The Art of Occlusal and Esthetic Waxing (eBook)

Functions of Teeth

The main functions of teeth are mastication, speech, and esthetics. Waxing teeth should be perfected in a way that allows the final restoration to maintain and improve the ability to perform these functions. Teeth do not perform these functions alone; they work in harmony with the other components of the masticatory system, namely the arches, the temporomandibular joints, muscles involved both directly and indirectly in mastication, and the vessels and nerves supplying these tissues. All components of the masticatory system are under neurologic control. The masticatory system can be defined as the functional unit of the body primarily responsible for mastication, speaking, and swallowing.1 This chapter focuses on one component of the masticatory system, the teeth, as this is the main component that can be controlled by the dentist. All the teeth in the mouth assist in performing these three functions. Posterior teeth are mainly important for mastication, but they still affect esthetics and speech indirectly, as their proper alignment provides support for the adjacent soft tissues and muscles of the face and maintains the vertical dimension of occlusion (ie, the distance between the maxilla and the mandible when teeth are in occlusion; Fig 1-1). Thus, proper form and position of posterior teeth influence the overall facial appearance, prevent tooth migration, and maintain the stability of the dentition. Anterior teeth mainly function in providing beautiful esthetics and proper pronunciation while still assisting in mastication through incising and biting foods as well as guiding the mandible through lateral movements. The ability of the teeth to achieve their functions is based mainly on three factors: (1) tooth morphology, (2) tooth alignment, and (3) occlusion (ie, the way the opposing teeth fit together, which is largely dependent on the first two factors).

Fig 1-1 Vertical dimension of occlusion (arrows).

Important Definitions2-4

Centric relation The relationship between maxillary and mandibular teeth when the mandibular condyles are in the most physiologic orthopedically stable position, which is the most superoanterior position in the articular fossae. This position is independent of tooth contact and does not necessarily coincide with maximal intercuspation. In this position, the muscles of mastication function more harmoniously and with less intensity.

Maximal intercuspation The complete intercuspation of the opposing teeth, sometimes referred to as the best fit of the teeth independent of the condylar position. This position is dictated by the teeth themselves, determined when the patient habitually self-closes into complete tooth intercuspation. Maximal intercuspation coincides with centric relation in only 10% of the population.

Protrusive mandibular movement The movement of the mandible forward from the intercuspal position.

Retrusive mandibular movement The movement of the mandible posteriorly from the intercuspal position.

Lateral mandibular movement The movement of the mandible to the right and left sides from the intercuspal position.

Mandibular excursive movements Movement occurring when the mandible moves away from maximal intercuspation in any direction, which includes protrusive, retrusive, and lateral mandibular movements.

Working side The side toward which the mandible moves in lateral excursion.

Nonworking side The side away from the direction of mandibular movement in lateral excursion.

Axial loading The process of directing forces along the long axes of teeth, which are the most tolerated forces by teeth.

Tooth Morphology4-8

The following description of tooth morphology is mainly focused on the tooth surfaces that are created during occlusal waxing, which first-year dental students are expected to learn to perfect their wax-ups and understand the significance of the morphologic features on proper function. A description of the morphologic features needed to create anterior esthetic wax-ups is available in the second chapter of this book.

Morphology of the occlusal thirds of posterior teeth (occlusal surfaces)

Occlusal surfaces are formed of several elevations and depressions. These elevations and depressions form an uneven surface; therefore, when an elevation on an occlusal surface comes in contact with a depression on the occlusal surface of the opposing tooth during mastication, the food is efficiently ground. Proper knowledge of the specific form of these elevations and depressions is of great importance during occlusal waxing and tooth restoration to achieve proper masticatory function. In addition to their role in grinding food, occlusal surfaces should be restored in a way that achieves axial loading (ie, directs the occlusal forces along the long axes of the teeth). These are the only forces that do not harm the periodontium and supporting structures. On the other hand, if the teeth are subjected to lateral forces, this usually leads to frictional wear, occlusal instability, and possible tooth mobility.

Elevations of the occlusal surface

Cusps

A cusp is an elevation on the crown portion of a tooth making up a divisional part of the occlusal surface. Cusps resemble pyramids in shape, and the highest point of the pyramid-shaped cusp is called the cusp tip (Fig 1-2). Cusps are present in the posterior teeth and the canines. Premolars have two cusps, except the mandibular second premolar, which may have three cusps, and molars may have three to five cusps. Cusps are named according to their location (eg, the maxillary first molar cusp that faces the mesial and lingual surfaces is called mesiolingual cusp). The part of the tooth between the buccal and lingual cusp tips is approximately 50% to 60% of the buccolingual dimension of the tooth and is where the forces of mastication are applied; this part is called the inner aspect of the tooth or the occlusal table, and the part outside the cusp tips is called the outer aspect of the tooth (Fig 1-3). The inner and outer aspects of the teeth are made up of inclines that extend from the cusp tips to either the central groove or the height of contour on the facial or lingual surface. Depending on the role the cusps play in the dentition, they are divided into functional and nonfunctional cusps.

Fig 1-2 Pyramid-shaped cusp and cusp tip.

Fig 1-3 Inner and outer aspects of the tooth.

Functional cusps are given this name because they are the cusps responsible for mastication and maintaining the vertical dimension of occlusion. They also can be called working cusps and supporting cusps. From the occlusal aspect, their cusp tips are located approximately one-third the distance into the total buccolingual width of the tooth (Fig 1-4). Functional cusps are the lingual cusps of maxillary teeth and the buccal cusps of mandibular teeth. Thus, premolars have only one functional cusp, which occludes with the opposing marginal ridge(s), and molars have two functional cusps, one that occludes with the opposing central fossa and one that occludes with the opposing marginal ridge(s) when the teeth are in maximal intercuspation (Fig 1-5). This contact relation may result in slightly flatter cusp tips due to possible wear. If these cusps are not properly restored to contact, the opposing tooth may supererupt. An imaginary line can be extended through the functional cusp tips in each arch. In the mandible the line will pass through the buccal cusp tips and is named the bucco-occlusal line, and in the maxilla the line will pass through the lingual cusp tips and is therefore named the linguo-occlusal line. A third imaginary line can be extended through the central developmental grooves of posterior teeth and is named central fossa line.4 These three lines pass smoothly and continuously and are a major aid during cusp and central groove placement in occlusal wax-ups (Figs 1-6 to Figs 1-8).

Fig 1-4 Buccal cusps are the functional cusps of a mandibular first molar and thus occupy one-third of the buccolingual dimension. Lingual cusps are non-functional and occupy only one-sixth of the buccolingual dimension.

Fig 1-5 Supporting cusps occlude in the opposing central fossa. Nonsupporting cusps do not contact opposing teeth in maximal intercuspation.

Fig 1-6 Bucco-occlusal line (dotted line).

Fig 1-7 Linguo-occlusal line(dotted line).

Fig 1-8 Using bucco-occlusal, linguo-occlusal, and central fossa lines as a guide in occlusal waxing.

Nonfunctional cusps or nonworking cusps are the mandibular lingual cusps and the maxillary buccal cusps. They do not contact the opposing teeth in maximal intercuspation; therefore, they do not maintain the vertical dimension of occlusion or play a major role in mastication, and they tend to have a sharper cusp tip. They do however maintain the bolus of food on the occlusal table for mastication and minimize tissue impingement as well as guide the mandible in lateral movements; thus they are also named guiding cusps (see Fig 1-5). Nonfunctional cusps tips are located approximately one-sixth the distance into the total buccolingual width of the tooth...