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Centric relation

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In prosthodontics, replacing the missing, without affecting the other components of the masticatory system has two main reference the maximum intercupation and the centric relation.
In this lecture discussion of centric relation as reference is exposed.

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Centric relation

  1. 1. CENTRIC RELATION, THE BASIC REFERENCE
  2. 2. A. Introduction. B. 10 factors to manage the occlusion. C. Relevant terms. -Centric relation. - Maximum intercuspation position MIP. -Centric occlusion. D. Significance. E. Dental procedures which reference MIP or CO ? F. Manipulation of mandible for CR registration. G. Articulators.
  3. 3.  Masticatory system is composed of 2 main parts;  1. Active part represented by the NM system.  2. Passive part represented by the TMJ and the teeth.  The NM system is controlled by the CNS.  These 2 elements are connected instantly by sensitive receptors.
  4. 4.  In the absence of pathology, these elements work in synergy and harmony.  Maintaining this comfort done by balanced distribution of the elevator muscle forces between the teeth and the TMJ.  Pathology and disturbances affect mainly the teeth so that the relationship in the same and opposite jaw.  Disturbances of the occlusion affect the existent harmony with the TMJ and the NM system.  This disharmony leads to pathologic manifestations in the TMJ and the muscles.
  5. 5.  Treating the affected teeth or replacing the missing ones need to integrate them in the masticatory system. The treatment reference could be;  The MIP or the occlusion in centric relation CO.  In 10% of population the MIP coincide with the CR (occlusion in CR position)  Discrepancies between MIP and CR is frequently present and may lead to pathologic conditions in certain situation.
  6. 6.  The 10 must know factors of occlusion. Understand these factors and you will never have to treat occlusal problems by guessing. Dawson
  7. 7.  1. Centric relation: the maxillomandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective disks with the complex in the anterior superior position against the shapes of the articular eminencies.  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. (GPT-5).
  8. 8.  CR is anatomically determined; it is repeatable and reproducible. Ruth et al  Okeson, describes it as the most orthopedically and musculoskeletally stable position of the mandible.  others consider it to be the essence of optimal temporomandibular joint form and function.  It is the most reliable reference point for accurately recording the relationship of the mandible to the maxilla.
  9. 9.  Therefore, a determination of the CR is a prerequisite for the analyses of dental interarch, condylar position, and skeletal relationships.  A properly aligned condyle-disc assembly in centric relation can resist maximum loading by the elevator muscles with no sign of discomfort.
  10. 10.  At the most superior position, the condyle disc assembly are braced medially, thus CR also the midmost position.
  11. 11. In CR the mandible has Purely rotary move-ment about transverse Horizontal axis.
  12. 12.  Head position effect on mandible position.
  13. 13.  3.centric occlusion; the occlusion of opposing teeth when the mandible is in centric relation. This may or may not coincide with the maximal intercuspal position.
  14. 14. Centric occlusion
  15. 15.  Centric occlusion, coincidence between CR and ICP
  16. 16.  CR position and MIP are well reproducible reference positions of the mandible.  When using the "freedom in centric" concept, the occlusal range is about 0.5 mm.  Retruded contact position is very close to MIP in most people. Therefore it can be used as a "therapeutic compromise" for occlusal rehabilitation.  Utz KH1, Duvenbeck H, Oettershagen K.
  17. 17.  Regarding dental procedures, the mandible can assume two well-known positions as a reference for treatment: centric relation (CR) and maximum inter cuspation (MIC).  These usually are not coincident in the general population.  The MIC and CR are reproducible.  10% of the population have coincidence between CR And MIC.
  18. 18.  all simple procedures related to occlusal surfaces where the ( VDO) and the MIC position are not affected, in this case the reference is MIC. Fillings and single crown. Missing tooth replacement of limited span ( FPD or RPD).
  19. 19.  In which cases CR is considered as basic reference?  Missing of all the upper or lower posterior teeth or both.  Cases where the VDO is affected.  Severe dental wear.  Missing of all the upper or lower teeth or both.  Signs and symptoms in masticatory system (TMJ, NMS, Teeth) where the OCC. Is involved.  Cases need full mouth rehabilitation.  In orthodontic treatment where discrepancy between CR and MIC position is more than 3mm.
  20. 20.  All the cases where the CR is the reference the MIP will coincide with CR position.
  21. 21.  Avoiding damage caused by premature contact or occlusal interference put the codyles away from their position in CR  The ability of the dentist to modify the occlusion and reprogram the condylar position and muscle response is easily demonstrated clinically in occlusal treatment procedures.
  22. 22.  . After several jaw closures the muscles reprogrammed the condylar position to complement the prevailing occlusion.  This manipulation should avoid tooth contact. Otherwise prematurity will affect again the position of the condyles in CR.  This manipulation should be achieved without pain or stress which indicates relaxation of the lower lateral pt muscle responsible for mandible deviation.
  23. 23.  So that closure of the mandible is achieved by elevator muscles.  Guiding the mandible to CR position should never let the Pt. feels any stress or discomfort in the TMJ, otherwise a iatrogenic TMD may occur.
  24. 24.  Kontor et al, researched reproducibility and spatial patterning of CR record by using  1. swallowing.  2.Chin-point guidance.  3.Chin-point guidance with anterior jig.  4.Bilateral manipulation. ( Dawson )  Bilateral manipulation allowed the greatest reproducibility, followed by chin point guidance. Swallowing was the least consistent.
  25. 25.  Achieving dental work or occlusion analysis in the laboratory require mounting the models on the articulator.  An instrument which simulates closely the mandible and TMJ movements.  Different types of articulators are exposed, the development of these instruments is closely related to the continual development in understanding the anatomy, physiology, biomechanics of occlusion, NMS, TMJ,and mandibular movements.
  26. 26.  Improperly using the most sophisticated articulator results in poor job quality but attentive using of simple instrument can give acceptable results.  Without exact information we are only guessing.  Therefore, for an articulator to be acceptable, it must be anatomically correct and  should allow enough adjustment to accommodate a majority of patients.
  27. 27.  The following parameters need be considered:  1. Intercondylar distance  2. Condylar inclination  3. Mandibular arc of closure  4. Hinge axis position [Centric Relation (CR) and Centric Occlusion (CO)]
  28. 28.  Intercondylar distance.
  29. 29.  Condylar inclination
  30. 30.  Why use a semi adjustable articulator?  • Greater accuracy  • Savings in chairside adjustment time  • Eliminate iatrogenic occlusal interferences  • Improved doctor/laboratory relations  • Increase patient’s perception of care and skill level  • Revenue source for the practice and laboratory  …and finally, because it is the right thing to do.
  31. 31.  Chairside refinement is the last step to integrate the prosthesis in the masticatory complex which result in patient comfort due to the synergy between the different components of the masticatory system. This adjustment is controlled by the nervous sensitive receptors.
  32. 32. THANKS FOR YOUR ATTENTION

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