Masticatory Cycle

Physiological variants of tooth closure

Proper chewing of food is based on a physiological bite. Several variants are distinguished, each with a specific contact pattern:

• Edge-to-edge bite: in the anterior region, the incisal edges of the upper incisors do not overlap the lower ones, and closure occurs strictly edge-to-edge. The posterior teeth contact according to the classic orthognathic type.
• Physiological progenia: the lower dental arch is wider than the upper. It is characterized by reverse incisor overlap (lower incisors are positioned anterior to the upper ones), and contact is maintained even with slight forward protrusion of the mandible. In the posterior segment, the buccal cusps of the lower molars lie lateral to the upper ones. A point contact occurs: the anterior buccal cusp of the upper first molar rests against the posterior buccal cusp of the lower one.
• Biprognathia: is distinguished by a pronounced vestibular inclination (forward protrusion) of the central incisors of both jaws. Despite the inclination, the depth of overlap and contact between them are preserved. The posterior segments close in an orthognathic relationship.

Positioning of the mandible and articular heads

To understand biomechanics, it is important to distinguish the spatial positions of the mandible relative to the maxilla. The fundamental concept is centric relation of the jaws, a condition in which the mandible is in its central position. In this case, the midpoint of the chin strictly coincides with the sagittal plane, the height of the lower facial third is not impaired, and the articular heads occupy a posterior, unstressed position. The occlusal analogue of this condition is the distal contact position.

The location of the articular head may vary:

1. Musculoskeletally stable position (musculoskeletally stable (MS) position): superior-anterior position of the head. This is the most orthopedically stable and physiological state. It is precisely in this position that the fabrication of artificial dental arches must be performed.
2. Ligamentous position (ligamentous (LP) position): superior-distal position of the head. According to V.A. Khvatova's classification, it corresponds to centric relation.

Methods for determining the height of the lower facial third

Upon loss of antagonist teeth, occlusion is disrupted, which requires the restoration of proper facial proportions. Three main techniques are used in clinical practice:

• Anatomical method: is descriptive in nature and based on assessing the patient's appearance. The clinician relies on the normal prominence of the nasolabial folds, the absence of lip retraction, and their relaxed contact without tension.
• Anthropometric method: is based on the principle of proportionality. The face is conventionally divided into three equal parts (upper, middle, and lower). Since the middle segment remains relatively unchanged with age, its dimension is used as a standard for calculating the height of the lower third.
• Anatomophysiological method: is the most demanded in practice. It relies on the relative physiological rest position height of the mandible and the presence of a free interocclusal clearance. Normally, when the mandible is at rest, the distance between the anterior antagonist teeth is 2–4 mm.

Tactics for dental arch defects

The choice of method for determining centric occlusion or centric relation directly depends on the number and topography of remaining antagonist teeth:

• There are $\ge$ 3 pairs of antagonists (located in three different functional groups): the so-called three-point contact is formed. Jaw models can be articulated in centric occlusion directly, without additional devices.
• Contacts are preserved in only 1 or 2 groups: missing occlusal connections must be restored using wax bases with occlusal rims.
• Complete absence of antagonists: a two-stage approach is applied. First, the height of the lower facial third is determined in the physiological rest position, and then the centric relation of the jaws is recorded using wax rims.

Simulation of Jaw Movements: Occluders and Articulators

After fixing the central occlusion, the jaw casts are sent to the dental laboratory, where they are mounted in special devices to simulate biomechanics.

• Occluder: has limited functionality and is capable of reproducing only vertical movements (opening and closing of the mouth).
• Articulator: a complex device that simulates the full range of movements.

Articulators are divided into three groups:

1. Average-value (universal) articulators: operate with strictly constant, averaged angles. Adjustments to the prosthesis are made by the dentist directly in the oral cavity.
2. Semi-adjustable articulators: joint mechanisms are adjusted using interchangeable inserts.
3. Fully adjustable (individual) articulators: are adjusted strictly to the patient based on recordings of condylar and incisal path angles. Parameter registration is performed intraorally (using a functional recorder) or extraorally (using a facebow).

Class and exam questions

What else to study on this topic

• Anatomical features of the structure of dental arches and factors of their stability
• Characteristics of dental arch forms in the upper and lower jaws
• Differences between the dental, alveolar, and basal arches
• Precise definition of the occlusal plane and basic terminology (articulation, occlusion)
• Anthropometric facial landmarks for denture design

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

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.