Understanding Centric Relation in Dentistry: A Comprehensive Guide

Posted on Aug 24, 2024 in Biology

Factors Influencing Centric Relation Records: Requirements for Recording CR

Stable transfer base, Established vertical dimension, Accessories depending on the method of recoding CR

VERTICAL RELATION

Established by two things:

  • Musculature
  • Occlusal stops from the natural dentition or the occlusal rims

Types

  • Vertical relation at occlusion (VRO)
  • Vertical relation of rest position (VRR)

Anatomical Consideration

Closing Muscles

  • Masseter
  • Medial pterygoid
  • Temporal muscles

Opening Muscles

  • Inframandibular
  • Suprahyoid
  • Mylohyoid
  • Geniohyoid
  • Digastric
  • Platysma

  1. Significance

    Health of the periodontal ligament that supports the natural tooth and that of the mucosa of the basal seat for denture depends on the rest from the occlusal forces to maintain their health. It is assumed that when the muscles that close and open the jaws are in a state of minimum contraction, the jaw is held in a physiological rest position.

  2. Vertical Relation at Occlusion (VRO)

    It is the vertical separation of the mandible to the maxillae when the natural teeth are in occlusion or the dentures are in occlusion in an edentulous state.

  1. Vertical Relation at Rest (VRR)

    It’s a postural position controlled by the muscles that open and close the mandible. Further controlled by the position of the head which modifies the effect of gravity. On reclining, the effect of gravity does not pull the mandible down. The head of the patient should be upright and unsupported when observation of the VRR is made.

  2. Methods of Determining VR

    Mechanical Methods

    • Ridge relation
    • Distance of incisive papillae from the tip of mandibular incisors is 4 mm
    • Parallelism of the ridges
    • Measurement of former dentures
    • Pre-extraction records
    • Profile photographs
    • Cast of teeth in occlusion
    • Facial measurement

    Physiological Method

    • Physiological rest position
    • Phonetics and esthetics as a guide
    • Swallowing threshold
    • Tactile sensation

CENTRIC RELATION (GPT-8)

The most posterior relation of the lower to the upper jaw from which lateral movements can be made at a given vertical dimension.

  1. Importance of Centric Relation

    Centric relation is a reproducible and stable position. Therefore it is used as a reference when mounting dentulous and edentulous casts in articulator. Thus CR serves as a reference relationship for establishing an occlusion. When CR and CO of artificial teeth do not coincide or a freedom of centric is not present the stability of denture bases is in jeopardy and the patient will have unnecessary pain or discomfort. Edentulous patients use CR closures in mastication and in other mandibular activities such as swallowing. An accurate CR record will properly orient the lower cast to the opening axis of the articulator and orient CR to the hinge axis of both the articulator and the mandible.

  2. Complications in Recording CR

    The structure of TMJs are such that one joint can be displaced downward by uneven pressure when records are made and yet the condyles be in their most retruded position. This situation cannot occur on the articulator and thus a deflective occlusal contact may be the source of instability, soreness and resorption despite the correctness of the other relations. Realeff effect by Hanau: according to it, there is uneven resiliency in the soft tissues. This resiliency is present in both the mucosa and the TMJs, thus undue pressure in securing the relation must be avoided lest excessive displacement of soft tissues occur. Even though a balanced and equilized registration has been made it often is lost due to:

    • Cast mounting procedures
    • Processing of denture

  1. ECCENTRIC RELATION

    Any relationship of the mandible to the maxilla other than centric relation. The eccentric relations that are recorded and used in complete dentures are:

    • Protrusive
    • Right lateral
    • Left lateral

  2. METHODS TO RECORD

    • Functional or chew in
    • Graphic
    • Tactile or direct check record

SIGNIFICANCE OF CENTRIC RELATION

  1. It is the ideal arch to arch relationship and hence optimum position of jaws for the health, comfort and function of TMJ.
  2. It is a mandibular position where the condyle-disc assembly is seated/braced in anterior-superior position against the posterior slope of articular eminence. Although it was believed to be the upmost, rearmost position in the glenoid fossa (RUM).
  3. Errors in mounting of the cast can be detected, when centric relation is used as a horizontal reference position.
  4. An accurate centric relation record properly orients the lower cast to the opening axis of the articulator and the mandible.
  5. Accurately recorded centric relation when transferred to the articulator permits proper adjustments of the condylar guidances for the control of eccentric movements of the instrument.
  6. It is a reproducible position which can be repeatedly arrived at and thus serves as a reliable guide to develop centric occlusion in artificial dentures.
  7. It serves as a reference position for the institution of occlusal rehabilitation in dentulous condition.
  8. It serves as the reference position to relate and nomenclate several occlusal positions of upper and lower teeth

CONDYLES ARE HEALTHY AND COMFORTABLE ONLY IN CENTRIC RELATION

  1. If they are correctly aligned with their discs.
  2. Stopped by bony stop.

  1. Reasons for Advocating the Uppermost Position

    • Mechanical standpoint
    • Physiological standpoint

TMJ DISCOMFORT IS DUE TO

  1. Improper position of the condyle.
  2. Improper alignment of the disc.
  3. Pathosis of articulating surfaces.

RECORDING OF CENTRIC RELATION

There are two aspects:

  1. Assisting the patient to retrude the mandible.
  2. Using recordings.

METHODS OF RECORDING CENTRIC JAW RELATION

  1. Functional method
    • Needles house method
    • Patterson method
  2. Graphic method
    • Intra oral method
    • Extra oral method
  3. Physiological method
    • Tactile or interocclusal check record method
    • Pressure less method
    • Pressure method
  4. Radiographic method

  1. Factors Influencing Centric Relation Records

    • The resiliency of the supporting tissues
    • The stability of the record bases
    • The TMJ and its associated neuromuscular mechanism
    • The skill of the dentist
    • The health and cooperation of the patient
    • The maxillomandibular relationship
    • Posture of the patient
    • Character or size of the residual alveolar arch

  1. GRAPHIC METHOD

    These methods are so called because they use graphs to record the centric relation. Two types:

    1. Arrow point tracing
    2. Pantograph

  2. ARROW POINT TRACING

    Measured across a single plane. Central bearing device is used. Defined as “a device that provides a central point of bearing or support between the maxillary and mandibular dental arches. It consists of a contacting point attached to one dental arch and a plate attached to the opposing dental arch. The plate provides the surface on which the bearing point rests or moves and on which the tracing of the mandibular movement recorded.”

  3. CENTRAL BEARING DEVICE CONSISTS OF

    • Central bearing point
    • Central bearing plate

TYPES OF ARROW POINT TRACERS

Depending upon the placement of recording device:

  • Intra oral
  • Extra oral

EXTRAORAL ARROW POINT TRACING

Larger than intraoral because they are made farther from centers of rotation. Apex is more discernible. It is visible while tracing is being done. Patient can be guided properly.

  1. Types of Extra Oral Tracers

    • Hight tracers
    • Stransbery tracers
    • Sears trivet
    • Phillips tracer

  2. INTRAORAL ARROW POINT TRACING

    Located intraorally. Simple compared to extraoral.

  1. Disadvantage

    1. It cannot be observed during the tracing.
    2. Small, it is difficult to find the true apex.

  2. PHYSIOLOGICAL METHODS

    They are called as they are based on the proprioceptive impulses of the patient. Kinesthetic sense helps to direct mandibular movements. Visual insight and sense of touch of the dentist. No pressure is exerted on the inter-occlusal record.

  3. TACTILE SENSE OR INTEROCCLUSAL CHECK RECORD METHOD

    INDICATIONS

    • Abnormally related jaws
    • Displaceable, flabby tissues
    • Large tongue
    • Uncontrollable Mandibular movements
    • Patients already using complete dentures

    MATERIALS USED

    1. Waxes
    2. Impression compound
    3. Zinc oxide eugenol

  4. ERRORS IN CENTRIC RELATION

    When centric relation is not coinciding with centric relation of the patient.

    Due to

    1. Incorrect horizontal relation of the mandible to the maxilla
    2. Incorrect equalization of vertical contact

    POSITIONAL ERRORS

    1. Failure of the operator in his registration of the correct horizontal relationship.
    2. Failure of the operator to record equalized vertical contact.
    3. Application of excessive closure pressure by the patient at the time of recording.
    4. Changes in the supporting area.

    TECHNICAL ERRORS

    1. Ill fitting occlusion rims: if record bases are not stable.
    2. The slight shifting teeth which occurs between the stage of final arrangement in wax and the transfer to a permanent base material.
    3. A movement by the tooth or several teeth either horizontally, or vertically, introduces an error.