Tooth selection Dr. Waseem Bahjat Mushtaha Specialized in prosthodontics
Tooth selection The selection of teeth for complete dentures is classified into: I- anterior teeth selection. II- posterior teeth selection.
The anterior teeth are primary selected to satisfy esthetic requirements, whereas the posterior teeth are primarily selected to satisfy masticatory functional requirements . both the anterior and posterior teeth must function in harmony with, and be anatomically and physiologically compatible with the surrounding oral environment .
I-anterior teeth selection 1-maxillary anterior teeth: there are three basic considerations when selecting the maxillary anterior teeth. These are size, form and shade.
a- size of anterior teeth four anatomic landmarks are used as guide for the selection of the size of anterior teeth. 1-size of face: the average width of the maxillary cental incisor is estimated to one sixteenth(1\16) of the bi-zygomatic width. The combined width of the six maxillary anterior is slightly less than one third (1\3) of the bi-zygomatic width of the face. The face-bow can be used as a caliper to record the bi-zygomatic width of the face.
-The cuspid eminences:2 After the occlusal rim has been properly contoured for esthetic, the approximate location of the cuspid eminence (the apex of the upper natural canine) can be determined by extending parallel lines from the lateral surfaces of the ala of the nose, corners of the mouth onto the labial surface of the upper occlusion rim. The distance between the two marks following the contour of the arch is the combined width of the six maxillary anterior teeth.
3-the vertical distance between the ridges: the length of the teeth is determined by the available space between the existing ridges, when space is available, it is more esthetically acceptable to use a tooth, long enough to eliminate the display of the denture base.
4-the lip: when the lips are relaxed and apart, the labial surface of the maxillary anterior teeth should support the upper lip. Frequently the incisal edges of maxillary anterior teeth extend inferior to or slightly below the lip margin. During speech, the incisal edge of the maxillary anterior teeth contact the lower lip at the junction of the moist and dry surfaces of the vermilion border. This is best demonstrated when the letter f is pronounced by the patient. The properly contoured maxillary occlusion rim should include this position as the incisal edge position that will aid in determining the length of the teeth . also, the lower lip line and the high lip line marked on a properly contoured occlusion rim aid in determining the length of maxillary anterior teeth.
b-form of the teeth three factors are used as guides in the selection of the form of the anterior teeth: 1-the form and contour of the face: the general outline form of the tooth should conform to the general outline of the face when viewed from the frontal aspect. Also a tooth viewed from the mesial and distal aspect should conform to the contour of the facial profile.
a-the general outline of the face: it can be grouped into square, tapering, ovoid or combination. This can be determined by, imagining two lines on either side of the face running about 2.5 cm in front of the tragus of the ear and touching the angle of the mandible, 1-if these lines are parallel of the face is square. 2-if they converge towards the chin the face is tapered 3-if they diverge away from the chin, the face is ovoid. b-the facial profile: (to determine the facial profile observe the relative straightness of curvature of the profile). The labial surface of the teeth should be in harmony with the facial profile. If the face presents a curved profile than a tooth should be selected to harmonize with that profile.
2-sex curved facial features are always associated with femininity, while square features are always associated with muscularity. There for , the teeth of females are more ovoid or tapered rather than square , those for males are usually square.
3- shape of the dental arch The form of central incisors should not only harmonize with the form of the face but also with the form of the dental arch. Patients with predominantly square dental arches often have square teeth. The same principle of the harmony apply to the square tapering and ovoid types, i.e. V-shaped arch is associated with incisors that are narrower at the neck than at the incisal edge.
C-color or shade of anterior teeth 1- the color of the teeth, like the form must be in harmony with the surrounding environment if they are to appear natural. Harmony should exist between the color of the teeth and the color of the skin, hair and eyes. 2-the patient's age: The color of natural teeth change with age they get progressively darker as a result of deposition of secondary dentine. The general rule is that darker teeth are more suitable for old patients while, lighter teeth are suitable for young patients.
Most of natural teeth have the following characteristics: a- the neck of the tooth have a more pronounced yellow color than incisal third. b-the incisal edge is more translucent than the body of the tooth because it is composed entirely of enamel. c-the upper central incisors are the lightest teeth in the mouth. The upper lateral ,lower centrals and laterals are slightly darken. The upper and lower canines are more dark. d-teeth darken slightly with age.
2-mandibular anterior teeth Selection of the lower six anterior teeth is relatively a simple procedure. Each set of the upper anterior teeth has a corresponding set of lower anterior teeth which match in the size, form, and color.
II-posterior teeth selection The selection of posterior teeth involves shade, size, number and form: a-shade of posterior teeth: the shade of posterior teeth should harmonize with the shade of anterior teeth.
b-size and number of posterior teeth: the posterior teeth must support the cheeks and tongue and function in harmony with musculature in swallowing, speaking and mastication. 1-the buccolingual dimension: Of the artificial teeth should be less than that of natural teeth to reduce the size of the occlusal table, and stresses transmitted to the underlying supporting structures. This will also permit forces from the cheeks and tongue maintain the dentures in their position or the residual ridge. However, this reduction should not be accomplished at the expense of losing support for the cheek.
2-the anteroposterior dimension: Of posterior teeth is determined by the edentulous between the distal surface of the canine and the ascending are of the mandible which is usually situated slightly anterior to the retro molar pad. Placing teeth on the ascending area of the mandible will direct the masticatory forces at an inclined plane. Forces directed at an inclined plane are more dislodging than forces directed at right angles to the supporting structure. 3- the vertical length: Of posterior teeth depends on, the available interarch distance and the length of anterior teeth. The length of maxillary first premolars should be comparable to that of the maxillary canines to have proper esthetic effect.
c- form of posterior teeth: the form of occlusal surface of posterior teeth may be classified according to their cuspal inclination into: I-anatomic form II-non anatomic form.
I-anatomic form The anatomic form has cusps, grooves and inclined planes . the cusp angle ranges from 20-45 with average 33. II-the non anatomic form:(cuspless or flat teeth): Has flat occlusal surface with shallow grooves and no cusps. Selection of a suitable form of posterior teeth depends on individual requirements. The anatomic forms of posterior teeth are commonly used for patients having normal ridge relations and well developed ridges. Non anatomic form; however, may be used in the following condition: a- cross-bite ridge relation. In this condition, the lower posterior teeth are in a buccal relation to the upper posteriors which is the reverse of that in normal ridge relation. b-cases suffering from T.M.J. disturbances. c-in nervous or hysterical patients, or in patients where neuro-muscular co-ordination has been lost. d-in cases with advanced ridge resorption.
Tooth material Artificial teeth may be made from: 1-acrylic resin 2-porcelain 3-metal
1-acrylic resin teeth Properties of acrylic teeth: a-acrylic teeth unite with the denture base resin by chemical union, they can be used in cases having insufficient inter-arch distance. b-they are less harmful to the underlying tissues because of their great resilience. c-they have low specific gravity(light in weight), and can be easily modified by grinding. d-acrylic resin posterior teeth are used when oppose natural teeth or teeth whose occlusal surface have been restored with gold. This reduces the possibility of the artificial acrylic teeth, causing unnecessary abrasion and destruction to the opposing natural or metallic occlual surfaces. Acrylic resin teeth have low co-efficient of wear than gold and enamel. e-acrylic resin teeth on the other hand may become worn up with consequent loss of the established occlusal vertical dimension
II-porcelain teeth 1-Properties of porcelain teeth: Porcelain teeth unite to the denture base by mechanical means, anterior porcelain teeth have pins in their lingual surfaces while posterior teeth have holes in their under surfaces. The pins and holes act as a retentive device for mechanical union with the denture base. 2-porcelain teeth have high degree of abrasion resistance and therefore maintain their luster longer than acrylic resin teeth and preserve the established occlusal vertical dimension. 3-porcelain teeth are brittle, and thus susceptible to fracture. 4-the under surface of porcelain teeth cannot be ground much of fit limited inter-arch distance without losing their retentive device. Further more, any occlusal grinding must be followed by careful polishing in order to reduce the co-efficient of friction.
The combined use of porcelain teeth in the maxillary denture occluding with acrylic teeth in the mandibular denture has the following advantages: 1-sharp impact sound of porcelain teeth is reduced 2-chipping and fracture of porcelain teeth is reduced. 3-the occlusion can be easily adjusted by grinding only the acrylic teeth. 4-the efficient sharp cusps are retained. 5-as long as the glaze on the porcelain remains intact, a good quality acrylic teeth can resist wear, thus maintaining the vertical dimension.
Tooth arrangement Factors governing the position of artificial teeth: I-anterior teeth: Anterioposterior positioning of anterior teeth is important for esthetic and phonetic because of the support that teeth give to lips. Therefore it is important to place anterior artificial teeth in the same position or in a position as close as possible to that occupied by natural teeth to maintain proper support of these tissues and natural patient appearance
The crest of the residual ridges aids in positioning the artificial teeth if the natural teeth are recently extracted. As resorption progresses, the crests of the residual ridges do not remain in their relative position, due to the pattern of alveolar bone resorption. The loss of bone structure is usually greater on the buccolabial aspect of the maxillary ridge than one the palatal aspect. As a result the crest of residual ridge becomes more palatal than it was prior to extraction of the teeth. For this reason the incisive papilla is used as a useful anatomic guide in determining the anterioposterior and mediolateral position of maxillary anterior teeth, because of its constant relationship to the natural teeth.
A line drawn on the cast anterioposteriorly bisecting the midsagital suture, incisive papilla and the labial frenum coincide with the midline of the upper dental arch. Mediolaterally, the central incisors are set on either side of this line. Anterioposteriorly, the central incisors are positioned so that the distance from the center of the incisive papilla to the labial surface of the central incisors is about 8-10 mm.
II-posterior teeth Posterior teeth are generally placed to enhance the stability of the mandibular denture. This is done by arranging the teeth, so that the mandibular posterior teeth are over, or slightly lingual to the crest of the mandibular residual ridge. The retromolar pad is used as a guide in determining the buccolingual position of posterior teeth, a line drawn from the center of the retromolar pad to the canine area, marks crest of the mandibular ridge
Arranging the maxillary anterior teeth To establish points of reference to the correct arrangement of the teeth, extend the midline and the cuspid line on both sides to the margins of the cast and the mounting plaster. The midline coincides with the contact point between the two central incisors.
Procedure: 1-remove a small section of wax from the occlusal rim, corresponding to the position of the maxillary left central incisor from the occlusal rim. Then soften the wax where the occlusion rim has been cut away with a hot spatula until the wax pools.
The maxillary central incisor is placed in this position, so that, 1- its mesial surface coincides with the midline. 2-its long axis shows a slight distal inclination to the sagittal plane or to the perpendicular. 3-the incisal edge is on the occlusal plane. 4-the neck of the tooth should be slightly depressed. However the facial surface of the tooth is nearly perpendicular to the occlusal plane. 5-in a similar manner, place the adjacent central incisor to preserve the midline.
2-place the lateral incisors following the same procedure, so that, 1-the incisal edge is raised approximately 1\2,1mm from the occlusal plane. 2-its long axis shows more distal inclination than the central incisors. 3-the neck is depressed more than the central incisors. 4-the facial surface is in the line with the central incisors 3-place the maxillary canines in place, following the same procedure so that, 1-the long axis is almost vertical or it may have a slight distal inclination. 2-the incisal tip is on the occlusal plane. 3-the neck is prominent in order to support the corner of the mouth. 4-the distal surface of the canine is rotated in a posterior direction with the crest of the posterior alveolar ridge i.e. the canine is the turning point of dental arch . normally when positioned properly the mesiolabial aspect of the canine will be visible from the anterior view. 5-remove excess wax on the palatal surface with hot spatula. 6-examine the anterior teeth from- the occlusal aspect, the facial surfaces of the teeth should follow the original contours of the dental arch.
Arranging the mandibular anterior teeth Following the same procedures, 1- the mandibular central incisors : are placed with their long axis perpendicular to the occlusal plane with the necks depressed 2-the mandibular lateral incisors: Are placed with a slight distal inclination at the neck and with the facial surface at a right angle to the occlusal plane.
3-the mandibular canines: 1-Are placed with amore distal inclination and the necks are prominent. 2-the distal incline of the maxillary canine should occlude against the mesial incline of the mandibular canine. 3-the distal surface of the canine is rotated in a position direction to form the turning point of the dental arch. 4-the horizontal plane used for aligning lower anterior teeth is above the actual occlusal plane, a distance described as the vertical overlap or overbite, which consequently affects the incisal guidance 5-the mandibular incisors should never touch the maxillary incisors in centric occlusion and the incisal guide angle should be kept as low as possible to enhance free movement of the teeth in protrusive and lateral excursions.
Arranging the posterior teeth To determine the buccolingual position of posterior teeth, mark the crest of the mandibular ridge from the center of the retromolar pad to the canine area on both sides. -extend this line to the land area of the cast at the anterior and posterior borders on both sides. -mark a line on the occlusion rim corresponding to the line drawn on the crest of the ridge. This line serve as an aid in checking the alignment and position when setting the maxillary posterior teeth.
a-arranging the maxillary posterior teeth: maxillary first premolar: a position of wax is removed, the remaining wax is softened and the maxillary first premolar is placed in position so that: 1-the bucall surface of the first premolar harmonized with the canine. 2-both buccal and the palatal cusps are on the occlusal plane. The palatal cusp lies over the line on the occlusal rim, representing the crest of mandibular ridge. 3-the long axis of the tooth is at right angle to the occlusal plane. Maxillary second premolar: Place the maxillary second premolar so that: 1-its long axis is at right angle to the occlusal plane. 2-the buccal and palatal cusps are on the occlusal plane. 3-the palatal cusp is over the crest of the lower ridge. 4-the facial surface of the second premolar harmonizes with that of the first premolar and canine i.e. forming a straight line.
Maxillary first molar: Place the maxillary first molar so that: 1-its long axis has slight mesial inclination at the neck. 2-the mesio-buccal cusp is slightly off the occlusal plane. The distobuccal is slightly a greater off the plane. The mesio-palatal cusp is on the plane. The distopalatal cusp is slightly off the plane 3-the mesio-buccal cusp should be aligned with the two premolars. 4-the mesio-palatal is the only cusp touching the plane. .5-the palatal cusps lie over the crest of the lower ridge.
Maxillary second molar: 1-place the maxillary second molar in a similar manner. All cusps are raised from the occlusal plane. 2-follow the same procedure in placing the posterior teeth on the opposite side.
Arranging the mandibular posterior teeth: In centric occlusion, the buccal cusps of mandibular teeth fit into the central fossae of the maxillary teeth while the lingual cusps of the maxillary teeth fit into the central fossa of the mandibular teeth. this position establishes the proper buccal over jet i.e. the buccal cusps of maxillary teeth extend beyond the facial surfaces of the mandibular teeth.
Mandibular first molar: The mandibular first molar is the key-tooth in articulation and setting of mandibular posterior teeth. Procedure: 1-a section of the mandibular rim is removed to make room for the mandibular left first molar. The wax is soften to permit placement of the tooth. 2-the mandibular first premolar is placed in position. so that: A-the mesiopalatal cusp of the maxillary first molar fits into the central fossa of the mandibular first molar to establish the proper buccal over jet, and the mesiobuccal cusp of the upper first molar fits into the buccal groove of the lower first molar. b-the mesioplatal cusp of the upper first molar fits in the central fossa of the lower and the mesiobuccal cusp of the lower first molar fills the embrasure between the upper second premolar and first molar
mandibular second molar: following the same procedure. The mandibular second molar is positioned so that, the mesiobuccal cusp of the maxillary second molar fits into the buccal groove of the second molar, and the mesiobuccal cusp of the mandibular second molar fits into the embrasure between the maxillary first and second molar. Mandibular second premolar: Following the same procedure place the mandibular second premolar with the tip of the buccal cusp fitting into the embrasure between the maxillary second and first premolar and contact the mesial marginal ridge of the upper second premolar Mandibular first premolar: In some instances there may not be sufficient space for the mandibular first premolar, in such case grind the tooth mesially and distally until it fits into the available space. The tooth is positioned with the tip of the buccal cusp in contact with the mesial marginal ridge of the upper first premolar -following the same procedures , place the posterior teeth on the opposite side.