| Abstract|| |
Background: The occlusal plane position is considered to be the primary link between esthetic and function.
Aim: To evaluate the relationship between extraoral and intraoral soft tissue landmarks with the occlusal plane in dentulous subjects on both sides and to check for any variations.
Study and Design: An in vivo study and 100 subjects with all healthy permanent teeth in normal arch and alignment were selected from Modern Dental College and Research Centre, Indore.
Materials and Methods: Three custom made instruments (occlusal plane analyzer, buccinator groove relator, and level analyzer) were indigenously designed to check parallelism of the interpupillary line, ala-tragus line, buccinator groove with the occlusal plane. Relation of retromolar pad with the occlusal plane was checked with the metallic scale.
Statistical Analysis: Chi-square test.
Results: In 20.0% subjects, the occlusal plane was parallel to the interpupillary line. The posterior reference point for ala-tragus line was middle point on right side in 56% and left side in 58% subjects. Intraorally, right side 59% and left side 62% subjects had the occlusal plane at the same level as that of buccinator groove. Right side 48% and left side 45% subjects showed occlusal plane at the middle one-third of retromolar pad.
Conclusion: The occlusal plane is not generally parallel to interpupillary line. The occlusal plane is parallel to the ala-tragus line with middle point of tragus as posterior reference point on right and left sides. Both sides, the buccinator groove and the middle one-third retromolar pad are coinciding with occlusal plane level.
Keywords: Ala-tragus line, buccinator groove, Camper′s plane, interpupillary line, retromolar pad
|How to cite this article:|
Jain R, Shigli K. An in vivo study to correlate the relationship of the extraoral and intraoral anatomical landmarks with the occlusal plane in dentulous subjects. Indian J Dent Res 2015;26:136-43
The increase in life expectancy in last few decades has led to an increase in the number of people in need of complete dentures.  Complete denture prosthesis places the greatest number of important factors in control of operator.  Occlusal plane orientation is one of the most important planes to be determined in complete denture fabrication, as it is an important basis for tooth arrangement.  It has been advised that the artificial teeth should be placed in the position previously occupied by the natural teeth.  If the occlusal plane is placed too high, the tongue cannot rest on the lingual cusp of the mandibular denture to prevent denture displacement and could lead to accumulation of food in buccal and lingual sulci. If the occlusal plane is placed too low, it could lead to tongue and cheek biting. 
|How to cite this URL:|
Jain R, Shigli K. An in vivo study to correlate the relationship of the extraoral and intraoral anatomical landmarks with the occlusal plane in dentulous subjects. Indian J Dent Res [serial online] 2015 [cited 2019 Sep 18];26:136-43. Available from: http://www.ijdr.in/text.asp?2015/26/2/136/159138
Various authors utilized extraoral and intraoral landmarks, ,,,,,,,,,,,,,,,,,, Cook's plane,  HIP plane,  devices, ,,,,,, frontal and lateral photographs, ,,,,, and cephalometric analysis ,,,,,,,,,,,,,,,,,, for orientation of the occlusal plane.
When constructing complete dentures, the location of the occlusal plane allows for wide variations, often depending on individual dentist judgment.  Various concepts have been frequently reported in the literature for occlusal plane determination in complete denture fabrication, but its reliability still remains unclear. 
This study was undertaken with following aims and objectives:
- To assess the relationship between extraoral and intraoral landmarks-interpupillary line, ala-tragus line, buccinator groove and retromolar pad with the occlusal plane in dentulous subjects
- To determine consistency of soft tissue landmarks with occlusal plane on both sides and to check for any variations.
| Materials and Methods|| |
The study was conducted on 100 dentulous subjects of both sexes age ranging from 19 to 24 years from 250 dental students of the Modern Dental College and Research Centre, Indore (Madhya Pradesh, India) in 2 years duration. Subjects with all permanent healthy teeth in normal arch and alignment and those who gave their consent were included in the study. Exclusion criteria: Subjects with any orthodontic treatment or periodontal surgery, interdental spacing or crowding, grossly abraded or attrited teeth and teeth with gross dental restoration.
Occlusal plane analyzer
This instrument was custom made to check the relation of the occlusal plane to interpupillary line and ala-tragus line. A fox plane (Dr. Frank Fox, Dentsply/York Division, York, PA, USA) was taken. Four long screws were attached at four ends of the occlusal plane relator arm; all the screws were placed parallel to each other. Three stainless steel plates (two oblique and one horizontal) were then attached to these screws. Two plates (oblique) were made parallel to the occlusal plane relator arm. It served to analyze the parallelism of occlusal plane with ala-tragus line on right and left sides. The third plate (horizontal) was parallel to the frontal connecting arm which could relate the occlusal plane to interpupillary line. All the three plates were marked on its outer edge with arrowheads. The corresponding arrowhead marks were also placed on the outer edge of the fox plane. These marks helped to determine parallelism of the plates by measuring the distance between the plates [Figure 1].
Buccinator groove relator
It was a custom made instrument to compare the level of the buccinator groove (linea alba buccalis) with the occlusal plane. Vertical stainless steel impression plates with rounded ends (to prevent soft tissue injury) were attached on either side of the bite fork. Small holes were made on the vertical plates for the retention of the irreversible hydrocolloid impression material [Figure 2].
A U-shaped stainless steel plate was made to check the level of the buccinator groove with the occlusal plane on the impression made. The prongs of the U plate were at the same level and parallel to each other [Figure 3].
Step-1: Parallelism of occlusal plane to the interpupillary line
It was checked using occlusal plane analyzer. With the subject seated in an upright position, the occlusal plane analyzer was placed in subject's mouth and held in position by subject biting over it. The frontal metal plate was then made parallel to the interpupillary line. Finally, the parallelism was checked with the frontal connecting arm of the fox plane, by measuring the distance between two plates using digital vernier caliper. Same distance between right and left sides indicated parallelism [Figure 4].
Step-2: Parallelism of the occlusal plane to the ala-tragus line
Using a scale and marking pen, three points were marked on the tragus of the ear according to its location, that is, superior, middle and inferior. The occlusal plane analyzer was placed in the subject's mouth and held in position by subject biting over it. Anterior end of the oblique plate was raised until it coincided with ala of the nose. The distance between the arrowheads on the two plates was then measured using digital vernier caliper. Same distance between the two plates on both sides indicated parallelism. The posterior end was then checked to see if it coincided with the superior, middle or inferior point of the tragus and three readings were recorded for all three points. The average readings were noted down [Figure 5].
Step 3: Level of the occlusal plane with the linea alba buccalis/buccinator groove
For the buccinator groove, the buccinator groove relator and level analyzer were used. An indelible pencil was used to mark the linea alba buccalis in the subject's mouth which represented the occlusal plane [Figure 6]. The irreversible hydrocolloid impression material was placed on the vertical impression plates on both the sides till distal aspect of the buccal vestibules were filled. The subject was then asked to pucker his/her lips as in sucking with the lips slightly separated but without the loss of contact of teeth with the bite plate. This helped to transfer the mark of buccinator groove on the impression. The impression was then removed from the mouth, washed with water and excess trimmed.
The level analyzer was then used to check the level of the groove in relation to the occlusal plane. One of the prongs of the plate was kept on the top side of the bite plate (depicted maxillary occlusal plane) such that the second prong extended on the other side of the vertical plate, carrying the vestibular impression. The coincidence/noncoincidence of the groove to the occlusal plane was then noted [Figure 7].
|Figure 7: Checking the level of the buccinators groove using level analyzer|
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Step-4: Relationship of the occlusal plane with the retromolar pad
Using an indelible marking pencil, the retromolar pad area of the subject was divided into three equal zones. The stainless steel scale was then slided from the tip of the mandibular cuspid to the distolingual cusp of the mandibular second or third molar. The zone of contact between the scale and the retromolar pad was then recorded [Figure 8].
The procedure from step-1 to step-4 was done on both right and left sides.
The collected data were tabulated and analyzed using SPSS version 13. Chi-square test was applied to find out the results. The level of significance was set at (P ≤ 0.05).
| Results|| |
Occlusal plane and interpupillary line
It was observed that in 76 subjects (80.0%), the occlusal plane was not parallel to the interpupillary line [Figure 9] and [Table 1].
|Figure 9: Relationship of the occlusal plane to the interpupillary line in dentulous subjects|
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|Table 1: Relationship of the occlusal plane to the interpupillary line in dentulous subjects |
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Occlusal plane with the ala-tragus line
The study concluded that (56%) subjects on right and (58%) subjects on left sides had the middle point of the tragus as the posterior reference point showing parallelism to the occlusal plane. Statistically, there was no significant difference between right and left sides regarding the relationship of the occlusal plane with the ala-tragus line to superior, middle and inferior point of tragus (P = 0.122) [Figure 10] and [Table 2].
|Figure 10: Relationship of the occlusal plane to the ala-tragus line on right and left sides in dentulous subjects|
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|Table 2: Relationship of the occlusal plane to the ala-tragus line on right and left sides in dentulous subjects |
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Occlusal plane with the buccinator groove
On right side in (59%) subjects and on left side in (62%) subjects, the buccinator groove was at the occlusal plane level. Statistically, there was no significant difference (P < 0.05) between right and left sides regarding the relationship of the occlusal plane with the buccinator groove (P = 0.310) [Figure 11] and [Table 3].
|Figure 11: Relationship of the occlusal plane to the buccinator groove on right and left sides in dentulous subjects|
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|Table 3: Relationship of the occlusal plane to the buccinator groove on right and left sides in dentulous subjects |
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Occlusal plane with the retromolar pad
On right side in (48%) subjects and on left side in (45%) subjects the occlusal plane coincided with the middle one-third of the retromolar pad. Statistically, there was no significant difference (P < 0.05) between right and left sides regarding the relationship of the occlusal plane with retromolar pad (P = 0.703) [Figure 12] and [Table 4].
|Figure 12: Relationship of the occlusal plane to the retromolar pad on right and left sides in dentulous subjects|
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|Table 4: Relationship of the occlusal plane to the retromolar pad on right and left sides in dentulous subjects |
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Comparing the right and left sides
The variation of the occlusal plane with the ala-tragus line, buccinator groove, and retromolar pad were observed. The ala-tragus line showed the maximum variation (29%) followed by buccinator groove (23%) and retromolar pad (22%). This result suggests that the buccinator groove and the retromolar pad can be used as reliable landmarks intraorally [Figure 13] and [Table 5].
|Figure 13: Variation of the occlusal plane relationship with the ala-tragus line, buccinator groove and retromolar pad as observed on right and left sides in dentulous subjects|
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|Table 5: Variation of the occlusal plane relationship with the ala-tragus line, buccinator groove and retromolar pad as observed on right and left sides in dentulous subjects |
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| Discussion|| |
One challenging aspect of treating complete denture patients is establishing the patient's plane of occlusion.  The extraoral and intraoral soft tissue landmarks can be readily employed to establish lost occlusal plane. The study was carried out in dentulous subjects as several oral anatomic landmarks bear fixed positional relationship to some natural teeth, these landmarks serve as reliable guides in replacing the lost natural teeth with artificial teeth. 
Relation to the interpupillary line
The results of the study agree with Morley and Eubank  according to whom, patients may have eyes in different planes, the creation of an incisal plane perpendicular to facial midline produces a more reliable and repeatable position that does not depend on the interpupillary line.
The results are in agreement with Gupta et al.  who stated that slight canting was observed, and the occlusal plane was not always parallel to interpupillary line.
Our results do not agree with Malafaia et al.  who concluded that majority of the subjects showed parallelism between the bipupillary plane and tangent to the incisal edge of the maxillary central incisors irrespective of the gender. This disagreement could be the difference in methodology.
Relation to the ala-tragus line
The technique of using the ala-tragus line to establish the occlusal plane is well-documented. However, a debate exists within the prosthodontic community over the exact definition of ala-tragus line or Camper's line. A review of the literature reveals that considering the ala-tragus line, anterior landmark is common to all researchers, but most of the controversy revolves around which tragal reference is to be considered as a posterior landmark.  Okan et al.,  Sahoo et al.,  Ismail and Bowman,  Sharry et al.,  Lahori et al.,  Glossary of prosthodontics term-8  , Spratley  and Lefebvre CA  had given the various concepts.
Our results agreed with Merkeley;  Shigli et al.;  Gupta et al.;  Williams,  Ghosn et al.  according to whom the occlusal plane was parallel to the ear-nose plane drawn from the inferior border of ala of the nose to the middle point of the tragus of the ear.
The results agree with Abrahams and Carey  according to whom plane oriented using the ala-tragus line (upper border) was not parallel to the occlusal plane.
The results do not agree with AL Quran et al.;  Ogawa et al.;  Sadr and Sadr  according to whom the line connecting the lower border of the ala of the nose to the superior border of the tragus was most accurate in orienting the occlusal plane. The disagreement could be as they carried out the study using three-dimensional mandibular movement analyzing system, lateral cephalometric radiographs and photographs, respectively.
The results were different from Okane et al.;  Hartono;  van Niekerk et al.;  Rostamkhani et al.;  Singh;  Hindocha et al.  who concluded that the line connecting the lower border of the ala of the nose to the inferior margin of the tragus was nearly parallel to the occlusal plane because they used cephalometrics and electromyographic activity.
Relation to the buccinator groove
Fewer studies ,,, reported using the buccinator groove to determine the occlusal plane in comparison to the ala-tragus line and the retromolar pad.
The results agree with those of Merkeley;  Lundquist and Luther;  Gupta et al.;  Shetty  according to whom the buccinator groove was at the level of the occlusal plane.
Our results do not conform with Shigli et al.  who stated that buccinator groove was 0.94 mm below the Mandibular occlusal plane due to the difference in the methodology.
Relation to the retromolar pad
Various authors have used the retromolar pad in relocating the occlusal plane in the edentulous patients to nearly an ideal position. ,,,,,,
Results are in agreement with Gupta et al.  according to whom posteriorly the lower occlusal plane was confirmed at middle one-third of the retromolar pad.
Results do not conform to Yasaki  according to whom the posterior end of the rim terminated at the anterior margin of the retromolar pad because the retromolar pad was divided into anterior and posterior parts.
Our results do not conform with Lundquist and Luther;  Carey and Dent;  according to whom the occlusal plane terminated at the lower half of the retromolar pad in majority of the subjects because of difference in the methodology.
The results do not agree with Shetty  according to whom the occlusal plane terminated in upper half of the retromolar pad in most of the subjects because the retromolar pad was divided into upper and lower half in edentulous subjects.
The results do not conform with Ismail and Bowman  who stated that the occlusal plane should be modified by placing the second molars at the level of the upper third of the retromolar pad rather than middle or lower third because study was carried out cephalometrically.
Variability of the ala-tragus line, buccinator groove, and retromolar pad
The results indicated that the ala-tragus line showed the maximum variation (29%) followed by the buccinator groove (23%) and the retromolar pad (22%). According to the results obtained - The ala-tragus line was a less reliable landmark as compared to buccinator groove and the retromolar pad for determining the occlusal plane. Since the buccinator groove and the retromolar pad showed less variability, they can serve as a guide in the determination of the occlusal plane.
The soft tissue landmarks used for locating the occlusal plane are liable to change with aging as well as with the judgment of the individual operator.  The locations of soft tissue landmarks are extremely difficult to measure accurately. These results are presented with the knowledge that some errors have been incorporated unintentionally. However, a determined effort was made to reduce soft tissue distortion to a minimum. 
| Conclusion|| |
The results of the present study suggested that more than one anatomical landmark should be used for determination of the occlusal plane. These parameters along with a judicious clinical judgment can lead to establishing the ideal occlusal plane level for an edentulous patient. ,
Further studies can also be carried out considering higher age group, larger sample size, facial types, and considering both sexes so that norms for the occlusal plane can be specified.
| Acknowledgments|| |
My special thanks to Dr. Prashant S Virgi and Dr. Neha Agrawal (Reader, Department of Community Dentistry, Modern Dental College and Research Centre, Indore) for statistical analysis.
Financial support and sponsorship
Conflict of interest
There are no conflict of interest.
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Dr. Ruchi Jain
Department of Prosthodontics, Modern Dental College and Research Centre, Indore, Madhya Pradesh
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]