Form and position of occlusal contacts
One conceptual model would be to represent opposing cusps as spheres laid out in the pattern of occlusion. The spheres would lie in the deepest areas of the masticatory surfaces, the contact areas. Given a pronounced occlusal relief, a sphere rests in a stable position and thereby displays three-point (tripod) contact. If the tooth had no cusps, the sphere would roll to and fro.Thus, centric stops in their contact areas do not contact the antagonists at a single point, but each tooth has at least two contact areas on which centric stops of the opposing teeth lie.
These contact areas basically lie on the line of the central developmental grooves, with a lingual tendency in the mandible and a rather buccal tendency in the maxilla. The centric stops lie on the buccal cusps in the mandibular dentition and on the lingual cusps in the maxillary dentition. This produces a double row made up of the contact areas and the centric stops on the occlusal surfaces of the rows of teeth. The sum of forces that occur will load each individual tooth centrically.
Cusp-fossa-marginal ridges occlusion means an occlusal relation in which the centric stops do not contact the opposing tooth centrically in a tooth-to-two tooth relationship, but the opposing cusps occlude in the contact areas in both the interdental embrasures and the central fossae. Centric occlusion only comes about when the row of teeth is closed within itself and forces can be transferred via the approximal contacts. Although the majority of centric stops do not contact the opposing teeth centrically, the sum of the acting forces nevertheless puts centric load on the teeth within the closed row of teeth.
Cusp-fossa occlusion denotes a contact relationship in a constructed tooth-to-tooth relation in which one tooth only contacts its antagonist. This can result in axially directed, stable loading.
To clarify this, further observations are first made based on an individual tooth.Teeth with totally flat masticatory surfaces would be conceiv-able.To achieve centric loading, these teeth would have to be guided exactly in full surface contact, or the masticatory surfaces would have to be so large that grinding movements out of this central position would have only minimal influence on any centric loading. These tooth forms can be found in ruminants: short teeth with a large, flat surface. However, this makes it very difficult to process tough food. A surface with cusps is better suited to crushing fibrous and tough as well as grainy foods. With a cusp-bearing chewing surface, the food a person eats can be chewed up more efficiently. After all, humans are omnivores, which means we do not favor a single, specific type of food.
The simplest intercuspation would thus be an opposing pair of teeth, comprising one tooth with one cusp, that occluded in a trough-shaped antagonist—similar to a pestle and mortar arrangement. If the single-cusp tooth were then far smaller than the trough-shaped tooth, food could be crushed and ground up by large lateral movements to and fro. However, this food could still escape to the sides and would have to be propelled back by the tongue and cheek. The greater the difference in shape, the more the periodontium is stressed during lateral movements.
Therefore, a structural shape in which the opposing teeth represent both pestle and mortar is more favorable. This results in the anatomical form of masticatory surfaces as the occlusal relief becomes fully refined (Fig 5-51). The grinding or frictional area is doubled, and the two tooth surfaces can be shaped to fit together more closely. A lateral movement from the outside inward simply needs to be performed, in which the outer pestle grinds the food and pushes it into the inner mortar, where the food can also be crushed. The tongue then pushes the food back between the pestle and mortar surfaces.To compensate for the transverse forces during lateral movement, the teeth merely need to be appropriately tipped against the direction of movement.
The efficiency of such a pestle and mortar system can be further enhanced if the contact areas do not interdigitate as flat planes but only touch at a few points. The well-developed natural pattern of occlusion displays punctiform contact with the opposing teeth (Fig 5-52). The occlusal contacts are positioned so that centric loading of the teeth can be achieved.
Figs 5-53 to 5-59 further illustrate the form and position of occlusal contacts.