Evaluation of bone availability for grafts in different donor sites, through computed tomography

Abstract Objective To quantify the bone volume that can be safely withdrawn from 3 donor sites: (1) the mandibular symphysis, (2) the oblique mandibular line and (3) the skullcap. Methodology For the symphysis, 200 tomographic exams were evaluated by the extension of the anterior loop of mental foramen, by the nerve, by the distance of the foramens, by the distance between the vestibular cortical and the lingual plates and by the distance between the apexes, or lower anterior teeth, and the mandibular base, using the “distance” tool of the I-CAT Vision, in the panoramic and parasagittal reformations. For the oblique line, 70 TCFC exams were analyzed retrospectively in panoramic and parasagittal reformations, evaluating the thickness of the vestibular cortical and the distance between the cortical and the mandibular canal. For the cranial bone, a hexagonal donor site located in parietal area was considered. Results The average dimensions of the bone blocks that can be safely removed from the region of the mandibular symphysis are: 32.27 mm in length, 4.87 mm in height and 4 mm in thickness, providing a volume of 628.61 mm3 available for grafting. In the oblique line, the available bone volume for grafting was 859.61 mm3. In the region of the cranial vault, multiplying the average bone thickness by the area of the hexagon, an average volume of 2,499 mm3 was obtained. Conclusions Comparing the donor sites, the bone availability in the cranial vault is 3 times greater than in the mandibular posterior region, and at least 2 times greater than in the mandibular symphysis.

mental foramen (MF) -, and a well-executed surgical technique will reduce complications during the surgical procedure and increase its success rate. 3,4 The region of the mandibular body and ramus, constituted by the cortical and trabecular bones, is one of the most used intraoral donor sites for this purpose, primarily for its bone quality. This provides osteogenesis, osteoconduction, osteoinduction and osteointegration, as well as low morbidity and few postoperative sensorial complaints when compared with other donor sites. Besides having a high concentration of bone morphogenetic proteins, 5 this region has low volume loss and excellent incorporation in the short term. Another advantage is that the donor and the recipient sites are in the same surgical field, reducing the surgical time and the necessary amount of anesthetic and allowing the surgery to be performed at outpatient level. However, the access may reveal difficulties related to visibility and limitations on the graft size and shape, 6 impairing the bone volume. 7 Most studies on this subject 1,8-12 report an advantage of the skullcap toward the other sites because it is a corticalized bone that undergoes less resorption, leading to more predictable results for the installation of implants, both in the maxilla and mandible, with lower postoperative morbidity. The disadvantages are related to the need for general anesthesia, to the potential complications and to the patient acceptance of cranial surgery more than to its surgical difficulty. 13 As in any type of surgery, careful planning is essential; therefore, three-dimensional analysis using computed tomography is very useful. Thus, this visualization capacity was used to quantify the bone availability, since studies that inform and discuss the bone volume that can be removed were not found in the scientific literature.

Mandibular symphysis
The sample size calculation was done according to some inclusion criteria. This study was approved by the Research Ethics Committee of the University Center. mm, as can be seen in Table 1.      Initially, the area of this hexagon was calculated.

External oblique line
Next, the cortical, medullary and total bone thickness (cortical + medullary) were measured at 9 points ( Figure 5), obtaining the mean bone thickness. By multiplying the area of the hexagon by the bone thickness, the volume of bone that can be removed for grafts from that region was obtained.
For the thickness measurements, on the MPR screen of the software, in the window corresponding to the sagittal reformations, the blue line, which determines the coronal reformations, was positioned exactly on the coronal suture ( Figure 6). Therefore, a coronal reformation was obtained at the level of the coronal suture, in which a vertical line corresponding   to the median sagittal suture was drawn using the distance tool (vertical line) with 30, 40 and 50 mm from the right side, respectively, obtaining a distance guide to the median sagittal suture, to perform the thickness measurements (Figure 7).
In each of these positions, cortical, medullary and total (cortical + medullary) bone thickness measurements were performed at the level of the coronal suture. Afterwards, the blue line, which determines the coronal reformations, was moved first 10 mm, then 20 mm posteriorly and the cortical, medullary and total (cortical + medullary) bone thickness were measured again ( Figure 6). In summary, cortical, medullary and total (cortical + medullary) bone thickness were measured at 3 points at the level of the coronal suture, at 3 points 10 mm posteriorly and at 3 points 20 mm posteriorly, as shown in Figure 2.

Results
The ages of the 50 patients (25 women and 25 men) whose exams were used in this study ranged from 18 to 71 years old, with an average age of 35.6 years. Table 3 shows the averages of cortical, medullary and total (cortical + medullary) bone thickness measurements used to calculate the bone volume.

Mandibular symphysis
The imaging test of the symphysis is necessary to verify if there is enough bone to be used as graft. 13 With the frequent use of CBCT, which offers more With the use of the CBCT, considering these safety margins and a correct planning, our study reveals that an adequate patient selection and a reduction in postoperative complications are predictable. [16][17][18][19]21 The symphysis may provide adequate bone grafts to increase a site previously occupied by two to six teeth. It will never offer enough bone to raise an arch.
If the increase in the complete dental arch is required or if the extent of the alveolar bone loss is significant, another source of bone should be considered. 13

External oblique line
The use of autogenous bone from the mandibular body and ramus has been proved to be effective in reconstructive surgeries of the maxillary bones. 22 However, no studies report safe bone volume obtained in this region. 5,22 Furthermore, the posterior region of the mandible, unlike the mandibular symphysis, does not present defined limits for bone removal, so no protocol delimits the exact donor site and there is no standard for the available volume.
In this study, we used the molar teeth as reference for the anterior limit, 23-25 specifically the distal of first molar, 6,22 which is considered a safe limit to prevent interference with the mental nerve ramus.   Shintani 22 (2015) consider the mandibular lingula as the posterior limit. In the studies by Capelli 6 (2003), incisions were made at the base of the coronoid process, as well as in the reports by Haggerty et al. 25 (2015), in which the extension in the posterior direction can also include this region.
For the lower limit, the reference considered is the junction between the anterior and posterior osteotomies, with an average height of 1 cm 6 or the junction of the osteotomies that extend from 10 to 12 mm below the external oblique line or 4 mm above the mandibular canal. 25 In this study, the internal cortical of the mandibular base is considered the lower limit. In Line X, or anterior limit, the average height was 16.31 mm and in Line Y, or posterior limit, it was 18.36 mm.
The resulting average graft height was 17.33 mm.
The average distance between the anterior and posterior limits (Line X and Line Y) was 18.98 mm, as listed in Table 2 Comparing the results of this study with those found in the literature, a significant difference in the methodology should be considered, since in all the previously mentioned studies performed direct measurements in dry skulls. This means that these thickness measurements considered the external cortical, the medullary bone and the inner cortical bone. In the methodology of this study, on the other hand, only the external cortical and the medullary layer were measured, since they are the ones that are effectively used in the grafts. As it can be seen in Table 3, the mean thickness of the cortical + medullary bone of the 9 evaluated points was 5.95 mm.
The bone volume of the other donor sites were 628.61 mm 3 in the mandibular symphysis and 859.33 mm 3 in the external oblique line region. The available bone volume in the skullcap region, calculated in this study was 2,499 mm 3 (Table 3). Comparing it with the volumes available in the intraoral donor sites of the symphysis and of the posterior region of the mandible, it is reported that the skullcap can offer bone volume almost 3 times greater than the latter and at least 2 times more than the former. In addition, as it allows the withdrawal of several blocks, the skullcap can be used for reconstructions that need more extension.

Conclusion
All sites discussed in this article are excellent options for the removal of autogenous bone grafts for the reconstruction of defects and for the bone resorption of the jaws. The choice of the site will depend on the type of defect.
Compared with intraoral donor sites, the bone Evaluation of bone availability for grafts in different donor sites, through computed tomography J Appl Oral Sci. 2020;28:e20190435 9/9 availability of the skullcap is 3 times greater than that of the posterior region of the mandible and at least 2 times greater than that of the mandibular symphysis.