Dental Arches and Occlusion

Definition: The relationship between upper and lower teeth when they achieve maximal intercuspation, with the condyles resting in the most unstrained retrusive position within the glenoid fossa.
Each tooth, excluding the lower central incisors and upper third molars, occludes with two opposing teeth. This arrangement aids in maintaining normal occlusion even after the loss of a tooth.
The upper and lower central incisors align at the midline where their mesial outlines coincide. Similarly, the distal outlines of the upper and lower third molars align.

The permanent upper and lower first molars are considered the key to occlusion due to several reasons:

They are the first permanent teeth to erupt in the oral cavity.
They are the largest teeth in the oral cavity; the upper first molar is the largest buccolingually, while the lower first molar is the largest mesiodistally.
They have no deciduous predecessors to interfere with their eruption.
Their position in the dental arch is guided by the preceding deciduous teeth.
The upper first molar's position is more stable since it is attached to a bone fixed to the base of the skull.

Distobuccal cusps of the upper first and second molars do not make contact during occlusion. Their triangular ridges are positioned in the buccal embrasures between the lower first and second molars and between the lower second and third molars, respectively. The distobuccal cusp of the upper third molar is also free from occlusion, with its triangular ridge aligning with the distal slope of the distobuccal cusp of the lower third molar.
Mesiolingual cusps of the upper first, second, and third molars rest in the central fossae of the lower first, second, and third molars, respectively.
The distolingual cusp of the upper first molar contacts the mesial marginal ridge of the lower second molar. In the upper second and third molars, if present, the distolingual cusps strike the distal marginal ridges of the lower second and third molars, respectively.

Mesiobuccal cusp of lower first molar strikes the distal marginal ridge of upper second premolar and mesial marginal ridge of upper first molar. For the second molar, the mesiobuccal cusp strikes the distal marginal ridges of upper first molar and mesial marginal ridge of upper second molar. The mesiobuccal cusp of the third molar is located in the mesial triangular fossa of upper third molar.
Distobuccal cusps of the lower first, second, and third molars rest in the central fossae of the upper first, second, and third molars, respectively.
The distal cusp of the lower first molar rests in the distal triangular fossa of the upper first molar.
Mesiolingual cusps of the lower first, second, and third molars are free from occlusion and are located at the embrasures between upper second premolar & first molar, between upper first & second molars and between upper second & third molars respectively.
Distolingual cusps of the lower first, second, and third molars are free from occlusion, and their triangular ridges are located at the lingual developmental grooves of upper first, second, and third molars, respectively.

Incisal edges of upper incisors are free from occlusion and cover the incisal third of the opposing teeth. The incisal edges of lower incisors strike the opposing teeth at the junction of the incisal and middle thirds on the lingual surfaces of opposing teeth.

The incisal edge does not make contact and is labial to the incisal thirds of the lower central and lateral incisors (mesial half).
The cingulum is free from occlusion and is located above the lingual embrasure between the lower central and lateral incisors.
Mesial outline is in line with the mesial outline of lower central incisor.
Distal outline is centered above the labial surface of lower lateral incisor.

Incisal edge is free from occlusion and has labial relation to the incisal thirds of lower lateral incisor (distal half) and lower canine (mesial half).
Cingulum is free and above the lingual embrasure between lower lateral incisor & canine.
Mesial outline is centered above the labial surface of lower lateral incisor.
Distal outline is centered above the labial ridge of lower canine.

Upper posterior teeth are slightly distal to the lower teeth.
Buccal cusps of the upper teeth are free from contact, and their triangular ridges lie in buccal relation to those of the lower teeth.
Lingual cusps of the upper teeth strike the occlusal surfaces of lower teeth.
Buccal cusps of the lower teeth strike the occlusal surfaces of upper teeth.
Lingual cusps of the lower teeth are free and lingual to the upper teeth.

The buccal cusp of the upper first premolar is free from contact, and its triangular ridge is located in the buccal embrasure between the lower first and second premolars. The buccal cusp of the upper second premolar is free from contact, and its triangular ridge is located in the buccal embrasure between the lower second premolar and first molar.
The lingual cusp of the upper first premolar strikes the distal marginal ridge of the lower first premolar. For the upper second premolar, the lingual cusp strikes the distal triangular fossa of the 2-cusp type lower second premolar or the distal slope of the distolingual cusp of the 3-cusp type lower second premolar.
The mesial outline of the upper first premolar aligns with the buccal cusp of the lower first premolar; for the upper second premolar, it aligns with the buccal cusp of the lower second premolar.
The distal outline of the upper first premolar aligns with the buccal cusp of the lower second premolar; for the upper second premolar, it aligns with the mesiobuccal cusp tip of the lower first molar.

The mandible moves from centric occlusion downward to allow separation of upper and lower teeth and then move forward to make the lower anterior teeth in anterior relation by 1-2 mm to the upper teeth.
During this action the incisal ridges of lower incisors which were in contact with the lingual incisal thirds of the upper incisors show tip to tip incisal edges by slight depression and protrusion of the mandible (functioning side), also the posterior teeth show anterior relation of the lower teeth to the upper teeth (balancing side). This movement is called anterior masticatory cycle or masticatory cycle of anterior teeth.

In these movements, the mandibular dental arch is placed in a right or left lateral relation to centric occlusion with the maxillary teeth.
The working side shows the buccal cusps of maxillary and mandibular teeth to be in contact.
The balancing side shows contact of lingual cusps of the maxillary teeth with the buccal cusps of mandibular teeth. Then both sides move to centric occlusion and alterations from one side to the other were performed by slight depression of the mandible and then side movement to right or left. This movement is called posterior masticatory cycle or masticatory cycle of posterior teeth.