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ISSN (Print) 1013-9052
EISSN 1658-3558

The Saudi Dental Journal,
P.O. Box 52500,
Riyadh 11563,
Kingdom of Saudi Arabia 
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Mandibular cephalometric characteristics of a Saudi sample of patients having impacted third molars

Ali H. Hassan
Faculty of Dentistry, King Abdulaziz University, Saudi Board in Orthodontics - Western Region, Saudi Arabia

Keywords

Third molar; Impaction; Etiology; Mandibular geometry; Cephalometrics; Saudi


Abstract

Objective: To evaluate the cephalometric characteristics of mandibles of Saudi patients having impacted third molars and to compare them to those of patients having normally erupted third molars. Material and methods: One hundred and twenty-one Saudi adult subjects (59 females and 62 males; age: 20-40 years) were divided into two groups based on the status of the mandibular third molars: [1] impaction group and (2) normal group. Means and standard deviations of 21 cephalometric measurements related to mandibular geometry were measured and compared between the two groups using the unpaired t-test. Males and females in the impaction group were also compared with their equivalent subgroups in the normal group using the unpaired t-test.  Results: Anteroposteriorly, space distal to second molar, ramal width and mandibular body length were significantly less in the impaction group than in the control group. In addition, posterior teeth were more upright in the impaction group. Vertically, posterior alveolar height was significantly less in the impaction group. The Y-axis was significantly increased in the impaction group. The significance of these measurements was variable between males and females.  Conclusions: Third-molar impactions in the Saudis living in the Western region of Saudi Arabia were more likely to occur when inadequate retromolar space is present. This can be attributed to certain mandibular skeletal and dental features, among which the increased width of mandibular ramus and backward inclination of posterior teeth seem to be the most influencing factors in both sexes.

1. Introduction

Third molar is the most frequently impacted tooth (Andreasen, 1997). The registered frequency of its impaction was highly variable among the different populations with the highest reg­istered in a Singapore Chinese population at 68.6% (Quek et al., 2003). In a Saudi population living in the central region, the prevalence was reported to be 32.2% (Haidar and Shal- houb, 1986). In the Western region of Saudi Arabia, however, a higher frequency was reported at 40.5% (Hassan, 2010). Interestingly, impaction was significantly more common in the mandibles than in the maxillae (Hassan, 2010).

Many international studies were conducted to determine the etiology of third-molar impaction. Lack of space between the distal surface of the second molar and the ramus (retromo- lar space) was found by many investigators to be a significant etiological factor for mandibular third-molar impaction (Bjork et al., 1956; Ricketts, 1972; Schulhof, 1976; Forsberg et al., 1989). Several other skeletal and dental factors were also blamed to contribute to the impaction of third molars. These include, the size, the growth amount and direction of the man­dible (Broadbent, 1943; Bjork et al., 1956; Bjork, 1963; Rich­ardson, 1977), the remodeling and the width of the ramus, the rate of maturation of third molars, the inclination of pos­terior dentition and the size of dentition relative to the jaws (Begg, 1954; Bjork et al., 1956; Bjork, 1963; Ricketts, 1972; Richardson, 1977; Forsberg, 1988).

Bjork (1963) in a longitudinal cephalometric study found that the space distal to the second molar was considerably reduced in most of the mandibular third-molar impaction cases. This agrees with the findings of (Ricketts, 1972; Schulhof, 1976; Forsberg et al., 1989). Bjork (1963) identified two skeletal and two dental factors that may cause third-molar impaction: vertical direction of condylar growth, a small total mandibular length as the distance from the chin point to the condylar head, backward directed eruption of the dentition and retarded maturation of third molars. However, Broadbent (1943) found that the inability of the mandible to achieve its full growth potential may contribute to the impaction of third molar. Capelli (1991) suggested that third-molar impaction is more likely to occur in vertically growing mandibles. A long ascending ramus, short mandibular length, and greater mesial crown inclinations of the third molars seem to be indicative of third-molar impaction. Breik and Grubor (2008) concluded that subjects with brachyfacial facial growth pattern demon­strated two times lower incidence of third-molar impaction than subjects with dolichofacial growth pattern.

Ricketts (1972) believed that the direction of tooth eruption plays a critical rule for third molar. This agrees with the find­ings of Bjork (1963) and Bjork et al. (1956) who stated that dis­tal direction of eruption is associated with lack of space for third molar. It also agrees with the observations of Begg (1954) who attributed impaction to insufficient forward move­ment of the teeth of modern man due to the lack of interprox- imal attrition that was observed in ancient skulls. Richardson (1977) concluded that the space distal to the mandibular second molar increases over the five years following full tran­sition form the primary to permanent dentition and this in­crease is due to equal but highly variable contribution from the remolding resorption of the anterior border of the ramus and the mesial movement of the first molar.

An unfavorable path of eruption might also be blamed for mandibular third-molar impaction. Richardson (1977) found that the developmental initial mesial angulation of third mo­lars to the mandibular plane was observed more in subjects with impacted third molars than in those with normally erupted third molars.

Extraction of permanent second molars (Cavanaugh, 1985; Gooris et al., 1990), first molars (Bayram et al., 2009), or pre­molars (Kim et al., 2003) was found to reduce the frequency of third-molar impaction due to the increased eruption space accompanying the mesial movement of the molars during space closure.

These findings point out that mandibular third-molar impaction is associated with certain dental and skeletal man- dibular features that are controversial and different among the different populations. This is beside the fact that different samples and methods of analyses were used in the supporting studies. In Saudi Arabia, the etiology of the relatively high fre­quency of mandibular third-molar impaction remains a mys­tery. Therefore, it seems important to investigate the etiology of third-molar impaction in Saudis using thorough cephalo- metric analysis.

The objective of this study was to evaluate the cephalomet- ric characteristics of mandibles having impacted third molars in a sample of Saudi patients living in the western region of Saudi Arabia, and to compare them with those of patients hav­ing normally erupted third molars.

2. Material  and methods

This study was approved by the Ethical Research Committee at King Abdulaziz University, Faculty of Dentistry (KAU-FD). Records of patients, registered for the treatment at the Faculty of Dentistry, King Abdulaziz University, during 2003-2004 were reviewed, from which 121 patients (59 females and 62 males; age: 20-40 years with a mean age of 23.89 years) were selected. The inclusion criteria were: (1) non-syndromic pa­tients, (2) no history of orthodontic treatment, (3) presence of initial orthopantomogram (OPG) and lateral cephalometric radiograph (LC), (4) presence of complete normal mandibular dentition, and (5) presence of mandibular third molars which have complete root formation and either fully erupted or impacted.

Subjects were divided into two groups based on the status of the mandibular third molars: (1) impaction group (IG), which included patients having one or two of incompletely erupted mandibular third molar with radiographic evidence of apical closure or near closure (n = 71) and (2) normal group (NG), which included patients having normally erupted third mandibular third molars (n = 50). Groups were further divided into male and female groups: male impaction group (MIG) (n = 41), male normal group (MNG) (n = 21), female impaction group (FIG) (n = 30) and female normal group (FNG) (n = 29).

All cephalometric radiographs were traced and analyzed manually. Fourteen landmarks and 21 linear and angular measurements related to mandibular geometry were identified (Tables 1 and 2, Figs. 1 and 2).

To assess the intra-examiner reliability, 15 randomly se­lected lateral cephalograms were re-traced and re-analyzed, two weeks after the first measurements. The method error was calculated using Dahlberg's double determination formula (Dahlberg, 1940).

Statistical analysis was performed using SPSS software package (SPSS for Windows 98, version 16.0, SPSS Inc, Chicago, IL, USA). Means and standard deviations of all the variables were calculated. An unpaired t-test was used for the statistical analysis at a significance level of P < 0.05 to compare variables between the two groups; IG and NG. Males and females in the impaction group were also compared to their equivalent subgroups in the NG using unpaired t-test (P < 0.05).

3. Results

The range of the method errors was between 0.2 and .95° for the angular measurements and between 0.25 and 1 mm for the linear measurements. (Table 3).

Means and standard deviations of the measured variables are presented in Table 4.

Comparing the anteroposterior dimensions of the mandibles (MAPD) between the IG and NG has shown the following: the retromolar space (Xi-M2) was significantly decreased in the IG as compared to NG and it was in the range of 21.28 ± 3.06 (P < 0.001). Skeletal pattern was Class I in both groups, although A and B points were significantly retruded in the IG as compared to the NG (P < .05). SN inclination angle relative to FH was significantly increased in the IG than in the NG (P < 0.05). Mandibular body length was significantly smaller in the IG than in the NG (P < 0.05). Ramal width was also sig­nificantly increased in the impaction group as compared to NG (P < 0.05).

Comparing the vertical dimensions of the mandibles (MVD) has shown the following: Condylar axis length (Dc-Xi) was not significantly different between the two groups. (P > 0.05) Y-axis was also significantly increased in the IG than in the NG (P > 0.05). Posterior alveolar height of the mandible was significantly increased in the IG (P > 0.05). Mandibular first molar was significantly more up­right in the IG than in the NG (P > 0.001).

Effective mandibular length (Co-Gn), ramal length (Co-Gn), and condylar axis length were all insignificantly different between the two groups. In addition, lower face height and angle, mandibular arc angle, and mandibular plane angle were also insignificantly different between the two groups.

In the FIG, only four mandibular measurements were sig­nificantly different than the FNG; retromolar space, ramal width, posterior alveolar height and inclination of mandibular first molar (Table 5).

In the MIG, retromolar space, A and B projection, effective mandibular length, mandibular body length, corpus length, condylar axis length, Y-axis, and ramal width were signifi­cantly different than in the MNG (Table 6).

4. Discussion

Knowing the development of third molar, its prognosis, its eruption pattern, its possible effect on the dentition during and after orthodontic treatment as well as the effect of ortho­dontic treatment on the third molar eruption is an important issue for orthodontists to formulate a successful long term treatment plan.

The etiology of mandibular third-molar impaction was investigated by many investigators and several dental and skel­etal factors were blamed. Lack of space in the retromolar re­gion seems to be the main factor, which can be attributed to either the failure of the mandible to attain its adequate size or the tooth size-jaw size discrepancy. In addition, narrow alveolar arch can be a retarding factor for the eruption of third molar. Finally, late third molar maturation combined with early physical maturation can also be a contributing factor (Svendsen and Maertens, 1997). These etiological factors seem to be highly variable and depend in many aspects on the stud­ied population. The current study was designed to evaluate the cephalometric characteristics of mandibles in patients having third-molar impaction in order to find out if mandibular geometry contributes to the impaction as an etiological factor in the selected Saudi population living in the western region. A comprehensive evaluation of the geometry of the mandible was performed using 21 cephalometric variables, taken form differ­ent analyses to evaluate the anteroposterior and the vertical dimensions of the mandible, as well as its relationship to the cranial base.

The minimum sample size needed for this study was esti­mated to be 45 patients in each group. It was calculated based on the result of a pilot study on 40 patients using the following equation:

n = (2s2Xt2)/D2

where n = minimum number of subjects needed to achieve sig­nificance at 0.05; s = average standard deviation for the two groups; t = t-test value at P = 0.05; D = half of the means standard deviation of the two groups.

The number of subjects was increased to provide adequate number for the evaluation of males and females separately.

The present study found that geometry of the mandible, which is a reflection of the growth and maturity of the mandi­ble, seems to be different in many aspects in people having third-molar impaction which might be blamed for the impac- tion. The present results indicate that patients with impacted mandibular third molars had smaller retromolar space (21.28 mm) when compared to those with normally erupted third molars (27.6 mm) (P < 0.001). This is in agreement with previous studies (Bjork et al., 1956; Ricketts, 1972; Schulhof, 1976; Forsberg et al., 1989). Moreover, Ricketts, 1972 and Schulhof, 1976 have concluded that a retromolar space less than 21 mm is associated with impaction group and 31 mm or more is associated with normally erupted third molars. In the present study, the retromolar space was found to be 21.28 mm in the impaction group and 27.6 mm in the normal ones. Legovic et al. (2008), however, concluded that the pres­ence of adequate space for mandibular third molars does not guarantee its normal development. The author believes that these numbers can be used as references to clinically predict third-molar impaction at a later age, around the age of 18 years when most of the remodeling of the ramus is completed and the third molars are ready to erupt. In addition, they can be used as references to evaluate the space availability in the posterior dental segments, especially when attempting molar distalization.

In the present study, different linear measurements were used to evaluate the MAPDs. Ramal width seems to contribute strongly to the impaction of third molars as it was significantly increased in the impaction group as compared to the normally erupted group. The increased ramal width was attributed to the failure of remodeling resorption of the anterior border of the ramus (Bjork, 1963; Richardson, 1977; Behbehani et al., 2006) which provides inadequate sagittal space distal to second molars in the mandibles (Richardson, 1977).

At the same time, mandibular body length (Go-Gn) was smaller in the impaction group (79.28 ± 5.23 mm) (P < 0.002). This agrees with the findings of Broadbent (1943), Bjork (1963), and Capelli (1991) and disagrees with the findings of Dierkes (1975) and Kaplan (1975) who did not show any significant difference in the mandibular length between subjects with impacted and erupted third molars. However, the present results showed that effective mandibular length was insignificantly decreased in the IG. This disagrees with the findings of Bjork (1963).

MVDs were also investigated using different measurements, which included condylar axis length, ramal length, and vertical alveolar height. In the present study, the increased vertical alveolar height was the only vertical measurement that was associated with the impaction of third molars.

Inclination of lower posterior teeth (L6-MP angle) was also assessed in the two groups. Interestingly, the angle was signif­icantly reduced in the impaction group. This agrees with the findings of several investigators (Ricketts, 1972; Bjork, 1963; Begg, 1954; Richardson, 1977; Shiller, 1979; Capelli, 1991). They, except Bjork, demonstrated that the initial angulation of the lower third molar to the mandibular plane can be a fac­tor in predicting impaction. Those studies based their conclu­sion on evaluating the path of eruption of the third molar itself which was difficult to evaluate in the present study, in which the first molar inclination to mandibular plane was used instead.

Assessing the rotational and angular measurements of the mandibles revealed that the orientations of mandibular plane, occlusal plane and gonial angle are indifferent between people having impacted third molars and those who do not. This dis­agrees with Sakuda et al. (1976), and Leighton and Hunter (1982) who demonstrated larger mandibular plane angle and occlusal plane angles to Sella-Nasion (S-N) in patients having dental crowding, who are expected to have impacted third molars, as compared to patients having spacing.

The present findings also disagree with Behbehani et al. (2006) who found that small mandibular plane and gonial an­gles are associated with an increased risk for mandibular impaction. The only measurement that was found different was the Y-axis angle (to SN) which was larger in the IG when compared to the NG. This agrees with Breik and Grubor (2008) who found that brachyfacial subjects have a lower incidence of mandibular third-molar impactions. This was explained by the fact that greater growth potential of the mandible is expected in brachyfacial subjects. However, it seems that growth pattern has no effect on the impaction of third molar in the Saudis living in the Western region. This is because the other measurements that determine the growth pattern such as the lower face height and mandibular arc angle and mandibular plane angle were found to be indifferent be­tween the IG and the NG. In addition, the Saudis in the wes­tern region are characterized by large mandibular plane, occlusal plane and Y-axis angles (Hassan, 2006).

Breik and Grubor (2008) findings contradict those of Behbehani et al. (2006) in explaining the effect of the rotational growth of the mandible on the resorption of the anterior bor­der of the ramus and consequently on third-molar impaction. Breik and Grubor (2008) believe that growth potential is great­er in brachyfacial growth pattern, which allows more remodel­ing resorption of the anterior border of the ramus. Behbehani et al. (2006) believe that vertical growth of condylar, which is associated with forward mandibular growth, predisposes to less resorption of the anterior border of the ramus and conse­quently to greater third-molar impaction. The present findings disagree with the two theories. Although the ramal width was found increased in the IG, the growth pattern was indifferent between the IG and the NG.

The sample was further divided into males and females to see if there is any gender difference in mandibular geometry as related to the impaction of third molars. Both impaction groups (FIG and MIG) were compared to the corresponding normal groups.

Patients in both MIG and FIG had smaller retromolar spaces, larger ramal width and more backward inclination of the posterior teeth, when compared to their corresponding normal groups. However, most of the remaining differences between IG and NG were found among males. Males with im­pacted third molars were found to have more retruded A and B points and an increased Y -axis than those with normally erupted third molars.

In addition, MAPDs were significantly smaller only in MIG, which disagrees with the findings of Richardson (1977) who found smaller mandibular length in the FIG as compared to the FNG. They also disagree with Kaplan (1975) who found no significant sex predilection when comparing impacted group to the erupted group. This observed sex variability among the different studies seems to be due to the variability of the timing of mandibular skeletal maturity between males and females and among the different populations. Interest­ingly, the present findings indicate that MAPDs can be consid­ered as influencing factors on the impaction of mandibular third molars in males but not in females, possibly due to the presence of late mandibular growth in males, which continues until the age at which third molars are about to erupt. In females, mandibular growth rate decreases tremendously at menarche, which is greatly affected by environmental factors such as health and socioeconomic conditions and energy balance related to physical activities, rather than genetics. In Saudi females, the estimated age for menarche is 13.05 (Babay et al., 2004), which is different from many other populations (Thomas et al., 2001). This variability in the timing of menar- che among the different populations can explain the variability seen in the literature regarding the sex predilection as related to third-molar impaction.

The present study represents the first study to evaluate the geometry of the mandible as related to third-molar impaction in the Saudi population. Future studies are required to evalu­ate other possible factors such as the tooth mass which if eval­uated together with MAPDS (Tooth size-jaw size discrepancy) can give more accurate understanding of the effect of MAPDs on third-molar impaction in the Saudi population and to find predictors for impaction in the Saudi population. In addition, future studies are required to investigate the etiology of upper third molars.


5. Conclusion

The high frequency of mandibular third molar in the Saudis living in the Western region can be attributed to certain skeletal and dental features. Lack of retromolar space distal to second molar seems to be the main etiological factor for third-molar impaction. Other specific dental and skeletal mandibular features can also be blamed for the impaction of third molars. These include:

-    the presence of wide mandibular ramus in both sexes.
-    upright lower posterior dentition in both sexes
-    shorter MAPDs especially in males.
-    hyperdiverged Y-axis especially in males
-    increased posterior alveolar height especially in females.

However, angular and rotational measurements of the mandibles such as mandibular and occlusal planes, mandibu- lar arc and gonial angles seem to be irrelevant to the impaction of third molars.

These features should be considered when assessing third molars, especially when formulating a long-term orthodontic treatment plan.

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Tables and Figures

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