Numerous approaches have been used to assess bone tissue density such as conventional radiography, dual-energy X-ray absorptiometry, digital image analysis, ultrasound, and CT. Most of these methods are impractical for routine clinical use. CT is an established non-invasive method for acquiring bone images prior to dental implant placement. Quantitative computed tomography (QCT) has the major advantage of enabling the trabecular and cortical bone densities to be evaluated separately. It allows precise three-dimensional anatomic localization and furnishes direct density measurements, expressed in HU .
The bone density in the midline area was found to be low in both the maxilla and mandible relative to other interradicular areas. It could be possibly due to development of the mandible in two halves (right and left bodies). This separation present at the midline symphysis menti is gradually eliminated between the 4th and 12th months after birth, when ossification converts the syndesmosis into a synostosis, uniting the two halves. The presence of the synostosis joint in this region could be the possible cause of a lesser density in this region . The presence of less density in the midline area of the maxilla explains the splitting of the midpalatal suture during rapid maxillary expansion and creation of midline diastema. The same finding was observed by Moon et al  in their study. They compared the bone density in various regions of the palate and found the lowest density in the vicinity of the midpalatal suture.
The density in the maxilla and mandible increased progressively from the midline towards the posterior region, which could be explained by distribution of occlusal force during mastication. The maximum biting forces are found to increase from the anterior towards the posterior teeth . The highest density between the first and second molars in the basal bone could be explained by the presence of the zygomatic buttress. The zygomatic buttress is a strong bony pillar that provides pressure absorption and transduction in the facial skeleton . The increased bone density in this area could be responsible for a stronger bone structure. The presence of least density at the tuberosity region could be due to the absence of direct mechanical stimulation in that region. The highest density at the retromolar area could also be explained by the presence of thick oblique ridges in that area as well as attachment of the muscles of mastication in that area. Furthermore, cortical bone thickness in the mandible showed a gradual increase from the anterior to the posterior region [12, 13]. Thus, the results suggest that the mandibular posterior area contains denser and thicker cortical bone.
The basal bone was found to be denser than the alveolar bone for both the maxilla and mandible (Additional file 1: Graphs 1 and 2). This difference can be attributed to the transmission of masticatory forces to the basal bone through the teeth. However, the bone density at the retromolar bone was found to be significantly higher at the alveolar region compared to the basal region, which could be because of the presence of thick oblique ridges in that region. The mandible was found to have higher density values than the maxilla (Additional file 1: Graphs 3 and 4), which could be explained by the difference in loads (compression, tension, and torsion) to which the maxilla and mandible are exposed . Functional loading dictates the osseous anatomy of opposing jaws. The mandible is subjected to substantial torsion and flexion caused by muscle pull and masticatory function. Thick and dense mandibular cortices are needed to resist the torsional and bending strain. The maxilla, however, is loaded predominately in compression. It has no major muscle attachments and transfers much of its load to the rest of the cranium. Because of the entirely different functional role, the maxilla ispredominantly trabecular with thin cortices.
The sample in the present study showed almost a similar bone density value pattern in the maxilla and mandible as observed by Park et al  in a Korean population, except that they found the highest bone density in the canine premolar region of the maxilla. In general, bone density values were found to be higher in an Indian population than in a Korean population [14, 15]. Differences in metabolic or lifestyle factors account for a larger share of the racial differences in bone mass. Lifestyle factors such as dietary calcium intake, physical activity, smoking, and alcohol intake have been found to influence bone density . Another reason which could be possible for the variation in the observed bone mineral density values is the difference in methodological approach such as the use of different slice thickness, software, CT machines, etc. .
When a pairwise comparison of density at the seven interradicular areas was done, differences between bone densities of any two areas were found to be very highly significant. This makes knowledge of site-specific bone density important prior to planning anchorage strategies and placement of mini-implants.
In general, the rate of tooth movement is inversely related to the bone density. As the bone density decreases, the rate of tooth movement increases . In the current investigation, the alveolar process supporting the mandibular molars has been found to be denser than that supporting the maxillary molars, thereby offering more resistance to tooth movement. This could explain as one of the reasons for mandibular molars having a higher anchorage value than the maxillary molars. The high-density bone is formed as the leading roots are moved mesially. After a few months of mesial translation, the trailing roots engage the high-density bone formed by the leading root and the rate of tooth movement declines . In the areas of low bone density, it is necessary to augment the anchorage using transpalatal arch, implants, etc. as per requirement.
Bone mineral density has been also used to establish a treatment plan to ensure thestability of implants in dentistry. During early stages, bone density appears to be the key determinant for stationary anchorage of mini-implants in the sites with inadequate cortical bone thickness because primary retention of mini-implants is achieved by mechanical means rather than through osseointegration . The mechanical distribution of stress occurs primarily where the bone is in contact with the implant . The smaller the area of the bone contacting the implant body, the greater is the overall stress, when all other factors are equal. The bone density influences the amount of bone in contact with the implant surface. Since less dense bone is found in the posterior maxilla, it will offer less area of contact with the body of the implant. Consequently, a greater implant surface area is required to obtain a similar amount of bone-implant contact in soft bone compared with denser bone quality. In the present study, the bone density at the maxillary tuberosity was approximately 950 HU and comparatively weak. Therefore, when placing microscrew implants in the maxillary tuberosity, longer implants should be used.
Whenever the mini-implants are placed in the thick, dense cortical bone, insertion torque increases and thereby chances of fracture or breakage of implant increases and more amount of bone is damaged . Therefore, while placing the mini-implants in the thick and dense cortical bone area, it is advisable to use pre-drilling method.
The presence of the thick cortical bone in the posterior mandible and the high bone density as observed in this study might show that bone damage is possible from overheating during drilling. Tehemar stated that heat generation increases during drilling in dense bone. The success of a dental implant can be affected adversely if greater than 47°C of heat is generated as it is known to cause bone necrosis. Bone necrosis is found to be the result in proportion with increase in temperature and exposure time to heat . Therefore, when placing the mini-implants into the retromolar and posterior areas in the mandible, clinicians must be careful not to generate heat. Heat generation can be prevented by irrigating abundantly with a saline solution, not applying too much pressure on the bone, and not using a worn drill. Also, a large-diameter drill can be used instead of a small-diameter drill.