Arrangement of the posterior teeth Dr.Mohammad Al Sayed 4/5/2008

Orientation of the occlusal plane: The arrangement of the posterior teeth involves the application of principles similar to those followed in the arrangement of anterior teeth. The artificial posterior teeth should be placed as nearly as possible where the natural teeth were . Orientation of the occlusal plane: * The anterior end of the occlusal plane is oriented according to esthetics. The vertical level at which the anterior teeth are set is the level of the anterior end of the occlusal plane. This level is usually 1-2 mm below the resting upper lip. * The posterior end of the occlusal plane should be located so that it would be in level with the junction between the middle and the distal third of the retromolar pad. This guideline will place the occlusal plane that is familiar to the tongue.

* The inclination of the occlusal plane is an important factor in stability or instability of dentures. Ideally, the occlusal plane should be parallel to both ridges. * The vertical orientation and inclination is also governed by the relative amount of bone lost from the two ridges. The occlusal plane should be placed at a level that would favor the weaker of the two ridges. ( If more bone has been lost from the maxilla than from the mandible, the occlusal plane must be closer to the maxilla than the mandible. If more bone has been lost from the mandible than from the maxilla, the occlusal plane must be closer to the mandible than the maxilla). However the level of the occlusal plane must be placed as nearly as possible to the position of the occlusal plane of the natural teeth.

:Buccolingual position of posterior teeth * The buccolingual position of the posterior teeth and the posterior arch form are determined anteriorly by the positions of the anterior teeth. Posteriorly, it is determined by the shape of the basal seat provided by the maxilla and mandible. The mandibular posterior teeth should be placed so that the antero-posterior groove (central) is placed on the crest(centre) of the residual ridge. The crest of the mandibular residual ridge is marked on the edentulous cast by a line drawn from the crest of the ridge in the 1st premolar and canine area up to the mesial of the retromolar pads. If the teeth are placed buccal to the ridge, the dentures may tip on the opposite side when pressure is applied to these teeth. The teeth should not be placed lingual to the residual ridge because this will cause lack of space for the tongue, and the dentures will be displaced when the tongue moves.

* The posterior part of the arch form will be determined to a great extent by the neutral zone between the cheeks and the tongue. Neutral zone is the area between the tongue on one side and the cheeks and lips on the other, where opposing soft tissue displacing forces create a zone of neutral or minimal muscular force, the teeth should be placed as far as possible with respect for these muscle forces. * Post extraction changes may lead to a gradual shifting of the neutral zone with the tongue spreading laterally. In other words the buccolingual space available for placing teeth is reduced. This is of particular importance in the case of the lower denture. The artificial teeth used are considerably narrower than the natural teeth and this allows them to be placed so that the denture is stable.

Position of maxillary posterior teeth: The maxillary 1st premolar: The maxillary first premolar is placed with its long axis perpendicular to the occlusal plane (vertically upright).The buccal cusps are raised 0.5mm from the occlusal plane. The maxillary 2nd premolar: The long axis is perpendicular to the occlusal plane ( parallel to the vertical axis). The buccal cusp is 0.5 mm above the occlusal plane and the palatal cusp touches the occlusal plane.

molar

The maxillary 1st molar: The long axis (viewed from side ) may slightly incline mesially. When viewed from the front the tooth slopes buccally. The mesiobuccal cusp of the first molar is raised 0.5 mm above the occlusal plane. The mesiopalatal cusp is in contact with the occlusal plane. The distobuccal cusp is raised about 1 mm above the occlusal plane.

The maxillary 2nd molar: The long axis ( viewed from side, cervical area) may slightly incline mesially. The mesiobuccal cusp of the 2nd molar is raised 1 mm above the occlusal plane. The distobuccal cusp 1.5 mm above the occlusal plane. No cusp touches the occlusal plane. When viewed from the front the tooth slopes buccaly (more than the 1st molar).

The maxillary teeth viewed from the occlusal surface should follow the following guidelines: - The distobuccal surface of the canine, the buccal surfaces of the premolars and the mesiobuccal surface of the 1st molar should follow a straight line. Buccal surfaces of the 2nd molar are slightly inwards and are placed so that all buccal surfaces of the 1st and 2nd molars follow a straight line.The posterior teeth should be placed so that the central grooves of the maxillary posterior teeth follow the line of the crest of the mandibular residual ridge.

The position of the mandibular posterior teeth: The placement of the mandibular posterior teeth determines how well the teeth occlude, both in centric occlusion and in lateral and protrusive excursions. The mandibular 1st molar is first placed in position followed by the 2nd molar, and then the two bicuspids. After placing each mandibular tooth in the correct position the incisal guide pin should be checked to insure that it is in contact with the guide table.

Position of the mandibular 1st molar: The mesibuccal cusp of the maxillary 1st molar fits into the buccal groove of the mandibular 1st molar. The mesiolingual cusp of the maxillary 1st molar fits into the central groove of the mandibular 1st molar. This provides proper buccal overjet.

Position of the mandibular 2nd molar: The mandibular 2nd molar is placed with the mesiobuccal cusp fitting between the maxillary 1st and 2nd molars and contact with their marginal ridges. The lingual cusps of the maxillary 2nd molar fit into the central groove of the mandibular 2nd molar.

Position of the mandibular 2nd premolar: The mandibular 2nd premolar is placed with the buccal cusp fitting between the maxillary 1st and 2nd premolars and in touch with their marginal ridges. The lingual cusp fits in between the palatal cusps of the maxillary bicuspids.

Position of the mandibular 1st premolar: The buccal cusp of the mandibular 1st premolar fits between the maxillary canine and the 1st premolar. Usually this tooth is not placed until the anterior and posterior teeth are placed. If there isn’t sufficient space for the placement of this tooth it may be trimmed and made narrower to fit into the available space rather than alter the placement of anteriors.

Occlusal contacts during lateral movements: In centric occlusion the buccal cusps of the mandibular teeth contact the central fossae of the maxillary teeth while the lingual cusps of the maxillary teeth fit into the central fossae of the mandibular teeth. Occlusal contacts during lateral movements: Contacts on the working side i.e. the side to which the mandible is moving. Working or functional occlusion occurs when the buccal cusps of the maxillary teeth meet the buccal cusps of the mandibular teeth, and the palatal (lingual) cusps of maxillary teeth meet the lingual cusps of the mandibular teeth. However the relationship is not cusp tip to cusp tip, but cusp tip into cusp “valley” with each maxillary cusp distal to the corresponding mandibular cusp. There should be contact of anterior maxillary and mandibular teeth on the working side.

Balancing occlusion: The balancing side is the opposite side to the side which the mandible has moved. Balancing occlusion occurs simultaneously on the opposite side from the working occlusion.`In balancing occlusion, the palatal cusps of the maxillary teeth contact the buccal cusps of the mandibular teeth. Balancing occlusion functions to maintain the dentures in position during excursive movements. There will be no contact of the anterior teeth on the balancing side.

Balancing contacts in protrusive occlusion: Balancing contacts in protrusive excursions permit the posterior teeth to touch when the anterior teeth are in contact. The distal inclines of the maxillary buccal cusps contact the mesial inclines of the mandibular buccal cusps.

Compensating curve of spee: Anatomic curvature of the occlusal alignment of the natural teeth beginning at the tip of the lower canine and following the buccal cusps of the natural premolars and molars is called curve of spee. The anterior height of the lower posterior teeth can be checked clinically. The height of the 1st mandibular premolar usually coincides with the height of the resting lower lip at the corner of the mouth.

Curve of spee Curve of wilson

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