During central closure in the normal dentition the lingual cusps of the maxillary posterior teeth and the buccal cusps of the mandibular posterior teeth make contact with the occlusal fossa or the marginal ridges of the opposing teeth.
They grind food like a molar during mastication and are called functional (supporting) cusps.
On the other hand, the buccal cusps of the maxillary molars and premolars and the lingual cusps of the mandibular posterior teeth do not contact the opposing teeth.
These cusps prevent food from overflowing, confine food within the sulcus, and protect the buccal mucosa and the tongue by keeping them away from the functional cusps. Since these cusps do not make direct contact with opposing teeth, they are called non-functional cusps.
The type, number and distribution of occlusal contacts is called an occlusal scheme. The occlusal scheme can be classified by the location of the occlusal contact made by the functional cusp on the opposing tooth in centric relation. There are two types of occlusal schemes: cusp-fossa and cusp-marginal ridge.
The – cusp-marginal ridge – the cusp-marginal ridge relation is the type of occlusal scheme in which the functional cusp contacts the opposing occlusal surfaces on the marginal ridges of the opposing pair of teeth, or in a fossa. Therefore, a cusp-marginal ridge occlusion is basically a one-tooth-to-two-teeth arrangement.
This is the most natural type of occlusion and is found in 95% of all adults. Since the majority of adults exhibit the cusp-marginal ridge type of occlusion, it is an occlusal pattern widely utilized in daily practice. It can be used for single restorations.
The waxing technique used for cusp-marginal ridge occlusion was originally devised by E.V. Payne was the first wax-added (addition) technique for functional waxing. The same technique, modified by the use of color-coded waxes, has become a widely used method for teaching functional waxing.
The – cusp-fossa – the cusp-fossa relation is an occlusal pattern in which each functional cusp is positioned (contacts) into the occlusal fossa of the opposing tooth. It is a tooth-to-tooth arrangement. Although considered to be an ideal occlusal pattern, it is rarely found in its pure form in natural teeth.
Each centric cusp should make contact with the occlusal fossa of the opposing tooth at three points. The contact points are on the mesial and distal ridge (incline) and the inner facing incline of the cusp, producing a tripod contact. Since the cusp tip itself never comes in contact with the opposing tooth, the cusp tip can be maintained for a long time with a minimum of wear.
The technique used for producing wax patterns with an exclusively cusp-fossa occlusion was developed by P.K.
Thomas. It is important to keep in mind, however, that the same technique, utilizing the same sequence of morphologic development, can be used with excellent results for developing a cusp-marginal ridge occlusal relationship.When the cusp-marginal ridge arrangement is the desired end result, cusp placement is altered slightly.
Occlusal forces are directed parallel to the long axis of the tooth. These forces are near the center of the tooth – placing very little lateral stress on the tooth.
Since this type of occlusion is rarely found in natural teeth, it usually can be used only when restoring several contacting teeth and the teeth opposing them, as well as for full mouth reconstruction.
The basic purpose of wax modeling and developing optimal occlusal contacts between the maxillary and mandibular dental arch is to reconstruct the lost anatomic shape and function without trauma to the supporting structures and with a uniform distribution of forces during mastication. All this depends on the type of occlusal scheme and the proper distribution of the occlusal contacts.
A good restoration adapts perfectly to the prepared tooth and the gingival margin,restores the proximal contacts, the continuity of the dental arch, the interocclusal relationship and the lost harmony of the dental arch. With the wax modeling we restore (reconstruct) the shape, size, proportion and all characteristics of the destroyed or fully completely lost tooth crown.