Journal of Dentistry, 6, No. 2, 1978, pp. 147-160.
Printed
in
Great Britain
Tooth positions on complete dentures* David M. Watt, PhD, FDS RCS (Edin) Department
of Restorative
Dentistry,
University
of Edinburgh
ABSTRACT Complete dentures should be constructed to fill the denture space, which may be defined as ‘that space in the edentulous mouth which was formerly occupied by the teeth and their supporting tissues which have since been lost’. The artificial teeth should be placed in the approximate positions occupied by the natural teeth. Various guides to the placement of artificial teeth m these positions are discussed: 1. The form of the lips, the nasolabial and horizontal labial angles and the effect of nose form and tooth inclination on these angles. 2. The relationship of the maxillary incisors to the incisive papilla. 3. General rules for the arrangement of anterior teeth in cases of jaw and lip malformation. 4. The location of the remnant of the linguogingival margins in the edentulous mouth and placement of upper teeth in relation to it. 5. Placement of lower teeth in relation to the lower ridge, tongue, cheek and lips.
INTRODUCTION In this paper some biometric guides are presented which aid the designing of complete dentures and facilitate the placement of the artificial teeth in the approximate positions which the natural teeth formerly occupied. It is generally believed that dentures function best when the teeth occupy these positions (Hooper, 1934; Fish, 1948; Landa, 1954; Swensen, 1959; Pound, 1960; Lee, 1962; Ballard, 1963; Martone, 1963). POSITION OF UPPER ANTERIOR TEETH There are various positive indications as to the correct position of anterior teeth. The average distance from the middle of the incisive papilla to the middle of the labial surface of the upper central incisor is approximately 10 mm. It is seldom less than 8 mm. After the teeth have been extracted resorption of the buccal plate makes the papilla appear to approach the top of the ridge, and when the resorption is extreme the papilla approaches the front of the ridge. Nevertheless, the labial surface of the artificial central incisor should be positioned approximately 10 mm in front of the midpoint of the papilla or slightly more if the natural teeth were proclined and slightly less if retroclined. However, unless the impression has restored the full width of the sulcus the teeth cannot be correctly positioned. The correct lip position and form must therefore be determined at the impression stage in order to have sufficient sulcus width on the cast. The following guides to the restoration of the lip form are of value: 1. The middle of the curved junction between the columella of the nose and the philtrum of the lip should lie about half-way between the tip of the nose and the groove behind the ala. 2. In the average individual the sagittal angle between the columella and the lip is *Presented at the Annual Conference of the British Society for the Study of Prosthetic Dentistry in April 1977.
148
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b
Fig. 1. Two
of the factors that influence the sagittal nasolabial angle: the inclination of the teeth and nose form. a, Where the teeth are proclined the nasolabial angle tends to be a right angle, but it tends to be.more than a right angle when the teeth are retroclined. 6, When the columella is prominent and
lies at a lower level than the ala, the angle is greater than a right angle. When the columella and ala are on the same level the nasolabial angle tends to be a right angle.
Fig. 2. a, Collumella
and ala on the same level. b, Prominent
columella
and
high ala.
approximately a right angle, but several factors influence the actual angle in any individual. Two of the factors are shown in Fig. I: a. If the teeth are proclined the nasolabial angle tends to be approximately 90”, but if the teeth are retroclined the angle tends to be more than a right angle. If no pre-extraction records are available the inclination of the teeth can be deduced from the slope of the labial surface of the residual alveolar ridge. b. If the columella is prominent and at a lower level than the ala, a nasolabial angle of more than a right angle is indicated, but if the columella and ala are at the same level this indicates a right angle. The colwnella and ala of the nose illustrated in Fig. 2u are on the same level, indicating a nasolabial angle of a right angle, but in Fig. 2b the patient has a low prominent columella
Watt: Tooth
positions on dentures
b a Fig. 3. a, In this patient both tooth inclination and nose form tend to reduce the nasolabial angle to less than a right angle. 6, In this patient both tooth inclination and nose form tend to increase nasolabial angle to more than a right angle.
the
a
b
fig. 4. The upper lip is identical in both diagrams, but in b the upper lip looks more prominent because the lower lip is unsupported.
and high ala, indicating a nasolabial angle of more than a right angle. When both factors work together towards reducing the nasolabial angle it is not uncommon to find that that angle is less than a right angle. Fig. 3u shows a patient with ala and columella on the same level and the teeth proclined; the nasolabial angle is less than a right angle. The patient shown in Fig. 3b has both factors tending to increase the nasolabial angle. The columella is prominent and the teeth are retroclined, and in this case the nasolabial angle is quite obtuse. The vertical dimension is another factor which affects the nasolabial angle. Where it is high the angle is increased and the inferior labial angle is flattened. Where it is low the nasolabial angle is decreased. The inferior labial angle is also decreased in the same way. These then are some of the biometric guides to the restoration of lip form which guide the positioning of anterior teeth. There is a tendency to underfill the upper lip because a correctly supported upper lip looks much too prominent when the lower lip is not filed out. This is illustrated in Fig 4a and 4b In both cases the upper lip is identical and the only difference is that the lower lip is unsupported in Fig. 4b. Another angle which is important for the correct restoration of the lips is the horizontal labial angle (Fig. 5). This angle varies between 90 and 120”. In narrow-faced individuals it tends to be nearer 90” and in broad-faced individuals a flatter angle is more usual. A common fault of complete maxillary dentures is that they do not restore the correct contour of the front of the upper lip (Rayson et al., 1970; Ismail, 1971). Dentures are often wrongly built out in the canine regions in an attempt to satisfy patients who complain about deep nasolabial grooves between the alae and the angles of the mouth. This toe of restoration flattens the horizontal labial angle and produces a monkey-like appearance. By ftiing out the lip in the midline the nasolabial grooves can be reduced just as effectively as by building up the denture in the canine region. The horizontal labial angle is thus restored and the appearance of the patient is improved.
150
Fig. 5. The horizontal labial angle varies from 90 to 120”. The line (arrowed) between the glossy oral mucosa and the transitional epithelium can be clearly seen.
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Fig. 6. The level of the occlusal plane is determined by asking the patient to keep the lips in light contact while separating the jaws slightly. A wax knife is passed between the lips, and the level of the occlusal plane is thus marked on the labial surface of the block.
In Fig. 5 the difference between the dull transitional epithelium of the lip and the glossy oral mucosa can be seen. The junction (arrowed) between these two represents the line of contact of upper and lower lips. If this glossy oral mucosa cannot be seen when the lips are slighly apart they are not sufficiently supported. An inadequately supported lip folds inwards so that only the transitional epithelium can be seen. if the lips are correctly restored they will meet at the line of the junction between the oral and transitional epithelium. Another aid to anterior tooth position is that the upper incisal edges should be approximately opposite the junction of the closed lips when the mandible is in the resting or slightly open position. The position of the teeth and the mobility of the lips influence the amount of tooth exposed during smiling. In this respect it is important to appreciate that the tooth/lip relationship varies widely between different individuals (Ellinger, 1968; Nassif, 1970). Nevertheless, when most dentures are designed it is customary to place the incisal edges of the upper incisors 2-3 mm below the resting position of the upper lip. This is not a particularly good method as it makes little allowance for individual variations. Other methods have been described (Ismail and Bowman, 1968; Lundquist and Luther, 1970), but the author prefers the following method for positioning the occlusal plane and hence the level of the incisal edges of the upper anterior teeth. It has the advantage that allowance is made for variations in the mobility of the upper lip and as a result the denture looks more natural. At the time of registration of the jaw relationship, and after the upper occlusion rim has been adjusted to provide a suitable support for the upper lip, ask the patient to close the lips gently and separate the jaws as if stifling a yawn. Then pass a wax knife between the lips and make a mark on the vestibular surface of the upper rim to indicate the level of the occlusal plane (Fig. 6). Patients with mobile lips who normally expose more of their anterior teeth when they smile have a greater downward movement of the upper lip during this procedure, and hence the level of the occlusal plane is placed at a more suitable position. Conversely,
Watt: Tooth
positions on dentures
fig. 7. The upper anterior teeth should be set so that their incisal edges follow the curve of the smiling lower lip.
Fig. 8. Patient with a cleft lip showing severe scarring and distortion. Note how the teeth have been set to follow the contour of the lower border of the upper lip in order to mask its distortion. Note also how the irreguiar setting of the teeth has been carried out to accommodate differences in the thickness of the lip so that the skin surface presents a more regular contour.
patients who have relatively immobile lips do not move them much during this localization of the occlusal plane, and will therefore show correspondingly less of their teeth on smiling. Thus, allowance is made automatically for variations in the mobility of the upper lip and a more natural appearance of the denture is produced. The position of upper anterior teeth can be checked at the trial denture stage by asking the patient to make ‘f’ and ‘v’ sounds. The words ‘fish’ and ‘victor’ should be pronounced, and if the teeth are in the correct position the incisal edges of the upper incisors will contact the lower lip at the junction between the transitional epithelium and the oral mucosa. In general, the upper anterior teeth should be set so that their incisal edges follow the curve of the smiling lower lip (Fig. 7). To facilitate this a smile curve should be marked on the occlusal rims. A pleasing effect is produced by this method of setting the upper anterior teeth because the line of the lower lip when the patients smiles is used as a visual reference line in the assessment of tooth position. Teeth which lie too near the lower lip are assessed as too long and teeth which lie too far away as too short. The lip contour may be disturbed by scarring as a result of an accident or surgery. Perhaps the most common cause of irregular lip contour is congenital cleft of the lip. In these patients the incisal edges of the anterior teeth should be arranged so that they tend to follow the contour of the lower border of the upper lip (Fig. 8). The irregularity of the lip is thus less obvious than if the teeth were set on a regular curve. In some cases scarring may produce differences in thickness of the lip although the contour of the border at the labial commissure is regular. It is important to set the teeth in such a way that the differences in thickness are accommodated so that the skin surface of the lip presents a regular contour. Thus, where part of the lip is thickened by scar tissue the anterior tooth opposite that part should be depressed, and where part of the lip is thin the adjacent anterior tooth should be set further forwards to restore the skin contour. It is important to realize that irregularities of the teeth can be considered normal in a patient with a cleft lip. Even if the lip has been repaired and its contour is quite smooth, it is usually wise to set the anterior teeth irregularly as the presence of a lip scar leads observers to expect an irregularity and if it is absent the teeth look artificial.
152
Fig. 9. Average changes in coronal traces through the canine and premolar regions at 1, 3, 6,12 and 30 months after the extraction of the teeth.
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Fig. 70. Average changes in coronal traces through the molar region at 1,3,6, 12 and 30 months after the extraction of the teeth. The buccolingual breadth @LB) of the dentulous alveolar ridge was remarkably constant for each tooth position in all the patients measured.
The distortion of the lips can be quite minor in patients with slight unilateral weakness of the facial muscles or with habitual crooked smiles. In these cases symmetrical placement of the teeth about the midline of the face exaggerates the distortion of the mouth and should be avoided. It is better to place the teeth in the centre of the mouth so that the mesio-incisal point lies midway between the right and left corners of the mouth when the patient smiles. Horizontal placement of the occlusal plane should be avoided if the mouth is tilted, as this, by providing a horizontal reference line, will exaggerate the crooked appearance of the mouth. In these cases it is better to arrange the occlusal plane parallel to a line joining the right and left comers of the mouth when the patient smiles. POSITION OF UPPER POSTERIOR TEETH Posterior teeth on maxillary dentures are frequently placed just under the crest of the ridge, yet studies of oral changes following tooth loss (Watt and MacGregor, 1976) have shown that the ridge crest does not necessarily correspond to the position of the alveolar process and varies greatly in position with the amount of resorption. The average post-extraction changes in coronal traces through maxillary canine and premolar regions are shown in Fig. 9. It can be seen that the whole breadth of the palate is affected by resorptive change in the canine region. This amount of tissue loss is seldom replaced by a corresponding amount of denture base, with the result that whistling during the pronunciation of sibilants is not uncommon in edentulous patients. In the molar region (Fig. IO) it can be seen that while change in the lateral parts of the palate are quite large, the greatest losses occur buccally and in the height of the ridge. Greater losses occur over longer periods (Likeman and Watt, 1974; Watt and Likeman,
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153
positions on dentures
Fig. 11. Extreme resorption of the maxillary Note the great breadth of the buccal sulcus.
ridge.
Tab/e 1. Analysis of measurements of the horizontal breadth of the alveolar process from the linguogingival margin to the mid-buccal point in 100 dentulous patients Position and plane of measurement Central incisor, sagittal Canine, coronal First premolar, coronal Second premolar, coronal First molar, coronal Second molar, coronal Third molar, coronal
Average change k s. d (mm) 6.3 8.5 10.0 10.6 12.8 11.6 10.1
* t * + f t k
0.91 1.06 1.03 1.40 0.98 1.14 1.33
1974). Fig. 11 shows the mouth of a patient with a very resorbed maxillary ridge. The front of the mouth is obviously distorted by retractors, but it can be appreciated that even without retraction there would still be a broad gap between the buccal surface of the ridge and the cheek posteriorly. This gap partly indicates the amount of buccal resorption of the ridge in this patient who had been edentulous for over 30 years. To construct the diagrams in Figs. 9 and 10 it was necessary to measure casts before and at intervals after extraction, so that the various increments of change could be plotted (Watt, 1960), but only one of these measurements need be considered here: the buccolingual breadth of the dentulous alveolar ridge. The position of this measurement can be seen on the right of the bottom diagram in Fig. 10 and is marked BLB. The constancy of this measurement for each tooth position was so striking in the original sample of 25 that later 100 patients were measured to check that the variation was really so small as was first observed (Watt et al., 1967). The results of these measurements are shown in TableI. The interesting thing about these data is that the standard deviations are only about 1 mm at each tooth position. This means that the average measurements pbs or minus about 1 mm will be correct for about 68 per cent of all patients measured, and the average measurement plus or minus about 2 mm will be correct for 95 per cent of all patients measured. Thus, provided that the linguogingival margin can be located in an edentulous patient, there is a reasonable guide for positioning the cheeks and lips correctly in the majority of edentulous patients because the measurements are relatively constant for every tooth position. With
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ractb
Aftw 9 months Fig. 72. Pre-
and post-extraction casts show how the vestige of the Iinguogingival margins can be identified as a fine cord-like elevation near the crest of the residual ridge. Mean measurements of the buccolingual breadth (arrows) of the alveolar process before extraction can be transferred to the edentulous cast to give an approximate guide to the breadth of the flanges of complete upper dentures.
reference to Fig. 10, which shows the average changes in the molar region, imagine the cheek in contact with the buccal surface of the ridge. Knowing that the buccolingual breadth in this region is about 12 mm, it can be deduced where the cheek should lie in an edentulous patient, provided that the remnant of the linguogingival margins can be located. This proved to be comparatively easy, as during the course of the study of oral changes following tooth loss (Watt, 1960) tattoo spots were placed on the mucosa of 8 patients before extraction. These spots were near the buccal and linguogingival margins and made it possible to identify the remnant of the linguogingival margin as a fine cord-like elevation of mucosa near the crest of the residual ridge. There can be no doubt that this structure does represent the vestige of the lingual margins, as the tattoo spots lay in the same relationship to it after healing had taken place. The progress of the spots in a series of casts studied over the course of 4% years can be seen in Fig. 12. These spots on the linguogingival margin correspond with the cord-like ridge in the edentulous mouth which represents the remnant of that margin. Thus, by using the average measurements of buccolingual breadth the location of the cheek can be determined fairly accurately. To put it another way, the sulcus breadth of the edentulous cast can be determined with reasonable accuracy. The outer surface of the sulcus represents the cheek position, and by using the average measurements in Table I it is possible to be fairly sure that the cast represents the dimensions of this part of the denture space. Unfortunately, however, measurements show that there is an outward movement of this structure so that it lies
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positions on dentures
Table II. Analysis
of the buccal movement
gingival vestige after
extraction
of the linguo-
of maxillary
teeth
in 25
patients Position and plane of measurement Incisor, sagittal First premolar, coronal Second premolar, coronal First molar, coronal Second molar, coronal Third molar, coronal
Average change + s. d. (mm) 1-6 2.6 2.8 2.9 3.6 2.9
f + + + f f
1.16 1.43 1.16 1.30 1.26 1.51
buccally and superior to the positions occupied by the linguogingival margins in the dentulous mouth. These measurements from the longitudinal study of morphological change are given in Table II. Owing to its outward movement, the use of the linguogingival vestige as a reference line on the edentulous cast causes an overestimate of the amount of denture base required to restore the shrunken ridges to their pre-extraction form. The mean amount of outward movement in the sample measured was less in the incisor than in the molar region and was small compared with gross errors which commonly occur in estimates of flange width and tooth positions in complete upper dentures. It is possible to make allowance for the errors produced by the outward movement of the linguogingival vestige by deducting the mean outward movement from the mean buccolingual breadth, but there is no guarantee that such a procedure might not increase the error in individual cases. Clinical experience shows that a slight overestimate of sulcus breadth is preferable to an underestimate, provided that there is no overextension of the sulcus depth. The additional breadth in the molar region can be accepted since in the dentulous patient contact between the cheek and alveolar process may not be present in the molar region. The thinning of the lips with age might more than compensate for the incisor overestimate. Thus, the mean measurements of pre-extraction buccolingual breadth of the alveolar process can be transferred directly to the edentulous cast to give an approximate guide to the breadth of the flange of complete upper dentures. There can be no doubt that an element of guesswork still exists, but the errors can be reduced to some extent by this method. The remnant of the linguogingival margin can be clearly seen on the edentulous ridge in Fig. 12. This ridge would cause little problem in denture construction, but the very resorbed ridge in Fig. I3 might present a difficult problem. Note that the rugae cross the ridge. The soft palatal tissue comprising the anterior part of the ridge is the tissue in the rugae area, while the posterior parts of the ridge contain no alveolar tissue but are composed of the soft tissue that normally lies at the sides of the palate. In this case the remnant of the linguogingival margin lies in the sulcus and all the upper teeth should be placed buccal to this to provide adequate tongue space for the patient. To make this possible the impression must be taken to restore the full breadth of the buccal sulcus. Too often a small tray is made for a case of this kind and a small impression is taken. The impression material dribbles over the edge of the tray and reproduces a narrow collapsed sulcus form because of the contraction of an unsupported buccinator. In order to make a denture for this patient it is necessary first
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Fig. 13. In this case there is much resorption and the rugae cross the ridge, which consists entirely of palate tissue. The remnant of the linguogingival margin (arrows) lies in the sulcus.
a
b
Fig. 74. a, Impressions may fail to reproduce the true breadth of the buccal sulcus, and when the teeth are set in their correct positions an unfavourable form of polished surface results. b, Correct sulcus width on the cast enables correct shaping of the polished surface.
to restore the full breadth of the sulcus by the guide lines indicated. It is then possible to place the teeth correctly and thus give the patient adequate tongue space. It is not possible to make satisfactory dentures for this kind of difficult edentulous patient unless the sulcus breadth is properly restored at the time the impressions are taken (Fig. 14). Fig. 2.5shows old unsatisfactory dentures and the new dentures which were satisfactory. These dentures were made for the same mouth. On the right the old denture has a narrow periphery, and on the left the new denture, which was constructed on the principles described, has a broad periphery. The old lower denture is underextended and the load per unit area on the mucosa under it would obviously be about double the load per unit area on the mucosa under the new denture which covers twice the mucosal area. The sulcus breadth required in the upper denture is proportional to the amount of tissue loss, and the artificial teeth are placed buccal to the ridge crest ln approximately the positions which were occupied by the natural teeth. Those who believe that the ridge crests acts as a fulcrum around which the denture tilts when an occlusal load is placed on one side might object to this positioning of the teeth, but the author’s view is that the denture is rather like a raft floating on water. The fulcrum around which it tilts when loaded will depend very much on the point of application of the load, but if a retainer is put on the
Watt: Tooth
positions on dentures
Fig. 75. Old and new dentures for the same patient. On the right the old dentures which the patient found unsatisfactory; on the left the new replacement dentures showing a broader periphery and a greater extension.
Fig. 16. When one side of a complete denture is loaded the denture tilts around a soft tissue fulcrum rather like a raft floating on water, but if the opposite end of the raft is held the load is more evenly distributed. The broad peripheral form of the denture ensures that it does not tilt by providing a buccal seal and a bulge against which the buccinator can act.
opposite end of the raft the fulcrum is shifted and the load is more evenly distributed (Fig. 16). The broad peripheral form of the denture acts as the retainer because it provides a buccal seal and also provides a bulge against which the buccinator can usefully act. In the maxilla the relationship between the buccinator and reflected mucosa in the molar region is fundamentally different to that in the mandible, because in the maxilla the part of the bone to which the buccinator is attached in the dentulous state resorbs when the teeth are lost. As a result, the maxillary buccinator attachment moves inwards, but the mandibular attachment does not since the external oblique line is relatively unaffected by tooth loss. In order to restore the maxillary denture space it is necessary to displace the buccinator outwards by the periphery of the denture. This can be done without causing displacement of the denture in function because the buccinator’s attachment is higher than the attachment of the oral mucosa. When the cheek is pushed outwards by the denture the curve of reflected oral mucosa lies at a lower level than the curve of the displaced buccinator (Fig. 17). Contraction of the buccinator thus improves the peripheral seal. If the impression is taken wrongly so that the width of the sulcus is not restored, the surplus of reflected mucosa is stretched upwards together with the buccinator, and the upper denture can then be displaced in function. Similar displacement of the mandibular denture will occur if it is extended on to the buccinator attachment. POSITION OF LOWER POSTERIOR TEETH The restoration of the upper part of the denture space has been considered in detail since the lower denture space cannot be properly delineated unless an upper appliance holds the lips and cheeks in their correct position. It is well to remember that the attachments of the mandibular muscles are not so much affected by the resorptive change, except labially, and as they are close to the periphery of the denture the lower part of the denture space can be defined more easily.
158
Journal of Dentistry,
a
b
Fig 77. The effect
of the cheek position and on the sulcus form. a and 6, Incorrect;
Fig. 78. The teeth
should
C
on the relationship c and d, correct.
buccolingual width of the artificial be approximatelv equal to the width
of the buccal cusps of the natural teeth and should occupy the same position over the residual ridge. The cross-section form of the lower denture is shaded. Note the upward and inward buccolingual surface and the rounded form of the buccal flange which provides a shelf on which the buccinator can rest.
between
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d the buccinator
muscle and mucosa
Fig. 79. The last molar should be positioned approximately a little finger’s breadth in front of the posterior edge of the denture to provide a posterior shelf. The tongue (stippled) lies on the lingual side of the shelf while the buccinator lies on the buccal side. Together they stabilize the denture.
The lower posterior teeth should be set directly over the lower ridge. In this position there is an optimum relationship between the forces of the tongue and cheeks. The buccolingual width of the artificial teeth should be approximately equal to the width of the buccal cusps of the natural teeth (Fig. 18). The discrepancy in width between upper and lower endentulous ridges is caused chiefly by resorption buccahy in the maxilla and partly by post-extraction resorption of the lingual plate of the mandible. However, if lower posterior teeth of natural buccolingual width are set in the pre-extraction position an undercut is produced into which the tongue bulges and raises the lower denture. The lingual polished surface of the lower denture should face upwards and inwards if muscle balance is to be achieved by the downward and outward pressure of the tongue. It is also important to ensure that teeth are not placed on the part of the denture base which lies over the posterior part of the mandibular ridge which curves upwards to the ascending ramus. This area constitutes the posterior shelf. The tongue lies on the lingual part
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159
positions on dentures
of this shelf while the buccinator overlies the buccal part of the shelf as it sweeps towards its posterior attachment to the pterygomandibular raphe on the lingual side of the mandibular ridge (Fig. 1Y).
POSITION OF LOWER ANTERIOR TEETH It is frequently said that lower incisors should be placed immediately
above the lower residual ridge, but when there is much bony resorption the remnant of the ridge is part of the lingual plate and the teeth should be set on the labial side of it-how far labially will depend on the migration of the mentalis muscle attachments and on the activity of the lips. The optimum position for the lower incisors can easily be found by replacing the anterior segment of the record block with a mouldable substance, such as soft wax or a functional impression material, and then asking the patient to speak with both upper and lower record blocks in the mouth. It is essential that the upper block is carved to restore the lips and cheeks to their former position before any attempt is made to locate the lower incisors by this functional method, because if the upper lip is collapsed the activity of the orbicularis oris is changed and the wax indicating the position of the lower incisors will be located in the wrong position.
CONCLUSION Artificial teeth on complete dentures should be placed in the approximate positions occupied by the natural teeth. To enable this to be done biometric guides should be used to restore the lips and cheeks to the positions which they occupied when the natural teeth were present. Dentures constructed in this way have a natural appearance and function extremely well.
REFERENCES
Ballard C. F. (1963) Variations of posture and behaviour of the lips and tongue which determine the position of the labial segments; the implications in orthodontics, prosthetics, and speech. Trans. Eur. Orthod. Sot. 39, 67-88. Ellinger C. W. (1968) Radiographic study of oral structures and their relation to anterior tooth position. J. Prosthet. Dent. 19, 36-45. Fish E. W. (1948) Principles ofFull Denture Prosthesis, 4th ed. London, Staples, p. 104. Hooper B. L. (1934) Functional factors in the selection and arrangement of artificial teeth. J. Am. Dent. Assoc.
21, 603-615.
lsmail Y. H. (1971) Changes in soft-tissue profile following extraction and complete denture treatment. J. Prosthet. Dent. 26, 1 l-20. Ismail Y. H. and Bowman J. F. f 1968) Position of the occlusal plane in natural and artificial teeth. J. Prosthet. Dent. 20, 407-411. Landa J. S. (1954) The troublesome transition from a partial lower to a complete lower denture. J. Prosthet. Dent. 4,42-5 1. Lee J. H. (1962) Dental Aesthetics. Bristol, Wright, p. 103. Likeman P. R. and Watt D. M. (1974) Morphological changes in the maxillary denture bearing area: a follow-up 14 to 17 years after tooth extraction. Br. Dent. J. 136, 500-503.
Lundquist
D. 0. and Luther W. W. (1970) Occlusal plane determination.
23.489-498.
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Martone A. L. (1963) Clinical applications of concepts of functional anatomy and speech science to complete denture prosthodontics. Part VIII. The final phases of denture construction. J. Prosthet. Dent. 13, 204-228. Nassif N. J. (1970) The relationship between the mandibular incisor teeth and the lower lip. J. Prosthet. Dent. 24, 483-491.
Pound E. (1960) Modern American concepts in esthetics. Znt. Dent. J. 10, 154-172. Rayson J. H., Rahn A. O., Wesley R. C. et al. (1970) Placement of teeth in a complete denture: a cephalometric study. J. Am. Dent. Assoc. 81, 420-424. Swensen M. G. (1959) Complete Dentures, 4th ed. St Louis, Mosby, pp. 51, 174. Watt D. M. (1960) Morphological changes in the denture bearing area following the extraction of maxillary teeth. PhD Thesis, University of Edinburgh. Watt D. M., Durran C. M. and Adenubi J. 0. (1967) Biometric guides to the design of complete maxillary dentures. Dent. Msg. Oral Top. 84, 109- 111. Watt D. M. and Likeman P. R. (1974) Morphological changes in the denture bearing area following the extraction of maxillary teeth. Br. Dent. J. 136, 225-235. Watt D. M. and MacGregor A. R. (1976) Designing Complete Dentures. Philadelphia, Saunders.
Printed
in
Great Britain
Tooth positions on complete dentures* David M. Watt, PhD, FDS RCS (Edin) Department
of Restorative
Dentistry,
University
of Edinburgh
ABSTRACT Complete dentures should be constructed to fill the denture space, which may be defined as ‘that space in the edentulous mouth which was formerly occupied by the teeth and their supporting tissues which have since been lost’. The artificial teeth should be placed in the approximate positions occupied by the natural teeth. Various guides to the placement of artificial teeth m these positions are discussed: 1. The form of the lips, the nasolabial and horizontal labial angles and the effect of nose form and tooth inclination on these angles. 2. The relationship of the maxillary incisors to the incisive papilla. 3. General rules for the arrangement of anterior teeth in cases of jaw and lip malformation. 4. The location of the remnant of the linguogingival margins in the edentulous mouth and placement of upper teeth in relation to it. 5. Placement of lower teeth in relation to the lower ridge, tongue, cheek and lips.
INTRODUCTION In this paper some biometric guides are presented which aid the designing of complete dentures and facilitate the placement of the artificial teeth in the approximate positions which the natural teeth formerly occupied. It is generally believed that dentures function best when the teeth occupy these positions (Hooper, 1934; Fish, 1948; Landa, 1954; Swensen, 1959; Pound, 1960; Lee, 1962; Ballard, 1963; Martone, 1963). POSITION OF UPPER ANTERIOR TEETH There are various positive indications as to the correct position of anterior teeth. The average distance from the middle of the incisive papilla to the middle of the labial surface of the upper central incisor is approximately 10 mm. It is seldom less than 8 mm. After the teeth have been extracted resorption of the buccal plate makes the papilla appear to approach the top of the ridge, and when the resorption is extreme the papilla approaches the front of the ridge. Nevertheless, the labial surface of the artificial central incisor should be positioned approximately 10 mm in front of the midpoint of the papilla or slightly more if the natural teeth were proclined and slightly less if retroclined. However, unless the impression has restored the full width of the sulcus the teeth cannot be correctly positioned. The correct lip position and form must therefore be determined at the impression stage in order to have sufficient sulcus width on the cast. The following guides to the restoration of the lip form are of value: 1. The middle of the curved junction between the columella of the nose and the philtrum of the lip should lie about half-way between the tip of the nose and the groove behind the ala. 2. In the average individual the sagittal angle between the columella and the lip is *Presented at the Annual Conference of the British Society for the Study of Prosthetic Dentistry in April 1977.
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b
Fig. 1. Two
of the factors that influence the sagittal nasolabial angle: the inclination of the teeth and nose form. a, Where the teeth are proclined the nasolabial angle tends to be a right angle, but it tends to be.more than a right angle when the teeth are retroclined. 6, When the columella is prominent and
lies at a lower level than the ala, the angle is greater than a right angle. When the columella and ala are on the same level the nasolabial angle tends to be a right angle.
Fig. 2. a, Collumella
and ala on the same level. b, Prominent
columella
and
high ala.
approximately a right angle, but several factors influence the actual angle in any individual. Two of the factors are shown in Fig. I: a. If the teeth are proclined the nasolabial angle tends to be approximately 90”, but if the teeth are retroclined the angle tends to be more than a right angle. If no pre-extraction records are available the inclination of the teeth can be deduced from the slope of the labial surface of the residual alveolar ridge. b. If the columella is prominent and at a lower level than the ala, a nasolabial angle of more than a right angle is indicated, but if the columella and ala are at the same level this indicates a right angle. The colwnella and ala of the nose illustrated in Fig. 2u are on the same level, indicating a nasolabial angle of a right angle, but in Fig. 2b the patient has a low prominent columella
Watt: Tooth
positions on dentures
b a Fig. 3. a, In this patient both tooth inclination and nose form tend to reduce the nasolabial angle to less than a right angle. 6, In this patient both tooth inclination and nose form tend to increase nasolabial angle to more than a right angle.
the
a
b
fig. 4. The upper lip is identical in both diagrams, but in b the upper lip looks more prominent because the lower lip is unsupported.
and high ala, indicating a nasolabial angle of more than a right angle. When both factors work together towards reducing the nasolabial angle it is not uncommon to find that that angle is less than a right angle. Fig. 3u shows a patient with ala and columella on the same level and the teeth proclined; the nasolabial angle is less than a right angle. The patient shown in Fig. 3b has both factors tending to increase the nasolabial angle. The columella is prominent and the teeth are retroclined, and in this case the nasolabial angle is quite obtuse. The vertical dimension is another factor which affects the nasolabial angle. Where it is high the angle is increased and the inferior labial angle is flattened. Where it is low the nasolabial angle is decreased. The inferior labial angle is also decreased in the same way. These then are some of the biometric guides to the restoration of lip form which guide the positioning of anterior teeth. There is a tendency to underfill the upper lip because a correctly supported upper lip looks much too prominent when the lower lip is not filed out. This is illustrated in Fig 4a and 4b In both cases the upper lip is identical and the only difference is that the lower lip is unsupported in Fig. 4b. Another angle which is important for the correct restoration of the lips is the horizontal labial angle (Fig. 5). This angle varies between 90 and 120”. In narrow-faced individuals it tends to be nearer 90” and in broad-faced individuals a flatter angle is more usual. A common fault of complete maxillary dentures is that they do not restore the correct contour of the front of the upper lip (Rayson et al., 1970; Ismail, 1971). Dentures are often wrongly built out in the canine regions in an attempt to satisfy patients who complain about deep nasolabial grooves between the alae and the angles of the mouth. This toe of restoration flattens the horizontal labial angle and produces a monkey-like appearance. By ftiing out the lip in the midline the nasolabial grooves can be reduced just as effectively as by building up the denture in the canine region. The horizontal labial angle is thus restored and the appearance of the patient is improved.
150
Fig. 5. The horizontal labial angle varies from 90 to 120”. The line (arrowed) between the glossy oral mucosa and the transitional epithelium can be clearly seen.
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Fig. 6. The level of the occlusal plane is determined by asking the patient to keep the lips in light contact while separating the jaws slightly. A wax knife is passed between the lips, and the level of the occlusal plane is thus marked on the labial surface of the block.
In Fig. 5 the difference between the dull transitional epithelium of the lip and the glossy oral mucosa can be seen. The junction (arrowed) between these two represents the line of contact of upper and lower lips. If this glossy oral mucosa cannot be seen when the lips are slighly apart they are not sufficiently supported. An inadequately supported lip folds inwards so that only the transitional epithelium can be seen. if the lips are correctly restored they will meet at the line of the junction between the oral and transitional epithelium. Another aid to anterior tooth position is that the upper incisal edges should be approximately opposite the junction of the closed lips when the mandible is in the resting or slightly open position. The position of the teeth and the mobility of the lips influence the amount of tooth exposed during smiling. In this respect it is important to appreciate that the tooth/lip relationship varies widely between different individuals (Ellinger, 1968; Nassif, 1970). Nevertheless, when most dentures are designed it is customary to place the incisal edges of the upper incisors 2-3 mm below the resting position of the upper lip. This is not a particularly good method as it makes little allowance for individual variations. Other methods have been described (Ismail and Bowman, 1968; Lundquist and Luther, 1970), but the author prefers the following method for positioning the occlusal plane and hence the level of the incisal edges of the upper anterior teeth. It has the advantage that allowance is made for variations in the mobility of the upper lip and as a result the denture looks more natural. At the time of registration of the jaw relationship, and after the upper occlusion rim has been adjusted to provide a suitable support for the upper lip, ask the patient to close the lips gently and separate the jaws as if stifling a yawn. Then pass a wax knife between the lips and make a mark on the vestibular surface of the upper rim to indicate the level of the occlusal plane (Fig. 6). Patients with mobile lips who normally expose more of their anterior teeth when they smile have a greater downward movement of the upper lip during this procedure, and hence the level of the occlusal plane is placed at a more suitable position. Conversely,
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fig. 7. The upper anterior teeth should be set so that their incisal edges follow the curve of the smiling lower lip.
Fig. 8. Patient with a cleft lip showing severe scarring and distortion. Note how the teeth have been set to follow the contour of the lower border of the upper lip in order to mask its distortion. Note also how the irreguiar setting of the teeth has been carried out to accommodate differences in the thickness of the lip so that the skin surface presents a more regular contour.
patients who have relatively immobile lips do not move them much during this localization of the occlusal plane, and will therefore show correspondingly less of their teeth on smiling. Thus, allowance is made automatically for variations in the mobility of the upper lip and a more natural appearance of the denture is produced. The position of upper anterior teeth can be checked at the trial denture stage by asking the patient to make ‘f’ and ‘v’ sounds. The words ‘fish’ and ‘victor’ should be pronounced, and if the teeth are in the correct position the incisal edges of the upper incisors will contact the lower lip at the junction between the transitional epithelium and the oral mucosa. In general, the upper anterior teeth should be set so that their incisal edges follow the curve of the smiling lower lip (Fig. 7). To facilitate this a smile curve should be marked on the occlusal rims. A pleasing effect is produced by this method of setting the upper anterior teeth because the line of the lower lip when the patients smiles is used as a visual reference line in the assessment of tooth position. Teeth which lie too near the lower lip are assessed as too long and teeth which lie too far away as too short. The lip contour may be disturbed by scarring as a result of an accident or surgery. Perhaps the most common cause of irregular lip contour is congenital cleft of the lip. In these patients the incisal edges of the anterior teeth should be arranged so that they tend to follow the contour of the lower border of the upper lip (Fig. 8). The irregularity of the lip is thus less obvious than if the teeth were set on a regular curve. In some cases scarring may produce differences in thickness of the lip although the contour of the border at the labial commissure is regular. It is important to set the teeth in such a way that the differences in thickness are accommodated so that the skin surface of the lip presents a regular contour. Thus, where part of the lip is thickened by scar tissue the anterior tooth opposite that part should be depressed, and where part of the lip is thin the adjacent anterior tooth should be set further forwards to restore the skin contour. It is important to realize that irregularities of the teeth can be considered normal in a patient with a cleft lip. Even if the lip has been repaired and its contour is quite smooth, it is usually wise to set the anterior teeth irregularly as the presence of a lip scar leads observers to expect an irregularity and if it is absent the teeth look artificial.
152
Fig. 9. Average changes in coronal traces through the canine and premolar regions at 1, 3, 6,12 and 30 months after the extraction of the teeth.
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Fig. 70. Average changes in coronal traces through the molar region at 1,3,6, 12 and 30 months after the extraction of the teeth. The buccolingual breadth @LB) of the dentulous alveolar ridge was remarkably constant for each tooth position in all the patients measured.
The distortion of the lips can be quite minor in patients with slight unilateral weakness of the facial muscles or with habitual crooked smiles. In these cases symmetrical placement of the teeth about the midline of the face exaggerates the distortion of the mouth and should be avoided. It is better to place the teeth in the centre of the mouth so that the mesio-incisal point lies midway between the right and left corners of the mouth when the patient smiles. Horizontal placement of the occlusal plane should be avoided if the mouth is tilted, as this, by providing a horizontal reference line, will exaggerate the crooked appearance of the mouth. In these cases it is better to arrange the occlusal plane parallel to a line joining the right and left comers of the mouth when the patient smiles. POSITION OF UPPER POSTERIOR TEETH Posterior teeth on maxillary dentures are frequently placed just under the crest of the ridge, yet studies of oral changes following tooth loss (Watt and MacGregor, 1976) have shown that the ridge crest does not necessarily correspond to the position of the alveolar process and varies greatly in position with the amount of resorption. The average post-extraction changes in coronal traces through maxillary canine and premolar regions are shown in Fig. 9. It can be seen that the whole breadth of the palate is affected by resorptive change in the canine region. This amount of tissue loss is seldom replaced by a corresponding amount of denture base, with the result that whistling during the pronunciation of sibilants is not uncommon in edentulous patients. In the molar region (Fig. IO) it can be seen that while change in the lateral parts of the palate are quite large, the greatest losses occur buccally and in the height of the ridge. Greater losses occur over longer periods (Likeman and Watt, 1974; Watt and Likeman,
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153
positions on dentures
Fig. 11. Extreme resorption of the maxillary Note the great breadth of the buccal sulcus.
ridge.
Tab/e 1. Analysis of measurements of the horizontal breadth of the alveolar process from the linguogingival margin to the mid-buccal point in 100 dentulous patients Position and plane of measurement Central incisor, sagittal Canine, coronal First premolar, coronal Second premolar, coronal First molar, coronal Second molar, coronal Third molar, coronal
Average change k s. d (mm) 6.3 8.5 10.0 10.6 12.8 11.6 10.1
* t * + f t k
0.91 1.06 1.03 1.40 0.98 1.14 1.33
1974). Fig. 11 shows the mouth of a patient with a very resorbed maxillary ridge. The front of the mouth is obviously distorted by retractors, but it can be appreciated that even without retraction there would still be a broad gap between the buccal surface of the ridge and the cheek posteriorly. This gap partly indicates the amount of buccal resorption of the ridge in this patient who had been edentulous for over 30 years. To construct the diagrams in Figs. 9 and 10 it was necessary to measure casts before and at intervals after extraction, so that the various increments of change could be plotted (Watt, 1960), but only one of these measurements need be considered here: the buccolingual breadth of the dentulous alveolar ridge. The position of this measurement can be seen on the right of the bottom diagram in Fig. 10 and is marked BLB. The constancy of this measurement for each tooth position was so striking in the original sample of 25 that later 100 patients were measured to check that the variation was really so small as was first observed (Watt et al., 1967). The results of these measurements are shown in TableI. The interesting thing about these data is that the standard deviations are only about 1 mm at each tooth position. This means that the average measurements pbs or minus about 1 mm will be correct for about 68 per cent of all patients measured, and the average measurement plus or minus about 2 mm will be correct for 95 per cent of all patients measured. Thus, provided that the linguogingival margin can be located in an edentulous patient, there is a reasonable guide for positioning the cheeks and lips correctly in the majority of edentulous patients because the measurements are relatively constant for every tooth position. With
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ractb
Aftw 9 months Fig. 72. Pre-
and post-extraction casts show how the vestige of the Iinguogingival margins can be identified as a fine cord-like elevation near the crest of the residual ridge. Mean measurements of the buccolingual breadth (arrows) of the alveolar process before extraction can be transferred to the edentulous cast to give an approximate guide to the breadth of the flanges of complete upper dentures.
reference to Fig. 10, which shows the average changes in the molar region, imagine the cheek in contact with the buccal surface of the ridge. Knowing that the buccolingual breadth in this region is about 12 mm, it can be deduced where the cheek should lie in an edentulous patient, provided that the remnant of the linguogingival margins can be located. This proved to be comparatively easy, as during the course of the study of oral changes following tooth loss (Watt, 1960) tattoo spots were placed on the mucosa of 8 patients before extraction. These spots were near the buccal and linguogingival margins and made it possible to identify the remnant of the linguogingival margin as a fine cord-like elevation of mucosa near the crest of the residual ridge. There can be no doubt that this structure does represent the vestige of the lingual margins, as the tattoo spots lay in the same relationship to it after healing had taken place. The progress of the spots in a series of casts studied over the course of 4% years can be seen in Fig. 12. These spots on the linguogingival margin correspond with the cord-like ridge in the edentulous mouth which represents the remnant of that margin. Thus, by using the average measurements of buccolingual breadth the location of the cheek can be determined fairly accurately. To put it another way, the sulcus breadth of the edentulous cast can be determined with reasonable accuracy. The outer surface of the sulcus represents the cheek position, and by using the average measurements in Table I it is possible to be fairly sure that the cast represents the dimensions of this part of the denture space. Unfortunately, however, measurements show that there is an outward movement of this structure so that it lies
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Table II. Analysis
of the buccal movement
gingival vestige after
extraction
of the linguo-
of maxillary
teeth
in 25
patients Position and plane of measurement Incisor, sagittal First premolar, coronal Second premolar, coronal First molar, coronal Second molar, coronal Third molar, coronal
Average change + s. d. (mm) 1-6 2.6 2.8 2.9 3.6 2.9
f + + + f f
1.16 1.43 1.16 1.30 1.26 1.51
buccally and superior to the positions occupied by the linguogingival margins in the dentulous mouth. These measurements from the longitudinal study of morphological change are given in Table II. Owing to its outward movement, the use of the linguogingival vestige as a reference line on the edentulous cast causes an overestimate of the amount of denture base required to restore the shrunken ridges to their pre-extraction form. The mean amount of outward movement in the sample measured was less in the incisor than in the molar region and was small compared with gross errors which commonly occur in estimates of flange width and tooth positions in complete upper dentures. It is possible to make allowance for the errors produced by the outward movement of the linguogingival vestige by deducting the mean outward movement from the mean buccolingual breadth, but there is no guarantee that such a procedure might not increase the error in individual cases. Clinical experience shows that a slight overestimate of sulcus breadth is preferable to an underestimate, provided that there is no overextension of the sulcus depth. The additional breadth in the molar region can be accepted since in the dentulous patient contact between the cheek and alveolar process may not be present in the molar region. The thinning of the lips with age might more than compensate for the incisor overestimate. Thus, the mean measurements of pre-extraction buccolingual breadth of the alveolar process can be transferred directly to the edentulous cast to give an approximate guide to the breadth of the flange of complete upper dentures. There can be no doubt that an element of guesswork still exists, but the errors can be reduced to some extent by this method. The remnant of the linguogingival margin can be clearly seen on the edentulous ridge in Fig. 12. This ridge would cause little problem in denture construction, but the very resorbed ridge in Fig. I3 might present a difficult problem. Note that the rugae cross the ridge. The soft palatal tissue comprising the anterior part of the ridge is the tissue in the rugae area, while the posterior parts of the ridge contain no alveolar tissue but are composed of the soft tissue that normally lies at the sides of the palate. In this case the remnant of the linguogingival margin lies in the sulcus and all the upper teeth should be placed buccal to this to provide adequate tongue space for the patient. To make this possible the impression must be taken to restore the full breadth of the buccal sulcus. Too often a small tray is made for a case of this kind and a small impression is taken. The impression material dribbles over the edge of the tray and reproduces a narrow collapsed sulcus form because of the contraction of an unsupported buccinator. In order to make a denture for this patient it is necessary first
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Fig. 13. In this case there is much resorption and the rugae cross the ridge, which consists entirely of palate tissue. The remnant of the linguogingival margin (arrows) lies in the sulcus.
a
b
Fig. 74. a, Impressions may fail to reproduce the true breadth of the buccal sulcus, and when the teeth are set in their correct positions an unfavourable form of polished surface results. b, Correct sulcus width on the cast enables correct shaping of the polished surface.
to restore the full breadth of the sulcus by the guide lines indicated. It is then possible to place the teeth correctly and thus give the patient adequate tongue space. It is not possible to make satisfactory dentures for this kind of difficult edentulous patient unless the sulcus breadth is properly restored at the time the impressions are taken (Fig. 14). Fig. 2.5shows old unsatisfactory dentures and the new dentures which were satisfactory. These dentures were made for the same mouth. On the right the old denture has a narrow periphery, and on the left the new denture, which was constructed on the principles described, has a broad periphery. The old lower denture is underextended and the load per unit area on the mucosa under it would obviously be about double the load per unit area on the mucosa under the new denture which covers twice the mucosal area. The sulcus breadth required in the upper denture is proportional to the amount of tissue loss, and the artificial teeth are placed buccal to the ridge crest ln approximately the positions which were occupied by the natural teeth. Those who believe that the ridge crests acts as a fulcrum around which the denture tilts when an occlusal load is placed on one side might object to this positioning of the teeth, but the author’s view is that the denture is rather like a raft floating on water. The fulcrum around which it tilts when loaded will depend very much on the point of application of the load, but if a retainer is put on the
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positions on dentures
Fig. 75. Old and new dentures for the same patient. On the right the old dentures which the patient found unsatisfactory; on the left the new replacement dentures showing a broader periphery and a greater extension.
Fig. 16. When one side of a complete denture is loaded the denture tilts around a soft tissue fulcrum rather like a raft floating on water, but if the opposite end of the raft is held the load is more evenly distributed. The broad peripheral form of the denture ensures that it does not tilt by providing a buccal seal and a bulge against which the buccinator can act.
opposite end of the raft the fulcrum is shifted and the load is more evenly distributed (Fig. 16). The broad peripheral form of the denture acts as the retainer because it provides a buccal seal and also provides a bulge against which the buccinator can usefully act. In the maxilla the relationship between the buccinator and reflected mucosa in the molar region is fundamentally different to that in the mandible, because in the maxilla the part of the bone to which the buccinator is attached in the dentulous state resorbs when the teeth are lost. As a result, the maxillary buccinator attachment moves inwards, but the mandibular attachment does not since the external oblique line is relatively unaffected by tooth loss. In order to restore the maxillary denture space it is necessary to displace the buccinator outwards by the periphery of the denture. This can be done without causing displacement of the denture in function because the buccinator’s attachment is higher than the attachment of the oral mucosa. When the cheek is pushed outwards by the denture the curve of reflected oral mucosa lies at a lower level than the curve of the displaced buccinator (Fig. 17). Contraction of the buccinator thus improves the peripheral seal. If the impression is taken wrongly so that the width of the sulcus is not restored, the surplus of reflected mucosa is stretched upwards together with the buccinator, and the upper denture can then be displaced in function. Similar displacement of the mandibular denture will occur if it is extended on to the buccinator attachment. POSITION OF LOWER POSTERIOR TEETH The restoration of the upper part of the denture space has been considered in detail since the lower denture space cannot be properly delineated unless an upper appliance holds the lips and cheeks in their correct position. It is well to remember that the attachments of the mandibular muscles are not so much affected by the resorptive change, except labially, and as they are close to the periphery of the denture the lower part of the denture space can be defined more easily.
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Journal of Dentistry,
a
b
Fig 77. The effect
of the cheek position and on the sulcus form. a and 6, Incorrect;
Fig. 78. The teeth
should
C
on the relationship c and d, correct.
buccolingual width of the artificial be approximatelv equal to the width
of the buccal cusps of the natural teeth and should occupy the same position over the residual ridge. The cross-section form of the lower denture is shaded. Note the upward and inward buccolingual surface and the rounded form of the buccal flange which provides a shelf on which the buccinator can rest.
between
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d the buccinator
muscle and mucosa
Fig. 79. The last molar should be positioned approximately a little finger’s breadth in front of the posterior edge of the denture to provide a posterior shelf. The tongue (stippled) lies on the lingual side of the shelf while the buccinator lies on the buccal side. Together they stabilize the denture.
The lower posterior teeth should be set directly over the lower ridge. In this position there is an optimum relationship between the forces of the tongue and cheeks. The buccolingual width of the artificial teeth should be approximately equal to the width of the buccal cusps of the natural teeth (Fig. 18). The discrepancy in width between upper and lower endentulous ridges is caused chiefly by resorption buccahy in the maxilla and partly by post-extraction resorption of the lingual plate of the mandible. However, if lower posterior teeth of natural buccolingual width are set in the pre-extraction position an undercut is produced into which the tongue bulges and raises the lower denture. The lingual polished surface of the lower denture should face upwards and inwards if muscle balance is to be achieved by the downward and outward pressure of the tongue. It is also important to ensure that teeth are not placed on the part of the denture base which lies over the posterior part of the mandibular ridge which curves upwards to the ascending ramus. This area constitutes the posterior shelf. The tongue lies on the lingual part
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positions on dentures
of this shelf while the buccinator overlies the buccal part of the shelf as it sweeps towards its posterior attachment to the pterygomandibular raphe on the lingual side of the mandibular ridge (Fig. 1Y).
POSITION OF LOWER ANTERIOR TEETH It is frequently said that lower incisors should be placed immediately
above the lower residual ridge, but when there is much bony resorption the remnant of the ridge is part of the lingual plate and the teeth should be set on the labial side of it-how far labially will depend on the migration of the mentalis muscle attachments and on the activity of the lips. The optimum position for the lower incisors can easily be found by replacing the anterior segment of the record block with a mouldable substance, such as soft wax or a functional impression material, and then asking the patient to speak with both upper and lower record blocks in the mouth. It is essential that the upper block is carved to restore the lips and cheeks to their former position before any attempt is made to locate the lower incisors by this functional method, because if the upper lip is collapsed the activity of the orbicularis oris is changed and the wax indicating the position of the lower incisors will be located in the wrong position.
CONCLUSION Artificial teeth on complete dentures should be placed in the approximate positions occupied by the natural teeth. To enable this to be done biometric guides should be used to restore the lips and cheeks to the positions which they occupied when the natural teeth were present. Dentures constructed in this way have a natural appearance and function extremely well.
REFERENCES
Ballard C. F. (1963) Variations of posture and behaviour of the lips and tongue which determine the position of the labial segments; the implications in orthodontics, prosthetics, and speech. Trans. Eur. Orthod. Sot. 39, 67-88. Ellinger C. W. (1968) Radiographic study of oral structures and their relation to anterior tooth position. J. Prosthet. Dent. 19, 36-45. Fish E. W. (1948) Principles ofFull Denture Prosthesis, 4th ed. London, Staples, p. 104. Hooper B. L. (1934) Functional factors in the selection and arrangement of artificial teeth. J. Am. Dent. Assoc.
21, 603-615.
lsmail Y. H. (1971) Changes in soft-tissue profile following extraction and complete denture treatment. J. Prosthet. Dent. 26, 1 l-20. Ismail Y. H. and Bowman J. F. f 1968) Position of the occlusal plane in natural and artificial teeth. J. Prosthet. Dent. 20, 407-411. Landa J. S. (1954) The troublesome transition from a partial lower to a complete lower denture. J. Prosthet. Dent. 4,42-5 1. Lee J. H. (1962) Dental Aesthetics. Bristol, Wright, p. 103. Likeman P. R. and Watt D. M. (1974) Morphological changes in the maxillary denture bearing area: a follow-up 14 to 17 years after tooth extraction. Br. Dent. J. 136, 500-503.
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Martone A. L. (1963) Clinical applications of concepts of functional anatomy and speech science to complete denture prosthodontics. Part VIII. The final phases of denture construction. J. Prosthet. Dent. 13, 204-228. Nassif N. J. (1970) The relationship between the mandibular incisor teeth and the lower lip. J. Prosthet. Dent. 24, 483-491.
Pound E. (1960) Modern American concepts in esthetics. Znt. Dent. J. 10, 154-172. Rayson J. H., Rahn A. O., Wesley R. C. et al. (1970) Placement of teeth in a complete denture: a cephalometric study. J. Am. Dent. Assoc. 81, 420-424. Swensen M. G. (1959) Complete Dentures, 4th ed. St Louis, Mosby, pp. 51, 174. Watt D. M. (1960) Morphological changes in the denture bearing area following the extraction of maxillary teeth. PhD Thesis, University of Edinburgh. Watt D. M., Durran C. M. and Adenubi J. 0. (1967) Biometric guides to the design of complete maxillary dentures. Dent. Msg. Oral Top. 84, 109- 111. Watt D. M. and Likeman P. R. (1974) Morphological changes in the denture bearing area following the extraction of maxillary teeth. Br. Dent. J. 136, 225-235. Watt D. M. and MacGregor A. R. (1976) Designing Complete Dentures. Philadelphia, Saunders.
