September 2012, Volume 62, Issue 9

Original Article

Differences in various measurements on panoramic radiograph among erupted and impacted lower third molar groups

Irfan Qamruddin  ( Department of Orthodontics, Baqai Dental College, Karachi. )
Wasif Qayyum  ( Department of Oral Diagnosis and Oral Medicine, Ziauddin University, Karachi. )
Syed Mahmood Haider  ( Department of Oral and Maxillofacial Surgery, Karachi Medical & Dental College & Abbasi Shaheed Hospital, Karachi. )
Seema Wasif Siddiqui  ( Department of Community Dentistry, Ziauddin University, Karachi. )
Faisal Rehan  ( Department of Periodontology, Baqai Dental College, Karachi. )


Objective: To determine the differences in various linear and angular measurements between erupted and impacted lower third molar group on conventional panoramic radiographs.
Methods: The quasi-experimental study based on non-probability convenience sampling and done at the National Institute of Oral Diseases, Karachi, comprised 140 patients having full dentition with bilaterally present mandibular third molars varying between ages of 18-30 years. After clinical examination, 280 lower third molars were divided into two groups; Group A had erupted, while Group B comprised impacted mandibular third molars. All radiographs were traced and measured for five variables to compare the two groups. For statistical purpose, SPSS version 10, and t test were used.
Results: Retromolar space measured from Xi (centre of ramus) was 31.60±3.33 mm in Group A and 27.02±4.17mm in Group B. Retromolar space measured from anterior edge of ramus (AER) was 16.30±2.51mm in Group A and 11.21±3.69 in Group B. Mesiodistal width was 13.39±1.44mm in Group A and 13.80±1.58 in Group B. Retromolar space/width ratio was 1.22±0.20 for Group A and 0.82±0.34 for Group B. Mean angulations in Group A was 5.98±5.9° whereas 34.56±27.89°in Group B.
Conclusion: The probability of eruption of the lower third molar increases if retromolar space measured from anterior edge of ramus (AER-7) and centre of rumus point (Xi-7) is 13mm and 25mm respectively, provided the space/width ratio is greater than 1 and angulation is also vertical. Mesiodistal width of the tooth has no significant role in impactions.
Keywords: Third molar group, Anterior edge of ramus, Centre of ramus. (JPMA 62: 883; 2012).


Impaction may be defined as the failure of complete eruption into a normal functional position of one tooth within normal time due to lack of space in the dental arch, caused by obstruction by another tooth or development in an abnormal position.1
The most often congenitally missing as well as impacted teeth are the third molars, which are present in 90% of the population with 33% having at least one impacted third molar.2 They account for 98% of all the impacted teeth.3
The mandibular third molar is the most frequently impacted tooth.4 The incidence varies from 9.5% to 25% in different populations.5 Several factors have been reported to be responsible for the high rate of impaction of mandibular third molars. These include deficient space in the dental arch,6 unfavourable angulations and aberrant path of eruption, density of overlying soft and hard tissues, and the late eruption sequence.4 Mesiodistal width of the third molar may also play a role in the tendency for impactions.7
The ability to predict the eruption or impaction of third molars is of considerable value in clinical dentistry. If they erupt, they may be beneficial for orthodontic anchorage, prosthetic abutments, or transplantation.8 If they get impacted, they can cause adjacent root resorption, inflammatory process (pericoronitis), temporomandibular joint dysfunction and late incisor crowding;9 thus prophylactic removal greatly reduces risk to a patient as opposed to extraction later in life.10 Early germectomy can also reduce the risk of intra-operative and post-operative complications associated with surgery on a fully developed third molar such as nerve damage with resultant paresthesia, dry socket, inflammation, bleeding, and pain.11 Therefore, assessment of germ position and progress of mandibular third molar eruption is necessary for better patient management.
Many studies have been done to predict the probability of third molar eruption. Henry and Morant12 suggested third molar space index, while Schulhof13 used centre of ramus (Xi) point to measure the available space for eruption of the mandibular third molar. Most of the authors have used lateral cephalometric radiograph, but because of least amount of superimposition, orthopantomogram (OPG) is considered a more accurate radiograph to evaluate the position of the third molar.9,14-17
Although accurate prediction of third molar eruption or impaction is not possible before 20 years of age, but the differences in various angular and linear measurements performed on radiographs of impacted and erupted lower third molars can be used as a reference for early prediction of lower third molar eruption or impaction. Therefore, the purpose of this study was to determine the differences in various measurements among the erupted and impacted lower third molars on conventional panoramic radiograph. It was hypothesised that retromolar space, mesiodistal width and initial angulations of the lower third molar have a significant role of their own in the impaction and eruption of the tooth.

Patients and Methods

The study was conducted at the National Institute of Oral Diseases, Karachi. The study design was quasi-experimental whereas the study population was based on non-probability convenience sampling. A sample size of about 142 third molars in each group was calculated to detect a two-sided difference of 1mm in Xi -7 assuming a standard deviation (SD) of 3 mm at 5% level of significance and 80% power. The final sample has 140 patients having full dentition with bilaterally present mandibular third molars varying between ages of 18 and 30 years. Of them, 74 were females and 66 were males. Patients with worn-out dentition, previous orthodontic treatment or dentofacial anomalies were excluded. Two groups of mandibular third molars were formed on the basis of dental examination of the patients. Group A had those mandibular third molars which were fully erupted into functional position whereas Group B comprised those mandibular third molars which were below the occlusal plane/erupted up to the occlusal plane but not fully functional because of their aberrant angulations. Conventional panoramic radiographs of all the patients were taken by means of an orthopantomograph machine (Rotograph plus, model MR05, CEI Bologna Italy with the magnification of × 1.2). The radiographs were traced on overlying acetate paper and analysed by means of various measurements (Figure-1).

Retromolar space was measured by drawing a line from Ricketts Xi point i.e. centre of ramus to distal surface of the lower second molar.
Retromolar space from anterior edge of ramus AER-7 was measured up to the distal surface of the lower second molar along the occlusal plane.
Mesiodistal width of the lower third molar (MDW) was calculated, while the space/width ratio (SWR) was obtained through AER-7/MDW.The angulations of the lower third molar (Ang-8) were measured using the method of Ventä.18 The angle between the occlusal line of the lower third molar and the occlusal plane was measured. The angulations were classified as: Vertical (±10 degrees); Mesioangular (11±70 degrees); Distoangular (11±70 degrees); and Horizontal (greater than ±71 degrees).
All the measurements were recorded by one author and then reconfirmed by the other authors for reliability of data, which was then analysed with independent t test using SPSS version 10 for detecting the difference between the two group means.


Of the 140 subjects with a total of 280 mandibular third molars, 168 teeth were impacted, while 112 molars were erupted (Table-1).

In Group A, the mean distance between Xi-7 was 31.60±3.33 mm, whereas in Group B it was 27.02±4.17 mm. There was significant difference (p=0.00) between the groups (Table-2).

In terms of AER-7, the difference was again significant (p=0.00). It was insignificant (p=0.117) for MDW, while the difference was significant for SWR (p=0.00) and ANG-8 (p=0.00).


Mandibular third molars exhibit the highest rate of impaction as well as the highest rate of agenesis.19 Hellman20 found the rate of mandibular third molar impaction up to 9.5% while Björk5 found 17% to 22% in the two groups of patients in his research.
Although third molar impaction has multifactorial elements, but inadequate retromolar space for eruption is considered to be the major factor.11,16 The distance from Xi to the distal surface of the lower second molar is the most useful measurement for the evaluation of lower third molar space.21 In the current study, the mean value for Xi-7 was 31.60 mm for Group A and 27.02 mm for the Group B which was close to findings by others.13 In Group A, 100 (71.4%) of mandibular third molars had Xi-7 value >30mm while 40 (28.57%) had Xi-7 value between 25-29mm which shows that the minimum required value for successful eruption of the lower third molar is 25mm if the angulation is favourable (Figure-2).

In Group B, 28.5% had Xi-7 value >30mm while 62 (44%) mandibular third molars had Xi-7 value between 25-29mm, and 38 (27.38%) had Xi-7 value <25mm which shows that retromolar space is not the only factor responsible for the lower third molar impaction. Quiros22 used panoramic radiograph and found the range of 35-39mm for the successful eruption of the lower third molar. In another study conducted on Iraqi population using digital panoramic radiographs, the mean value for Xi-7 was 29.2mm for marginal eruption group and 35.2mm in males and 34.4mm in females with full eruption.1818
In our study, the mean distance from AER to the distal surface of second molar was 16.30mm for the Group A and 11.21mm for Group B. Significant difference was observed between the two groups. Ventä10 also reported that the probability of eruption is 100% if the retromolar space is at least 16.5mm.Quiros22 also found the range between 14-17mm in the erupted group. Minimum retromolar space required for the eruption of lower third molar measured from AER-7observed in our study was 13mm (Figure-3)

which was almost similar to the findings of Hattab at el8 who reported 13.9mm for females and 14.3mm for males.
Mesiodistal crown widths of lower third molars were more or less similar in both the groups, i.e. 13.39mm for Group A and 13.80mm for Group B; with average difference of 0.41mm. Other studies,8,10 also confirmed that third molars were larger in the impacted group, but the difference was insignificant.
The SWR was greater in the erupted group with the mean value of 1.22 whereas in the impacted group, it was 0.82 (Figure-4).

This is in accordance with previous research23 which found this ratio to be 0.85 in the marginal eruption group and 1.94 in the full eruption group. In this study, 76.2% of the impacted cases had ratio of less than 1, while 23.8% cases had SWR of more than 1. However, in the erupted group, 98.21% of the lower third molars had SWR >1. Dierkes24 stated that some third molars get impacted even in the presence of sufficient space.
It has been demonstrated that the greater the inclination, the greater the probability of impaction.25 Mean angulations of the lower third molars among the impacted group was found to be 34.56°, while in erupted group, it was 5.98°. Significant difference of 27.17° between the two groups was observed. It has been suggested in another study that if a third molar has a low initial inclination and adequate space, then eruption is possible.26 Among Group A, 100 (71.42%) of lower third molars were vertical, while 40 (28.57%) were mesioangular. A maximum angulation of lower third molar in the erupted group was found to be 22°. Among Group B, 80 (57.14%) of lower third molars were mesioangular; 39 (27.85%) were vertical; 18 (13.09%) showed horizontal angulation; and 3 (2.38%) were distoangular. Queck27 in his study also reported the highest frequency of mesioangular impactions as well as Uthman23 made the same conclusion in his study.


Early removal of those mandibular third molars which are unlikely to erupt in a normal position can prevent the associated pathology and surgical complications. Results from the study can be useful in assessing the prognostic value of mandibular third molars on conventional orthopantomogram.


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