Comparative evaluation of characteristics of spongiosa in alveolar and basal bones of human jaws: A dry skull study

Abstract

Background:

To give immediate strength to the implants, which are inserted into the bone, the bone should be hard and strong. The bone, in particular the trabecular width (TW) of the cancellous bone, is thin and therefore weak.

Aim:

To compare the human jaw microanatomy of the alveolar and basal bones for implant stability to find whether the trabecular bone of the basal parts is wider and stronger than the alveolar parts of the jaws.

Materials and Methods:

Strategic areas were identified and marked in both the jaws for the study of spongiosa in the alveolar and basal bones. The jaw bones were sectioned along the marked orientation. The collected specimen was grounded and smoothed. After processing of the specimen, it was observed under a stereo microscope to assess the spongiosa (microanatomy).

Statistical Analysis Used:

Comparative observational study using standard of deviation.

Results:

In the maxilla, the TW in the anterior nasal spine of the alveolar bone was 62.2 ± 13.7 μm and of the basal bone was 31.7 ± 8.89 μm. This variation in the TW, i.e. widest in the alveolar zone and least in the basal zone, was found to be true for all the strategic areas of the skull.

Conclusions:

In the maxilla as well as mandible, when we started comparing bone marrow space diameter (BMSD) and TW in the alveolar and basal bones, on specific strategic locations individually, BMSD increases and TW decreases. Thus, the TW of the cancellous bone moving from the alveolar to the basal part was thin and weak.

INTRODUCTION

Dental implants have been widely used in the edentulous areas.[1] Although conventional implantology (endosseous implants) is most popular and is based on available alveolar bone, it possesses certain limitations, such as two-stage surgery, prolonged treatment time, and requirement of systemically healthy patients with sufficient bone height. Hence, to overcome these limitations, the science of cortical implantology is now considered a guiding tool in the future era. Assessment of bone quality is very important for evaluating the healing phase after the installation of an implant. According to the trajectory theory of Meyer and Culmann: “routes of stress coincide with trabecular patterns,”[2] while Wolff’s law states that: “the internal structure and external shape of a bone develops in response to the change in function and forces acting upon it.”[3]

During insertion, the obstruction offered by the bone will be reflected at the bone–implant interface and can likewise demonstrate the nature of the bone through which it passes. Hence, bone structure in these strategic areas of the alveolar and basal bones is an important factor, which influences the mechanical strength of implant installation.

In dental implant research, bone microanatomy plays a great role. Bone microarchitecture consists of two types – cortical and cancellous bones. The trabeculae or “trabecular” bone is the primary anatomical and functional unit of the cancellous bone. Cortical bone helps attain primary implant stability, but the role of the cancellous bone is remarkable as the cancellous bone has a higher bone turnover rate than the cortical bone[4] and has a direct contact with most of the implant surface.[5] Accordingly, it influences the healing and osseointegration process at the implant–bone surface.[6] Bone strength has a significant role in determining implant success. To improve the prediction of bone strength, the measurements of trabecular density and trabecular microstructure should be combined.[7] Precise clinical assessment of bone structural and mechanical properties is essential in planning dental implant treatment and implant thread design.[8] The task can be performed on two-dimensional plain, such as by histomorphometry analysis of a bone specimen under a stereo microscope.

MATERIALS AND METHODS

This ex vivo study was performed on five dried human skulls that were collected from various medical colleges in Kolkata. The study was conducted in the department of periodontics. The dried human skulls were vertically sectioned at the desired anatomical locations with a carborundum disc connected to a dental micromotor straight handpiece. In the maxilla, specific strategic locations were recognized as follows: anterior nasal spine, nasal floor, and canine pillar. In the mandible, specific strategic locations were recognized as follows: symphysis, para-symphysis, and lingual cortex of the body.

The samples were then grounded and smoothed for microscopic observation. The samples in their desired plane were held securely on the flat surface of a rapidly rotating coarse abrasive wheel. Water was directed constantly onto the wheel to prevent charring. The sample was ground down nearly to the level of the desired section. The coarse wheel was then exchanged for a fine abrasive lathe wheel, and the cut surface of the sample was ground again until the level of the desired section (0.5–1 mm) was reached. Then, the specimen was rubbed on a ground glass with pumice-glycerin paste to remove the scratches. The section was dipped in ether, dried, and stored in tight containers for further stereo microscopic studies [Figure 1].

Figure 1.

Photographs depicting the strategic locations of the maxilla and mandible together with longitudinal sections. (a) Anterior nasal spine, (b) nasal floor, (c) canine pillar, (d) symphysis, (e) parasymphysis, (f) lingual cortex body region

Each specimen was placed on a slide under the stereo microscope [Figure 2], and the sample was focused at ×2. Then, the photomicrographs were taken to record trabecular width (TW) and bone marrow space diameter (BMSD). The photomicrographs thus obtained were transferred to a laptop and analyzed by ImageJ software (National Institutes of Health (NIH), Washington, DC (United States), founded in 1997 by Wayne Rasband). An imaginary reference line was taken at the level of the apex of the root. The section of the bone coronal to this line was considered the alveolar bone, whereas the section apical to the line was considered the basal bone [Figure 3].

Figure 2.

Stereo microscope

Figure 3.

Longitudinal section under ×2 showing stereo microscopic image of the strategic location

For measuring BMSD and TW, three diameters at their greatest dimensions were recorded in the region of interest. The average of these three measurements was taken as BMSD in one region of interest. Similarly, BMSD of all regions of interest was measured. In this way, BMSD and TW of all the strategic locations were recorded for the both alveolar and basal bones.

RESULTS

The mean and standard deviation of the BMSD and TW (in μm) for the six separate areas selected for the study of the dry skull are summarized in Tables 1 and 2, respectively. The calcified part between the two adjacent marrow spaces was the trabeculum. It was observed that in the alveolar zone, there were wider calcified structures within which the marrow spaces were situated wide apart. The trabeculum seen in the basal zone was found to be thinner than that observed in the apical zone. The marrow spaces in the basal zone were situated close apart. This variation in the TW, i.e., widest in the alveolar zone and least in the basal zone, was found to be true for all the strategic areas of the skull.

Table 1.

The mean and standard deviation of the bone marrow space diameter (μm) for the six separate areas

Jaw	Regions	Mean±SD (Alveolar bone)	Mean±SD (Basal bone)
Maxilla	Anterior nasal spine	112±29a,b	274±80a,c
	Nasal floor	92.2±29.6a	204±50b
	Canine pillar	115±24.9b	207±45.7b
Mandible	Symphysis	144±30.8c	352±62.3c,d
	Parasymphysis	143±37.6c	306±66c
	Lingual cortex body region	255±61d	391±79d

Different letters indicate statistically significant differences between two regions for each of the bone types (P≤0.05). SD – Standard deviation

Table 2.

The mean and standard deviation of the trabecular width (μm) for the six separate areas

Jaw	Regions	Mean±SD (Alveolar bone)	Mean±SD (Basal bone)
Maxilla	Anterior nasal spine	62.2±13.7a	31.7±8.89a
	Nasal floor	64.2±15.3a	41.4±7.71b
	Canine pillar	116±35b	41.6±8.93b
Mandible	Symphysis	110±22.9b	63.9±17.2c
	Parasymphysis	107±19b	62.4±14.8c
	Lingual cortex body region	109±27.3b	49.9±18b

Different letters indicate statistically significant differences between two regions for each of the bone types (P≤0.05). SD – Standard deviation

BMSD (in μm) of the alveolar and basal bones across different regions of the jaw showed a statistically significant difference [F (2.7, 93) = 84, P ≤ 0.001 and F (3.9, 133) = 47, P ≤ 0.001, respectively, Figures 4 and 5]. Similarly, the TW (in μm) of the alveolar and basal bones across different regions of the jaw showed a statistically significant difference [F (3.6, 121) = 44, P ≤ 0.001 and F (3.6, 124) = 33, P ≤ 0.001, respectively, Figures 6 and 7].

Figure 4.

Bar graph depicting the bone marrow space diameter (in μm) of the alveolar bone across different regions of the jaw

Figure 5.

Bar graph depicting the bone marrow space diameter (in μm) of the basal bone across different regions of the jaw

Figure 6.

Bar graph depicting the trabecular width (in μm) of the alveolar bone across different regions of the jaw

Figure 7.

Bar graph depicting the trabecular width (in μm) of the basal bone across different regions of the jaw

DISCUSSION

The present study was performed to get an overview on the similarities and differences of the microarchitecture of the human dry skulls, particularly in predefined strategic areas of the maxilla and mandible, to assess the bone quality and bone quantity with data related to TW and marrow space diameter. This may help understand the orientation and differential pattern of the trabecular bone and bone marrow spaces, which influences the primary stability when inserting the implant.

The study was conducted on 210 samples collected from five dried human skulls. In the maxilla, specific strategic locations were recognized as the anterior nasal spine, nasal floor, and canine pillar, whereas in the mandible, the strategic sites were symphysis, parasymphysis, and lingual cortex of the body region.

In the maxilla and mandible, when we started comparing BMSD and TW in the alveolar and basal bones, on specific strategic locations individually, BMSD increases and TW decreases. This holds true for all the selected strategic sites. It was also reported by Park et al.[9] that the density of the cancellous part of the maxilla ranged approximately between 150 and 500 HU for the alveolar bone and 150–400 HU for the basal bone. In the mandible, the cancellous part of the alveolar and basal bones had densities of 300–500 HU and 170–440 HU, respectively. Thus, the cancellous part of the basal bone showed a lesser density than the cancellous part of the alveolar bone.

It was clearly evidenced that all six areas of the maxilla and mandible did not reveal any similarity in terms of numerical values of the TW and BMSD. Statistical analysis showed that the differences were found to be highly significant in terms of TW and marrow space dimension.

The term “bone quality” has been used extensively in the literature to describe different aspects of bone characteristics with variable definitions. The most important factor that influences bone quality is the trabecular bone by Ibrahim et al.[10] The trabeculae or “trabecular” bone is the primary anatomical and functional unit of the cancellous bone. Cortical bone helps attain primary implant stability, but the role of cancellous bone is also remarkable. This is because the cancellous bone has a higher bone turnover rate than the cortical bone and has a direct contact with the majority of the implant surface. Accordingly, it influences the healing and osseointegration process at the implant bone surface.

The root of a tooth or dental implant is embedded within the jawbone, which in turn provides support to the masticatory system. This jawbone plays a significant role in the percentage of the bone to the implant contact. The contact is mediated through a microanatomical component of the trabecular bone to the implant surface. The trabecular thickness with its mineral components also plays an important role in the initial stability of the dental implant soon after its insertion. This is reflected in the clinical outcome of the implant surface. Here, the alveolar bone area of the jaw bone was structurally less voluminous than the basal bone area. Hence, to adjust to the less voluminous part of the jaw bone, the trabeculae were required to be thicker in the alveolar bone area.

The mechanical distribution of stress occurs primarily where the bone is in direct contact with the implant. Smaller the area of the bone contact with the implant, greater the overall stress. Thus, the density of the bone is important, not only for initial immobilization of the implant during healing but also for the wide distribution of stress to the bone.[11] Since, in the alveolar bone area, TW was thicker, the marrow space became smaller. Conversely, in the basal bone area, the TW was lesser and the marrow space became greater. To give immediate strength to the implants, which are inserted into the jawbone, the bone should be hard and strong. The bone, in particular the TW of the cancellous bone moving from the alveolar to basal part, was thin and weak. Hence, it was not able to perform its duty to the desired level of implant stability and withstand the masticatory load. Hence, to overcome this limitation, the science of cortical implantology is now considered a guiding tool in the future era.[12]

In this study, dry human skulls were considered. This so happened because there was no scope to collect human jaw due to ethical issues, and this might have yielded a negligible amount of dimensional error along with mild distortion of architectural pattern.

The future in vivo studies are needed to reveal the microanatomical features after insertion of corticodental implants in strategic locations.

CONCLUSIONS

The study design comprised dried human skull that was effective, easy, and reproducible for revealing the microanatomical features before the insertion of the dental implants. The TW was found to be the widest in the alveolar zone and the least in the basal zone for both the maxilla and mandible. The marrow space diameter was found to be the least in the alveolar zone and the widest in the basal zone for both the maxilla and mandible. It was observed that the greater the TW, the lesser is the marrow space size, and conversely, the bigger the marrow space, the thinner is the TW for both the maxilla and mandible. There was a distinct inverse relationship of TW and marrow space diameter at the alveolar and basal zones for both the maxilla and mandible. The TW and marrow space diameter of the anterior and posterior regions of the maxilla and mandible are different. There was no definite decreasing trend of TW and marrow space diameter of six different areas selected for the study. The present study was done on five dried human skulls. A greater number of samples are required for further research on this topic, which will lend more authenticity to the results of the same.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.