This principle of change in radiological density over time can be used as an indicator of bone turnover. The pre-operative AGV indicated that the maxilla was significantly denser than the mandible (Table
1; F = 11.92, p = 0.0007). This finding contrasts with previous findings that the density of the mandible is higher than that of the maxilla[
25]. This can be due to the difference in position of the AOI, as the bone density measured on radiographs in the mouth is normally affected by surrounding structures in the length of the pathway of the x-ray and also the thickness of bone[
22]. The AOI in our study was more apically located and included more of the dense palatal bone on the radiographic image. However, in the present study, the pre-operative difference in bone density of the maxilla and mandible is not of any significance, as the changes in density were measured over time with the post-operative value as the starting point. This pre-molar area included more bone volume and the denser soft tissue from the palate, as well as the thicker cortical bone of the palate. The pre-operative values were used for calculating the percentage change in bone density. The possible influence of differences in bone density between the maxilla and mandible and between animals before the defects were created was excluded by calculating the changes relative to the pre-operative densities. Through this approach, the post-operative values were normalized to zero. It was not surprising that the post-operative percentage density for the area of interest was similar for both the mandible and maxilla following the removal of the same volume of bone (Table
1). Any change to a number greater than the post-operative percentage AGV was regarded as a percentage increase in bone regeneration.
The increase in density of the mandible after the surgical intervention – was not significantly different from that in the maxilla at three-, six- and three-to-six weeks (Table
1). This finding contrasts with that of a study using histomorphometric methods where the mandible healed twice as fast as the maxilla[
26]. Similar results were found in our study comparing the rate of healing between the mandible and maxilla during the first three weeks period. The percentage increase in density in the mandible was 8.9% and 4.3% in the maxilla (Table
1) which represents a difference of approximately 106%. At six weeks the difference in percentage of the rate of healing between the mandible and maxilla decreased to an average of 20%. The time sequence of bone regeneration following an extraction is started with clot formation on the same day which is replaced by granulation tissue in the first week. The granulation tissue is replaced by connective tissue on day 20. Osteoid formation is evident from day seven and on day 38, two-thirds of the extraction socket is filled by connective tissue[
27]. The sequence of the regeneration process indicates active cellular activity directly post-operative in the defect area which implies a quicker increase in density of the AOI on the radiological image. The tempo of the process of regeneration decreases from day 20 as connective tissue and osteoid is developing. As the mandibular alveolar bone is denser than the maxillary alveolar bone, it can be expected that more regeneration tissue will be present in the mandible bone, therefore the difference in the rate of healing between the maxilla and mandible in the first three weeks. The mandible is subjected to higher mechanical forces and consequently has a higher rate of healing than the maxilla[
27]. The dynamism imposed by muscle force on the bone causes complex patterns of stress and strain in the mandible, such as sagittal and transverse bending and deformation from shear and torsion[
28]. In contrast, the maxillary and pre-maxillary bones are primarily exposed to forces generated by occlusal contact with the mandibular teeth[
29]. The bone quality for both anterior and posterior jaw regions are predominantly types 2 and 3 (Lekholm-Zarp classification)[
30]. The anterior part of the mandible has the densest bone, followed by the posterior mandible, anterior maxilla, and posterior maxilla[
31]. The area of interest was the pre-molar region of both the mandible and maxilla. Both regions were Lekholm-Zarp type II or III classification with the density of the cortical and trabecular bone in the mandible the highest[
13,
16]. One may speculate that the denser quality of bone in the mandible provided more bone cells for osteogenesis at the site of trauma. Bone cells respond to mechanical stimulation and induce new bone formation
in vivo and also increase the metabolic activity and gene expression of osteoblasts[
31]. However, the molecular events involved in the translation of mechanical stimulation into cell proliferation and bone formation are not yet well understood[
32]. The more mechanical stimulation of the mandibular bone cells due to the denser bone and more forces influencing the mandible[
28,
29] may also contribute to the increase in the rate of bone healing, especially following the creation of defects at the postoperative three-week period.