0099-2399/90/1610-0480/$02.00/0 JOURNAL OF ENDODONTICS Copyright 9 1990 by The American Association of Endodontists
Printed in U.S.A. VOL. 16, NO. 10, OCTOBER 1990
An Anatomical Study of the Position of the Mesial Roots Of Mandibular Molars Kathleen T. Frankle, DDS, MS, Werner Seibel, PhD, and Thom C. Dumsha, MS, DDS
In order to determine the position of the anatomical apex of the mesial roots of the first and second molars, the horizontal distance from the outer surface of the buccal cortical plate to the apex and the vertical distance from the superior border of the neurovascular bundle to the apex were examined in 33 human cadaver mandible halves. The vertical distances were compared with measurements taken from periapical radiographs between the apex of each mesial root and the superior border of the mandibular canal prior to sectioning. The means of the horizontal buccal distances of the second molar were found to be significantly greater than those of the first molar. The means of the vertical distances of the second molar were found to be significantly less than those of the first molar. A high correlation was found between the vertical anatomic and radiographic measurements. In the majority of the sections, the neurovascular bundle was found in the lingual half of the mandible.
and impingement on the neurovascular bundle. Access to the root apex is a function of the buccolingual position of the root which cannot be assessed radiographically. Therefore, it would be advantageous to know the average horizontal distance from the cortical plate to the root tip when evaluating a patient for surgery. It would also be of great value to know the average distance between the root apex and the neurovascular bundle for any given tooth and to determine whether a correlation exists between this anatomical distance and the radiographic distance. This information could be utilized by the clinician to assess the potential for nerve impairment when performing mandibular posterior surgery. Littner et al. (5) radiographed the molar areas of 46 dry mandibles and were able to define the position of the mandibular canals in 42 specimens. The purpose of this study was to investigate quantitatively the anatomical location of the mesial apices of the first and second mandibular molars in anatomically prepared human cadaver specimens and to determine the correlation between anatomic and radiographic vertical measurements.
MATERIALS AND METHODS The greatest concern for the clinician when performing mandibular molar surgery is the position of the root apices within the mandible. Both the buccal cortical plate and the neurovascular bundle can represent formidable obstacles when performing surgery. Reports detailing the anatomical constraints of posterior teeth are frequent, while literature documenting lower molar apicoectomies is scant. Altonen and Matilla (1) evaluated 46 resected maxillary and mandibular molars and found complete healing in 71% of the cases. No reports of injury to the inferior alveolar nerve were given. By examining 327 apicoectomies involving lower molars, Harris (2) observed only one incident of nerve damage for a short duration. Persson (3) studied 26 apicoectomized molars (8 mandibular) and documented a 73% success rate. When only mandibular teeth were evaluated, the success rate was 63%. He found no postoperative impairment o f the inferior alveolar nerve. Ioannides and Borstlap (4) resected 86 molars, in 73% of which healing was complete. Three cases of transient paresthesias of the inferior alveolar nerve were noted, all of which disappeared within 6 months. Two major concerns which arise when performing posterior mandibular endodontic surgery are access to the root apices
Thirty-three anatomically prepared human cadaver mandible halves were selected which had a uniform plane of occlusion with no supereruption and had both first and second molars present. Only adult mandibles were used and no attempt was made to classify the mandibles by gender, race, or age. In a few instances, whole mandibles were sectioned to give two mandible halves. Mandible halves were selected in which no abnormal mesial-distal tipping or buccolingual tooth rotations were obvious. Soft tissue was dissected from the bony plates. Figures 1 and 2 show examples of occlusal views of two mandibular halves prior to sectioning. Lines were drawn through the axis of the mesial root of the first and second molars where sections were to be cut. Paralleling periapical radiographs were exposed of the first and second molars prior to the sectioning of each mandible. The X-ray head was directed at a 90-degree angle to the film and to the long axis of each tooth. In addition, the mandibles were positioned with wax so that the long axis of each tooth was parallel to the film. All films were processed under the same conditions. Kodak Ultraspeed D film was used and exposed with a GX-770 (Gendex) X-ray machine using a built-in long cone at 70 kV, 32 impulses, and 10 mA. In most cases, 480
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extended to the outer surface on the buccal. Another line (B) perpendicular to line A was dropped to the level of the superior position of the neurovascular bundle. The following measurements were recorded for the first and second molar of each cross-sectioned mandible half (Fig. 3): (a) the horizontal distance from the mesial root apex to the outer surface ofbuccal cortical plate which represents the distance between the dots on line A; and (b) the vertical distance from the mesial root apex to the level of the superior border of the neurovascular bundle on line B. Also, the position of the neurovascular bundle was determined with reference to its buccal or lingual position within the mandible.
Radiographic Measurements The vertical distances from the mesial root apices of each first and second molar to the superior border of the mandibular canal were measured. Analyses o f variance were performed on both the anatomical distances and the radiographic distances, in order to determine any significant difference between the first and second molars. A Pearson's correlation coefficient was utilized to determine the degree o f correlation between the anatomical and the radiographic vertical measurements at the level of the first and second molars. A
FiG 1. Occlusal view of mandible half prior to sectioning. Lines are drawn through regions to be sectioned.
xeroradiographs (Xerox 110) were also exposed in identical fashion. Using the periapical radiograph as a guide tO the curvature of the mesial roots, the mandible half was sectioned vertically in a buccolingual plane through the long axis of the mesial root of each first molar such that a slice through the anatomical apex was approximated. Each mandible half was then similarly positioned to obtain a second vertical section directed through the long axis of the mesial root of the second molar. A 4-inch Dremel circular table saw mounted on a fixed stand and a fine blade (Jewelers Flatting Saw; Thurston Manufacturing Co., Providence, R I ) o f 0.016-inch thickness was used for the vertical sectioning. The two mandibular cross-sections were photographed using a Canon AE-1 Program Camera with Kodachrome 64 and 1:1 magnification.
Anatomical Measurements Each of the 33 cross-sections for each molar tooth was examined. Figure 3 illustrates an example o f a cross-section with the lines drawn where distances were measured. All measurements were made with a Boley gauge and a magnifying lens. Initially, a vertical line through the long axis of the tooth was approximated. A perpendicular line (A) was subsequently drawn at the level of the mesial anatomic apex and
FIG 2. Occlusal view of mandible half prior to sectioning. Lines are drawn through regions to be sectioned.
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Bu
A B
first and second molars were found to be 5.3 m m and 3.6 mm, respectively (Table 3). Analysis of variance was performed on this data and a significant F ratio of 9.59 was obtained (p _ 0.003) as presented in Table 4. The vertical radiographic distances of the first and second molars were measured using periapical radiographs and xeroradiographs taken prior to the sectioning of the mandibles. Figure 4 represents an example of a radiograph that was measured. In 1 of the 33 specimens, the canal space could not be assessed radiographically and therefore only the horizontal distance was measured. The distances between the radiographic apex of the mesial root and the superior border of the canal were found to be 6.0 m m for the first molar and 3.7 m m for the second molar (Table 5). Analysis of variance was performed to determine any significant difference in these radiographic means and an F ratio of 10.18 was determined as presented in Table 6 (p = 0.002). A Pearson's correlation coefficient, r, was applied to the data to determine whether a significant correlation existed between the vertical anatomic distances and the vertical radiographic distances. A Pearson's r of 0.85 for the first molar and 0.86 for the second molar was found (p < 0.05). A Pearson's correlation coefficient, r, was applied to the data to determine whether a significant correlation existed between the vertical anatomic distance and the horizontal anatomical distance existed. No significant correlation was found (r = -0.34, p > 0.05). TABLE 1. Mean horizontal anatomical distance (mm) within the mandible from mesial root apex to outer surface buccal cortical plate
FIG 3. Model of vertical cross-section through mandible depicting lines drawn and distances measured. Bu, buccal; L, lingual. Lines A and B are identified in text.
probability value of less than 0.05 was considered statistically significant for all statistics applied to this study.
Specimen
Mean
SD
No.
First molar Second molar
4.18 7.35
2.12 2.05
32 32
TABLE 2. Analysis of variance of mean horizontal distance
Source
df
SS*
MS
F
Between Within
1 62
160.97 269.83
160.97 4.35
36.991-
Total
63
430.81
9 SS, sum of squares; MS, mean of squares.
RESULTS When studying the mandibles before sectioning, we observed an apparent substantial increase in thickness of the buccal aspect as one proceeds posteriorly within the mandible in the majority of the 33 mandible halves used. Examination of the anatomical sections revealed that the mean horizontal distance between the mesial apices of the first and second molars and the outer surface of the buccal cortical plate was 4.2 and 7.4 mm, respectively (Table 1). Analysis of variance was performed to detect any significant difference between these horizontally measured components of the first and second molars. A significant F ratio of 36.99 was determined (p _< 0.001) as presented in Table 2. In 1 of the 33 specimens, the horizontal component was immeasurable and therefore 32 specimens were used in assessing the horizontal aspect. The vertical distances of the anatomical sections between the anatomical apices and the neurovascular bundle of the
1- P ~ 0.001.
TABLE 3, Mean vertical anatomical distance (mm) from the mesial root apex to the superior border of neurovascular bundle
Specimen
Mean
SD
No.
First molar Second molar
5.33 3.55
2.36 2.24
32 32
TABLE 4. Analysis of variance of mean vertical anatomical distance
Source
df
SS
MS
F
Between Within
1 62
50.77 328.26
50.77 5.29
9.59*
Total
63
* p = 0.003.
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FtG 4. Example of radiograph from which measurements were taken. TABLE 5. Mean vertical radiographic distance (mm) from the mesial root apex to the superior border of the mandibular canal Specimen
Mean
SD
No.
First molar Second molar
5.96 3.69
2.86 2.83
32 32
TABLE 8. Analysis of variance of mean vertical radiographic distance Source
df
SS
MS
F
Between Within
1 62
82.36 501.34
82.36 8.09
10.18*
Total
63
583.70
* p = 0.002.
The position of the neurovascular bundle in the first molar region was observed to be 81% of the time in the lingual half o f the mandible and 16% o f the time in the buccal half. In one specimen (3%) it existed in the center of the mandible. In none of the specimens was the neurovascular bundle adjacent to the apex of the first molar. In one specimen, as seen in Fig. 5, the neurovascular bundle exited in the mandible inferior to the mesial apex of the first molar. At the level of the second molar, the neurovascular bundle was observed in the lingual half of the mandible with a frequency of 84% and was not observed to be located in the buccal half in any of the specimens. In three specimens (9%) the neurovascular bundle was found adjacent to the apex of the second molar and in two specimens (6%) the bundle was observed in dead center. Figure 6 represents an anatomical section through the mesial root of the first molar and demonstrates the neurovascular bundle in the lingual half of the mandible. DISCUSSION In this study the mesial root apex of the second molar was found to be located significantly further from the outer surface of the buccal cortical plate than the mesial apex of the first molar. Our findings quantitatively substantiate previous studies which demonstrated that there is a significant increase in width of the alveolar process as one proceeds posteriorly within the mandible (6-9). The external oblique ridge is
FIG 5. Vertical section through mesial root apex (arrow) of first molar depicting mental foramen (M) at same level. B, buccal; L, lingual.
present in the second molar region and represents the union of the ascending ramus and the body of the mandible. This would account for the increased mean horizontal distance of the second molar with respect to the first molar and would therefore make access to this area more difficult. Figures 1 and 2 represent examples o f the range of the widths of alveolar processes of mandibles in this study. In both cases, it is suggested from the positions of the crowns of teeth that the second molar roots may reside in a more medial position than the first molar roots. This was found to be true as the horizontal anatomical distances between these first and second molar apices and the outer surface of buccal cortical plate in Figure 1 were found to be 9.1 and 11.9 mm, respectively. The corresponding distances of other molars in Figure 2 were found to be 1.3 and 4.1 mm. With the knowledge of the mean horizontal distances (4.2 and 7.4 ram) and the wide ranges in these examples, it may be possible to obtain an approximation of the location of the apices of the first and second molars with regard to the outer surface of buccal cortical plate. This might allow one to predict the difficulty in obtaining adequate access to an affected root. In addition to access, impingement on normal anatomical landmarks is also a major concern when evaluating a patient for apical surgery. In the mandible this relates to proximity of the mandibular canal to the apices of the teeth. There was
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Frankleetal.
FIG 6. Vertical section through mesial root apex of first molar depicting neurovascular bundle in lingual half of mandible. Distances shown are of total width (13 mm) from apex to buccal aspect (3.6 mm) and from apex to level of neurovascular bundle (N). Note lesion at apex. B, buccal; L, lingual.
a high degree of correlation between the radiographic and anatomical means of this distance related to the first and second molars, indicating that a periapical radiograph can be a good predictor of distance to the neurovascular bundle. All periapical radiographs from which measurements were obtained gave a clear view of the position of the mental foramen (Fig. 4). Xeroradiographs were also helpful as an adjunct in extrapolating the limits of the canal space. In specimens in which there were radiographically poorly defined canal spaces, well-defined anatomical bundles existed within the marrow spaces. These were just as distinct as the neurovascular bundles within the well-defined canals. This is in agreement with Carter and Keen (10) who found a single neurovascular bundle in the majority of dissected mandibles. Our findings also confirmed with more precision an earlier study by Olivier (1 l) defining the relationship of the mandibular canal to the apices of the tooth. The mesial apices of the second molar were significantly closer to the canal than the apices of the first molar. This is consistent with the fact that the mandibular canal begins its ascent at the level of the second molar. Rajchel et al. (9) found that the canal is at its lowest point at the distal half of the first
molar and not at the mental foramen. From these results, it would seem that there would be more concern for impingement on the neurovascular bundle in the region of the second molar than the region of the first molar. At present, there has been no evidence that apical surgery on mandibular second molars, in contrast to first molars, results in an increased incidence of nerve impairment. Few studies have reported a high incidence of paresthesia associated with apical surgery on mandibular molar teeth. Several reasons could account for this. It has been established that the failure rate of conventional endodontics is higher on anterior teeth than on posterior teeth (12). Since the apicoectomy is frequently used where conventional endodontics has failed, the majority of the studies dealing with surgical success rates and complications have examined anterior teeth. The relative thin buccal cortical plate in the anterior region and the ease of visibility offer a favorable environment in which this procedure can be performed. Apicoectomy on lower molar teeth is an infrequently encountered surgical procedure. Increased difficulty with access, fear of impingement on neurovascular bundle and inexperience of the surgeon may lead a practitioner to recommend extraction in lieu of recommending an apicoectomy on a mandibular molar. As a result, nerve impairments associated with lower molar apicoectomies are too few to predict the incidence and etiology. Because of the strong correlation in the present study between the vertical radiographic measurement and the actual anatomical measurement, paralleling technique periapical radiographs are a valid tool to assess the position of the inferior alveolar nerve. Thus, the potential for nerve damage can be assessed by the clinician when apical surgery is performed. Furthermore, our findings quantitatively demonstrate that access to the second molar can present a greater problem than access to the first molar because of the significantly greater horizontal distance between the apex of the second molar and the buccal cortical plate. Therefore, fear of impingement on the anatomical structures and perceived difficulty with surgical access do not seem to warrant avoidance of the first mandibular molar apical surgery. On the other hand, it is suggested from this study that the second molar may present a significantly more difficult surgical procedure from the standpoint of access and potential nerve damage.
We wish to express our appreciation to Mr. Ron Wade, Director of Maryland State Anatomy Board, for his assistance in obtaining cadaver specimens, Dr. Larry Jack for his photographic assistance, and Ms. Jacqueline Bayly for manuscript preparation. This study was, carried out in partial fulfillment of the requirements for the Master of Science in Oral Biology at the University of Maryland. Dr. Frankle was a postgraduate endodontie resident, University of Maryland Dental School, Baltimore, MD, and is currently in private practice in Washington, DC and Maryland. Dr. Seibel is an associate professor of anatomy at the University of Maryland. Dr. Dumsha is director, Postgraduate Endodontic Program, University of Maryland.
References 1. Altonen M, Matilla K. Follow-up study of apicoectomized molars. Int J Oral Surg 1976;5:33-40. 2. Harris M. Apicoectomy and retrograde amalgam in mandibular molar teeth. Oral Surg 1979;48:405-407. 3. Persson G. Periapical surgery on molars. Int J Oral Surg 1982;11:96100.
Vol. 16, No. 10, October 1990 4. Ioannides C, Borstlap WA. Apicoectomy on molars: a clinical and radiographical study. Int J Oral Surg 1983;12:73-9. 5. Littner M, Kaffe A, Tamse A, Dicapua P. Relationship between the apices of the lower molars and mandibular canal--a radiographic study. Oral Surg Oral Med Oral Path 1986;62:595-602. 6. MacMillan HW. The structure and function of the alveolar process. J Am Dent Assoc 1924;11:1059-70. 7. Sicher H, Tandler J. Anatomie fur Zahnarzte. Wien, Berlin: Springer, 1928:343-7. 8. Gutmann J, Harrison J. Posterior endodontic surgery: anatomical consid-
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erations and clinical techniques. Int Endod J 1985; 18:8-34. 9. Rajchel J, Ellis E, Fonseca R. The anatomical location of the mandibular canal: its relationship to the sagittal ramus osteotomy. Int J Adult Orthod Orthognathic Surg 1986;1:37-47. 10. Carter RB, Keen EN. The intramandibular course of the inferior alveolar nerve. J Anat 1971 ;108:433-40. 11. Olivier E. Le canale dentaire inferiur et son nerf chez radulte. Ann Anat Pathol Norm Med-Chir 1927;4:975-87. 12. Strindberg LZ. The dependence of the results of pulp therapy on certain factors. Acta Odontol Scand 1956;14:21.
Printed in U.S.A. VOL. 16, NO. 10, OCTOBER 1990
An Anatomical Study of the Position of the Mesial Roots Of Mandibular Molars Kathleen T. Frankle, DDS, MS, Werner Seibel, PhD, and Thom C. Dumsha, MS, DDS
In order to determine the position of the anatomical apex of the mesial roots of the first and second molars, the horizontal distance from the outer surface of the buccal cortical plate to the apex and the vertical distance from the superior border of the neurovascular bundle to the apex were examined in 33 human cadaver mandible halves. The vertical distances were compared with measurements taken from periapical radiographs between the apex of each mesial root and the superior border of the mandibular canal prior to sectioning. The means of the horizontal buccal distances of the second molar were found to be significantly greater than those of the first molar. The means of the vertical distances of the second molar were found to be significantly less than those of the first molar. A high correlation was found between the vertical anatomic and radiographic measurements. In the majority of the sections, the neurovascular bundle was found in the lingual half of the mandible.
and impingement on the neurovascular bundle. Access to the root apex is a function of the buccolingual position of the root which cannot be assessed radiographically. Therefore, it would be advantageous to know the average horizontal distance from the cortical plate to the root tip when evaluating a patient for surgery. It would also be of great value to know the average distance between the root apex and the neurovascular bundle for any given tooth and to determine whether a correlation exists between this anatomical distance and the radiographic distance. This information could be utilized by the clinician to assess the potential for nerve impairment when performing mandibular posterior surgery. Littner et al. (5) radiographed the molar areas of 46 dry mandibles and were able to define the position of the mandibular canals in 42 specimens. The purpose of this study was to investigate quantitatively the anatomical location of the mesial apices of the first and second mandibular molars in anatomically prepared human cadaver specimens and to determine the correlation between anatomic and radiographic vertical measurements.
MATERIALS AND METHODS The greatest concern for the clinician when performing mandibular molar surgery is the position of the root apices within the mandible. Both the buccal cortical plate and the neurovascular bundle can represent formidable obstacles when performing surgery. Reports detailing the anatomical constraints of posterior teeth are frequent, while literature documenting lower molar apicoectomies is scant. Altonen and Matilla (1) evaluated 46 resected maxillary and mandibular molars and found complete healing in 71% of the cases. No reports of injury to the inferior alveolar nerve were given. By examining 327 apicoectomies involving lower molars, Harris (2) observed only one incident of nerve damage for a short duration. Persson (3) studied 26 apicoectomized molars (8 mandibular) and documented a 73% success rate. When only mandibular teeth were evaluated, the success rate was 63%. He found no postoperative impairment o f the inferior alveolar nerve. Ioannides and Borstlap (4) resected 86 molars, in 73% of which healing was complete. Three cases of transient paresthesias of the inferior alveolar nerve were noted, all of which disappeared within 6 months. Two major concerns which arise when performing posterior mandibular endodontic surgery are access to the root apices
Thirty-three anatomically prepared human cadaver mandible halves were selected which had a uniform plane of occlusion with no supereruption and had both first and second molars present. Only adult mandibles were used and no attempt was made to classify the mandibles by gender, race, or age. In a few instances, whole mandibles were sectioned to give two mandible halves. Mandible halves were selected in which no abnormal mesial-distal tipping or buccolingual tooth rotations were obvious. Soft tissue was dissected from the bony plates. Figures 1 and 2 show examples of occlusal views of two mandibular halves prior to sectioning. Lines were drawn through the axis of the mesial root of the first and second molars where sections were to be cut. Paralleling periapical radiographs were exposed of the first and second molars prior to the sectioning of each mandible. The X-ray head was directed at a 90-degree angle to the film and to the long axis of each tooth. In addition, the mandibles were positioned with wax so that the long axis of each tooth was parallel to the film. All films were processed under the same conditions. Kodak Ultraspeed D film was used and exposed with a GX-770 (Gendex) X-ray machine using a built-in long cone at 70 kV, 32 impulses, and 10 mA. In most cases, 480
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Position of Molar Roots
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extended to the outer surface on the buccal. Another line (B) perpendicular to line A was dropped to the level of the superior position of the neurovascular bundle. The following measurements were recorded for the first and second molar of each cross-sectioned mandible half (Fig. 3): (a) the horizontal distance from the mesial root apex to the outer surface ofbuccal cortical plate which represents the distance between the dots on line A; and (b) the vertical distance from the mesial root apex to the level of the superior border of the neurovascular bundle on line B. Also, the position of the neurovascular bundle was determined with reference to its buccal or lingual position within the mandible.
Radiographic Measurements The vertical distances from the mesial root apices of each first and second molar to the superior border of the mandibular canal were measured. Analyses o f variance were performed on both the anatomical distances and the radiographic distances, in order to determine any significant difference between the first and second molars. A Pearson's correlation coefficient was utilized to determine the degree o f correlation between the anatomical and the radiographic vertical measurements at the level of the first and second molars. A
FiG 1. Occlusal view of mandible half prior to sectioning. Lines are drawn through regions to be sectioned.
xeroradiographs (Xerox 110) were also exposed in identical fashion. Using the periapical radiograph as a guide tO the curvature of the mesial roots, the mandible half was sectioned vertically in a buccolingual plane through the long axis of the mesial root of each first molar such that a slice through the anatomical apex was approximated. Each mandible half was then similarly positioned to obtain a second vertical section directed through the long axis of the mesial root of the second molar. A 4-inch Dremel circular table saw mounted on a fixed stand and a fine blade (Jewelers Flatting Saw; Thurston Manufacturing Co., Providence, R I ) o f 0.016-inch thickness was used for the vertical sectioning. The two mandibular cross-sections were photographed using a Canon AE-1 Program Camera with Kodachrome 64 and 1:1 magnification.
Anatomical Measurements Each of the 33 cross-sections for each molar tooth was examined. Figure 3 illustrates an example o f a cross-section with the lines drawn where distances were measured. All measurements were made with a Boley gauge and a magnifying lens. Initially, a vertical line through the long axis of the tooth was approximated. A perpendicular line (A) was subsequently drawn at the level of the mesial anatomic apex and
FIG 2. Occlusal view of mandible half prior to sectioning. Lines are drawn through regions to be sectioned.
482
Frankle et al.
L
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Joumal of Endodontics
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II \
Bu
A B
first and second molars were found to be 5.3 m m and 3.6 mm, respectively (Table 3). Analysis of variance was performed on this data and a significant F ratio of 9.59 was obtained (p _ 0.003) as presented in Table 4. The vertical radiographic distances of the first and second molars were measured using periapical radiographs and xeroradiographs taken prior to the sectioning of the mandibles. Figure 4 represents an example of a radiograph that was measured. In 1 of the 33 specimens, the canal space could not be assessed radiographically and therefore only the horizontal distance was measured. The distances between the radiographic apex of the mesial root and the superior border of the canal were found to be 6.0 m m for the first molar and 3.7 m m for the second molar (Table 5). Analysis of variance was performed to determine any significant difference in these radiographic means and an F ratio of 10.18 was determined as presented in Table 6 (p = 0.002). A Pearson's correlation coefficient, r, was applied to the data to determine whether a significant correlation existed between the vertical anatomic distances and the vertical radiographic distances. A Pearson's r of 0.85 for the first molar and 0.86 for the second molar was found (p < 0.05). A Pearson's correlation coefficient, r, was applied to the data to determine whether a significant correlation existed between the vertical anatomic distance and the horizontal anatomical distance existed. No significant correlation was found (r = -0.34, p > 0.05). TABLE 1. Mean horizontal anatomical distance (mm) within the mandible from mesial root apex to outer surface buccal cortical plate
FIG 3. Model of vertical cross-section through mandible depicting lines drawn and distances measured. Bu, buccal; L, lingual. Lines A and B are identified in text.
probability value of less than 0.05 was considered statistically significant for all statistics applied to this study.
Specimen
Mean
SD
No.
First molar Second molar
4.18 7.35
2.12 2.05
32 32
TABLE 2. Analysis of variance of mean horizontal distance
Source
df
SS*
MS
F
Between Within
1 62
160.97 269.83
160.97 4.35
36.991-
Total
63
430.81
9 SS, sum of squares; MS, mean of squares.
RESULTS When studying the mandibles before sectioning, we observed an apparent substantial increase in thickness of the buccal aspect as one proceeds posteriorly within the mandible in the majority of the 33 mandible halves used. Examination of the anatomical sections revealed that the mean horizontal distance between the mesial apices of the first and second molars and the outer surface of the buccal cortical plate was 4.2 and 7.4 mm, respectively (Table 1). Analysis of variance was performed to detect any significant difference between these horizontally measured components of the first and second molars. A significant F ratio of 36.99 was determined (p _< 0.001) as presented in Table 2. In 1 of the 33 specimens, the horizontal component was immeasurable and therefore 32 specimens were used in assessing the horizontal aspect. The vertical distances of the anatomical sections between the anatomical apices and the neurovascular bundle of the
1- P ~ 0.001.
TABLE 3, Mean vertical anatomical distance (mm) from the mesial root apex to the superior border of neurovascular bundle
Specimen
Mean
SD
No.
First molar Second molar
5.33 3.55
2.36 2.24
32 32
TABLE 4. Analysis of variance of mean vertical anatomical distance
Source
df
SS
MS
F
Between Within
1 62
50.77 328.26
50.77 5.29
9.59*
Total
63
* p = 0.003.
Position of Molar Roots
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FtG 4. Example of radiograph from which measurements were taken. TABLE 5. Mean vertical radiographic distance (mm) from the mesial root apex to the superior border of the mandibular canal Specimen
Mean
SD
No.
First molar Second molar
5.96 3.69
2.86 2.83
32 32
TABLE 8. Analysis of variance of mean vertical radiographic distance Source
df
SS
MS
F
Between Within
1 62
82.36 501.34
82.36 8.09
10.18*
Total
63
583.70
* p = 0.002.
The position of the neurovascular bundle in the first molar region was observed to be 81% of the time in the lingual half o f the mandible and 16% o f the time in the buccal half. In one specimen (3%) it existed in the center of the mandible. In none of the specimens was the neurovascular bundle adjacent to the apex of the first molar. In one specimen, as seen in Fig. 5, the neurovascular bundle exited in the mandible inferior to the mesial apex of the first molar. At the level of the second molar, the neurovascular bundle was observed in the lingual half of the mandible with a frequency of 84% and was not observed to be located in the buccal half in any of the specimens. In three specimens (9%) the neurovascular bundle was found adjacent to the apex of the second molar and in two specimens (6%) the bundle was observed in dead center. Figure 6 represents an anatomical section through the mesial root of the first molar and demonstrates the neurovascular bundle in the lingual half of the mandible. DISCUSSION In this study the mesial root apex of the second molar was found to be located significantly further from the outer surface of the buccal cortical plate than the mesial apex of the first molar. Our findings quantitatively substantiate previous studies which demonstrated that there is a significant increase in width of the alveolar process as one proceeds posteriorly within the mandible (6-9). The external oblique ridge is
FIG 5. Vertical section through mesial root apex (arrow) of first molar depicting mental foramen (M) at same level. B, buccal; L, lingual.
present in the second molar region and represents the union of the ascending ramus and the body of the mandible. This would account for the increased mean horizontal distance of the second molar with respect to the first molar and would therefore make access to this area more difficult. Figures 1 and 2 represent examples o f the range of the widths of alveolar processes of mandibles in this study. In both cases, it is suggested from the positions of the crowns of teeth that the second molar roots may reside in a more medial position than the first molar roots. This was found to be true as the horizontal anatomical distances between these first and second molar apices and the outer surface of buccal cortical plate in Figure 1 were found to be 9.1 and 11.9 mm, respectively. The corresponding distances of other molars in Figure 2 were found to be 1.3 and 4.1 mm. With the knowledge of the mean horizontal distances (4.2 and 7.4 ram) and the wide ranges in these examples, it may be possible to obtain an approximation of the location of the apices of the first and second molars with regard to the outer surface of buccal cortical plate. This might allow one to predict the difficulty in obtaining adequate access to an affected root. In addition to access, impingement on normal anatomical landmarks is also a major concern when evaluating a patient for apical surgery. In the mandible this relates to proximity of the mandibular canal to the apices of the teeth. There was
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FIG 6. Vertical section through mesial root apex of first molar depicting neurovascular bundle in lingual half of mandible. Distances shown are of total width (13 mm) from apex to buccal aspect (3.6 mm) and from apex to level of neurovascular bundle (N). Note lesion at apex. B, buccal; L, lingual.
a high degree of correlation between the radiographic and anatomical means of this distance related to the first and second molars, indicating that a periapical radiograph can be a good predictor of distance to the neurovascular bundle. All periapical radiographs from which measurements were obtained gave a clear view of the position of the mental foramen (Fig. 4). Xeroradiographs were also helpful as an adjunct in extrapolating the limits of the canal space. In specimens in which there were radiographically poorly defined canal spaces, well-defined anatomical bundles existed within the marrow spaces. These were just as distinct as the neurovascular bundles within the well-defined canals. This is in agreement with Carter and Keen (10) who found a single neurovascular bundle in the majority of dissected mandibles. Our findings also confirmed with more precision an earlier study by Olivier (1 l) defining the relationship of the mandibular canal to the apices of the tooth. The mesial apices of the second molar were significantly closer to the canal than the apices of the first molar. This is consistent with the fact that the mandibular canal begins its ascent at the level of the second molar. Rajchel et al. (9) found that the canal is at its lowest point at the distal half of the first
molar and not at the mental foramen. From these results, it would seem that there would be more concern for impingement on the neurovascular bundle in the region of the second molar than the region of the first molar. At present, there has been no evidence that apical surgery on mandibular second molars, in contrast to first molars, results in an increased incidence of nerve impairment. Few studies have reported a high incidence of paresthesia associated with apical surgery on mandibular molar teeth. Several reasons could account for this. It has been established that the failure rate of conventional endodontics is higher on anterior teeth than on posterior teeth (12). Since the apicoectomy is frequently used where conventional endodontics has failed, the majority of the studies dealing with surgical success rates and complications have examined anterior teeth. The relative thin buccal cortical plate in the anterior region and the ease of visibility offer a favorable environment in which this procedure can be performed. Apicoectomy on lower molar teeth is an infrequently encountered surgical procedure. Increased difficulty with access, fear of impingement on neurovascular bundle and inexperience of the surgeon may lead a practitioner to recommend extraction in lieu of recommending an apicoectomy on a mandibular molar. As a result, nerve impairments associated with lower molar apicoectomies are too few to predict the incidence and etiology. Because of the strong correlation in the present study between the vertical radiographic measurement and the actual anatomical measurement, paralleling technique periapical radiographs are a valid tool to assess the position of the inferior alveolar nerve. Thus, the potential for nerve damage can be assessed by the clinician when apical surgery is performed. Furthermore, our findings quantitatively demonstrate that access to the second molar can present a greater problem than access to the first molar because of the significantly greater horizontal distance between the apex of the second molar and the buccal cortical plate. Therefore, fear of impingement on the anatomical structures and perceived difficulty with surgical access do not seem to warrant avoidance of the first mandibular molar apical surgery. On the other hand, it is suggested from this study that the second molar may present a significantly more difficult surgical procedure from the standpoint of access and potential nerve damage.
We wish to express our appreciation to Mr. Ron Wade, Director of Maryland State Anatomy Board, for his assistance in obtaining cadaver specimens, Dr. Larry Jack for his photographic assistance, and Ms. Jacqueline Bayly for manuscript preparation. This study was, carried out in partial fulfillment of the requirements for the Master of Science in Oral Biology at the University of Maryland. Dr. Frankle was a postgraduate endodontie resident, University of Maryland Dental School, Baltimore, MD, and is currently in private practice in Washington, DC and Maryland. Dr. Seibel is an associate professor of anatomy at the University of Maryland. Dr. Dumsha is director, Postgraduate Endodontic Program, University of Maryland.
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