Dental Occlusion and Cusp Types

Occlusion

• Occlusion refers to the way the upper and lower teeth come together.
• Centric Occlusion: Also known as maximum intercuspation, it's the position where teeth make the most contact. It's determined by the teeth.
• Centric Relation: This is the reproducible position of the mandible (lower jaw) with or without teeth. The articular disc is in place and muscles are relaxed, determined by the joint.
• Convex surfaces touching each other. Points of contact are limited to specific areas.
• Normal Intercuspation: Ideal tooth contacts and relationships contribute to stability. Occlusal forces are directed along the tooth's long axis during intercuspation. Cusp arrangements allow lateral jaw movement without interference.
•  Exception: The mandibular central incisor and maxillary third molar are exceptions to the general pattern.
• Arch Shape: The maxillary arch is generally larger than the mandibular arch (contained).

Cusp Types

• Stamp cusp: Occludes into the fossa of the opposing tooth or marginal ridge. Maxillary = palatal cusp; Mandibular = buccal cusp.
• Shear cusp: Deflects tongue/cheek from occlusal table. Occludes in embrasures or grooves. Maxillary = buccal cusp; Mandibular = lingual cusp.

Normal Characteristics

• Overjet and Overbite: Normal ranges are 2-3mm.
• Cusp-Fossa Relationship: Mesiolingual cusp of upper molars to central fossa of lower molars.
• Cusp-Marginal Ridge Relationship: Cusp of 15 to the marginal ridge of 45 and 46.
• Cusp-Embrasure/Groove Relationship: Cusp tips of 14 to the embrasure between 44 and 45.

Tooth Contacts

• Tooth Contacts: Prevent buccal-lingual and mesiodistal drifting of teeth.

Alignment of Dentition

• Plane of Occlusion:
• Curve of Spee: Curved line from anterior to posterior touching the tips of all mandibular teeth (lateral view).
• Curve of Wilson: Curved line from right to left.
• Sphere of Monson: 3D representation of the curve of occlusion (combined Spee and Wilson). This variation is because of varying tooth inclinations.

Inter-Arch Tooth Alignment

• Mesial/Distal Inclines: Maxillary mesial inclined planes and mandibular distal inclined planes.
• Buccal Cusp Position: Buccal cusps of max teeth are lateral to buccal cusps on mandible. This prevents tongue or cheek biting. It also guides jaw during lateral movements.

Malocclusions (Classes)

• Class I (Neutroocclusion):

• First Molar Relationship: Mesiobuccal cusp of upper 6 to mesiobuccal groove of lower 6.

• Canine Relationships: Mesial inclined plane of the upper 3 occluding to the distal inclined plane of the lower 3.

• Class II:

• Canine Relationship: More complex.

Muscles of Mastication

• Muscles responsible for elevation, depression, protrusive and retrusive movements of the mandible.

Lateral Movements

• Working Side: Side towards which the mandible is moving.
• Non-Working Side: Opposite side.
• Muscles Involved: Masseter, temporalis, medial pterygoid, lateral pterygoid.

Muscle Descriptions

• (Note: Specific origins and insertions are included in your original text, and are repeated here for detail)*

• Masseter:

  • Anterior fibers: elevate mandible
  • Posterior fibers: protrude mandible
  • Unilateral (one-sided) contraction: pulls mandible laterally toward working side.

• Temporalis:

  • Origin: temporal fossa
  • Insertion: coronoid process of mandible

• Medial Pterygoid: Elevates and protrudes mandible

• Lateral Pterygoid: Pulls mandible forward during opening. Controls return of condyle to fossa during closing.

  • Origin: lateral surface of lateral pterygoid plate of sphenoid bone.
  • Insertion: condyle of mandible.