Medical knowledge | Dentistry » Devlin-Reddick - Biological guides to the positioning of the artificial teeth in complete dentures

O SS T H O D O N T I C S P R O S T H O D O NPTRI C Biological Guides to the Positioning of the Artificial Teeth in Complete Dentures H. DEVLIN AND G HOAD-REDDICK Abstract: Setting teeth for complete dentures is traditionally done away from the clinic in the dental laboratory. This has unwittingly given the impression that arranging tooth position is a mechanical process in which the clinician has little say. Many technicians are given few instructions, but a detailed prescription is crucial to the success of the denture. This article describes those considerations the dentist should address in communicating with the laboratory technician. A ‘denture space’ impression technique is described to assist the dentist in the correct prescription for posterior teeth placement. Dent Update 2001; 28: 492–495 Clinical Relevance: The function of the lips, cheeks and other muscles of the oral tissues influence the optimal tooth position of artificial teeth in complete dentures. The aim of

this paper is to explain the biological forces that influence tooth position, and describe clinical techniques that can be used to provide denture stability and retention. W hen providing complete dentures the base should always be optimally extended, the polished surface correctly shaped and the teeth placed in the most favourable position. The patient and the dentist may have conflicting agendas: the dentist will consider factors promoting denture stability and retention, and which provide good aesthetics without compromising function; the patient may be more concerned with aesthetics (at the expense of function) or may prefer a modified copy of their previous denture rather than radically different dentures with significant improvements. Most edentulous patients have H. Devlin, PhD, MSc, BSc, BDS, Senior Lecturer in Restorative Dentistry, and G. Hoad-Reddick, PhD, MSc, BDS, Senior Lecturer in Dental Education,The University Dental Hospital of Manchester. 492 existing complete

dentures, which can be used as a template. However, if there is no existing denture, or if a patient has expressed dissatisfaction with previous dentures, the dentist may need to use biometric principles. These use anatomical landmarks to guide in the optimal placement of teeth on the denture. Using biometric principles, the upper artificial teeth can best support the lips and cheeks if they are placed in the position previously occupied by the natural teeth. In addition, a peripheral seal is formed between the denture flange and the cheeks and lips, which enhances denture retention and stability. Pre-extraction photographs and models can be invaluable guides but, if they are not available, the dentist must rely on biological guides to assist in the optimal placement of the artificial anterior teeth for function and aesthetics. GUIDES TO THE POSITION OF THE OCCLUSAL PLANE A majority of British dentists consider the occlusal plane to be parallel to the interpupillary line and the

alar-tragal line,1 but they differ as to where the points are located on the relevant cartilages. The alar-tragal line lies between the lower border of the ala of the nose and the upper border of the tragus of the ear.2 The angulation relative to the horizontal provided by the alar-tragal line allows the denture teeth to articulate in a harmonious manner, sympathetic to the movement of the mandibular condyle down the articular eminence. Other allowances are built into the set-up by the technician to provide optimal balanced articulation – cusp angles, compensating curves and incisal guidance. The condylar guidance can be programmed into an adjustable articulator to allow optimal arrangement of the teeth. The alternative viewpoint questions the need for classical anatomical articulation because during mastication most occlusal contacts occur within a few millimetres either side of retruded contact position. Further research is needed to resolve this issue. Ismail and Bowman3 found

that positioning the occlusal plane at the level of the upper third of the retromolar pad brings it close to the level of the pre-extraction natural dentition. The lower wax rim, used in the jaw registration, should be constructed in the laboratory to the height of the upper part of the retromolar pad.4 This position of the plane allows satisfactory function, as it usually lies mid-way between the two residual ridges. Dental Update – December 2001 P R O S T H O D O N T I C S Figure 1. The occlusal plane of the lower complete denture is level with the upper part of the retromolar pad. The occlusal plane is low enough for the tongue to position food on the occlusal platform of the lower teeth. Tuckfield5 recommended placing the occlusal plane parallel to the crest of the lower ridge for maximum denture stability. Although this advice has been repeated in textbooks, it has not been thoroughly investigated experimentally. There is reluctance amongst dentists to raise the occlusal

plane level because it will cause denture instability if the tongue is unable to place food comfortably on the occlusal platform. At rest, the dorsum of the tongue should lie at least at the level of the occlusal plane (Figure 1) or overlying the lingual cusps to provide denture stability. This position can be difficult to assess clinically as the tongue position will vary according to the denture wearing experience of the patient. In addition, patients who tend to retch with dentures will often retract their tongue on opening their mouth to guard the pharynx unconsciously. Residual ridge resorption is about four times greater in the mandible than in the maxilla,6 and in the patient who has been edentulous for a long time this can result in the lower denture appearing large and bulky. Fortunately, the height of the lower denture can be reduced slightly because the rest vertical dimension is diminished following extraction of the teeth. The pattern of resorption varies in the two

arches, with bone being lost laterally in the region of the maxillary tuberosity and medially in the posterior lingual region of the mandibular arch. Thus the upper ridge appears to become Dental Update – December 2001 narrower in relation to the lower as resorption follows its insidious progress. When the teeth are placed upon the denture this previous relation should be borne in mind: if the molar teeth of the upper denture are placed in the position previously occupied by the natural teeth, they might not be placed over the resorbed ridge crest. Retention and stability are not compromised because of the large surface area of the palate and the peripheral seal of the denture flange with the cheek. It is often stated that the lower molar teeth of the complete denture should be placed in the ‘neutral zone’, the area previously occupied by the natural teeth. This may be an oversimplification as natural lower molar teeth are often lingually inclined to such an extent that, if this

situation were replicated in the dentures, instability would result. Placing the lower molar teeth over the residual ridge should provide optimum stability. If the molar teeth are wide they may overhang the tongue, and during tongue movements the dentures will tend to be unseated. Choosing a narrow posterior tooth mould will help to prevent this problem. THE INCISOR TEETH With age, the reduction in muscle tone affects the relation of the upper lip and the incisor teeth. As a result the amount of upper central incisor showing below the upper lip reduces from an average of 2 mm. The exact length of visible tooth will vary with different jaw relationships and lip lengths, but is generally more in younger patients; older patients may prefer to keep the upper incisor edges level with the resting lip. The incisal edges of the upper incisors should follow the curve of the lower lip when smiling. The upper incisors are positioned to provide lip support, which is generally accepted to require

an average nasolabial angle of 90o. However, Brunton and McCord7 showed the nasolabial angle in their dentate subjects averaged about 110o, therefore a slightly more obtuse angle may be necessary. If the lip is not adequately supported, the delicate shape of the philtrum is lost. It is the crowns of the incisors which should support the lip: if the flange is thickened to provide support, not only will the lip be effectively shortened, but also the area directly beneath the nose will appear swollen. THE CANINE TEETH A coronal plane passes through the tip of the maxillary canine and the posterior part of the incisive papilla. When the patient is viewed from the front, a vertical line drawn from the inner canthus of the eye to the alar cartilages on each side passes through the maxillary canine tips. The canine eminence should be considered when the denture is being waxed up to provide support for the correct shape of the angle of the mouth. BIOMETRIC GUIDELINES The palatal gingival

vestige is a raised fibrous ridge on the palate. It is considered a remnant of the palatal gingivae, and is often used as a guide to the position of the maxillary artificial teeth.8,9 Using the palatal gingival vestige as a fixed-point, measurements of the average horizontal bone loss following extraction of the maxillary teeth allows the pre-extraction position of the teeth to be determined on the edentulous cast. The labial surface of Figure 2. The prominent palatal gingival vestige present in a 14-year-old patient with anodontia. Clearly the palatal gingival vestige cannot be the remnant of the palatal gingival margin. (Arrows indicate a structure resembling the palatal gingival vestige.) 493 P R O S T H O D O N T I C S Figure 3. A structure resembling the palatal gingival vestige is present in neonatal children before the eruption of the teeth (arrows indicate a structure resembling the palatal gingival vestige). the incisors and canines are placed 6 and 8 mm, respectively,

labial to the palatal gingival vestige, with the buccal surface of the premolars 10 mm and that of the molars 12 mm buccal to the vestige. However, this palatal fibrous ridge is present in patients with congenital absence of development of the teeth and associated periodontium (Figure 2). In addition, the palatal gingival vestige can be identified in newborn infants (Figure 3) before the eruption of teeth. The palatal gingival vestige cannot therefore be used as an absolute guide to positioning of teeth in the arch. The incisive papilla may likewise be used as a reference point. The tips of the central incisor teeth are generally placed 8–10 mm in front of the central point of the papilla. The position of the incisal edges of the anterior teeth relative to the incisive papilla can be positioned accurately using a Schottlander Alma Gauge (Davis, Schottlander and Davis Ltd., Letchworth Garden City, Herts, UK). The vertical metal pointer is placed over the incisive papilla and a

horizontal scale used to measure the distance in front of the papilla (Figure 4). Figure 5. Silicone impression material is applied to the denture and the patient performs functional movements. THE DENTURE SPACE IMPRESSION With the Finished Acrylic Denture There are many techniques which attempt to define the ‘denture space’ (that area of stability where the lower denture extension is in harmony with the surrounding musculature). Wright10 described a technique in which a lowviscosity silicone material is applied to the polished and fitting surfaces of the denture, which is then inserted into the patient’s mouth (Figure 5). Functional movements, such as chewing or speaking, are performed until the silicone has set. The tongue will remove the paste where the flange is too thick or where the teeth are positioned too far lingually. Areas of overextension are highlighted (Figure 6). The silicone can be easily peeled off, as no adhesive is used, and after the necessary denture

adjustments have been carried out, the procedure is repeated. Figure 6. The denture is removed when the silicone impression material has set. Acrylic visible on the denture periphery may indicate overextension. the upper trial denture in place. In the laboratory, indices are constructed around the impression so that, when the silicone is removed, the space for the denture teeth is identified. The resultant impression of the denture space can be copied in wax and the teeth set within its confines. DISCUSSION Certain changes occur in ageing of the facial tissues, which can affect facial contour and which are not accounted for in the biometric denture principles. For Defining the Denture Space During Denture Construction Figure 4. The Schottlander Alma Gauge is used to determine the optimal position of the incisal edges of the upper anterior teeth. 494 To ensure that the teeth do not conflict with the surrounding muscular environment, a light-cured acrylic baseplate can be

constructed with a posterior vertical flange, or stop, at the correct occlusal vertical dimension. Attached to the baseplate are wire loops, which retain a putty silicone impression material (Figure 7). The putty is moulded by the patient (Figure 8) with Figure 7. An acrylic baseplate is constructed, with wire loops to retain the impression material. Dental Update – December 2001 P R O S T H O D O N T I C S Figure 8. With the upper trial denture in place, the silicone putty is moulded by the patient talking and swallowing. When set, the denture space impression is removed and copied in wax in the laboratory. The technician is requested to position the artificial teeth in the wax template. instance, Pellacani and Seidenari11 used ultrasound to show that facial skin of elderly people was thicker than that of younger individuals in most areas. In addition, there is a wide variation in the amount of facial skin adipose tissue in the healthy population that must tend to invalidate

the application of fixed measurements to tooth position. The rigid application of rules to the positioning of teeth can prevent the development of satisfactory alternative tooth arrangements. For example, ABSTRACT A NEW ASPECT OF PRACTICE MANAGEMENT? How Can You Protect Yourself from Employee Dishonesty? J.M De St Georges and D. Lewis Jr Journal of the American Dental Association 2000; 131: 1763–1764. It has been reported that 40% of American dental offices will suffer fraud or embezzlement by an employee. The average amount of these losses is a staggering $105,000. Most dentists have suffered this loss in silence, but it is reported that more are now seeking redress through the courts. This brief article highlights the concerns and suggests some ways to protect against such actions. A full Dental Update – December 2001 patients with a prominent pre-maxilla may request that the upper artificial incisors are positioned more palatally than indicated by their pre-extraction position

and the labial flange of the denture either omitted or reduced in thickness. The dentist can adopt two main treatment strategies when providing replacement complete dentures.
In the first, the patient receives a denture based on biometric principles where the anatomical guides are used to position the artificial teeth for optimum function.
With the second treatment strategy, the dentist copies the good features of the patient’s existing dentures and incorporates improvements where there are deficiencies in existing denture design. These can be related to the biometric principles described earlier in this paper, which influence the optimal position of artificial teeth in complete dentures. Both treatment strategies are appropriate in particular circumstances. practice audit is both costly and timeconsuming. It is cheaper and simpler to ask an accountant to design a brief self-audit for you to use. The third, and cheapest, option is to carry out immediate, cursory and random

audits. It is suggested that 15 patient record cards should be drawn at random each week. Experience shows that, if the financial details on these cards matches the day-books and banking details, there is probably, although not definitely, not a problem. The authors suggest other ways of checking and preventing fraud, including a very interesting table of the common characteristics of embezzlers. It would appear that the best, longest-employed and most hard-working member of the team could be the prime suspect! Peter Carrotte Glasgow Dental School R EFERENCES 1. Williams DR. Occlusal plane orientation in complete denture construction. J Dent 1982; 10: 311–316. 2. Hobkirk JA Complete Dentures Bristol: Wright, 1986. 3. Ismail YH, Bowman JF Position of the occlusal plane in natural and artificial teeth. J Prosthet Dent 1968; 5: 407–411. 4. Celebic A, Valentic-Peruzovic M, Kraljevic K, Brkic H. A study of the occlusal plane orientation by intra-oral method (retromolar pad). J Oral

Rehab 1995; 22: 233–236. 5. Tuckfield WJ The problem of the mandibular denture. J Prosthet Dent 1953; 3: 8–28 6. Tallgren A The effect of denture wearing on facial morphology. A 7 year longitudinal study Acta Odontol Scand 1969; 25: 563–592. 7. Brunton PA, McCord JF An analysis of nasolabial angles and their relevance to tooth position in the edentulous patient. Eur J Prosthodont Restor Dent 1993; 2: 53–56. 8. Likeman PR, Watt DM Morphological changes in the maxillary denture bearing area. A follow-up 14 to 17 years after tooth extraction. Br Dent J 1974; 136: 500–503 9. Watt DM, Likeman PR Morphological changes in the maxillary denture bearing area following extraction of teeth. Br Dent J 1974; 136: 225–235. 10. Wright SM The polished surface contour: a new approach. Int J Prosthodont 1991; 4: 159– 163. 11. Pellacani G, Seidenari S Variations in facial skin thickness and echogenicity with site and age. Acta Derm Venereol 1999; 79: 366–369 COVER PICTURES Do you have an

interesting and striking colour picture with a dental connection, which may be suitable for printing on the front cover? Send your transparencies to: The Executive Editor Dental Update, George Warman Publications (UK) Ltd, Unit 2, Riverview Business Park, Walnut Tree Close, Guildford, Surrey GU1 4UX. Payment of £75 will be made on publication. 495