Posterior Teeth
Occlusal surfaces
Before the individual teeth are described in detail, features common to the posterior teeth are examined.
One common feature is the masticatory or occlusal surfaces (facies occlusalis), ie, the whole occlusal relief with its cusps, grooves, and fossae (Figs 4-41 to 4-43). The specific arrangement, in this case the particular shape of the occlusal surface, develops according to the rule of form and function and should be reproduced in all prosthetic teeth to maximize masticatory function.
Parts of the occlusal surface
Cusps (cuspides dentis) are the elevations on a masticatory surface made up of several surface components; they form the basic structure of the occlusal surface. Their arrangement on the cross section of the crown determines the shape of the tooth. A distinction is made between the basal part and the tip of the cusp. Cusps are classified as crushing or shearing, depending on their function. The crushing cusps of a tooth lie lingually in the maxilla and buccally in the mandible and are referred to as supporting cusps. The shearing cusps lie buccally in the maxilla and lingually in the mandible and are known as nonsupporting cusps.
The cusp tip forms the contact with the opposing teeth and takes on the actual cusp function of a contact and supporting cusp. There is a varying degree of intercuspation of occluding teeth, depending on the height of the cusp tip. The cusp tips of an occlusal surface are always displaced toward the middle of the occlusal surface, and the masticatory surfaces are always drawn in at the cusp tips.The widest circumference of a tooth with masticatory surfaces is always below the occlusal surface.
The triangular ridges are triangle-shaped enamel ridges that run mesially and distally from the cusp tips, along which the cusp tips of the opposing teeth slide during chewing.They are therefore the working surfaces (sliding surfaces) of a cusp for the opposing teeth.
The cusp crest or inner cusp slope runs as a ridge of enamel from the cusp tip centrally to the middle of the tooth and divides the triangular ridges.
The cusp slopes are ridges of enamel receding in a lingual or buccal direction, which run from the cusp tip to the outer surface of the tooth and thus define the buccal or lingual contour of the tooth. They are the external equivalents of the cusp crests.
The cusp ridges are angular enamel ridges that run around the whole occlusal surface from buccal to lingual and join the cusp tips. In the approximal area, they form the marginal ridges.
The approximal marginal ridges are the most strongly developed enamel crests of the cusp ridge in the approximal areas.The marginal ridges of adjacent teeth form the interdental embrasure into which the cusp tips of the opposing teeth insert. The approximal marginal ridges thus form the contact with the adjacent tooth but also the functional surface for the opposing teeth.
The central developmental groove (longitudinal groove; fissura longitudinalis) separates the buccal from the lingual cusps. It runs as a deep fissure distomesially, centrally over the occlusal surface, and parallel to the dental arch.
The main developmental groove (transverse groove; fissura transversalis) separates the mesial from the distal cusps in teeth with multiple cusps. It crosses the central groove, running from lingual to buccal or vice versa. Grooves are glide paths for cusps and allow crushed food to flow away.
The central fossa or pit (fovea centralis) is a deep depression in the middle of the occlusal surface on teeth with multiple cusps, where several cusp crests run together. Central fossae or pits are the preferred sites for the development of caries.
Supplemental or accessory developmental grooves are deep fissures that separate the ap-proximal marginal ridges from the cusps. The central developmental groove branches at the marginal ridges to form distinct supplemental grooves. Deep fossae or pits (fovea mesialis and distalis) form in the mesial and distal branching points.
The occlusal surfaces interlock. Reference is made to the cusp-fossa relationship, where a supporting cusp rests in the occlusal fossae of the opposing tooth or on two opposing marginal ridges and where the mandibular posterior teeth are displaced by half a cusp-width.
The cusp surfaces are inclined toward the occlusal plane; this is a theoretical plane in which the occlusal surfaces meet in terminal occlusion. When the occlusal relief is deep and the cusp is steep, this angle of cusp inclination is large; it can vary between 20 and 40 degrees.