The incidence of mandibular third molar impactions in different skeletal face types

SCIENTIFIC ARTICLE Australian Dental Journal 2008; 53: 320 324 doi: 10.1111/j.1834-7819.2008.00073.x The incidence of mandibular third molar impactions in different skeletal face types O Breik,* D Grubor* *Royal Dental Hospital of Melbourne, Victoria. ABSTRACT Background: The cause of mandibular third molar impaction is said to be due to inadequate space between the distal of the second mandibular molar and the anterior border of the ascending ramus of the mandible. The amount of space is determined primarily by facial growth. This study aimed to assess whether different patterns of facial growth lead to a different incidence of mandibular third molar impaction. It was hypothesized that those with predominantly horizontal (brachyfacial) would have lower incidence of mandibular third molar impaction compared with those with a predominantly vertical growth pattern (dolichofacial). Methods: Ninety-eight dental records were sourced from the records of orthodontic patients in the Royal Dental Hospital of Melbourne and all lateral cephalometric radiographs and orthopantomograms were assessed. The degree of impaction was determined by the Pell and Gregory system, and the facial type categorized by the facial axis angle. Results: The overall rate of mandibular third molar impaction was 58.76 per cent. Those with a facial axis angle >93 (brachyfacials) demonstrated an almost two times lower incidence of mandibular third molar impaction as compared to subjects with a facial axis angle <87 (dolichofacials). Conclusions: Within the limitations of the study, it was concluded that the greater horizontal facial growth pattern of brachyfacial subjects over dolichofacial subjects provides increased space for full eruption of the mandibular third molars. Key words: Third molar, impaction, dolichofacial, brachyfacial, skeletal face type. (Accepted for publication 16 February 2008.) INTRODUCTION Impacted third molars are common. The impaction rate is higher for third molars than for any other teeth. 1 A study by Dachi and Howell in 1961, 2 analysed 3874 radiographs and determined that impaction of third molars was more prevalent in the maxilla than in the mandible. The incidence was determined as 21.9 per cent for maxillary third molars and 17.5 per cent for mandibular third molars. No gender differences were noted in the incidence of impaction of third molars. The cause of third molar impaction has been suggested to be due to inadequate space in the retromolar area, between the distal of the second molar and the anterior border of the ascending ramus of the mandible. 3,4 Broadbent 5 believed that when a third molar became impacted, it was due to an inability of the mandible to achieve its full growth potential. Begg 6 claimed that there was insufficient forward movement of the dentition of modern man due to a lack of interproximal attrition which was observed to be greater in ancient skulls. Forsberg 7 demonstrated that failure of eruption and degree of arch crowding were proportional. Bjork 3 showed that third molar impaction was not only associated with a reduced amount of growth, but also with a downward as opposed to forward growth direction. Third molar impactions are rarely observed after second molar extraction, suggesting an increase in eruption space. 8 Recent studies have also demonstrated that premolar extraction therapy as part of orthodontic treatment leads to a reduced frequency of third molar impaction in both the maxilla and mandible. 9 From these studies, it seems that the failure of mandibular third molars to erupt is most affected by a lack of space in the alveolar arch between the distal of the second molar and the ascending ramus. Hence, an appreciation of mandibular and facial growth may assist in predicting mandibular third molar eruption. The facial skeleton grows in a forward and downward direction. In a mesofacial growth pattern, there is a relative harmony in these two directions, leading to 320 ª 2008 Australian Dental Association

Mandibular third molar impactions in different skeletal face types Table 1. Percentage of mesoangular impactions as a percentage of total impactions Mesoangular impactions Table 2. Percentage of horizontal impactions as a percentage of total impactions Horizontal impactions Percentage Brachyfacial Fac. Axis >93 86.21 Mesofacial Fac. Axis 87 90 92.68 Dolichofacial Fac. Axis <87 80.49 Percentage Brachyfacial Fac. Axis >93 10.34 Mesofacial Fac. Axis 87 90 7.32 Dolichofacial Fac. Axis <87 14.63 a facial profile which is described cephalometrically by a90 (±3) facial axis angle, or a 26 (±3) mandibular plane angle. Brachyfacial is the term used to describe the person with a short anterior face height and a wide face ( the short face syndrome ), and was described by Opdebeek and Bell. 10 Both the mandibular plane angle and the gonial angle are relatively small and the ramus height is increased, cephalometrically >93 facial axis angle, or <29 mandibular plane angle. Dolichofacial is the term used to describe a long anterior face height and a narrow face ( the long face syndrome ). There is a clockwise rotation of the mandible leading to a large mandibular plane angle and gonial angle but a relatively short ramus. Cephalometrically <90 facial axis angle, or >26 mandibular plane angle. The aim of this study was to evaluate the incidence of mandibular third molar impactions as compared between long-faced (dolichofacial) and short-faced (brachyfacial) subjects. It is hypothesized that brachyfacial subjects will have a significantly lower incidence of mandibular third molar impactions than dolichofacial subjects. MATERIALS AND METHODS A list of patients who had visited the orthodontic department at the Royal Dental Hospital of Melbourne in 2004 and 2005 was sourced. The criteria for inclusion of files in the study was: (1) adequate records with complete history of orthodontic and surgical treatment; (2) pre-orthodontic treatments OPG with complete dentition and mandibular third molars which have root formation at least two-thirds complete. Generally, patients are above the age of 16; (3) preorthodontic treatment lateral cephalometric radiograph taken at the same time as the OPG. In total, 98 files were deemed appropriate for inclusion. The data recorded were age of the patient, gender, eruption or degree of impaction of mandibular third molars, and the facial axis angle. The degree of impaction of each of the mandibular third molars was classified according to the Pell and Gregory classification system. 11 This classification system involves two main classes: 1, 2, 3 and A, B, C. Classes 1, 2 and 3 relate to the relationship of the third molar to the anterior border of the ramus. Class 1 when mesiodistal (MD) width of tooth is completely anterior to the ramus, Class 2 when partly within the ramus, and Class 3 when completely within the ramus. Classes A, B and C relate to the occlusal height as compared to the adjacent second molar. Class A when level with the adjacent tooth, Class B when between the occlusal and cervical margins of the adjacent tooth, and Class C when the occlusal is below the cervical margin. The facial type was determined by a measure of the facial axis angle. All angles were calculated using the Dimaxis computer system by the same investigator (OB). The facial axis angle was measured as the posterior angle created by the lines Ba-Na and Pt-Gn. The mean was 90 ±2. An angle of >93 was regarded brachyfacial, and an angle of <87 was regarded dolichofacial. Out of the nine subjects, 39 had a facial axis angle regarded brachyfacial, 30 had a facial axis angle regarded dolichofacial, and 28 were mesofacial. The results were then tabulated and recorded in graphs for analysis. RESULTS The results demonstrated an overall rate of mandibular third molar impaction of 58.76 per cent. As regards to gender differences, the results showed a rate of 43 per cent erupted in males as compared with 45 per cent erupted in females. The results illustrate a potentially significant difference between the percentage of erupted third molars within the brachyfacial and dolichofacial subjects. Figure 1 shows the percentage of erupted mandibular third molars between the groups of brachyfacial (Fac. Axis >93), mesofacial (Fac. Axis 87 90) and dolichofacial (Fac. Axis <87). Brachyfacial Mesofacial Dolichofacial Fig 1. Percentage of erupted third molars between the different facial axis groups. ª 2008 Australian Dental Association 321

O Breik and D Grubor DISCUSSION Failure of mandibular third molars to erupt is most affected by a lack of space in the alveolar arch between the distal of the second molar and the ascending ramus. It is mandibular growth that is associated with the provision of adequate space for correct positioning of the mandibular third molars. Bjork et al. 3 noted that in cases of mandibular third molar impaction, the alveolar arch space behind the second molar was reduced in 90 per cent of cases. It was also demonstrated that the space necessary for the third molar was diminished by three separate skeletal factors. These are a short mandibular length (measured from gonion to the condylar head), vertical direction of condylar growth, and by backward directed eruption of the dentition. It was concluded that out of the above factors which contributed to diminished arch space, the most influential was the vertical direction of growth of the condyle, which was seen in dolichofacial patients. The growth was in a predominantly vertical component in those with impacted mandibular third molars. 12 14 The second most influential factor was mandibular length. A short mandibular length predisposed to mandibular third molar impaction. 3,14,15 However, Kaplan 13 and Dierkes 16 did not show significant differences in mandibular length between subjects with impacted and erupted third molars. According to a study by Eroz et al., 17 the mandibular length was shorter in the long-face facial type, consistently supporting the hypothesis that dolichofacial patients have an increased risk of third molar impaction. Richardson 14 also demonstrated that the initial angulation of the lower third molar to the mandibular plane can be a factor in predicting impaction. This was not shown by Bjork, 3 but was also demonstrated by Shiller 18 and Capelli. 12 Our results are consistent with our hypothesis that the incidence of mandibular third molar impaction is greater in patients with a facial axis angle that is < 87, which is consistent with that of a dolichofacial (long face) profile. The results demonstrated an almost two times greater risk of impaction in the mesofacial and dolichofacial subjects. An interesting result was that the difference in incidence of impactions between the dolichofacial subjects and the mesofacial subjects was insignificant. A t-test analysis was performed with the results. A comparison between brachyfacial and dolichofacial subjects returned a p value of 0.000158, implying statistical significance of the results. A comparison between dolichofacial and mesofacial returned a p value of 0.211, implying a statistically insignifant difference. However, any statistical conclusions should be accepted with caution when taking into consideration the small sample size. A study by Leighton and Hunter 19 demonstrated that crowding was associated with larger mandibular plane angle and occlusal angles to Sella turcica Nasion (S N) compared with patients with spacing. Spacing was associated with the anterior segment of the mandible being positioned further forward or less downward implying a smaller mandibular plane angle. Similar results have been described by Sakuda et al. 20 The above conclusions demonstrate that in shortfaced patients, in whom the direction of growth is more forward than downward, there is a more horizontal occlusal plane length requiring greater resorption from the anterior border of the ramus during growth, and subsequently resulting in a less crowded occlusion and greater space for the eruption of third molars. Nanda et al., 21 also noted that the amount of time of growth differed between different facial types. It was shown that brachyfacial patients exhibited a prolonged period of facial growth in contrast to dolichofacial patients. This may also account for the greater amount of resorption of the anterior border of the ramus. The rate of impaction of mandibular third molars in our study was 58.76 per cent. This result is significantly higher than the results of the Dachi and Howell study, 2 which demonstrated the rate of mandibular third molar impaction to be 17.5 per cent. The difference can be explained by the limited number of subjects within our study and the sample type. Being orthodontic patients, they are more likely to suffer from a malocclusion and potential crowding, and so are likely to have a higher incidence of mandibular third molar impaction than a random population sample. The similar rate of impaction between genders is consistent with the study by Dachi and Howell. 2 Over 80 per cent of the impactions in all facial types were of the mesioangular type. This is similar to results from other studies, 22 where the predominant impaction type is the mesioangular type. The incidence of horizontal impactions was found to be greater in the dolichofacial subjects than the brachyfacial and mesofacial subjects. However, the difference was not statistically significant due to the small sample size. The limitations of this study include the limited sample size due to the difficulty in acquiring information of patients with at least two-thirds of the third molar root completed, who have had a lateral cephalogram, an OPG taken and have had no previous orthodontic treatment. Eruption time and impaction status is an unpredictable phenomenon and may be more dynamic than previously anticipated. The age range of the subjects used for this study was between 16.5 years and 21 years of age. Mandibular growth is completed normally by the age of 16 17 years of age. 23 The third molar is typically at its later stages of development by the age of 18 according to Graveley. 24 However, recent literature has demonstrated that a 322 ª 2008 Australian Dental Association

Mandibular third molar impactions in different skeletal face types third molar that is impacted at the usual time of eruption may upright and erupt later in life. Sewerin and von Wowern 25 demonstrated changes in the positioning of the third molars between the ages of 20 and 24. Richardson 26 concluded that between the ages of 18 and 21, many of the unerupted third molars changed position appreciably, although rarely leading to clinical eruption. A study of Jordanian subjects performed by Hattab et al., 27 showed that by the age of 19, some previously impacted teeth became erupted into functional occlusion. Kruger et al. 28 also showed full eruption of third molars impacted at the age of 19, by the age of 26. The study by Venta et al. 29 determined that changes like this can be seen up to the age of 32. Hence, in this study the classification of impaction describes only the impaction status at the time of taking the radiograph, and not the potential final status of the third molar. Further avenues for research may be to assess whether the changes in impaction status are more likely to be seen in dolichofacial or brachyfacial subjects. The study by Nanda et al. 21 noted that brachyfacial patients exhibited a prolonged period of facial growth in contrast to dolichofacial patients. It would be interesting to observe if this additional growth means that it is more likely for changes in impaction status to occur in brachyfacial subjects over time. Also, assessment of impaction incidence as it relates to mandibular length is another interesting avenue to study. CONCLUSIONS Within the limitations of this study, our hypothesis that brachyfacial subjects have a lower incidence of mandibular third molar impactions was validated. The additional space required for the eruption of the third molar may be available due to the greater growth potential of the mandible in brachyfacial subjects or the more forward direction of growth of the mandible leading to a more horizontal occlusal plane, which altogether may contribute to greater resorption of the anterior border of the ramus. Further research may give us a greater understanding of third molar impaction and mandibular growth. ACKNOWLEDGEMENTS We would like to acknowledge Dr. Judy Lewis, Head of the Department of Orthodontics, Royal Dental Hospital of Melbourne, for her advice and guidance. REFERENCES 1. Bishara SE, Andreason G. Third molars: a review. Am J Orthod 1983;83:131 137. 2. Dachi SF, Howell FV. A survey of 3874 routine full-mouth radiographs. II. A study of impacted teeth. Oral Surg Oral Med Oral Pathol 1961;14:1165 1169. 3. Björk A, Jensen E, Palling M. Mandibular growth and third molar impaction. Acta Odont Scand 1956;14:231 271. 4. Shilling G. Development and eruption of the mandibular third molar and its response to orthodontic therapy. Angle Orthod 1973;43:271 278. 5. Broadbent BH. The influence of the third molars on the alignment of teeth. Am J Orthod 1943;29:312 330. 6. Begg PR. Stone age man s dentition. Am J Orthod 1954;40:298 312, 373 383, 517 531. 7. Forsberg CM. Tooth size, spacing, and crowding in relation to eruption or impaction of third molars. Am J Orthod Dentofacial Orthop 1988;94:57 62. 8. Gooris CGM, Årtun J, Joondeph DR. Eruption of mandibular third molars after second molar extractions: a radiographic study. Am J Orthod Dentofacial Orthop 1990;98:161 167. 9. Kim T, Årtun J, Behbehani F, Artese F. Prevalence of third molar impaction in orthodontic patients treated non-extraction and with extraction of 4 premolars. Am J Orthod Dentofacial Orthop 2003;123:138 145. 10. Opdebeek H, Bell WH. The short face syndrome. Am J Orthod 1978;73:499 511. 11. Pell GJ, Gregory BT. Impacted mandibular third molars: classification and modified techniques for removal. Dent Digest 1933;39:330 338. 12. Capelli J. Mandibular growth and third molar impaction in extraction cases. Angle Orthod 1991;61:223 229. 13. Kaplan RG. Some factors related to mandibular third molar impaction. Angle Orthod 1975;45:153 158. 14. Richardson ME. The etiology and prediction of mandibular third molar impaction. Angle Orthod 1977;47:165 172. 15. Ricketts RM. Studies leading to the practice of abortion of lower third molars. Dent Clin N Am 1979;23:393 411. 16. Dierkes DD. An investigation of the mandibular third molars in orthodontic cases. Angle Orthod 1975;45:207 212. 17. Eröz B, Çeylan I, Aydemir S. An investigation of mandibular morphology in subjects with different vertical facial growth patterns. Aust Orthod J 2000;16:16 21. 18. Shiller WR. Positional changes in mesio-angular impacted mandibular third molars during a year. J Am Dent Assoc 1979;99:460 464. 19. Leighton BC, Hunter WS. Relationship between lower arch spacing crowding and facial height and depth. Am J Orthod 1982;82:418 425. 20. Sakuda M, Kuroda Y, Wada K, Matsumoto M. Changes in crowding of teeth during adolescence and their relation to the growth of the facial skeleton. Trans Eur Orthod Soc 1976:93 104. 21. Nanda SK. Patterns of vertical growth in the face. Am J Orthod Dentofac Orthop 1988;93:103 116. 22. Obiechina AE, Arotiba JT, Fasola AO. Third molar impaction: evaluation of the symptoms and pattern of impaction of mandibular third molar teeth in Nigerians. Odonto-Stomatologie Tropicale 2001;93:22 25. 23. Raley L, Chapnick P, Baker G. The impacted third molar. Dent J 1977;43:364 366. 24. Graveley JF. A radiographic survey of third molar development. Br Dent J 1965;119:397 401. 25. Sewerin I, von Wowern N. A radiographic four-year follow-up study of asymptomatic mandibular third molars in young adults. Int Dent J 1990;40:24 30. 26. Richardson M. Changes in lower third molar position in the young adult. Am J Orthod Dentofac Orthop 1992;102:320 327. ª 2008 Australian Dental Association 323

O Breik and D Grubor 27. Hattab FN. Positional changes and eruption of impacted mandibular third molars in young adults. A radiographic 4-year follow-up study. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1997;84:604 608. 28. Kruger E, Thomson WM, Konthasinghe P. Third molar outcomes from age 18 to 26: findings from a population-based New Zealand longitudinal study. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2001;92:150 155. 29. Venta I, Turtola L, Ylipaavalniemi P. Radiographic follow-up of impacted third molars from age 20 to 32 years. Int J Oral Maxillofac Surg 2001;30:54 57. Address for correspondence: Dr Dragan Grubor Senior Lecturer Oral and Maxillofacial Surgery Melbourne Dental School The University of Melbourne 720 Swanston Street Carlton, Victoria 3053 Email: dgrubor@unimelb.edu.au 324 ª 2008 Australian Dental Association