To place implants in an atrophic posterior mandible, vertical and horizontal bone reconstruction using autogenous, allogeneic, or alloplastic grafting is necessary. In particular, autografting has been integral to osteogenesis, osteoinduction, and osteoconduction1-3.

Normally, donor sites for auto bone harvesting are the iliac crest and tibia extraorally and mental symphysis, maxillary tuberosity, ramus, and extosis intraorally. Bone from the mandible is less resolvable than that from extraoral donor sites. Furthermore, due to the proximity between the donor and recipient sites during implant procedures, the time for operation and anesthesia can be shortened; thus minimizing post-operative discomfort and complications. Among the intraoral donor sites, the anterior ramus, which has been used for vertically and horizontally recessed ridge augmentation prior to implant placement, has generally been chosen since its introduction by Misch and Dietsh4. Bone grafting from the anterior ramus also allows harvesting of block bone and particle bone at the same time, again shortening the operation time, reducing the amount of anesthesia, and minimizing post-operative discomfort and complications thanks to the proximity between the sites. Disadvantages include the risk of bleeding, fracture, and sensory disturbances.

In particular, sensory disturbances of the inferior alveolar nerve occur in association with bilateral sagittal splint ramus osteotomy (BSSRO), intraoral vertical ramus osteotomy, ramus osteotomy, ramus angle reduction, mandible fracture, and mandibular third molar extraction. This is because of the anatomical position of the nerve in inferior alveolar canal (IAC). The rates of sensory disturbance of the inferior alveolar nerve have varied from 3.9% of third molar extractions to 39% of BSSROs. Silva et al.5 reported a sensory disturbance rate of 8.3% when harvesting mandibular bone.

The position of the inferior alveolar nerve is determined using dried mandibular bone from cadavers. Gowgiel6 studied the inferior alveolar nerve canal and blood vessels of 29 adults with natural dentition. They reported that the nerve runs near the lingual cortical side from its entrance in the ramus to the mental foramen, and that it is positioned about 1 cm above the inferior border of the mandible.

According to Li et al.7, the nerve was positioned along the lingual inferior portion of the canal, and blood vessels were found above the canal.

Cone beam computed tomography (CBCT) is reportedly useful in determining the position of IAC. Kamburoglu et al.8 noted that CBCT boasts of much higher accuracy than that of calipers. In investigating the position of the canal around the mandibular first molars, they concluded-after evaluating 50 patients with intact dentition-that IAC is positioned about 4.9 mm from the buccal cortical bone and 17.4 mm from the upper cortical bone of the mandible.

Previous studies have focused on the general lingual-buccal course of the inferior alveolar nerve canal using cadavers, panorama radiographs, and computed tomography (CT) related to the mandibular third molar and BSSRO. Nonetheless, not much has been reported on the course of the inferior alveolar nerve canal during ramus bone harvesting. Clear understanding of the anatomical position of IAC is essential in reducing the risk of nerve damage, which can be avoided by establishing a reference point (R point) prior to bone harvesting.

In this study, the R point for the mandibular ramus was established using CBCT, and the position and course of IAC were analyzed three-dimensionally. The results are expected to aid in preventing damage to the inferior alveolar nerve during mandibular ramus bone harvesting.

Although there are few reports on this issue, inferior alveolar nerve damage is one of the most important com-plications after mandibular ramus bone harvesting9. von Arx and Kurt10 reported disturbances of the inferior alveolar nerve including bleeding, and Silva et al.5 found complications due to sensory disturbances after mandibular ramus bone harvesting at a rate of 8.3%. These sensory disturbances occurred when the osteotomy was close to the inferior alveolar nerve canal. Yamamoto et al.11 reported that the risk of nerve damage increased during BSSRO at distances less than 0.8 mm between the internal side of the buccal cortical bone ramus and the inferior alveolar nerve canal. When harvesting bone from the mandibular ramus, the proximity to the inferior alveolar nerve canal was considered to increase the risk of nerve damage. Thus, understanding the anatomical course of the inferior alveolar nerve canal is important.

In the mandibular ramus, the position of the inferior alveolar nerve canal has been investigated according to the type of operation. Nerve damage was reported to occur at a rate of 3.9% in third molar extractions, suggesting the importance of the relationship between the third molar and the inferior alveolar nerve canal. In previous research on this relationship, the distance between the third molar and the inferior alveolar nerve canal has been measured by panorama radiographs and CT, and the risk of nerve damage has been evaluated as well. In ramus angle reduction, the risk and extent of operation were considered by measuring the distance between the angle and the nerve canal. In the ramus, findings of many previous operations have been used as preoperative evaluation indices to establish proper R points. Currently, however, there is a lack of research providing R points for bone harvesting from the mandibular ramus in terms of the anatomical position of the inferior alveolar nerve canal.

In this study, we defined the R point as the contact point between the occlusal plane and the anterior border of the mandible because this point can be found easily during bone harvesting. A plane crossing the mandibular foramen and mental foramen was set as the sagittal plane because this plane is very close to the actual course of the nerve canal that the distance from the buccal cortical bone can be measured easily. With this reference, the anatomical course of the inferior alveolar nerve canal and the distance between the buccal cortical bone and the alveolar crest could be measured using CBCT to evaluate the course of the inferior alveolar nerve canal.

We used the panoramic curve function in the Simplant soft-ware to establish the sagittal plane connecting the mandibular foramen and the mental foramen in the trabecular bone. We then set the R point where the occlusal plane met the anterior border of the mandible and where the coronal plane-which was vertical to the occlusal and sagittal planes-met the R point.

We connected the openings in the trabecular bone because the openings in the cortical bone are in a region where the inferior alveolar nerve canal changed course. As such, there might be some error in describing the course in the trabecular bone.

The distance between the R point and the center of the inferior alveolar nerve canal was 6.19±1.21 mm, and that between the lower buccal cortical bone and the inferior alveolar nerve canal was 5.39±1.56 mm. These values suggest that the bone became thinner as it went from the upper side of the ramus to the lower buccal cortical bone. In other research, Reich12 reported the distance between the buccal cortical bone and the inferior alveolar nerve canal in the retromolar area to be 2.835 mm. Levine et al.13 measured 4.9 mm around the mandibular second molar area. According to Ha et al.14, the distance from the outside of the buccal cortical bone of the mandibular first molar bucco-mesial root to the inferior alveolar nerve canal was 6.6±0.9 mm. The more posterior the measurement was, the longer the distance; from these studies, there was no way of determining an absolutely safe position for the 4 mm-thick bone grafts suggested by Misch15. Thus, the thickness of the buccal side of the inferior alveolar nerve canal and the distance from the upper alveolar crest to the inferior alveolar nerve canal should also be considered.

Nkenke et al.16 reported that the distance from the upper alveolar crest to the inferior alveolar nerve canal in the retromolar area was 11.0±2.2 mm. Levine et al.13 measured 17.4 mm around the mandibular second molar area. In this study, we measured the distance between the R point and the inferior alveolar nerve canal in terms of ND, HD, and VD, and the values were 13.07±2.45 mm, 14.24±2.41 mm, and 10.12±1.76 mm, respectively. As for the measurements of ND and VD every 1 mm from the R0 plane to 10 mm anterior, we determined ND to be 9.93±1.84 mm at R0; this distance increased to R+10, i.e., 11.39±2.03 mm. VD was 13.96±2.45 mm at R0, increasing as well to R+10 where the value was 13.90±2.74 mm. Smith et al.17 maintained that osteotomy with depth of 10 mm on the retromolar area should be safe from nerve damage. Our results confirmed that the R point was more than 10 mm away from IAC.

The course of the inferior alveolar nerve canal in the mandibular ramus is known to extend to the low anterior portion near the lingual cortical bone. Li et al.7 researched on the buccal-lingual and upper-lower positions of IAC in the mandibular ramus and reported that the canal coursed to the lingual side and the lower border of ramus. In this research, using a reference line connecting the mandibular foramen with the mental foramen, the center of IAC was measured. In the posterior area with depth of 10 mm, the center was buccally 0.36±0.56 mm from the reference, increasing to 0.61±0.68 mm at 1 mm anterior. After that point, it decreased to 0.36±0.85 mm at 10 mm anterior. These results indicate that the course is straight in the range of 0.61±0.68 mm along the line connecting the mandibular foramen and the mental foramen to the center of the inferior alveolar nerve canal. With that, the distance from the outside of the buccal cortical bone to IAC was measured from the posterior 10 mm to the anterior 10 mm. In the posterior, the distance was 3.7±1.51 mm; at the R point, it was 5.39±1.56 mm, increasing to 6.26±1.25 mm in the anterior. These values indicate that the risk of nerve damage increases as bone is harvested from the points anterior to the R point to the posterior.

In this study, the position of the inferior alveolar nerve canal position at the R point and the distance from the outside of the buccal cortical bone to the inferior alveolar nerve canal showed no statistically significant difference by gender. In terms of the ND, HD, and VD of the canal in the sagittal plane, the values for men were significantly larger than those for women. There was 1 female patient who had a large volume of posterior bone loss, resulting in shortened distances that were thought to have affected these results.

The anatomical position of the inferior alveolar nerve canal appears to differ by age. According to Levine et al.13, in older patients, the distance from the outside of the buccal cortical bone to the inferior alveolar nerve canal was shorter. Lavelle18 reported that alveolar bone around missing teeth had extensive bone loss, which was considered a normal physiological phenomenon. In our study, because 85% of the patients (17 out of 20) were below 30 years old, the data were not sufficient to evaluate the effects of age.

Recently, new methods and equipment have been developed to avoid nerve damage during osteotomy. Piezoelectric equipment converts electric energy into mechanical energy and cuts bone using vibration. These instruments can cut hard tissue but not soft tissue. Piezoelectric equipment can be effective and useful for the prevention of intraoperative nerve damage, which is possible even after careful evaluation of IAC via panoramic radiographs or CT.

This study sought to contribute to decreased incidence rates of nerve injury during bone harvesting, to improve understanding of the anatomical structure of the inferior alveolar nerve, and to simplify measurement of the distance between the exterior buccal cortex and IAC.

For application to standard practice, it is important to note that this study targeted patients who had retained their mandibular first molars. Further investigation of the course of the inferior alveolar nerve in patients who are completely edentulous or who have lost all of their molars is required.

This study sought to decrease the incidence of nerve injury during bone harvesting and to improve understanding of the anatomical structure of IAC and the distance between the exterior buccal cortex and IAC. Using CBCT for patients who had retained their mandibular first molars and central incisors on both sides, we measured the course of IAC, established a contact point between the occlusal plane and the anterior border of the mandible, and evaluated the distance between the buccal cortex, upper alveolar crest, and inferior alveolar nerve canal. The following results were obtained:

When the results above are compiled, it is apparently safe to harvest buccal cortical bone to a depth of 10 mm in the region 10 mm anterior to the R point between the occlusal plane and the anterior border of the ramus.