Anterior occlusion and sagittal paths
Forward and downward displacement of the mandible is called anterior occlusion. This movement is ensured by the simultaneous (bilateral) contraction of the lateral pterygoid muscles.
The pattern of dental closure during mandibular protrusion:
- Anterior segment: the cutting edges of the incisors meet precisely edge-to-edge.
- Posterior segment: disocclusion (complete separation of the teeth) occurs or contact is maintained on the distal cusps of the last molars.
Simultaneous closure of the anterior teeth and the last molars on both sides is called Bonwill's three-point contact. The presence of such contact depends on the prominence of the posterior tooth cusps, the depth of the incisal overlap, and the curvature of the sagittal occlusal curve (Curve of Spee).
Mandibular forward movement is described by two trajectories:
- Sagittal incisal path — the sliding of the lower incisors along the palatal surfaces of the upper ones. The angle formed by this path and the occlusal plane is normally 40–50° (according to Gysi) and directly depends on the degree of incisal overlap and the inclination of their axes.
- Sagittal condylar path — the movement of the condylar heads downward and forward along the slopes of the articular tubercles. The angle of the sagittal condylar path (Gysi angle) averages 33° (individual norm 20–40°).
Lateral occlusions and transversal movements
Lateral occlusions occur when the mandible is displaced to the right or left. This is mediated by the unilateral contraction of the lateral pterygoid muscle (m. pterygoideus lateralis). It is important to remember: the muscle always contracts on the side opposite to the direction of displacement.
The biomechanics of the temporomandibular joint (TMJ) during lateral displacement are asymmetric:
- Working side (side of displacement): movement occurs in the lower compartment of the TMJ, where the condyle simply rotates around its own axis.
- Balancing side: movement takes place in the upper compartment of the TMJ. The condylar head along with the disc is displaced downward, forward, and medially along the medial and upper walls of the articular fossa toward the apex of the articular tubercle.
The trajectory of the condyle on the balancing side is called the lateral condylar path. The angle between this path and the sagittal plane is termed the Bennett angle (normally 15–17°).
Concepts of occlusal contacts
During lateral mandibular shifts, teeth can occlude in different ways. Modern dentistry identifies three main concepts:
- Bilateral balanced contacts (classical Gysi-Hanan theory). On the working side, homologous cusps contact each other, while on the balancing side, non-homologous cusps do. This is rarely seen in natural dentition, but the concept is critical for stabilizing complete removable dentures in edentulous patients.
- Group function (group guidance). An entire group of teeth contacts on the working side — the buccal cusps of premolars and molars.
- Canine guidance (canine protection). Only the opposing canines contact on the working side, while all other teeth are disoccluded.
In group function and canine guidance, occlusal contacts on the balancing side are completely absent. These guidance patterns are normal for the majority of natural dental arches.
Briefly: answers to topical questions
What is the difference between the sagittal occlusal curve of Spee and the transversal curve of Wilson?
The sagittal curve of Spee and the transversal curve of Wilson differ in direction, formation mechanism, and anatomical landmarks.
| Characteristic | Curve of Spee | Curve of Wilson |
|---|---|---|
| Direction | Anteroposterior (sagittal) | Transverse (transversal) |
| Localization | Along the incisal edges of anterior teeth and buccal cusps of posterior teeth | Across the occlusal surfaces of the premolars and molars on the right and left sides |
| Formation mechanism | Reflects the anatomical concavity of the mandibular dental arch | Formed due to the varying levels of buccal and palatal cusps resulting from tooth inclination |
In which TMJ compartment do hinge movements occur, and in which do sliding movements take place?
In the temporomandibular joint, hinge (rotational) movements occur in the lower compartment, while sliding (translational) movements occur in the upper compartment.
- Lower compartment — performs rotational movements of the mandibular condyles around the frontal or vertical axis (e.g., at the onset of mouth opening or on the working side during lateral displacement).
- Upper compartment — provides sliding of the condylar processes and articular discs forward and downward along the slope of the articular tubercle (during mandibular protrusion or on the balancing side during lateral displacement).
Which muscles are involved in mandibular depression?
Mandibular depression and mouth opening involve a complex of antagonist muscles.
- Mylohyoid muscle — forms the floor of the oral cavity.
- Geniohyoid muscle — participates in mouth opening.
- Digastric muscle — both anterior and posterior bellies are involved.
- Stylohyoid muscle — takes part in jaw depression.
- Lateral pterygoid muscle (m. pterygoideus lateralis) — contributes to mouth opening.
For effective jaw depression, the hyoid bone must be rigidly fixed by the musculature located inferior to it.
To avoid confusion regarding muscle function during lateral movements, remember the "cross" rule: jaw moves left — the right lateral pterygoid muscle contracts; jaw moves right — the left one contracts.
Class and exam questions
Which muscles provide anterior occlusion?
Forward movement of the mandible is carried out due to the simultaneous (bilateral) contraction of the lateral pterygoid muscles.
What is the Gysi angle and what is its value?
This is the sagittal condylar path angle formed between the trajectory of the condyle and the occlusal plane. According to Gysi, its average value is 33°.
What type of contacts is characteristic of natural teeth during lateral movement?
Natural dentitions feature canine guidance (contact of canines only) or group function (contact of buccal cusps of posterior teeth) on the working side, with an absence of contacts on the balancing side.
What else to study on this topic
- Complete composition of Hanan's articular pentad
- Influence of the sagittal occlusal curve (Curve of Spee) on dental contacts
- Factors determining the magnitude of the sagittal incisal path
- Detailed anatomy and function of the superior and inferior heads of the lateral pterygoid muscle
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