0099-2399/88/1407-0325/$02.00/0 JOURNAL OF ENDODONTICS Copyright �9 1988 by The American Association of Endodontists

SCIENTIFIC ARTICLES

Printed in U.S.A. VOL. 14, NO. 7, JULY 1988

Root Form and Canal Anatomy of Mandibular Second Molars in a Southern Chinese Population

Richard Thomas Walker, BDS, MSc, FDS, RCPS

Racial differences in dental crown morphology have long been recognized, but the frequency of geneti- cally determined variations in root form and canal anatomy has not been established. Features of root and root canal morphology occurring at particularly high frequency in Mongoloid populations have not been systematically documented. The implications that such features may have upon clinical endodon- tics have not been fully reported.

In this study 100 mandibular second molars ex- tracted from Hong Kong Chinese patients were ex- amined visually and radiographically. For each tooth, the number of roots, root canals, and apical foramina were noted. Fifty-two percent of the man- dibular second molars examined were found to have complete root fusion and 55% of the teeth were found to have only two canals. The high prevalence of fusion of roots in this sample of Chinese teeth emphasizes the way in which racial divergence may have a direct influence upon the endodontic mor- phology of the teeth of the adult dentition.

Textbooks of endodontics present descriptions of the root and canal forms of permanent teeth (1, 2). The studies upon which these descriptions are based were conducted in Europe and North America and involved teeth of a predominantly Cau- casoid origin. In the United States endodontic studies may involve the examination of teeth of more than one racial group, and where reports have been made with reference to race clear differences have been found. For example, there is a higher prevalence of mandibular first premolars with two canals in the American black population (3, 4).

Commonly used textbooks of dental anatomy state that the mandibular second molar has an appearance which resem- bles the first molar, and occasionally the roots may be fused (5-7). Anthropologically based studies of the Arctic Peoples indicate that the fusion of roots in the mandibular second molar is a prevalent condition (8, 9). The extent to which this tendency directly influences the endodontic morphology of these teeth has not been researched.

325

This study was undertaken to determine the frequency of mandibular second molars with fused roots in a sample of teeth of Chinese origin and to investigate the relationship of this trait to the root canal morphology of the teeth.

MATERIALS AND METHODS

Research workers in root canal anatomy frequently make use of extracted teeth in experimental and descriptive studies. All teeth extracted at the Prince Philip Dental Hospital are collected and catalogued (10). Teeth are fixed in 10% buffered formalin saline and are placed into small containers bearing the patient's hospital registration number, sex, and age at the time of extraction. Information on all of the extracted teeth is entered into a computerized database in order to facilitate access to all data. A random study sample of 100 teeth was established by referring to a computer print out of all those patients, who, during the course of treatment, had lost man- dibular second molars. Hospital records were consulted to verify the identity of the teeth and confirm that they had been extracted from Chinese patients.

Each specimen was examined beneath a quartz halogen light with the aid of a hand lens. The number of single- and multirooted teeth was recorded. Following Turner (9), a root was not considered separate and distinct unless it was inde- pendent of other roots for at least one-half of its length.

Each tooth was mounted in an apparatus designed and constructed for radiographing teeth in vitro (11). This con- sisted essentially of a perspex base upon which was mounted first the standard film holding, beam-aiming device used in the standardized technique of intraoral radiography, and sec- ond a support for the tooth. The teeth were radiographed from different horizontal angles using buccal, buccal oblique, and proximal views. At least two radiographs were taken of each tooth. The radiographs were examined on a viewer using a magnifying lens. Data relating to numbers of root canals and apical foramina were collected. All radiographs were indexed and filed to facilitate cross-referencing of specimens, radiographs, and tabulated data.

It was considered desirable to select specimens from the study sample of extracted teeth for cleating using modifica- tions of the techniques previously described (12-14). Ten

326 Walker

teeth were subjected to surface cleaning and placed in 4 to 6% sodium hypochlorite for 30 min to dissolve away surface organic debris. Conventional access cavities were cut using high- and low-speed burs, and the root canal systems were flushed with 4- to 6% sodium hypochlorite using a 22-gauge needle and 10-ml syringe. This process was repeated several times before washing with water and allowing them to dry overnight. The teeth were perfused with Indian ink and de- calcified in 5 to 10% hydrochloric acid over a 7- to 12-day period. The injected teeth were then dehydrated in successive solutions of 95% and 100% alcohol over a 3-day period. Finally the teeth were placed into a storage medium of methyl salicylate. These specimens were photographed before and after being rendered transparent to provide a permanent visual demonstration of their root canal form.

RESULTS

Forty-eight percent of the 100 mandibular second molars had completely separated mesial and distal roots. An aston- ishingly high figure of 52% of the teeth exhibited root fusion. In all instances of fusion, there was complete fusion of the buccal aspect of the root surface with longitudinal grooving of the lingual side (Figs. 1 and 2).

Although there was a tendency in these mandibular second molars for root fusion, only one specimen was identified as having a single root canal (Fig. 3). Fifty-five percent of the total had only two canals and 72% possessed two canals at the apex (Fig. 4).

Examples of some of the other anatomical configurations identified in this radiographic study are presented (Fig. 5).

Joumal of Endodontics

FIG 3. Buccal and proximal radiographic views of right mandibular second molar with a single C-shaped canal and apical foramen (original magnification x2).

FIG 1. Buccal view of right mandibular second molar with fused roots (original magnification x2).

FIG 4. Buccal and proximal radiographic view of left mandibular second molar with two canals and two apical foramina (original magnification x2).

The diverse nature of the canal forms of those teeth with incomplete separation of roots was evident when selected cleared specimens were examined (Fig. 6).

The number of root canals and apical foramina were compared with the findings of other investigators (15-17) in Tables l and 2.

FIG 2. Lingual view of right mandibular second molar with fused roots (original magnification x2).

DISCUSSION

In the mandibular second molars with fused roots, the depth of separation on the lingual side varied considerably from mild grooving (Fig. 7) to almost complete separation of the roots (Fig. 8). The complete failure of division of the roots on the buccal side, associated with the partial division on the lingual side, gives the roots a horseshoe shape when viewed cross-sectionally. The fusion of the mandibular second molar roots is similar to the Neanderthal condition described by Keith (18, 19) and the East Greenland Eskimo form (8). Clearly, the southern Chinese have a tendency to develop this condition; however, the exact ancestral mechanisms by which

Vol. 14, No. 7, July 1988 Root Form and Canal Anatomy 327

FIG 5. Radiographs of mandibular second molars with multiple canals (original magnification x2).

the form was acquired remain obscure. It can however be stated that, when comparisons are drawn, there is a similarity in this condition between the European Neanderthals and modem races of Mongoloid origin. An early photograph of the Naulette mandible taken from the work of Keith (19) depicts the horseshoe shape of the socket (Fig. 9).

Weine (2) states that the mandibular second molar com- monly has two roots. The mesial root most frequently has two canals and two separate apical foramina; the distal root having one canal and one apical foramen. This statement implies that the tooth tends to have a canal configuration which consists of a total of three canals and three separate foramina.

When recent studies of the canal anatomy of this tooth are analyzed, this is not found to be the case (Tables 1 and 2). Vertucci's study of teeth of mainly Caucasoid origin (17) reveals that the mandibular second molar has three canals but tends to have only two apical foramina. This would indicate that, although the mesial roots has two canals, they merge to form only one apical foramen.

Direct comparison of all of the studies performed on man-

dibular second molars is made difficult because many of them have not examined this tooth as a single entity but have combined the first and second mandibular molars (20-23). This study and that of Pineda and Kuttler (15) present a different picture of the canal morphology of these teeth. In both reports there appears to be a prevalence of teeth with two canals and only two apical foramina. The figures compare favorably and differences are not significant at the 5% level of significance (• = 0.28). The Mexican sample (15) was likely to be predominantly Mongoloid in nature and a com- parison of the combined results of this study and the Mexican study with the in vitro work of Vertucci (17) indicates that there is a significant difference in the frequency of mandibular second molars with two canals at the 0.1% level of significance (x 2 = 29.2). The underlying explanation for this may be based upon racial influences. The present work has emphasized the characteristic prevalence of fusion of the roots in this sample of Chinese teeth. It seems reasonable to assume that where there is incomplete separation of roots there may also be incomplete division of root canals, leading to a lesser number of identifiable canals.

328 Walker Journal of EndodonUcs

FiG 6. Cleared mandibular second molars with fused roots (original magnification x2).

TABLE 1. Number of root canals

Study Year No. of Teeth

No. of Root Canals (%)

1 2 3 4

Pineda and Kuttler (15) 1972 300 Hartwell and Bellizzi* (16) 1982 416 Vertucci (17) 1984 100 Walker (11 ) 1987 100

- - 58 36.4 5.6 1.0 4.1 89.4 5.5 - - 27 65 8.0 1.0 55 41 3.0

�9 C l in ica l s t u d y .

TABLE 2. Number of apical foramina

Study Year No. of Teeth

No. of Apical Foramina (%)

1 2 3 4

Pineda and Kuttler (15) 1972 300 Vertucci (17) 1984 100 Walker (11 ) 1987 100

- - 78.6 17.9 3.5 - - 65 30 5.0 12 72 15 1.0

FIG 7. Lingual view of left mandibular second molar with mild grooving of roots (original magnification x2).

Confluence of canals in the horseshoe or C-shaped root produces the C-shaped canal. Cooke and Cox (24) in a review of their clinical records suggest that the frequency of this root condition is in the region of 8%. In this sample of Chinese

Vol. 14, No. 7, July 1988 Root Form and Canal Anatomy 329

percent of the 100 teeth had one apical foramen and 72% had two separate apical foramina.

Financial provisions for this research were made in the form of a research grant from the University of Hong Kong.

I acknowledge the help and advice received from Professor C. E. Renson and Dr. N. Jablonski in the preparation of this article. My thanks also go to Mr. R. Leung, Audio-Visual Officer, Dental Illustration Unit, Hong Kong University.

Dr. Walker is senior lecturer, Department of Conservative Dentistry, Dental Faculty, University of Hong Koog, Hong Kong.

FIG 8. Lingual view of right mandibular second molar with almost complete separation of roots (original magnification x2).

FIG 9. TOOth sockets of the Naulette mandible (see Ref. 19).

teeth the C-shaped root form was as common as the normal two-rooted one. The tendency for canals to converge in these fused roots also increases the likelihood of there being only one apical foramen.

CONCLUSION

One hundred mandibular second molars of Chinese origin were examined visually and radiographically, and selected specimens were rendered transparent to demonstrate the con- figurations of root canals. Fifty-two percent of these teeth were single-rooted and 55% had only two root canals. Twelve

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