Information regarding anatomical structures is essential for endodontic surgery because visualization and access to the surgical field may be compromised when this information is lacking.1 When performing apical surgery in mandibular molars, the dentist's greatest concern is the position of the root apices within the mandible2 and the thickness of the buccal bone plate,3 i.e., the distance from the outer surface of the buccal cortical plate to the root.

Frankel et al. reported that the bone thickness of the buccal side in anatomic sections of 33 human cadaver mandible halves were 4.2 mm and 7.4 mm in the first and second molars, respectively.2 The width of the alveolar process significantly increases with posterior progression in the mandible. The buccal bone is thicker in the posterior region of the mandibular arch that forms the external oblique ridge. The root tips of the premolars and first molars are usually close to the buccal bone plate, while those of second and third molars are close to the lingual plate. For this reason, the approach becomes more difficult as one progresses further into the posterior region. In some cases, apical access is very difficult through both cortical and medullary bones.1

Generally, apical surgery on the mandibular second molar is challenging because the thick buccal bone and tooth location in the arch make accessibility difficult.4 Therefore, knowledge regarding the depth of the root apex and the apical 3 mm from the buccal bony surface, which is the level of root end resection, is of major importance when planning periapical surgery.

When planning endodontic surgery, cone-beam computed tomography (CBCT) may provide three-dimensional information on the anatomy of the jawbone and root configuration. Three-dimensional imaging allows clear identification of the anatomical relationship of the root apices to the adjacent anatomical structures in various directions.567

Presently, data are not available on the distance of the root apex from the buccal bone surface in relation to factors such as age, sex, adjacent teeth, and root configuration, and these factors need to be evaluated when planning apical surgery. Therefore, the aims of this study were to analyze the distance from the buccal bone surface to the root apex in the mandibular molar teeth by using CBCT images and to evaluate the differences according to various influencing factors.

Apical surgery is often indicated when nonsurgical endodontic treatment fails to remove the etiology of the apical pathosis.8 The rationale of surgical endodontic treatment, especially apicoectomy, is based on removing the etiology of the apical pathosis by using surgical means.9 In apical surgery, osteotomy entails removal of the buccal bone to expose the root end; the approach for osteotomy must be deliberate and precise so that the osteotomy is performed exactly on the apices.10 In terms of approaching the apex and obtaining the operating field, the thickness of the buccal bone is a crucial factor, especially in the molar area.

For the mandibular first molar, the mean distances from the root apices to the buccal bone surface for the mesiobuccal and disto-buccal roots were 4.38 mm and 3.94 mm, respectively, in the present study and were similar with those of another study.3 These values indicate that the mesio-buccal and disto-buccal root apices of the mandibular first molar are relatively easy to access with surgical burs.9 However, the mean dimension from the disto-lingual root apex to the buccal bone surface for the mandibular first molar is much deeper than that from the buccal root apices (10.34 mm in the present study). Given these values, long surgical burs and careful attention to access the disto-lingual root apex may be required for the mandibular first molar.

For the mandibular second molar, the mean distances between the buccal bone surface and root apices for the mesial and distal roots were 8.78 mm and 9.60 mm, respectively. These values indicate that the mandibular second molar root apex is located much deeper than the buccal root of the first molar. The external oblique ridge is present in the second molar region, and this would account for the greater distance of the second molar apex to the buccal bone surface compared to that of the buccal root of the first molar.2 Given these values and the buccal muscle tension at the corner of the mouth, a routine apicoectomy procedure via the buccal approach may not provide successful outcome in these cases; therefore, different surgical techniques, such as intentional replantation, may be considered.4 The bony lid technique is an option if the operator is going to conserve the buccal bone from destructively creating a bony window; this procedure involves placing the buccal plate back into its initial position after it has been separated into blocks.1112

The thickness of the buccal bone over the root has been investigated in several studies.3913 The bone thickness was measured from the root apex in most of these reports. However, in this study, in addition to the apex, the 3 mm level from the apex was measured from the buccal bone surface. Morphology studies revealed that the apical 3 mm of the root contains canal irregularities, such as accessory or lateral canals, in most teeth.141516 Other studies showed that root resection of the apical 3 mm at a 0 degree bevel angle removes the majority of anatomical entities, which are potential causes of failure. Any remaining ramifications and lateral canals are sealed during retrograde filling of the canal, which extends 3 mm coronally.91017 Therefore, root end resection by 3 mm has become a standard procedure in apical surgery. The data regarding the distance between the buccal bone surface and the apical 3 mm level of the root in the present study may provide valuable clinical information for surgical endodontics.

In this study, we did not find statistically significant differences in buccal bone thickness with regard to gender or the presence/absence of mandibular third molar. Therefore, recommending or planning different apical surgery protocols based on these factors appears to be unnecessary.

The distance from the buccal bone surface to the mesial root of the mandibular second molar was significantly different based on the presence or absence of the mandibular first molar. The buccal bone thickness was significantly less when the first molar was missing compared with when the first molar was present. At least two factors may explain this observation. First, the teeth in the buccal segments of the dental arches may have a tendency to migrate anteriorly, i.e., possible mesial drift of the second molar when the first molar is missing.18 Second, the alveolar bone volume decreases after the first molar is extracted.1920

On the other hand, we found no difference in the thickness of the buccal bone over the second molar based on the presence or absence of the third molar in the present study. A previous study suggested that minimal changes occur in the mylohyoid and external oblique ridges area, which becomes increasingly prominent.21 This may explain why the absence of the mandibular third molar did not affect the buccal bone thickness of the adjacent second molar teeth compared with the absence of the mandibular first molar.

In the present study, the distance from the root apex to the buccal bone surface was significantly shorter in patients older than age 70. This decreased dimension may be explained by age-related bone loss in women and men, which is largely due to gonadal steroid deficiency and physiological secondary hyperparathyroidism later in life.2223 Other factors also play key roles, including vitamin D deficiency, intrinsic defects in osteoblast function, impairment of the growth hormone/insulinlike growth factor level, reduced peak bone mass, ageassociated sarcopenia, and various sporadic secondary causes.2223 However, more investigation may be necessary to generalize this finding of reduced dimension in patients older than 70 because of the limited number of the sample size in the present study.

For CBCT measurements, some discrepancies may exist between the values calculated on the CBCT images and the actual values of the subjects. In a previous study, the mean absolute errors between CBCT and direct measurements were small (0.30 and 0.13 mm for buccal bone height and buccal bone thickness, respectively), and the values obtained from the different measurements were not significantly different.13 These studies suggest that, although CBCT measurements are not identical to the actual measurements, the CBCT values are acceptable and would not have a significant effect on data interpretation.13

For apical surgery in mandibular molars, the distance from the buccal bone surface to the apex and apical 3 mm is significantly affected by the location, patient age, an adjacent missing anterior tooth, and root configuration.