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Occlusion (Part - 1)

Normal, Centric, Eccentric, and Balanced Occlusion, Classification, Keys, Concept, and Static and Dynamic occlusion

Occlusion (Part - 1)

Normal, Centric, Eccentric, and Balanced Occlusion, Classification, Keys, Concept, and Static and Dynamic occlusion

Occlusion is how the maxillary and mandibular teeth come together. It is the contact relationship between both sets of teeth when the jaws are in a fully closed (occluded) position. It is also the relationship between the teeth in the same arch.

Development of occlusion and factors involved

  • Occlusion develops along with the eruption of the primary teeth in a child. It is also the time for the development of oral motor behaviors and masticatory skills. 
  • Development of permanent dentition occlusion depends on the primary teeth shedding, except for the permanent molars, as they erupt distally to the primary dentition.
  • Neuromuscular patterns, the associated musculature, TMJ functioning, tongue functioning and posturing, orofacial behaviors, and habit patterns also affect occlusion development.

Why is occlusion important?

  • When the dentition is not occluding correctly, the teeth may not be able to perform their functions properly. This causes abnormal occlusal stress on the dentition. Incorrect tongue, lip, mandibular resting posture patterns, and parafunctional habits often make this stress excessive, resulting in occlusal disharmony leading to occlusal trauma.
  • When the teeth are not occluded correctly, they lose the ability to self-cleanse - The correctly occluded teeth are self-cleansing by the action of the cheek and lip musculature and with the neutralizing flow of saliva over the tooth surface.
  • Adequate knowledge of occlusion is very much required to manage the symptoms of occlusal disorders, otherwise leading to long-term discomfort of the patient. Maintaining proper occlusal harmony is important to increase the longevity of restorations and prevent fracture, wear, and pain.

Normal Occlusion

  • Normal occlusion is a concept that refers to the ideal occlusion in which 138 occlusal contacts for permanent dentition are achieved when 32 teeth come in contact. This forms the basis of treatment but rarely exists.

Centric Occlusion

  • The occlusion of opposing teeth when the mandible is in centric relation (CR). This may or may not coincide with the maximal intercuspal position (MI) (GPT8).
  • Maximal intercuspal position (MI): The complete intercuspation of the opposing teeth independent of condylar position, sometimes referred to as the best fit of the teeth regardless of the condylar position – called also maximal intercuspation (GPT8).  
  • Centric relation (CR): The maxillomandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective discs with the complex in the anterior-superior position against the slopes of the articular eminences. This position is independent of tooth contact. This position is clinically discernible when the mandible is directed superior and anteriorly. It is restricted to a purely rotary movement about the transverse horizontal axis (GPT8).

Eccentric Occlusion

  • When the contact relationship between the maxilla and mandible teeth is any other than centric relation, it is referred to as eccentric occlusion.

Balanced Occlusion

  • The bilateral, simultaneous, anterior and posterior occlusal contact of teeth in centric and eccentric positions (GPT8)

Classification of Occlusion

Types Description
Type I Maximal intercuspation occurs in harmony with the verifiable centric relation.
Type I (A)Maximal intercuspation occurs in harmony with adaptive centric posture.
Type II
Condyles must displace from verified centric relation for maximal intercuspation to occur.
Type II (A)
Condyles must displace from adaptive centric posture for maximal intercuspation to occur.
Type III
Centric relation or adaptive centric posture can not be verified. The TMJs accept loading without causing discomfort, so the relationship of maximal intercuspation to correct condyle position can not be immediately determined. The condition is diagnosed as transitory and treatable to achieve centric relation or adaptive centric posture.
Type IV
occlusal relationship is in a stage of progressive disorder because of the pathologically unstable and actively progressive deformity of TMJs.
Type IV (A)
occlusion may be described specifically as
(1) progressive open occlusion
(2) Progressive asymmetry
(3) progressive mandibular retrusion

Andrews Six Keys of Occlusion

  • Molar relationship
  • Crown angulation
  • Crown inclination
  • Absence of rotation
  • Tight contact
  • Curve of spee

Concepts of Occlusion

  • It varies with every specialty of dentistry based on the presence of natural or artificial teeth, static and functional characteristics of an occlusion and practical goals of the dentist for diagnosis and treatment of the occlusion.
  • When the patient has natural teeth present, the definitions are based on a static view of the dentition, where we describe the occlusion based on the fit of particular parts of individual maxillary teeth with specified parts of mandibular teeth.
  • In the case of the complete denture, the concept of "balanced occlusion" is used to define the term occlusion, considering bilateral contacts in all functional excursions. This concept is specially used due to the problem of instability of dentures.
  • The functional relationship is more stressed as it is used in diagnosing and treating functional disturbances of the masticatory system, which can be related to malocclusion, occlusal dysfunction, disorders of oral motor behavior, and bruxism.

Static and dynamic occlusion

  • Static occlusion is the contact between the teeth when the mandible is not moving.
  • Dynamic occlusion is the contact between the teeth when the mandible is moving.

Classification of static occlusion

British standards institute classification of incisor relationship
The relationship between the maxillary and mandibular incisors when in centric occlusion.
Class I
The lower incisal edges occlude with or lie immediately below the cingulum plateau of the upper incisors.
Class II
The lower incisal edges occlude posterior to the cingulum plateau of the upper incisors.
Further divided into two divisions:
Division I: the upper central incisors are proclined, usually resulting in an increased overjet
Division II: the upper central incisors are retroclined, usually resulting in a decreased overjet
Class III
The lower incisal edges occlude anterior to the cingulum plateau of the upper incisors.
Angles molar classification
The relationship between the maxillary and mandibular first molars when in centric occlusion.
Class I
The mesiobuccal cusp of the upper first permanent molar occludes with the mesiobuccal groove of the lower first permanent molar.
Class II
The mesiobuccal cusp of the upper first permanent molar occludes anterior to the mesiobuccal groove of the lower first permanent molar.
Class III
The mesiobuccal cusp of the upper first permanent molar occludes posterior to the mesiobuccal groove of the lower first permanent molar.
Canine classification
The relationship between the maxillary and mandibular canines in centric occlusion.
Class I
The mesial slope of the upper canine lies within the canine-first premolar embrasure.
Class II
The mesial slope of the upper canine lies in front of the distal slope of the lower canine.
Class III
The mesial slope of the upper canine lies behind the distal slope of the lower canine.

Dynamic occlusion

  • In dynamic occlusion, we study the contacts the teeth make when the mandible moves. These contact lines are made when the jaw moves sideways, forwards, backward, or at an angle.
  • You can get these lines on articulating paper when the patient grinds their teeth in every direction.
  • You need to understand guidance pathways (Posterior and Anterior guidance) and mandible movement (Protusion movement and lateral excursion) to study dynamic occlusion.

To be continued......

References

  1.  Textbook of Prosthodontics Hardcover – 1 June 2017 by Nallaswamy Veeraiyan
  2. Thomson, Hamish - Occlusion-Elsevier Science (2014)
  3. Occlusion Made Easy - Gerard Duminil with Olivier Laplanche, Jean-Philippe Re and Jean-Francois Carlier - (2016)
  4. Textbook Of Prosthodontics - Rangarajan - 2e 2017
  5.  Glossary of Prosthodontic Terms, Edition 8, J Prosthet Dent.
  6.  Recording the retruded contact position: a review of clinical techniques - Wilson and Banerjee - Published in 2004

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