[Figure 12-01] The occlusal plane represents the mean of the curvatures of the incisal and occlusal surfaces of the teeth.

[Figure 12-02] The anteroposterior occlusal curve or the curve of Spee begins at the tip of the mandibular canine. It follows the buccal cusps of the premolars and molars, continuing along the anterior border of the mandible to the most anterior portion of the condyle.

[Figure 12-03] In the frontal view, the mediolateral curve or curve of Wilson is determined by the buccal and lingual cusps of the left and right hemiarches, being concave in the mandible and convex in the maxilla.

[Figure 12-04] Monson’s sphere: According to Monson⁴, the curves of Spee and Wilson relate to a sphere with a radius of approximately 10 cm, centered at the Crista Galli. Although the geometric principles of this theory have been used as a reference in prosthetic reconstructions for several decades, the dimension and position of such a sphere have not been scientifically proven.

The curves of Spee and Wilson have different attributes for dentate as opposed to edentulous patients. For dentate patients, according to the philosophy of mutually protected occlusion¹⁰, the curves of Spee and Wilson allow the anterior functional guidance to disclude the posterior teeth during lateral and protrusive movements of the mandible^9,11. For edentulous patients, within the philosophy of bilateral balanced occlusion^12,13, the curves of Spee and Wilson are considered curves to compensate for the condylar trajectory, and promote stabilizing bilateral contacts for complete dentures during the same mandibular movements. The morphologic arrangement of such curves must be strongly related to the trajectory of the articular eminence, the degree of overjet, and the cuspal height¹⁴ [Figures 12-05A–C and 12-06A–C].

[Figure 12-05A–C] In dentate patients, the curves of Spee and Wilson allow the posterior teeth to be nonoccluded by the functional guidance during lateral and protrusive movements of the mandible.

[Figure 12-06A–C] In edentulous patients, the curves of Spee and Wilson are considered to compensate for the condylar trajectory. They should be respected to promote stabilizing bilateral contacts for complete dentures during the same mandibular movements.

Occlusal forces

Knowledge of the pattern of occlusal forces, mediated by the neuromuscular system during functional and parafunctional activities, is essential to determine the risks and prognosis of treatment due to its potentially harmful effects on the temporomandibular joints (TMJs), tooth structure, supporting tissue, and restorative materials, both in the short and long term.

The forces acting on the teeth can vary in magnitude, frequency, duration, and direction^9,11,15,16. The responses of tooth-supporting tissue to such forces depend on the shape and length of the roots, the composition and orientation of the periodontal ligament (PDL) fibers, and the extent of the bone support¹⁷.

The estimation of occlusal forces depends on several factors such as the measurement technique, the location of the measuring device in the dental arch, the number of teeth included in the study, the dimensions of the bite force transducer, and whether the measurement performed is unilateral or bilateral¹⁸. In the literature, the measurement of occlusal forces presents significant variations of about 150%, from 50 kilogram-force (kgf) to 122 kgf in the molar region^19,20.

In the sagittal view, the maxillomandibular relationship can be mechanically described as a Class III lever [Figure 12-07]. The fulcrum of this lever would be the TMJs, the force would be the jaw elevator muscles, and the resistance would be the food bolus. As the elevator muscles are located in the posterior region of the mandible, close to the fulcrum, the posterior teeth are subject to a significantly higher incidence of forces.

[Figure 12-07] The mandible behaves like a Class III lever in the sagittal view. Thus, as they are located far from the muscle power^21–23, the anterior teeth are subjected to forces of 10% to 30% compared with the molars.

Mansour and Reyik²¹ concluded that the magnitude of mean bite forces varied by a factor of one to nine in the anteroposterior direction of the dental arch. Thus, for an average bite force on a mandibular central incisor of 10 kgf, a force of 92 kgf in the region of the mandibular first molar was verified. Other authors have found a ratio of strength values between anterior and posterior teeth of 20%²² to 30%²³.

An acceptable way of comparing functional and parafunctional contact patterns is to assess the magnitude of force applied to the teeth in kilograms (kg) per second (s) per day of each activity⁹, thus considering the sum of the total duration of occlusal contacts.

Occlusal forces should be evaluated and compared during both functional (i.e. mastication and swallowing) and parafunctional activities. According to Gibbs et al²⁴, loads of about 26.6 kg occur during mastication for a brief period of 115 milliseconds (ms)²⁴. With an estimated 1,800 masticatory cycles per day²⁵, the sum of masticatory forces will result in a total of 5,500 kg-s/day [Tables 12-01 and 12-02].

[Table 12-01] Comparative data of occlusal forces present in mastication and swallowing functional activities.

(Modified from Okeson9 and Rugh & Solberg29.)

[Table 12-02] Comparative data on the magnitude of occlusal forces present in functional and parafunctional activities, the type of contraction involved, the direction of forces, and the influence of proprioceptive reflexes.

(Modified from Okeson9 and Rugh & Solberg29.)

The loads that act during swallowing and their time of action should also be considered. They occur approximately 590 times a day, 146 times during eating, 394 times between meals, and only 50 times during sleep²⁶, due to the reduction in saliva production in this rest period²⁷. During swallowing, loads of approximately 30.2 kg act for a period of time of 522 ms²⁴, totaling about 9,300 kg-s/day. Thus, according to the reported data, the sum of the functional forces of mastication and swallowing is 14,800 kg-s/day⁹.

A brief analysis of the forces involved during parafunctional activities becomes important because it influences risk analysis and the prognosis of restorative treatment. According to Nishigawa et al²⁸, bruxism can generate expressive and prolonged forces on the stomatognathic system. These authors observed a maximum value of 81.2 kgf in the region of the first molars, 12% higher than the maximum force of daytime voluntary clenching (79 kgf) of the analyzed patients, with episodes lasting up to 41.6 s.

Parafunctional activities generate forces of a value approximately twice that of daily functional forces^24,29. Muscle actions during mastication and swallowing consist of rhythmic isotonic contractions, with periods of relaxation, while contractions during parafunction are prolonged and isometric³⁰. The direction of mandibular movements and occlusal forces is mainly vertical during function, but with a predominance of horizontal vectors during bruxism^31,32. Still, the proprioceptive reflexes seem to have a limited action during parafunctional activities³³.

Favorable biomechanical criteria for occlusal plane planning

Simultaneous bilateral contacts

The presence of occlusal contacts of equal intensity, bilateral and simultaneous in the maximal intercuspal position (MIP), is a fundamental requirement for the distribution and dissipation of the functional forces on the teeth, periodontium, and alveolar bone. When this position coincides with the centric relation (CR), forces with favorable directions impinge on the TMJs. Understanding this interrelationship of contacts between the cusps and the opposing fossae or marginal ridges in MIP is critical because it occurs at the end of each masticatory cycle^34–36, during swallowing^37–39, and during bruxism⁴⁰.

According to Wiskott and Belser¹⁶, the symmetric distribution of occlusal contacts may be more important than their number for the balance and stability of the stomatognathic system. However, in natural dentitions, it is common to find an asymmetric distribution in terms of location and number of contacts in MIP, with variations according to tooth position in the dental arch, time of day, and bite force. Regarding the position in the dental arch, the first and second molars are the teeth that have the most frequent contact^41,42. The number, position, and intensity of occlusal contacts vary over a 24-hour period^43,44. The number of contacts decreases throughout the day, depending on the physical state of the masticatory muscles and the patient’s mental state⁴³. It was also found that the number of occlusal contacts increases considerably with bite force⁴⁵.

The dentist should know that in a physiologic state, stomatognathic systems rarely show the pattern of tooth contacts seen in textbooks or illustrations of idealized occlusions. However, the wide morphologic variations found in patients are, in most cases, compatible with the masticatory function and occlusal stability. Depending on the signs and symptoms, there is no indication for treatment.

The criteria recommended for a therapeutic occlusion are based on clinical observations, and research has been carried out to establish favorable and feasible biomechanical parameters for dentists and dental laboratory technicians (DLTs)^{11,16,41,45–48}. The objective is not to bring the lost tooth structure back to its original location but to reconstruct it esthetically and functionally in a way that is compatible with the patient’s individual characteristics [Figure 12-08].

[Figure 12-08] The therapeutic model of position and distribution of occlusal contacts is classically recommended as a reference in cases of indication of occlusal reorganization.

Direction of occlusal forces

The posterior teeth can support vertical forces of varying intensities, distributing them to the alveolar bone through the PDL during mandibular closure for mastication and swallowing in MIP. On the other hand, occlusal contacts on ridges introduce horizontal forces that are not so well tolerated by the structures mentioned above^49–52. According to Beyron⁵³, the direction of occlusal forces is more decisive than their magnitude, and the more the forces deviate from the long tooth axis, the worse the resistance to them from the supporting tissue. As an adaptive response, the neuromuscular system has an intricate sensory proprioceptive mechanism to detect and create reflex patterns of mandibular movement to avoid these harmful contacts, which can, conversely, generate muscle dysfunction⁵⁴.

The anterior teeth do not present adequate angulation to receive the functional forces in a vertical direction; however, the previously mentioned concept of levers has a mechanical disadvantage because they are located far from the point of application of the occlusal forces. Its primary function is to guide the functional movements of the jaw through its efficient “proprioceptive navigation” system. The clinician should be aware of signs of adaptation to these forces, which may be intense or in an unfavorable direction, such as increased mobility, inclination, and migration of the teeth⁵⁵ [Figure 12-09A,B].

[Figure 12-09A,B] The direction of the occlusal forces acting on the anterior and posterior teeth is different. The anterior teeth receive less intense forces as they are located far from the action of the jaw elevator muscles, and these forces are directed obliquely to their long axis. The posterior teeth receive loads of greater magnitude as they are closer to the muscle force vectors, and such forces must be transmitted as close as possible to their long axis.

Tooth inclination

The inclination of the posterior teeth is related to biomechanical factors and masticatory efficiency¹¹. The buccolingual inclination allows the long axis of the teeth to receive the occlusal forces almost parallel to the dominant force vector of the mandibular elevator muscles [Figure 12-10A,B].

[Figure 12-10A,B] The buccolingual inclination of the posterior teeth is related to the primary force vector of the mandibular elevator muscles.

These inclinations also facilitate the access of the food bolus to the occlusal table through the activity of the tongue and buccinator muscles, in addition to providing stability to complete dentures^56–59 [Figure 12-11A,B].

[Figure 12-11A,B] The position and slope of the occlusal plane should provide masticatory efficiency through the combined action of the tongue and cheeks during the food grinding process.

Occlusal plane morphology

Some basic concepts of occlusal plane morphology are discussed below regarding the recommended occlusal characteristics in order to support the reader in planning, for the diagnostic wax-up, or for adjusting the occlusal plane.

Centric and noncentric cusps

The occlusal anatomy of the posterior teeth can be divided into centric (or supporting) cusps and noncentric (or guiding) cusps^33,60,61. The mandibular buccal and maxillary lingual cusps of the molars and premolars are usually the centric cusps responsible for maintaining centric contacts and supporting the vertical dimension of occlusion (VDO). They have a rounded configuration and are centralized with opposing fossae and marginal ridges. The maxillary buccal and mandibular lingual cusps are called noncentric cusps. They have a more pointed anatomy and are located external to the occlusal table of the opposing teeth, so that the horizontal and vertical overlap between them and the supporting cusps prevents the biting of the tongue and cheeks during mastication [Figure 12-12A,B].

[Figure 12-12A,B] The occlusal anatomy of the posterior teeth can be divided into centric (or supporting) cusps and noncentric (or guiding) cusps. The mandibular buccal and maxillary lingual cusps of the molars and premolars are usually centric, presenting a rounded configuration and located centrally with the opposing fossae and marginal ridges. The maxillary buccal and mandibular lingual cusps are noncentric and have a more pointed anatomy, being located external to the occlusal table of the opposing teeth.

Noncentric cusps also help to keep the food bolus on the occlusal table during mastication and provide an escape zone for the bolus, thanks to their angulation. It should be clear to the DLT and the dentist during planning and when making adjustments on the prosthesis that both the centric and noncentric cusps should not interfere with the lateral and protrusive functional movements of the mandible (more details in Chapter 10).

The relationship of posterior occlusal contacts

The nature of the ideal static relationship of occlusal contacts has not yet been scientifically and definitively determined⁶², with two types of occlusal contacts described in the literature: 1) Cusp-marginal ridge; and 2) Cusp-fossa.

Cusp-marginal ridge

The relationship of occlusal contacts between the cusp tip and the marginal ridges of the opposing teeth is the standard occlusal scheme of the natural dentition of Angle Class I patients, also called the “one-tooth/two-teeth relationship.” In these patients, most contacts occur as described above, with only the mesiolingual cusps of the maxillary first and second molars, the mesiobuccal cusps of the mandibular first molars, and the distobuccal cusps of the mandibular second molars occluding against the opposing fossae^33,64–66 [Figure 12-13].

[Figure 12-13] Cusp-marginal ridge relationship. In these patients, most contacts occur between the cusp tip of a tooth against the marginal ridge of its opposing tooth, with only the mesiolingual cusps of the maxillary first and second molars, the mesiobuccal cusps of the mandibular first molars, and the distobuccal cusps of the mandibular second molars occluding against the opposing fossae^33,64–66.

According to the gnathological school, tooth contacts on marginal ridges could stimulate food impaction or even open interproximal contacts⁶⁷, and therefore the therapeutic relationship between the cusp and opposing fossae is considered to be more stable^68,69.

Cusp-fossa

The occlusal contact relationship between the cusp tip and the fossa of the opposing tooth is considered therapeutic. It can reduce the tendency toward food impaction and provide long-term stability of interproximal and occlusal contacts^67–70. These contacts between opposing teeth can be constructed using the principle of tripods or in the form of centered point contacts [Figure 12-14].

[Figure 12-14] The occlusal contact relationship between the cusp tip and the opposing fossa can reduce the tendency toward food impaction and provide long-term stability of interproximal and occlusal contacts.

Tripoidism is the occlusal relationship established by opposing teeth in which three equidistant contact points on the cusps’ external ridges close to their tip distribute the occlusal forces by making simultaneous contact with the three inclined planes of the ridges that contain them. Tripoidism aims to try to reproduce the idealized morphologic characteristics of a healthy young tooth.

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