]Dental Occlusion part 1

Occlusion By Prof. Mohamad Helal

Head of Oral Biology Department

Occlusion Def. Contact relationship of the upper and lower teeth during various activities of the mandible (mastication, swallowing and speech) .

Centric occlusion •The relation of the upper and lower teeth

when they are in maximum intercuspation during swallowing and the terminal position of the masticatory stroke.

Centric relation •Is a position of the mandibule in which the condyles are resting in the most retrusive unstrained position in the glenoid fossa and related anteriorly to the distal slope of the articular eminence.

Why we study the occlusion?

The study of occlusion helps in the treatment of malocclusion, Temporomandibular Joint problems, in proper construction of dentures and other dental restorations (e.g., crown and bridge, etc…).

To study the occlusion it must be include the following items:

1. Dental arch formation.2. Compensatory curves of dental arches.3. Compensatory curves of individual teeth.4. Angulations of individual teeth in relation to various planes.5. Functional form of the teeth at their incisal and occlusal

thirds.6. Facial relation of each tooth in one arch to its antagonist in

centric occlusion.7. Occlusal contact and intercuspation of all teeth of one arch

with those in the opposing arch in centric occlusion.8. Occlusal contact and interaction of all teeth during various

functional movements.

1. Dental arch formation: Outline of the maxillary arch is larger than mandibular arch

• So, maxillary teeth overhang the mandibular teeth when the teeth are in centric occlusion

•Permits overbite and overjet relation ship

Overbite: vertical relation of the upper teeth to the lower teeth in centric occlusion .(increase efficiency of ant. teeth in bitingOver jet: horizontal relation of the upper teeth to the lower teeth in centric occlusion .(increase duration of occlusal contact in protrusive and lateral movements in incision and mastication )

Importance:•Prevent soft tissue biting•Guide the mandible through various lateral movement

Overbite enables the disclusion of the posterior teeth during food incision and increases the efficiency of the anterior teeth in biting well.

Overjet increases duration of occlusal contact in protrusive and lateral movements in incising and mastication.

From occlusal aspect the outline following labial and lingual surfaces confirmed to a parabolic curves.

• the dental arches have three segments:

1. Anterior segment : semicircular line passing along facial surfaces of anterior teeth.

2. Middle segment : straight line continuous with semicircular segment , include 1st , 2nd , M half of 1st molar.

3. Posterior segment : straight line which is parallel to the median plane e including distobuccal half of 1st molar, B of 2nd and 3rd molars.

Phases in development of dental arches :

1st phase: 6•6 years • 1st permanent molar take place post. to 2nd deciduous molar •Due to jaw growth Lead to : • 1- increase chewing efficiency 2- Serve to back up the deciduous teethMandibular one erupt firstly and max. one to established occlusion•1st permanent molar corner stone

2nd phase: 1,2•Central and lateral lingual to the deciduous predecessors.

3rd phase: 4,5

•1st and 2nd premolar take place of deciduous molar. •Primate space the size of premolars are smaller than the larger teeth they replace.

(Leeway space)

4th phase: 3,7The canines (Keystones) and 2nd molars emerge. The forcing of the canine bet. Incisors & 1st premolars in both jaw with backing effect of 2nd molar

5th phase: 8•3rd molar distal to the 2nd molar.

Leeway space

• The erupting premolars are smaller in mesiodistal dimension than the primary molars.

• The difference in sizes between the premolars and primary molars as well as the deciduous canines called

“Leeway space’.

The average mesiodistal size of the deciduous molars in that area is 47mm, which when compared with the premolars dimension of 42.2 mm, indicates an average gain of 4.8 mm in available space.

The leeway space must be utilized for alignment of the lower incisors as these teeth erupt with an average of 1.6 mm of crowding. The remainder of the space will be utilized by the mandibular molar.  

This movement of the mandibular molars may correct an end-to-end molar relationship (normal for the mixed dentition) into a normal molar relationship in the permanent dentition, i.e., the mesiolingual cusp of the maxillary first molar occludes in the central fossa of the mandibular molar. This relationship is discussed later on.

2- Compensatory Curvatures of the dental arches (Curved Occlusal Planes):

The significant of these curve as guidance for mandible Movement to complement the condyle paths and for balance of mandible

•Curve of Spee• Sagittal planes.

• Incisal Ridges of the anterior teeth and the B cusps of the posterior teeth follow a curve which end at the anterior surface of the condyle.

• when the arches were observed from a point opposite 6.

Curve of Wilson

• Coronal plane.

• The occlusal surfaces of posterior teeth conform to a curved plane.

• The crowns of mandibular posterior teeth must incline to the lingual while the crowns of maxillary posterior teeth must incline toward the buccal.

• Deeper posteriorly molars inclination is greater than that of the premolar .

Curve of Monson

• Combination of curve of Spee and the curve of Wilson. • coronal and sagittal planes.

• concave for the mandibular arch and convex for the maxillary arch. • in centric occlusion form a segment of a sphere of 4 inches radius with the center of the sphere at the glabella

Bonwill Triangle

• Describe the mandible and found that the mandibular dental arch adapts itself to an equilateral triangle of 4 inches length.

• Apex at the mesial contact area of the lower central incisors

• Angles of the base at the centers of each condyle

Importance of compensatory curves:

Guidance for the mandible movement to complement the path of condyles for balance of the mandible.

3- Compensating curvatures of individual teeth :

long axis of post. Teeth are not To a horizontal plane but show axial curvature

importance : 1- stability of teeth to resist force of mastication . 7, 8 distally inclined to prevent any distal drift of the posterior teeth.

7,8 mesially inclined to strike the maxillary molar

3 .Compensating curvatures of the individual teeth:

When we observed the dissected human jaw with their roots exposed prove that the long axis of posterior teeth are not perpendicular to a horizontal plane but show axial curvature. This curvature is essential for stability of teeth in the dental arches to resist

forces of mastication .

•The maxillary 2nd &3rd molars have distal inclination to prevent any distal drift of posterior teeth while mandibular 2nd & 3rd molar are inclined mesially to strike the maxillary molars with a mesial direction of force

4 -Angulations of individual teeth in relation to various planes

The inclination of a tooth from a vertical axis in mesiodistal and faciolingual directions

1 -upper anterior: FL inclined lingualMD 1&2 inclined slightly mesial but canine toward distal

Def . Inclination of the tooth from a vertical axis in a MD and FL direction.

(Considered root inclination so the crown inclined in opposite direction ).

importance: Proper occlusal function ( each tooth must be placed at the angle that best withstand forces during function )

2-Maxillary premolars are slight lingual in FL, Distal in MD

3-Maxillary Molars roots have lingual inclination with distal inclination

4-Mandibular anterior have greet lingual root inclination in FL. In MD incisors are nearly straight or minor root inclination while canine has distal root inclination

5 -Mandibular premolars In MD have distal inclination but in FL 1st premolar incline lingually. But 2nd premolars is offset buccally

6-Mandibular molars moderate to great buccal and distal toot angulations

Frontal view of the angulation of mandibular teeth:

Frontal view of the angulation of maxillary teeth:

Sagittal view of the angulation of maxillary &

mandibular teeth:

FL angulations

All teeth have lingual root inclination except the 2nd PM and mandibular M have buccal root inclination

MD angulations All teeth have distally root inclination except the upper incisors teeth have slight mesially root inclination and the lower incisors have slight mesial or straight .