CENTRIC RELATION: CONCEPTS AND CONTROVERSIES

CENTRIC RELATION: CONCEPTS AND CONTROVERSIES[Type the document subtitle]

CONTENTS

Introduction Terminology Definition of Centric relation Anatomic Vs Significance based definition Chronology of change of definition of centric relation Interpretation of definition of Centric relation Anatomy of Temporomandibular joint as it pertains to centric relation Important features of Centric relation Significance of Centric relation How Centric relation is acquired from birth? Concepts of centric relation position Centric relation and Condylar movement Centric relation and Centric Occlusion Review of Literature Summary Conclusion

INTRODUCTION

One of the objectives of Prosthodontics is to restore missing dental and oral structures in such a way that there is a harmonious relationship among teeth, bones, joints and muscles. One of the most controversial aspects of this complex relationship has been referred to as centric jaw relation.

The term centric has been known in dentistry for many years and no other word in dentistry has been a source of controversy for years together, as has been the term centric. This term is derived from the word centre or centre oriented relation. A number of workers in the past have provided us with many different views about this seemingly complicated entity.

The importance of centric relation in complete denture prosthodontics has been well known for many years. Some dentist claim that a correct centric relation is the single most important measurement made in the construction of complete dentures. The human mandible can be related to the maxilla in several positions in the horizontal plane. Among these, centric relation is a significant and important position. This is because of its usefulness in relating the dentulous or edentulous mandible to the maxilla, where the teeth, muscles and the TMJ function are in harmony. In other words, it is a position of occluso-articular harmony.

The word centric is an adjective that should not be made to function as a noun.

TERMINOLOGY

Maxillomandibular relationship:Any spatial relationship of the maxilla to the mandible; any one of the infinite relationships of the mandible to the maxilla(GPT-8).

Terminal hinge axis /transverse horizontal axis:An imaginary line around which the mandible may rotate within the sagittal plane (GPT-8).

Terminal hinge relationThis is the range of retruded mandibular opening and closing movements while the transverse axis remains constant to the glenoid fossa and the condyle. This can happen only if the mandibular movement is of a pure hinge nature.

However, a pure hinge movement of the condyle occurs only when the mandible is in its retruded position. This means that in centric relation, a pure hinge movement occurs without translation of the condyle. Translation or combination of translation and hinge movement occur only when the condyle shifts anterior to centric relation. For this reason centric relation is known as the terminal hinge axis relation.

In centric relation, condyles rotate in fixed axis. As long as that rotational axis stays fixed at the most superior position against the eminentiae, mandible can open or close and still be in centric relation. If the condyle axis moves forward, it is no longer in centric relation. Retruded contact positionThat guided occlusal relationship occurring at the most retruded position of the condyles in the joint cavities. A position that may be more retruded than the centric relation position.

Centric occlusionThe occlusion of opposing teeth when the mandible is in centric relation. They may or may not coincide with the maximum intercuspal position (GPT-8).

Maximal intercuspal positionThe 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, also called as maximum intercuspation.

Centric relation record A registration of the relationship of the maxilla to the mandible when the mandible is in centric relation. The registration may be obtained either intraorally or extra orally.

DEFINITION OF CENTRIC RELATIONApart from the extensive research in the field of centric relation, also unusual is the change that centric relation definitions have undergone.

There have been six updates since its first publication in the Journal of Prosthetic Dentistry (Glossary of Prosthodontic terms, 1956).According toGPT 1- The most retruded relation of mandible to maxilla when the condyles are in the most posterior unstrained position in the glenoid fossae from which lateral movement can be made at any given degree of jaw separation.

Centric occlusion: Not defined

According to GPT 3 The most retruded physiologic relation of the mandible to the maxilla to and from which the individual can make lateral movements. It is a condition that can exist at various degree of jaw separation. It occurs around the terminal hinge axis.

The most posterior relation of the mandible to maxilla at the established vertical relation.

Centric occlusion: The centered contact position of the lower occlusal surfaces against the upper ones; a reference position from which all other horizontal positions are eccentric.

According to GPT 4 The jaw relation when the condyle are in the most posterior unstrained position in the glenoid fossae at any given degree of jaw separation from which lateral movement can be made.

Acc to GPT 5 The maxillomandibular relationship in which the condyle articulate with the thinnest avascular portion of their respective disks 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 superiorly and anteriorly. It is restricted to purely rotatory movement about the horizontal axis.

Centric Occlusion: The occlusion of opposing teeth when the mandible is in centric relation. This may or may not coincide with maximum intercuspation position.

Acc to GPT 8 Same as GPT-5 + additional 6 definitions

By Boucher (GPT 8) -The most posterior relation of the lower jaw to the upper jaw from which lateral movements can be made at a given vertical dimension.

By Ash (GPT 8) -A maxilla to mandible relationship in which the condyle and disks are thought to be in the midmost, uppermost position . The position has been difficult to define anatomically but is determined clinically by assessing when the jaw can hinge on a fixed terminal axis (upto 25 mm). It is a clinically determined relationship of the mandible to the maxilla when the condyle disk assemblies are positioned in their most superior position in the mandibular fossae and against the distal slope of the articular eminence

By Ramfjord (GPT 8) -A clinically determined position of the mandible placing both condyles into their anterior uppermost position. This can be determined in patient without pain or derangement in the TMJ.

By Lang (GPT 8) -The relation of the mandible to the maxilla when the condyles are in the upper most and the rearmost position in the glenoid fossae . The position may not be recorded in the presence of dysfunction of the masticatory system

THE GLOSSARY OF OCCLUSAL TERMS INTERNATIONAL ACADEMY OF GNATHOLOGY 1979:Centric relation:The relation of mandible to maxilla when condyles are in their rearmost, uppermost, midmost position in the glenoid fossa.

Centric relation can exist over a range of jaw opening and is not violated until the condyle leaves their posterior positions in the glenoid fossae, the unstrained hinge position of the mandible.

CONTROVERSIES REGARDING CENTRIC RELATIONAnatomical Vs Significance based definition

The two definitions of centric relation from the GPT 4 and GPT 5 appear to contradict each other. The earlier definition mentions of a most posterior position of the condyles in the glenoid fossa, while the latter definition speaks of an anterior-superior position of the condyle against the slopes of the articular eminence. Surprisingly the discrepancy between RUM position and anterior position is only approximately 0.2 mm. Theoretically, the difference is only about condylar position of centric.

Centric is better defined as When it is required to select one mandible to maxilla/ condyle fossa relationship that is most conducive to comfort, function and health of odontostomato-gnathic system, then without any controversy it would be centric relation position. This is the functional definition of centric relation which gives an indication as to why centric is important.

The GPT definition is purely a morphological definition, which purely describes the location of condyle in centric which is controversial.The morphological definition is only a guide to indicate the status of the condyle and to support the functional definition of centric.

Centric relation is easily understood if functional definition precedes the morphological definition.

Functional tells us Why centric is necessary Morphological helps us to secure this functional position.

CHRONOLOGY OF CHANGE DEFINITIONS OF CENTRIC RELATIONMc Collum (1920) - rearmost condylar position

He showed that the condyle had a pure rotational movement when the operator guided the mandible in the most retruded position in the glenoid fossa. He was the first to name this position as centric relation.

Fig1: Diagram showing condyle in the rearmost position in the glenoid fossa

Granger (1962)- upmost ,rearmost position .A second component namely a most superior position was considered necessary for bracing since the condyle was unstable when it was only in the most posterior position .

Fig. 2: Diagram showing the condyle in the uppermost, rearmostposition in the glenoid fossa

Stuart (1969)- Rearmost ,uppermost, midmost condylar position-(RUM) position A medial component was added for a stable condylar position (three dimensional position). It was considered a physiological condylar position harmonious with centric occlusion. RUM position was later accepted by the International Academy of Gnathology.

American Equilibrium Society (1977)-It challenged RUM position as it was considered to give pressure on the retrodiscal tissue at bilaminar zone and proposed the most anterior and upper most position of condyle opposite the slope of articular eminentia.

Celenza(1978) Stated that condyle disk assembly braced superiorly and anteriorly against the posterior slope of articular eminence.

American equilibration society (1987)They revised their previous definition and believed that the condyle articulate with the thinnest avascular portion of the disc in the anterior, most superior position of the dorsal slope of eminence.

GPT 5 (1987)- The maxillomandibular relationship in which the condyle articulate with the thinnest avascular portion of their respective disks with the complex in the anterior superior position against the shapes of the articular eminences.

New anterior superior definition

GPT 7 (1999)- 6 other definitions came along with it. Whatever may be the difference in opinion regarding the precise location of condyle in glenoid fossa, clinically centric relation is farthermost, retruded position taken by the mandible without producing signs of strains on the temporomandibular ligament.

This is the functional position of mandible at centric relation condition.

Position of condyle in glenoid fossa when this functional position is reached CONTROVERSIAL

Controversies are: Most superior position Most retruded superior position Most anterior superior position

The controversy regarding the most physiologic position of the condyles will continue until conclusive evidence exists that one position is more physiologic than the other.

INTERPRETATION OF DEFINITION Understanding various terms used in definitions

The rearmost position is relative term which denotes that the condyles can go backwards as far as the temporomandibular ligaments would permit without any strain.

It does not literally means the most retruded position in the glenoid fossa, since such a position will produce considerable amount of strain in ligaments and cause pain.

The term Unstrained refers to strain of the ligaments and not the strain of the muscles since its the ligament which limits the mandibular movements and not the muscles and hence only ligaments can suffer strain if the head of the condyle is taken posteriorly beyond centric relation position.Many assume and believe that it is the strain of the muscles which retrude the jaw. This is not true. During normal contraction of muscle, strain always occurs. The closing and retruding muscles are under some degree of strain in centric relation as centric is a power position. Rest position of the jaws is the only position where there is minimum tonic contraction of the muscles and truly an unstrained position.

Centric relation is the most distal position of the head of the condyle without causing strain on the ligaments.

The most anterior superior position of the condyle is the position used by the head of condyle when the mandible is in its retruded position, from where there is an anterior superior bracing of condyle against the distal slope of articular eminence.Anterior superior bracing against the distal slope of articular eminence is an intra articular position which cannot be clinically visualized.

Analysis of GPT-1 definition

GPT 1 defines centric relation as the most retruded unstrained position of condyles in the glenoid fossa at any given degree of jaw separation from which lateral movements can be made.It is divided into three parts:

1. It is the most retruded unstrained position of the condyles in the glenoid fossa. This is included in definition since this position is constant and can be recorded as it is desired throughout the life. No other position anterior to this can be recorded twice. Since the individual assumes this most retruded position voluntarily by the action of his mandibular musculature, this position is unstrained.

2. .at any given degree of jaw separation This implies that centric relation can be recorded in any vertical position of the mandible from one of the extreme overclosure to one of overopening.

3. from which lateral movements can be made This implies that it is impossible for individual to make lateral movements when mandible is opened to its greatest extent and that there must be no `forced` retrusion- a point from which the individual can certainly make no movements.

Centric relationJPD 1952However Granger in 1952 called this definition of centric relation as inadequate since it fails to consider the axial relationship, which is the only reason for the importance of centric relation.

According to him any given point on the surface of the head of the condyle does not remain in fixed relation to the meniscus. In every position however, the hinge axis does remain in same relation to the meniscus. And the only position in which it is possible to locate and reproduce the hinge axis is centric relation.

Therefore centric relation is the terminal hinge position of the mandible, in which hinge axis is constant to both maxilla and mandible.

Analysis of CR definition by DawsonDawson has interpretated the definition of centric relation by dividing it into 5 parts:

The relationship of the mandible to the maxilla/ when properly aligned condyle-disk assemblies/ are in most superior position/ against the eminentiae/ irrespective of the vertical dimension or tooth position.

1. The relationship of maxilla to mandible... Importance of determining correct maxillomandibular relationship before analyzing and planning treatment enables us to know the importance properly mounted diagnostic casts.

Since centric relation is mandible to maxilla/ relationship that is most conducive to comfort, function and health of odontostomato-gnathic system, casts mounted in centric relation enables us to accurately determine what must be done to bring the teeth into this harmony. Ignoring the position of TMJ when examining the occlusion is not acceptable.Just putting the cast together in maximum intercuspation does not provide the necessary information on how mandibular teeth relate to maxillary teeth when condyles are in CR position. Nor does it show what must be done to achieve harmony between occlusion and TMJ.

Analysis of mandible to maxilla when condyles are in CR presents a completely different picture from maximum intercuspation.Now it becomes obvious why molar are loose or wearing excessively? If TMJ in not in CR they cannot accept firm loading with complete comfort.

2. properly aligned condyle-disk assembly... If condyle is off the disk, upward pressure loads the condyle directly onto the vascular, innervated tissue and cause a response of tenderness or pain.

3. against the eminentiae TMJ = Hanging joints that should not be subjected to loading forces because they overly compress the joint structures is a MISCONCEPTION. Rather contraction of the elevator muscles keep condyle-disk assemblies loaded throughout the functional movements. Hence TMJ are Load bearing joints. Also this is against the eminentiae because when all the elevator muscles contract to pullthe mandible towards the origination of each elevator muscle, the condyle will be pulled tightly against the eminentiae.This is how the muscle always works by shortening their length to pull the attached bone towards the site of muscle origin.

4. irrespective of tooth position or vertical dimension This implies that if the condyles are in centric relation, they can rotate on a fixed axis to an opening of 20mm. Thus a bite record made at any point of opening on the correct CR arc is still in centric relation.If the casts are mounted on an articulation with correct condylar axis, the vertical dimension can be increased or decreased without any error. Thus the false conclusion that condyles cannot rotate on a fixed axis has led some clinicians to discredit the face bow recordings and articulators, claiming that the vertical dimension cannot be accurately changed on an articulator.. A provably false belief.

5. most superior position Most important condition to understand about CR is that in centric relation, the properly aligned condyle-disk assemblies are completely seated in the most superior position in their respective sockets.

ANATOMY OF TMJ AS IT PERTAINS TO CENTRIC RELATION

Cross-section of the Temporomandibular Joint

Temporomandibular JointIt is the articulation between the temporal bone and the mandible.The major components of the temporomandibular joint are the cranial base, the mandible and the muscles of mastication with their innervations and nerve supply. An articular disk separates the mandibular fossa and articular tubercle of the temporal bone from the condylar process of the mandible.

The compound synovial joint occurs between the squamous part of temporal bone and the mandibular condyle. A complete intra articular disc separates the two bones, matches the contour of their articular surface and subdivides the joint space into two synovial compartments.

Upper compartment: Arthroidal / gliding joint: Between the mandibular fossa and condyle-disk assembly. Lower compartment: Ginglymoid/ hinge joint: Between Condyle and articular disk

Compartments of the temporomandibular joint

Inferior compartment: Joint is pure ball and socket joint. Since disc is tightly bound to condyles by lateral and medial discal ligaments the only movement possible is rotation. Superior compartment: Lateral movements are accomplished by the sliding ball and socket joints as a unit on the glenoid fossa.This is because disc is not tightly attached to the articular fossa, so free sliding movements is possible between these surfaces Translation.In use, these two movements always occur simultaneously, however one joint may describe the gliding and rotational movements while other only rotational movements.

The centre of rotationIn each of these rotatory movements, the condyle moves or rotate about an axis.In pure vertical motion it revolves about a horizontal axis, in pure horizontal rotation it revolves about a vertical axis. In intermediate rotation it revolves about an axis at right angles to plane of rotation. All these axis meet at a point within the condyle. This is Centre of rotation.

When these points in two condyles are connected by an imaginary line, it forms HINGE AXIS.

Since hinge axis is located within the condyle, during any bodily movements of the mandible the hinge axis move along with it. If mandible is protruded by moving the meniscus upon the glenoid fossa, the hinge axis remains in same relation to the meniscus but not to the glenoid fossa. So hinge axis is said to be constant to the mandible.There is however, one and only one position in which hinge axis is constant to both mandible and maxilla. This is centric relation position.

In Centric relation position, mandible is opening and closing with pure rotatory movements in vertical plane around the hinge axis.Purely rotational movement, around the horizontal axis till the patient opens his mouth to about 20-25mm.This axis is called TERMINAL HINGE AXIS.Since this axis is constant to the mandibular teeth in every eccentric position and constant to both mandible and maxilla in centric relation, the axial relation of the teeth will be correct in every mastication procedure.The only position of the mandible in which pure rotatory movement can occur is that in which the meniscus is in the tough of the fossa, in the most retruded position to which it can be carried by patients own musculature.

GLENOID FOSSAThe mandibular condyle articulates at the base of the cranium with the squamous portion of the temporal bone.This portion is made up of Concave Mandibular Fossa called as ARTICULAR OR GLENOID FOSSA.

SQUAMOTYMPANIC FISSURE Posterior to mandibular fossa.Anterior to fossa convex bony prominence called ARTICULAR EMINENCE.Structure of the glenoid fossa

Anterior portion: This portion of the fossa is the principle bearing surface upon which the condyle presses through the disk and other structure.

Posterior portion: This portion of the fossa is more nearly perpendicular. The condyle does not bear directly in the fossa because it is separated by the synovial membrane and the articular disc. Middle part of the fossa is a fairly thin plate of bone whose upper surface forms the middle cranial fossa.

Immediately anterior to the fossa is a convex bony prominence called the articular eminence.

The degree of convexity of the articular eminence is highly variable but important because the steepness of the surface dictates the pathway of the condyle when the mandible is positioned anteriorly. The posterior roof of the mandibular fossa is quite thin indicating that this area of the temporal bone is not designed to sustain heavy forces. The articular eminence however consists of thick dense bone and is more likely to tolerate such forces.

ARTICULAR DISCArticular disc is composed of dense fibrous connective tissue devoid of nerves and blood vessels. This allows it to withstand heavy forces without damage or inducement of painful stimuli.The purpose of this disc is to separate, protect, and stabilize the condyle in the mandibular fossa during functional movements.

In SAGITTAL PLANE it is divided into 3 regions (according to thickness).1. Anterior 2. Posterior zone 3. Intermediate zone Posterior border is slightly thicker than anterior border; intermediate is the thinnest area of the disc.

LIGAMENTS OF THE JOINTLigaments do not enter actively into joint function but instead act as passive restraining device to limit or restrict border movements.

These include: Collateral (Discal) ligaments: They function to restrict the movement of the disc away from the condyle. So they permit the disc to be rotated anteriorly and posteriorly on the articular surfaces of condyle, hence responsible for hinging movement of temporomandibular joint.

Capsular ligament: This ligament acts to resist any medial, lateral or inferior forces that tend to separate or dislocate the articular surfaces.

Temporomandibular ligament: It is composed of two parts : Outer oblique part which resists excessive dropping of the condyle, therefore limiting the extent of mouth opening. It also influences the normal initial phase of opening, when condyle rotates around a fix point until this ligament becomes tight.

Beyond this if mouth is to be opened, the condyle needs to be moved downward and forward across the articular eminence.

In the erect posture and with vertically placed vertebral column, continued rotational movement can cause the mandible to impinge on vital submandibular and retromandibular structures of the neck. The outer oblique portion of TM ligament resists this impingement.

The inner horizontal portion of the TM ligament limits posterior movement of condyle and disc thereby preventing trauma to retrodiscal tissues.

Stylomandibular ligament : It limits excessive protrusive movements of the mandible.

ROLE OF MUSCULATUREPositional stability of the temporomandibular joint is determined by the muscles that pull across the joint and prevent dislocation of the articular surfaces. The directional forces of these muscles determine the optimum orthopedically stable joint position.

The major muscles that stabilize the joint are the elevators.

Muscles of mastication involved in centric positioningTemporalisThe functions of the posterior part of these muscles are to retrude the mandible and brace the condyle during lateral mandibular excursions.

The function of the middle parts is to elevate the mandible into centric position.

Masseter muscle The principle function of the masseter is to elevate the mandible vertically in order to obtain maximum intercuspation. The deep portion of the masseter muscle helps to retrude the mandible.

Medial pterygoid muscleIt is similar to the superficial masseter in fiber direction, and these two muscles function synergistically to form the muscular sling.

Lateral pterygoid muscleBesides its important role in opening of the jaws and protrusion of the mandible lateral pterygoid also has an important role in determining the position of the condyles relative to the eminence, and limits the degree of condylar retrusion.

Inferior lateral pterygoid muscle positions the condyles anteriorly against the posterior slopes of the articular eminences.

Superior lateral pterygoid remains inactive during mouth opening, it becomes active only in conjunction with the elevator muscles. These muscles are primarily responsible for joint stability and position.

RELOCATION OF CENTRIC RELATION POSITION OF THE CONDYLES FROM RUM TO THE ANTERO-POSTERIOR POSITION

The main reasons for the shift in focus from the rearmost, upmost and midmost position of the condyles to the anterosuperior position are as follows:

The roof of glenoid fossa is extremely thin and translucent. There is no articular cartilage in the glenoid fossa, with minute foramina for the passage of blood vessels and nerves. The thickened posterior zone of the articular disc occupies the glenoid space. This portion contains blood vessels and nerves and therefore is not suited for function of articulation. On the other hand, the superior portion of the condylar head is covered with articular cartilage extending forward over the anterior face of the condyles and is designed for stress. Similarly, the bony trabeculae on the curved surface of the posterior portion of the eminentia are oriented parallel to the direction of the articular eminence to withstand stress. The center of the articular disc that is interposed between the condyle and the posterior slope of the articular eminence is devoid of nerves and blood vessels, indicating a stress bearing portion of the functional area of the disc. On the other hand, the non-stress bearing thick periphery of the disc is rich in blood vessels and nerves. The disk is thickest posteriorly (2.9 mm) and thinnest in the middle part (1mm).

Hence the posterior portion of the eminentia articulating with the thin intermediate zone of the articular disc opposed by the anterior face of the condylar head appears to be the most logical functional arrangement for centric positioning of the condyles.

Thus the RUM position is not physiological to joint and superior anterior bracing of the condyle disc assembly against the slope of eminentia was the optimum condylar position in centric (Celenza, 1973).

IMPORTANT FEATURES OF CENTRIC RELATION

It is the ideal arch-to-arch relationship and hence optimum position of jaws for the health, comfort and function of the TMJ.

It is a retruded mandible position where the condyles are situated anterosuperiorly in the glenoid fossa as far as the ligaments of the TMJ and musculature would permit.

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. It is a starting point for the arrangement of artificial teeth to develop maximum intercuspation in complete dentures.

It also serves as a reference position for the institution of occlusal rehabilitation in dentulous conditions.

It serves as a reference position to relate and nomenclate several occlusal positions of upper and lower teeth. The terminal position of masticatory stroke end in centric relation. It is also a position where upper and lower teeth are braced against each other during deglutition.

It is a relationship of mandible to maxilla when both the condyles are in terminal hinge location. It is a position of terminal hinge closure.

SIGNIFICANCE OF CENTRIC RELATION Correct registration of centric relation is essential in construction of complete dentures. Many dentures fail because the occlusion is not planned or developed in harmony with this position. The maxillo- mandibular musculature is so arranged that a patient can easily move his mandible into centric relation. Thus CR serves as a reference relationship for establishing an occlusion. When the CR-CO of artificial teeth do not coincide or a freedom in centric is not present, the stability of the denture bases is in jeopardy and the edentulous patient is subjected to unnecessary pain or discomfort. Components of the masticatory system are the functional unit. In edentulous subjects the dental components are lost. Dentures restore the masticatory functioning, phonetics provided they are made at specific vertical and horizontal relation of the mandible to the maxilla. Unless these relations are properly ascertained, recorded and transferred to the articulator, the prosthesis may fail.The human mandible can be related to the maxilla in several positions in the horizontal plane. Centric relation relates the dentulous or edentulous mandible to maxilla in a position when teeth, muscles and temporomandibular joint function in harmony. It is a position of occluso-articular harmony. CR is the horizontal reference position of the mandible that can be routinely assumed by edentulous patients under the direction of the dentist. This makes it possible to verify the relationship of casts on the articulator when they are mounted in Centric Relation. Patients use CR closures in mastication and other activities, such as swallowing. When a bolus a food is prepared for swallowing, the teeth attempt to masticate it so that it has a semi fluid consistency as it passes into the esophagus. To do this, it is necessary to apply strong muscular force against the bolus. At this time, the condyles follow the paths of movement that the anatomic structure of the joint dictates, i.e., in an upward and backward direction. The condyle tries to seat itself in the glenoid fossa as far as it will go by its own muscular power. If the teeth intervene before this position is reached, there is a lateral component of force registered upon the teeth which subsequently causes pain in temporomandibular region.

The degree of this lateral force is directly proportional to: 1. The amount of force applied by the muscles of mastication 2. The degree the jaw is out of centric relation.

This signifies importance of accurate centric relation recording. Therefore the casts must be mounted on the articulator in this position so that the opposing teeth on complete dentures will meet evenly when the patient closes in CR.

Cohen;JPD; 1960; 10; 248Why is it that not all patient whose centric relation and centric occlusion are inharmonious are not heir to all the pathologic changes attributed to this condition?

There are several reasons:

The degree of tolerance in joint which permits the condyle to be out of its ideal position in final closure of mandible. Morphologic point of view, there are three types of glenoid fossa :

Type 1 anterior slope of the fossa is very light. Not much lateral component of force on the teeth or a pressure on the border of the meniscus .This type of joint has the greatest degree of tolerance.

Type 2 this is most generally encountered. The anterior wall has approximately a 30 degree inclination to the axis orbital plane. This joint has little tolerance.

Type 3 found occasionally. It has a very steep anterior wall and has no tolerance. This type of joint causes the most trouble since any slight eccentricity of the maxillomandibular relationship causes a pressure on the borders of the meniscus.

Granger. JPD; 1952; 2; 160He stated that centric relation is the terminal hinge position of the mandible which establishes the relation of the axis of the condyles to the teeth as they will close with muscular force against the resistance of a bolus of food in every contacting position.

As the teeth meet, they interdigitate, and pressure is exerted along their long axis. The hinge axis determines the arc on which they close and is related to the curve of the cusps. In the case of full dentures the proper centric closure seats and holds the denture firmly in place.

CR must be recorded correctly to permit accurate adjustments of the condylar guidance of the articulation for other eccentric movements. Condylar guidances are adjusted to form a pathway of condyle movement from a beginning point to the position of eccentric occlusal record. CR is the accurate beginning point for these other articulator adjustments.

HOW CR IS ACQUIRED FROM BIRTH?

Centric relation is learned after the teeth erupt. It is the first learned reflex determining the occlusal position of the mandible after the primary dentition is complete. Eccentric mandibular positions are learned as expedient mechanisms for avoiding occlusal disharmonies.

The mandible is moved and supported by a group of muscles, most of which receive their innervations from the fifth cranial nerve. They are classic examples of antigravity muscles.

When all the muscles capable of moving the mandible demonstrate no contractions other than those necessary to hold the bone in a balanced position against gravity, a state of equilibrium is maintained. The physiologist calls this the postural position of the mandible. The dentist calls it the physiologic rest position of the mandible.

Only this postural position is consistently observed prior to the eruption of the teeth. An occlusal sense develops as the erupting primary teeth first meet their antagonists of the opposite jaw. This occlusal sense is the formation of the neuromuscular reflex establishing centric relation.

After the teeth have erupted, the muscles learn one position of occlusion providing a maximum of occlusal contact and minimum of torque or lateral stress and strain on the roots of the teeth. This is the beginning of centric.

At the beginning, centric relation and centric occlusion are identical. Centric relation is the first established neuromuscular reflex concerning mandibular position when the teeth are in occlusion.

The anteroposterior limits of centric relation are defined first, since the primary incisors erupt first and restrict mandibular movements in this one direction only. Later, the teeth in the lateral segments of the dental arch inhibit mediolateral positioning, and thus help localize the limits of centric relation in this other direction. The vertical limits of centric relation are never so precisely defined.

CONCEPTS OF CENRTIC RELATON POSITION

There are two concepts: Douglas Allen Atwood , JPD ;1968;20 ;21 Anatomic Pathophysiologic

Anatomic: Centric relation is most retruded relation. A border position determined by the ligaments. Pathophysiologic: Centric relation is the most posterior unstrained jaw relation . A position which is not a border position and is established by muscle action .

S. David and R.M.J Gray; 2001; BDJ; 191; 235.Described centric relation three different ways , 1. Anatomically 2. Conceptually 3. Geometrically

Anatomically describe centric relation as position of the mandible to the maxilla with the intra-articular disc in place, when the head of the condyle is against the most superior part of the distal facing incline of the glenoid fossa.

The bone and fibrous articular surfaces are thickest in the anterior aspect of the head of the condyle and the most superior aspect of the articular eminence of the glenoid fossa.

Conceptual describes centric relation as that position of the mandible relative to the maxilla, with the articular disc in place, when the muscles that support the mandible are at their most relaxed and least strained position. This definition supports the concept of a qualitative relationship between a jaw position and another element of the articulatory system.

Geometrical described centric relation as position of the mandible relative to the maxilla, with the intra- articular disc in place, when the head of the condyle is in terminal hinge axis.Mandible opens firstly by a rotation of condyle and then a translation. Therefore, while closing in terminal closure it is purely rotational. At this phase of closure the mandible is describing a simple arc, because the centre of its rotation is stationary. This provides the terminal hinge point (of rotation) of one side of mandible; but because the mandible is a bone with two connected sides these two terminal hinge points are connected by an imaginary line:the terminal hinge axis.

CENTRIC RELATION AND CONDYLAR MOVEMENT

Anatomic mechanism of the centric relation is unknown.

Several theories have been given , the most accepted ones are :

THE MUSCLE THEORY THE LIGAMENT THEORY THE OSTEOFIBER THEORY THE MENISCUS THEORY

THE MUSCLE THEORY According to this theory, the centric relation is considered to be product of a defense reflex which causes the external pterygoid muscle to contract and thus halt the jaw every time the condyles or the interarticular disc approach the posterosuperior depth of glenoid fossae.

DRAWBACKS It could not explain the fact that centric relation is always the same at any vertical level. No anatomic explanation is provided for the posterior hinge movement, nor for the acuteness of the needle point tracing . If external pterygoid muscles were responsible for CR, the hinge axis would then go through anterior cervical insertion of these muscles, which it does not do, and needle point tracing would be elliptical. Woelfel, Hickey and Rinear found that the external pterygoid muscles are relaxed when the mandible is in centric.

THE LIGAMENT THEORY Given by Ferrein Acc to this theory, when the ligaments become tense they determine the limits of the retrusive movement. They are also capable of determining the terminal border movements.

When condyle within glenoid fossa is fully retracted, retrocondylar space behind it can be observed. It is possible to insert a surgical instrument in front of the tympanic plate and touch the posterior glenoid roof without disturbing the condyle from retruded position.Therefore the posterior wall of glenoid fossa does not constitute the condylar stop.

When condyles in centric relation are seen in lateral radiograph , it appears to be suspended or floated. These views reinforce the theory that soft radiographically translucent tissue is determining the final condylar position, and TMJ ligament are fulfilling these condition.The temporomandibular articulation in centric relation.

However, Ferrin, Posselt and Arstad found in cadavers, that temporomandibular ligament were tense when the jaw is in terminal retruded position.

DRAWBACKS The ligamentous fibers and the direction of the condylar sagittal path form an angle of almost 90 degree. This anatomic arrangement of temporomandibular ligament is not suited to halt the retrusive condylar movement. Ligamentous retrusive terminal stop provides no satisfactory location of the hinge axis.

Indeed, if the temporomandibular ligaments constitute the retrusive terminal stops, the posterior hinge opening must have its axis formed by the line connecting the ligamentous fiber insertions at the condylar necks. Nevertheless, the hinge axis appears to be centrocondylar .(Saizer and Rothman;McCollum)

Boucher (JPD, 1961, 11, 23)He found that the centric relation mandibular position does not change in the cadaver after section of the capsular ligament.

This theory does not explain satisfactorily the lateral border movements, because it cannot produce an acute gnathographic angle.It registers an elliptical tracing.

THE OSTEOFIBER THEORYGiven by Meyer. Acc to this theory, a retrusive terminal stop is formed by the soft tissue of the posterior part of the roof of the glenoid fossa. These fibrous stop acts as a buffer.

DRAWBACK Sicher(1965) pointed out that there is a thick layer of loose and vascularized connective tissue posterior to the condyles. Such tissue is readily adaptable to movement, either forward or backward. Rather than being a protector, such tissue needs to be protected.

THE MENISCUS THEORY Given by saizer The articular disk has definite zones. The thinner centric bearing area and the thicker anterior and posterior bands and the bilaminar zone.

The central bearing area of the inter-articular tissue remains interposed between the condylar articular surface and the articular eminence during simulated jaw movements.The bearing area is composed of densely woven collagen fibrils having no vascularity or innervations which indicate that zone is adapted to accept pressure.

Thickened posterior band possesses vascularity and innervations. Because articular eminence is an inclined plane, condyle disk assembly must be stabilized on this slope by muscular activity unless it is in a position of muscular equilibrium.The posterior movement of condyle on the eminence has been attributed to wedging of the thickened posterior band of disk between the distal surface of the condyle and the roof of the articular fossa.

The innervated posterior band possibly protects (by sensory feedback) the thin roof of the articular fossa from heavy pressure and provides a biomechanically stable relationship. It appears that any position posterior to this limit cannot be functional, as the condylar articular surface cannot engage the central bearing area of the disk and the eminence; nor can the position be biomechanically stable .

CENTRIC RELATION AND CENTRIC OCCLUSION

Centric occlusion (GPT 8) - the occlusion of opposing teeth when the mandible is in centric relation.

The understanding of centric relation is complicated by failure to distinguish between centric relation and centric occlusion.

Centric occlusion is a tooth-to-tooth position whereas centric relation is bone to bone relation. Centric relation serves as a reference position or baseline to nomenclate the various occlusal positions. Both may or may not be identical to each other.

Numerous studies have reported that the majority of patients with a natural dentition show discrepancy between the occlusal position of the mandible in CR and MI. This discrepancy is present in atleast 90% of dentitions.

In person with natural teeth, both centric relation and centric occlusion exist. After the removal of teeth, centric occlusion is lost, while centric relation remains and serves as a reliable guide to develop centric occlusion in artificial dentures.

In dentulous individuals, occlusion in centric relation (RCP retruded contact position) is not and need not be centric occlusion, although it would be ideal to have centric occlusion at centric occlusion.

In edentulous individuals however it is feasible that centric relation and centric occlusion are made to coincide.

When centric occlusion does not coincide or is not identical with centric relation, the condyles do not remain in their upper most position in the glenoid fossae, but take a position either anteriorly or laterally. This referred as centric slide.

Clinically, the difference between the two occlusal positions can easily be determined by closing the mandible in its CR position by manual guidance until the first tooth contact is established. This used to be called the retruded contact position (RCP) for many years and is now called centric relation contact position (CRCP).

The significance of the discrepancy is based on the presence of premature contacts, so that the patient is only able to find a stable occlusal position during closure in centric relation by sliding into MI. Premature contacts in general, and premature contacts during closing in CR in particular, might be trigger points for para-functional activities like clenching and bruxism. When the intercuspation of the teeth is in harmony with both correctly positioned and aligned condyle-disk assemblies, centric relation and centric occlusion are the same. This is the goal of occlusal treatment.

CENTRIC RELATION SHOULD COINCIDE CENTRIC OCCLUSION

In natural dentition, tooth interferences in centric relation initiate impulses and responses that direct the mandible away from deflective occlusal contacts into centric occlusion.

Impulses created by the closure of teeth into centric occlusion establish memory pattern that permit the mandible to return to this position without interferences. When natural teeth are lost, many receptors that initiate impulses resulting in positioning of the mandible are lost or destroyed.Edentulous patients cannot control mandibular movements or avoid deflective occlusal contacts in centric relation in the same manner as the dentulous patient can.

Deflective occlusal contacts in centric relation cause movement of denture bases and displacement of supporting tissues or direct the mandible away from centric relation . The centric relation must be recorded for edentulous patient so that centric occlusion can be established in harmony with centric relation .

CR not in harmony with CO

When mandible is in CR, opposing tooth do not contact evenly.

CR is not harmony with CO

For opposing teeth to meet evenly as in CO, the mandible must be moved away from CR

CR in harmony with CO

This can usually be achieved with centric relation and centric occlusion coinciding. In some patients a broader area of stable contacts near centric relation is necessary ,which is called freedom of centric or long centric

Posselt and Glickman reported that maximal intercuspal relation of the teeth is anterior to terminal hinge postion in 90% of analyzed individuals with full complement of teeth.

Posselt (JPD 1971/25/12) - centric occlusion placed the mandible an average of 1.2 mm anterior to its position in centric relation.

Beyron (DCNA1971, 15, 4) only 10 % of the individuals the centric relation and centric occlusion coincide. Centric occlusion occurs anterior to centric relation at varying but short distance.

William E Avant (1971) when no occlusion of teeth (natural /artificial) is involved then both centric relation and centric occlusion lose their significance. Yahia H.Ismail (JPD 1980 ,43 , 327 ) in centric occlusion condyles were centrally located antero-posterior in their fossae with equal anterior and posterior joint spaces .

REVIEW OF LITERATURE

Centric PositionRobinson JPD; 1951; 1; 384He stated that centric position is not only the maxillomandibular relation where the teeth should occlude in normal or good functional situation, but also where the condyle of mandible is in a balanced and unstrained position in the mandibular fossa. This position exists when the anterosuperior surface of the condyle is in close approximation with the posterior-inferior surface of the articular eminence.

Centric relationGranger. JPD; 1952; 2; 160He stated that centric relation is the terminal hinge position of the mandible which establishes the relation of the axis of the condyles to the teeth as they will close with muscular force against the resistance of a bolus of food in every contacting position.

As the teeth meet, they interdigitate, and pressure is exerted along their long axis. The hinge axis determines the arc on which they close and is related to the curve of the cusps. In the case of full dentures the proper centric closure seats and holds the denture firmly in place.

Physiologic jaw relations and occlusionShanahan JPD; 1955; 5; 319-324He stated the constant function of swallowing saliva is the basis for establishing the mandibular positions and occlusion.

In swallowing saliva, the mandible rises to its habitual closing terminal, then, as the saliva is swallowed mandible is forced backwards into the pharynx by the tongue, thus retruded to its physiological centric relation.

Stuart JPD;1960;10;304He stated that the success has been judged by how well the teeth close in centric occlusion, how well they balance bilaterally and protrusively.

If the teeth passed these tests, the examiner would know that the total setup of the cusps was identified with that basic jaw position called centric relation. Such balanced occlusion cannot be attained if started from a mandibular occlusal position not centrically related.Hinge axis and its practical application in determination of centric relationCohen ;JPD;1960;10;248The center of meniscus is devoid of blood vessels and nerves and is pressure bearing. Due this fact every joint has a degree of tolerance which permits the condyle to be out of its ideal position in final closure of mandible.

Morphologic point of view,there are three types of glenoid fossae.

Type 1 anterior slope of the fossa is very light. No much lateral component of force on the teeth or a pressure on the border of the meniscus .this type of joint has the greatest degree of tolerance.

Type 2 this is most generally encountered. The anterior wall has approximately a 30 degree inclination to the axis orbital plane .this joint has little tolerance.

Type 3 found occasionally. It has a very steep anterior wall and has no tolerance. This type of joint causes the most trouble since any slight eccentricity of the maxillomandibular relationship causes a pressure on the borders of the meniscus.

Anatomy of TMJ as it pertains to centric relationJamieson ;1962;JPD;12;473The indefinite apex of needle point tracing is often the result of degenerative changes in the structure of the temporomandibular joint which permit a greater latitude in movement. A degenerative change may occur in any part or all of the neuromuscular mechanism.

The indefinite apex may be an indication of acquired habitual mandibular movement which are the result of malocclusion of remaining natural teeth. Many of these patients, when edentulous for a period of time and conditioned by exercise of the muscles of mastication, will be able to produce more accurate tracing.

Anatomy of TMJ as it pertains to centric relationBoucher JPD; 1962;12;464He stated that centric relation is controlled by neuromuscular reflex which does not necessarily always function in the same anteroposterior position.

The terminal hinge position and the apex of the needle point tracings of the retruded mandible may be desirable positions from which to start the construction of dentures because they are reference positions, but this does not imply that it may be the ideal functional position of the mandible for all patients.

Shahahan and Leff 1963;JPD;13;871The theory that the mandible rotates about vertical axis in the region of condyles during lateral movements was investigated using central bearing plates.

They concluded that use of a central bearing point produces unnatural influences upon the lateral movements of the mandible.

Study of the mandibular movement from centric occlusion to maximum intercuspationLester Clark JPD;1967;18;19They concluded that: The mean anteroposterior component of slide measured with a position gnathometer was 0.440.54mm The mean vertical component was 0.470.64mm The mean lateral component of slide was 0.010.29mm

Radiographic study of condylar position in centric relation and centric occlusion Yahia H Ismail; JPD;1980;43;327 He radiographically determined the spatial differences in the condyle-to-fossa relationship when the mandible is in the centric relation and centric occlusions.

It was concluded that in centric relation position , both condyles were placed more posteriorly and superiorly in their fossae than in centric occlusion position .

In centric occlusion position , both condyles were symmetrically placed in their fossae with equal spatial distance anteriorly and posteriorly.

Greater spatial difference existed between the centric occlusion and centric relation positions on the left side , which was the orbiting (balancing ) side in most subjects .

Dawson 1995;JPD;74;619He defines adapted centric posture as the relationship of the mandible to maxilla that is achieved when deformed temporomandibular joints have adapted to the degree that they can comfortably accept firm loading when completely seated at the most superior position against the eminentiae.

Like centric relation , adapted centric posture is a horizontal axial position of the condyles. It occurs irrespective of vertical dimension or tooth contact .

It is also a midmost position, because even if the disk is totally displaced, the medial pole of the condyle adapts to the concavity of the fossa and maintains contact against its medial pole .

Condylar movement and centric relation in patients with internal derangement of the temporomandibular joint.Harper 1996;JPD;75;67Based on his study he concluded that centric relation in normal TMJ include a dynamic range of horizontal adaptation to the potential biomechanical and biologic stresses related to oral function.

The centric relation position or deranged reference position of the condyle in patient with TMJ internal derangement is a static position with decreased potential for adaptation.

SUMMARY

The term centric has been known in dentistry for many years and no other word in dentistry has been a source of controversy for years together, as has been the term centric.

The human mandible can be related to the maxilla in several positions in the horizontal plane. Among these, centric relation is a significant and important position. It is a position of occluso-articular harmony.

The two definitions of centric relation from the GPT 4 and GPT 5 appear to contradict each other. The earlier definition mentions of a most posterior position of the condyles in the glenoid fossa, while the latter definition speaks of an anterior-superior position of the condyle against the slopes of the articular eminence. Functional definition tells us Why centric is necessary Morphological helps us to secure this functional position.Position of condyle in glenoid fossa when this functional position is reached CONTROVERSIAL Most superior position Most retruded superior position Most anterior superior position

Based on understanding of anatomy and biomechanics of TMJ it is presently accepted that the RUM position is not physiological to joint and superior anterior bracing of the condyle disc assembly against the slope of eminentia was the optimum condylar position in centric.

Centric relation is a reference point for establishing the occlusion. It is the most retruded position of mandible in glenoid fossa which is reproducible and repeatable.

Centric relation can be described anatomically, conceptually and geometrically.

Anatomic mechanism of centric relation is explained through 4 theories : The muscle theory, The ligament theory, The osteofibre theory & The meniscus theory.

Positional stability of TMJ at centric relation is determined by elevator muscles, however it is the ligaments which act as passive restraining devices for the border movements.

Centric relation is bone to bone relation, while centric occlusion is tooth to tooth relation.

When the intercuspation of the teeth is in harmony with both correctly positioned and aligned condyle-disk assemblies, centric relation and centric occlusion are the same. This is the goal of occlusal treatment.

CONCLUSION The term centric relation has been used in dentistry for many years. Although it had a variety of definitions, it is generally considered to designate the position of mandible when condyles are in the terminal hinge position. Centric relation is the most reliable reference point obtainable in edentulous patient for accurately recording the relationship between mandible and maxilla and ultimate for controlling the occlusal contact pattern. The determination of centric relation is rightly considered one of the most important steps in complete denture construction.

It is apparent from the dental literature, that there are many opinions and much confusion concerning centric relation records. In normal cases, the occlusion, the temporomandibular joints, the bone, the soft tissue, and the musculature all produce the same relation to each other and any one of the many registration techniques may be used. A certain technique might be required for an unusual situation or a problem patient. In the final analysis, the skill of the dentist and the cooperation of the patient are probably the most important factors in securing an accurate centric relation record.

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