Occluders and Articulators

Occlusal plane and spatial curves

Analysis of the closure of the upper and lower dental arches begins with determining the occlusal plane. Visually, it is a straight line passing through the contact zone of antagonist teeth.

To ensure full chewing function, dental arches possess an anatomic curvature described by special curves:

1. Sagittal occlusal curve (Curve of Spee). Runs in the anteroposterior direction. Its main feature is reflecting the natural anatomic concavity of the mandibular dental arch.
2. Transverse occlusal curves (Wilson-Plese curve). Run in the transverse direction across the chewing surfaces of the premolars and molars on the right and left sides. They illustrate closure at the level of teeth 1.4/2.4, 1.5/2.5, and molars (e.g., 1.6/2.6).

Mechanism of formation of the Wilson curve: It is formed due to the physiological inclination of the teeth. In the upper jaw, teeth are inclined buccally, while in the lower jaw, they are inclined lingually. Consequently, the buccal and palatal (or lingual) cusps are located at different levels. Furthermore, the radius of curvature is strictly individual for each symmetrical pair of teeth. For the mandibular dental arch, this curve has a downward concavity and originates from the first premolars.

Patterns of articulatory movements

Articulation includes all possible movements of the mandible. The starting and ending point of these movements is always the state of relative physiological rest or centric occlusion.

Depending on where and how the mandible shifts, the following basic states are distinguished:

• State of relative physiological rest;
• Centric occlusion (centric relation);
• Anterior occlusions;
• Lateral occlusions (left and right);
• Distal contact position.

Features of the physiological rest state: This is the starting position. At this moment, the tone of the masticatory muscles is minimal, and the facial musculature is completely relaxed. The muscles that elevate and depress the jaw are in a state of equilibrium. At the same time, the teeth are not clenched—a space of 2 to 4 mm is maintained between the occlusal surfaces.

Characteristics of occlusion signs

Any type of occlusion in dentistry is described through three fundamental signs evaluated comprehensively:

• Dental sign: describes the position of the teeth at the moment of closure. In the lateral segments (posterior group), cusp-fossa contact (when a cusp of a tooth on one jaw fits snugly into the fossa of the antagonist) and cusp-cusp contact are distinguished. The latter can be of two types: closure with homologous or non-homologous cusps.
• Muscular sign: indicates which specific muscle groups are in a state of contraction at the moment of tooth contact.
• Articular sign: records the exact location of the articular heads of the temporomandibular joint (TMJ).

Centric occlusion: definitions and criteria

Historically, the term was introduced by the researcher Gysi (Gysi) in 1922. He described it as a multiple contact in which the lingual cusps of the upper posterior teeth fall into the intercuspal depressions of the lower ones. The modern definition is as follows: centric occlusion is multiple cusp-fossa contacts when the TMJ heads are in the central position within the articular fossae.

Main signs of centric occlusion:

• Dental: the greatest number of contacts between antagonist teeth is observed.
• Articular: the heads of the condylar processes are located strictly at the bases of the slopes of the articular tubercles of the temporal bones.
• Muscular: the elevators (temporal, masseter, and medial pterygoid muscles) are in tone.

Additional signs: The facial midline ideally coincides with the line between the central incisors. The upper incisors overlap the lower ones by exactly 1/3 of the crown height (in orthognathic bite). In the transverse direction, the buccal cusps of the upper teeth overlap the buccal cusps of the lower ones. A strict antagonist rule applies: an upper tooth occludes with the corresponding and distally positioned lower tooth, and a lower tooth with the corresponding and medially positioned upper tooth.

Classification and related terms

According to V.N. Kopeikin's classification, in addition to true centric occlusion, there is secondary centric occlusion. This is a forced position of the mandible. It occurs with maximum contraction of the elevator muscles when the body attempts to achieve maximum contact between the remaining (preserved) teeth.

There is also habitual multiple occlusion. In this condition, the teeth close as tightly as possible, but there is a fundamental difference from centric occlusion: the articular heads of the mandible may not be in the central position within the fossae. In foreign literature, the term Maximum intercuspal position (ICP) is most commonly used to describe maximum closure.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Detailed analysis of cusp-to-cusp and cusp-fossa contacts
• Pathogenesis of secondary centric occlusion formation according to V.N. Kopeikin
• Specifics of the distal contact position of the mandible
• Differences between homologous and non-homologous cusp contact

Other topics in "Chapter 5. Biomechanics of the Masticatory Apparatus"

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