws was found. The cortical bone thickness and density were greater in males than in females. These results disagree with other authors who found no sex differences (
7
,
10
,
15
-
17
). Ono et al. (
7
) asserted that there is not a significant sex difference regarding the alveolar cortical bone thickness at 4 mm from the alveolar crest but they found that cortical bone was thicker in males than females at vertical levels 1 to 2 mm and 5 to 9 mm apical to alveolar crest in the maxilla. Concerning the alveolar cortical bone density Chun and Lim (
16
) found no relationships with sex but this may be related to subject age (range 25-35 years). It has been reported that bone densities in Korean females peak around 35 years of age, slowly decrease until 50 years old and then rapidly decrease after 50 years of age. Up to 35 years of age, there are no differences in bone densities between Korean male and female.
As stated by other authors (
22
,
23
) the sex difference in cortical bone thickness and density, recorded in the current study, might be expected because males have larger bite forces and masticatory muscles than females.
In the current study, the alveolar cortical bone thickness of both jaws was greater in adults with a statistically significant difference at 4, 6, 8 mm from the alveolar crest. Farnsworth et al. (
10
) in their study found no age-related change distal to mandibular first molar but they found a statistically significant age-related difference in the maxillary buccal region similar results were found by Fayed et al. (
9
). Ono et al. (
7
) showed that cortical thickness mesial to the mandibular first molar and 3 to 8 mm apical to the alveolar crest was significantly thicker in adults than adolescents. Considering the results of the present study, the age does not seem to affect the density values, but comparable data are not available in the literature.
Although well-controlled studies have not been performed, it appears that TAD placed in younger or adolescent patients tend to fail more often than those placed in adults (
4
,
8
). Difference in cortical thickness between younger and older patients might be explained by allometry such as proportionate increases in overall body size and the size of the body parts (
10
).
In the present study it was found that the alveolar cortical bone thickness and density are greater in the mandible than in the maxilla. Same results were found by other authors that report a thicker and higher alveolar cortical bone density in the lower jaw than in the upper (
2
,
7
,
10
,
12
,
15
-
17
). Baumgaertel and Hans (
12
) observe a thicker buccal cortical bone in mandible.
Choi et al. (
15
) comparing bone density between the maxilla and mandible showed that the mandible had higher values and these differences were more significant in the posterior area of jaws.
Another interesting finding it was that alveolar cortical bone is thinnest in the anterior regions of both jaws and increases progressively toward the posterior. These results agree with those found by Baumgartel and Hans (
12
) who found a buccal cortical bone thinnest in the anterior sextants of both jaws and a progressive increase toward the posterior region, except distal to the maxillary second molars, where the buccal cortex average was thin. Farnsworth et al. (
10
) showed a cortical bone thickness decrease from posterior to anterior region. In our study, bone density showed the same increase from the anterior to the posterior except for the oral side, which had higher values in density in anterior area. Higher bone density values from the anterior to the posterior areas were found in other studies (
16
,
17
). Our study suggests that the posterior area may contain denser and thicker cortical bone. This pattern might be explained by the higher functional demands placed on the posterior teeth (
24
,
25
).
Concerning the buccal and oral side we found a thicker alveolar cortical bone and higher density in the oral side of both jaws. According to Farnsworth et al. (
10
) the mandibular buccal cortical bone is significantly thicker than the cortical bone in maxillary buccal, maxillary palatal and lingual regions. Sawada et al. (
14
) evaluating cortical bone thickness of the upper jaw found that the buccal cortical bone was thinner than the palatal cortical bone. Choi et al. (
15
) comparing bone density between the buccal and lingual sides in the mandible showed that the lingual side had higher values in the anterior area and vice versa in the posterior area. On the other hand, they did not find differences between the buccal and palatal sides in the maxilla.
In the current study gradual increase in the alveolar cortical bone thickness at different distances from the alveolar crest was found. These results agree with those found by Deguchi et al. (
2
) and Ono et al. (
7
) who observed that the cortical bone thickness tends to be thicker at greater heights and thinner at shallow levels. Also Sawada et al. (
14
) reported a tendency for the superior part of the alveolar process to be thicker than the inferior part. Park et al. (
17
) found, for the mandible, that all density values of the cortical basal bone were statistically higher than those of the alveolar bone. Our results indicate that bone thickness and density vary with the distance from the alveolar crest in the interradicular sites. According to Chun and Lim (
16
) mini-implants for orthodontic anchorage may be successfully placed in areas with equivalent bone density up to 6 mm apical to the alveolar crest.
Implant placement in the anterior regions of both jaws should be avoided for several reasons: in this area there is little cortical bone for anchorage of implants and little attached gingiva and there is often lack of sufficient interradicular distances (
5
).
Several factors affect the success rates of mini-implants: anatomic factors, oral hygiene technique used, design of the mini-implant and force used (
1
). Among these factors, the anatomy of site, especially the thickness and density of the cortical bone, seems to have a direct effect on success rate (
4
,
15
,
17
).
Alveolar cortical bone thickness and density appear to play an important role when planning a mini-implant placement. It is not indicated to perform a CT scan for mini-implants insertion. The present study evaluated the influence of different variables on alveolar cortical bone thickness and density providing the clinicians with useful data to reach a better primary mini-implant stability.
In conclusion.
● Males are characterized by a thicker and higher density alveolar cortical bone than females.
● Adults show a thicker alveolar cortical bone than adolescents;
● In the mandible the alveolar cortical bone is more compact and thicker than in the maxilla;
● High values of thickness and density characterize posterior regions of both jaws;
● The alveolar oral cortical bone is thicker than the buccal.
● There is a significant linear increasing of thickness and density from crest to base of alveolar crest.
