Sagittal and Transverse Mandibular Movements

Lateral mandibular movements

During lateral displacement of the jaw (transverse movements), complex movement trajectories occur in both the joint and dental areas. The biomechanics of these excursions are described by several key parameters:

• Lateral condylar path: reflects the trajectory of the condylar head (processus condylaris) of the temporomandibular joint (TMJ). On the balancing (non-working) side, the head moves downward, forward, and inward.
• Lateral incisal path: demonstrates the displacement of the incisal point (the area between the medial angles of the lower central incisors) relative to the median sagittal plane.
• Lateral incisal path angle (Gothic angle): is formed between the displacement lines of the incisal point to the right and left. Normally, its value ranges from 110–120°.
• Bennett angle ($\alpha$): is formed on the balancing side during lateral movement. It is projected in the horizontal plane and formed between the sagittal plane and the actual movement trajectory of the articular head displacing medially.

To understand the synchrony of displacement of all points of the mandible, the biomechanical Bonwill triangle is used. Its vertices are the centers of both articular heads and the incisal point.

Vertical movements and their muscular apparatus

Opening and closing of the mouth occur through the alternating work of antagonistic muscles.

Mouth closure (mandibular elevation) is provided by the temporalis, masseter, and medial pterygoid muscles. This process occurs against the background of smooth relaxation of the muscles that depress the jaw.

Mouth opening (mandibular depression) requires rigid fixation of the hyoid bone by the musculature located below it. The depression itself involves:

• Mylohyoid muscle (forms the oral diaphragm)
• Geniohyoid muscle
• Digastric muscle (both anterior and posterior bellies function)
• Stylohyoid muscle
• Lateral pterygoid muscles

Biomechanics of mouth opening is divided into three stages:

1. Initial: the articular heads rotate around their transverse axis.
2. Intermediate: the heads slide down and forward along the slope of the articular tubercle, reaching its apex.
3. Maximum opening: the heads perform additional rotation and are securely fixed at the anterior edge of the articular tubercle. The normal distance between the incisal edges of the incisors is 4–5 cm.

Development of occlusion and its classification

Occlusion refers to the character of dental arch closure in the state of central occlusion. Human teeth undergo two generations, forming three periods of development:

1. Primary (deciduous) dentition: is finally formed by the age of two. Includes 20 teeth (incisors, canines, molars).
2. Mixed dentition: starts around 6 years of age with the appearance of the first permanent molars (they erupt behind the second deciduous molars). Lasts until all primary teeth are completely shed. The number of teeth during this period is unstable.
3. Permanent dentition: completes its formation by approximately 14 years of age. Includes 28 to 32 teeth (premolar addition).

Globally, all types of occlusion are divided into three categories:

• Physiological: a congenital normal variant.
• Anomalous: a congenital deviation from the norm.
• Pathological: an acquired condition. It develops after eruption due to tooth mobility (against the background of periodontal disease), displacement, or loss.

Characteristics of orthognathic occlusion

Orthognathic occlusion is a classic example of physiological closure, ensuring ideal chewing function.

General signs of normality:

• The midlines between the central incisors of both jaws coincide and lie in the same plane.
• The incisal edges of the upper central incisors are located at the level of the lower edge of the red border of the upper lip (protruding by 1–2 mm).
• Each tooth contacts two antagonists. The exceptions are teeth 18, 28, 31, and 41, which have only one antagonist.
• The upper teeth occlude with the corresponding and posteriorly positioned lower teeth, and the lower teeth with the corresponding and anteriorly positioned upper teeth.

Specific signs (contacts):

• Frontal region: a tight incisal-cusp contact is formed. The upper incisors overlap the lower ones by no more than half the crown height.
• Posterior (molar/premolar) region: a fossa-cusp contact is formed. The buccal cusps of the upper teeth lie outside the lower ones. The palatal cusps of the upper teeth fall into the longitudinal fissure of the lower ones, and the buccal cusps of the lower teeth into the longitudinal fissure of the upper ones.

Relation to the occlusal plane: Only the incisal edges of the incisors, the tips of the canines, and the distal buccal cusp of the third molar touch the plane. The first and second premolars, as well as the first and second molars, lie below this plane. In the upper jaw, the central incisors and canines are lowered 2–3 mm below the occlusal plane, and the buccal cusps of the posterior teeth cross it. This arrangement creates the necessary physiological curvature of the dental arch in the anteroposterior and lateral directions.

Class and exam questions

What else to study on this topic

• Biomechanics of antagonistic muscle function during chewing
• Formation of mixed dentition and eruption times of permanent teeth
• Mechanisms of fossa-cusp contact formation
• Physiological curves of dental arches: sagittal and transverse
• Pathogenesis of pathological occlusion development upon tooth loss

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.