Concept of Long Centric Occlusion
Confusion exists over the term “centric occlusion”.
It is a term generally accepted as any contact between incising or masticating surfaces of upper and lower teeth.
An understanding of occlusion must be based on knowledge of the masticatory system and its function and dysfunction. Research on occlusion in recent years has given us a deeper insight into the physiology of this system and its functional adaptation. The masticatory system has ling been recognized as a functional unit, but the general view of the interaction between the occlusion of the teeth , their supporting tissues, the muscles, and the temporomandibular joints seems often in the past to have been based on a predominantly mechanical conception .
As far as back as the literature goes on this subject, there has been constant controversy and heated discussion as to what is the proper relationship of the occlusion to the various positions the mandible can assume.
To some persons, it is synonymous with maximum intercuspation of opposing teeth, while to others it means contact of the teeth when the mandible is in centric relation (CR), which is defined by the “Glossary of Prosthodontic term” as “the most retruded physiologic relation of the mandible to the maxilla to which and from which the individual can make lateral movements.
It is a position which can exist at various degrees of jaw separation and it occurs around a hinged axis. It is this position that many dentists use to re-establish a blemished occlusion, whether it be by grinding the natural teeth or by rebuilding the crowns of the teeth.
Other dentists claim that this is not the reference position at which teeth should be made to occlude. They claim that a slightly forward muscular guided position is where the teeth should be occluded. In maximum contact i.e. acquired centric or habitual centric position.
The work of posselt on the movements of the human mandi ble and the subsequent research of Beyron, Glickman(1968), Sheppard and Sheppard (1971), Celenza(1985),Dawson(1985) and Hobo and Iwata(1985)have shown that centric relation is a physiological relationship of the mandible (moveable part)to the maxilla(fixed part of the cranial base)., when the condyles are properly related to their articular discs and these condyle disc units are stabilized and braced against the posterior steps of the articular eminences of the glenoid fossae.
Now if during centric closure the condyles are deflected to positions other than centric relation as a result of deflective occlusal contacts ., it is referred to simply as centric occlusion(CO) and their deflection from CR to CO (habitual occlusion, acquired centric ) is known as a centric slide.
This centric slide usually occurs in the sagittal plane with the centric occlusion being slightly anterior and cranial to centric relation
These all contribute to functional malocclusion. About 90% of all people exhibit centric slides.
Therefore it is safe to say that about 90% of the population does not have coincidence between centric relation of the condyles and centric occlusion of the teeth. Thus this population has functional malocclusions.
The posterior guide the growth of directional movements of the mandible during final centric closure as the interfering inclined planes of the opposing teeth slide against each other. This is a tooth guided occlusion rather than a muscular or condylar guided occlusion and while it is not desirable from the functional standpoint, it may be harmful in many instances.
On an empirical basis, Beyron, Schuyler,and Pankey Mann found that if this slide between centric relation and centric occlusion is eliminated by providing a flat horizontal area at centric occlusal level , patient can easily move the Mandible back and forth according to the need and without having the mandible in interlocked intercuspal position and thus giving comfortable, harmless ,occlusion from functional standpoint and neuromuscular harmony.
The mandibular position which is known to be reproduced consistently by the dentulous patient , however, upon the loss of teeth and thereby proprioceptive information from them and the periodontium has been demonstrated not to show consistent results of mandibular position reproducibility without tooth contact , there is no lasting neuromuscular memory to guide the mandible to this position.
In fact this position if registered may show the significant change when dentures are inserted and the patient realizes, that there are occluding posterior teeth. The mandible will then move posteriorly to a position very close to the retruded border position.
It seems logical from the anatomy of the temporomandibular joint, the inclination of the glenoid fossae, the function of the muscles of mastication that stable occlusion should be designed to respect this posterior border position.
A free gliding non interfering occlusions from this point to any comfortable muscular position is not a difficult problem. Thus freedom from the precise posterior border position to an area of stable occlusion has given rise to the term “long centric”.
Long centric term implies that the patient has got the freedom to move his mandible back, back or forth in centric relation or centric occlusion at the same vertical dimension in antero-posterior direction (fig 1)
However freedom in centric is not just an antero-posterior direction. There is something known as “wide centric”. Also this implies that there is a freedom for the mandible to move in lateral dimension also and not just anteroposteriorly.
Therefore , freedom in centric is a flat area having both antero- posterior dimension .called as “long centric” and lateral dimension known as “wide centric”. (Fig 3)
The problem arises when few edentulous patients do not provide consistent jaw relation ,may be due to neuromuscular disharmony or patient has developed the habit of chewing anterior to the position selected from maximum intercuspation.
Now if we use anatomical cuspal teeth in maximum intercuspation at a centric relation position , and because of the interlocking effect of cusps , deflective contact results which ultimately leads to unsettling or movement of denture base and thus irritation and inflammation of supporting mucosa.
To deal with this kind of problem, non anatomical or cuspless teeth were introduced to allow teeth contact anywhere along the occlusal plane anteroposteriorly. Now the patient has got freedom to maintain intercuspation either in centric relation or anterior to it. This results in elimination of inclined plane forces, which create horizontal displacement of bases during function. But there are few disadvantages of monoplane occlusion using non anatomic teeth that lack esthetics, inability on part of patient to penetrate or shear food easily, more vertical forces of supporting mucosa etc.
Keeping all the facts in view, we can say that providing monoplane occlusion with cuspless teeth can be regarded as another option if we do not want to give “freedom in centric” for cuspal teeth in few edentulous patients according to situation needs.
CONCEPT OF LONG CENTRIC
When the jaws are firmly closed in the absence of occlusal interference to CR, the elevator muscles pull each condyle –disc assembly into its uppermost position,ligament, and bone and against the prominent distal lip of the disc. At this most superior, apex of force position, the small lateral pterygoid muscles are not required to resist the triad of much larger, stronger closing muscles( Masseter, Temporalis , and Medial Pterygoid).
This most retruded position of superiorly braced condyles permit the joints and their particular musculature and function comfortably under great pressure when the elevator muscles are contracted with force. Teeth that are perfectly harmonized to centric relation will not interfere with this braced position of either condyle during firm closure of jaw.
In many patients ,(approximately 50%).there is no difference in this superior positioning , whether the jaw is closed firmly with strong contraction of the elevator muscles or closed lightly to delicate contact
In many patients, however,(the other 50% approximately) the condyle position moves forward slightly when the elevator muscles apply light closing force. This very minimal protrusive movement is more apt to occur when the head is upright in a postural position and the closure from mandibular rest position occurs with minimum muscle force
If the slightly protruded closure occurs with a dentition that has steep inclines in contact in centric relation, the same steep inclines which do not interfere with a direct closure to CR may interfere with the slightly protruded closure from the postural rest position.
When there is a difference between the anteroposterior positions of the mandible during light versus firm closure, it is beneficial to provide a horizontal area of freedom forward to each centric relation contact.
Such an area of freedom i.e. between centric closure and light closure from rest when they are in postural position has been termed as “long centric” and it is precisely this difference between the two position that dictates the amount of “long centric” that any patient should have.
The term “long centric “is actually misleading because it suggests that centric relation is not a precise relationship. Further confusion has been added by describing long centric as “freedom in centric” It has been sufficiently determined that centric relation is a very precise relationship and not a vague area which permits horizontal movement within its boundaries. Thus freedom from CR would be more descriptive of the true meaning of long centric”.
Fillastre refers to the forward part of long centric as “rest centric relation”. He defines it as the maxillomandibular relation existing when the condyles are slightly anterior and or inferior to centric relation, determined by posture resilience of structures involved, strength of closure and neuromuscular factor. He further states that “rest centric could be considered as the earliest part of Protrusive glide, but since patients often close directly into a rest centric, it is desirable to define rests centric as a separate functional position”.
“Pin- point ” centric versus “area” centric relation
With proper operator guidance, tooth guidance the condyles should seat in a rearmost position in the mandibular fossae exactly at the time when maximum intercuspation of the teeth occurs in retruded contact position. This concept has been termed POINT CENTRIC.
In Long centric supporting cusps make contact with flat areas prepared in the restored teeth, not only when condyle is in CR but also when slightly anterior to CR.
Unlike long centric, which has some anterior dimension; point centric has zero dimension anterior dimensions.
One of the major unresolved question in the restoration of the occlusal form is whether the occlusion should be arranged that cusps need occlude very tightly in the opposing fossae or whether in fact cusps should meet loosely in those fossae when the jaw occludes from an open position.
If the jaw should be occluded in tight intercuspation, it is obvious that an “area” would be a mistake in therapy. If on the other hand, the preparation of a tight occluding relation is in error, one could expect a variety of signs and symptoms of disease to result from such a relation.
The fact which must be recognized is that the answer to this question is equivocal. The likelihood is great that for many patients, the relation of both the “area centric relation or the point centric relation will make little difference and signs and symptoms of disease will not occur whichever the choice. This is explained on the basis of physiological adaptability.
If the pin point occlusion is essential and area is provided, one might expect signs and symptoms of disease to develop. But many patients will, within a short period of time adapt in such manner that the muscles guide the jaw into that “pin point” position in order to avoid trouble.
Louis Boucher in an experiment made it clear that the muscles are responsible for limiting jaw position and not the ligaments as acclaimed by posselt. If the muscles are the limiting agents, it appears inherent in the physiology of muscles that contractile capacity varies from time to time and circumstance to circumstance. Were this not true, the higher jumper would be able to perform at the same level each day within a three or four day period which is not the case.
Some sort of “reasonable balance” of jaw muscles maintains centric position. Eccentric jaw positions are obviously the effect of muscle contraction, for movement from centric is performed by the contraction of certain muscles.
We cannot reconstruct dynamic occlusal form unless we understand pathways as well as positions. There is a constant trajectory from position through movement to position.
We must understand that the character of the path i.e. if it is a straight line, our task for reconstructing those occlusal dynamics is simple. We need only to develop a single instrument which would go from point A to point B in a straight line and nothing more would be needed.
In order to develop such an instrument we must be able to record the jaw movement itself, however if we record the movement at one time and the movement is not constant, then recording it once may not provide us with a dynamic occlusion facile enough to treat not only the majority of patients who will adapt but the minority of patients who may not. On that account and because muscle contraction is variable, jaw positions and jaw movements are also variable.
Therefore if we record the position and the movement today, we have no guarantee that an occlusion constructed to that path will treat successfully that minority of patients in whom we are so interested.
To broaden our success, we have two alternatives open to us. Either we record the position several times (a week apart) and strike some average using the outside variance as our limits or we can accept one of the movement paths which we have recorded and build in an arbitrary “looseness” which will likely include all of the movements the patient might make.
One of the means currently used to guide the patient into posterior position or retruded position is a manipulative technique, which employs dentist thumb.
The primary objection in the use of the thumb is the unpredictability of patient response to aggression. The patient who is pleasant, relaxed , confident in the capacity of dentist , might very well accept the efforts of the later to move his jaw back into a retruded position and on that account, relax his muscles in so far as he is able. However that same man under tense circumstances two or three weeks later may be unable to relax his muscles and thus place his jaw in a position different from the earlier time. Conversely a patient who is under great tension at the first visit may resist the efforts of the dentist to manipulate his jaws, when two or three weeks later he may have well gained enough confidence in the dentist to relax. If these circumstances did prevail, it is apparent that the position initially will vary from the position subsequently and restorations made under the first circumstance will not necessary function well under the second circumstance.
Of course, the simplest solution to such a problem is to keep the hand away from the patient’s jaw entirely and allow the patient to manipulate his own jaw backward
The variability is on the order of approximately 0.3-0.5mm.
Grasso’s work showed an area of centric which was not only “long” but was also in fact “wide” in a significant number of cases. Variability in jaw position was also apparent in the work of those who found consternation in their attempts to duplicate the terminal hinge axis.
For all these and a variety of other reasons , which are experimental , and in the face of an overwhelming absence of scientific evidence , one might conclude that the preferred therapeutic regime for restoration of occlusion in the largest number of patients would be to provide an “area of centric” rather then to depend upon teeth tightly intercuspated.
Basis for “freedom in centric concept”
In the concept of freedom in centric , centric relation and centric occlusion coincide, but there is a flat area in the central fossa upon which opposing cusps contact which permits a degree of freedom in eccentric movements uninfluenced by tooth inclines . There are anatomic and physiological reasons for accepting the concept of “long centric”
- The fit of the condyle into its disc is not like the fit of a machined ball in a bearing. Rather there is some front back play permitted by the disc that allows the condyle to hinge freely anywhere within the limits of the anterior and posterior lips of the disc. When the mandible is closed firmly, the strong contraction of the muscles of closure pulls the condyle to the back of the disc against its posterior lips. Light closure from the rest position may be of insufficient intensity to completely pull the condyle into such a terminal position and there will consequently be a slight difference between the firm terminal hinge closure of centric relation and light closure from the rest position.
- The research done by Posselt showed evidence that in a great majority of cases there was from 0.50mm to 1.50mm difference anteroposteriorly between the most retruded voluntary closure which most often coincide and the point of the Gothic arch tracing and an involuntary closure.
Dr. Joseph E. Grasso, in research work done at the University Of Alabama School Of Dentistry in 1967 showed evidence that the Gothic arch tracing is not an immutable point position but a relative position which varies from registration to registration and day to day. He showed also that the “variance in a mediolateral direction was greater than the anteroposterior variance”. There is also ample evidence that posture has
Our objective can be accomplished only if we recognize centric to be on
In the mechanical world we recognize the fact that if a body I motion contacts an inclined plane there will be a potential of shock at the point of contact in that intensity of shock would be increased by the more acute angulations of the plane contacted.
Our dentition possesses some characteristics not too dissimilar to the mechanical devices. Let us evaluate our posterior teeth in function with this thought in mind. With the normal tooth relationship, in the cycle of mastication, the lower buccal cusp first contacts the functional incline of the opposing upper tooth. There is little potential of trauma at this point of contact as the muscles of mastication are not only closing the mandible but are also shifting the mandible toward the lingual in a line which may be nearly parallel to the tooth plane being contacted.
But what happens when this lower buccal cusp enters the central fossae?
Here a close intercuspation associated with the point contact in centric would increase the potential of trauma above that which might be expected if there was a horizontal area of contact or freedom in centric.
As it is well known that proprioceptive impulses protect us from injury, in the closely intercuspation occlusion, this shock is instantaneous. It takes time for proprioceptive impulses to pass to nerve centers and back to muscles. A degree of freedom would give more ample time for this protection.
In fig 4, the potential trauma of the lower buccal cusp entering the closely interdigitated central fossae has a degree of similarity to automobile traveling into a similar angulations .A horizontal area at the bottom of the first incline would reduce the potential of shock or trauma.
A reduction in the steepness of the lateral inclines as in fig 5, associated with a horizontal area in the central fossae would further reduce the potential of trauma.
We also know that cycle of mastication may not terminate in centric but may pass through and beyond centric (fig6)
- The traumatic effect of the normal cycle of mastication when the occlusion presents a degree of freedom of movement in centric occlusion. The stress of the contact of the lower buccal cusp on the lateral incline of the opposing tooth is not potentially traumatic. The traumatic stress occurs between the balancing inclines contacting in centric occlusion and when the cycle of mastication passes through and beyond centric occlusion. With the freedom of movement in centric occlusion, there is an increased possibility that the prioprioceptive impulses can be effectively employed to reduce the shock on the balancing inclines when the cycle of mastication goes into and beyond centric occlusion.
B. The primary contact on the lateral functional incline is equivalent to that of A but the closure into centric occlusion is controlled by a progressively steeper parabolic tooth contour. The contact between the balancing inclines in centric occlusion would be abrupt and at an acute angle. If the cycle of mastication passed beyond centric occlusion, the rise on the balancing inclines would be very steep. Here the potential for prioprioceptive impulses to reduce the traumatic shock are greatly reduced, and the possibility of destructive pathologic stresses are greatly increased.
Unquestionably, the first potential area of trauma in the cycle of mastication is when the lower buccal cusp enters the central fossae of the opposing tooth. As the cycle of mastication continues through and beyond centric, the contact of the lower buccal cusp upon the buccal incline of the lingual cusp of the opposing tooth causes an opening of mouth.
Jaw relation while the muscle of mastication are contracting to close this relationship. The traumatic potential of this stress is extremely great. It is poorly distributed to a few teeth and is magnified by the unilateral application of force. It is reduced in eccentric position.
In conscious swallowing or swallowing of a small bolus of food, the mandible does not always reach CR. The initial occlusal contact may be made anywhere between CR and CO. Patients with no dysfunctional signs and symptoms and good adaptive potential probably have no compelling reason for freedom in centric.
On the other hand, patients with dysfunction have not for a variety of reasons adapted well to their existing occlusal relationships.
Freedom in centric would seem to be beneficial here since it increases the adaptive range needed to achieve optimal occlusal contacts.
This may be important in patients who should benefit from a less rigid need to adapt mandibular posture and occlusal contacts to the changing head positions and during conscious swallowing.
The mandible is stabilized in the maximal intercuspal position (MICP) when the teeth are occluded. More often than not, the MICP is slightly anterior to centric relation.
A freedom area distal to the pinpoint contact in an upper cusp seat affords the patient the option of closing in CR while maintaining axial forces on the teeth. In other words, the teeth can be closed in a more distal relationship without interfering from inclines.
In a lower cusp seat, freedom area is mesial to the
Lack of freedom, referred to as restraint, causes tension in the patient and o release this tension the patient bruxes. As long as the tension or restraint remains the patient continues to brux. The bruxing is usually done during sleep and most people who brux do not know they do it. Bruxism produces very damaging horizontal forces that cause occlusal trauma.
When the patient bruxes, he uses the supporting cusps to wear away the restraining inclines. Unfortunately, the supporting cusps are worn away faster than the restraining inclines are. In addition the patient loses mandibular stability in the retruded position, as his supporting cusps are worn away.
The only way the patient can eliminate occlusal restraint is with an abrasive diet.
But modern man does not have an abrasive diet and therefore it is up to the dentist to treat the problem by providing freedom of movement in centric relation.
Some have argued that unless
In a full denture Prosthesis, the freedom in centric occlusion is most strongly recommended if a cusp form of posterior tooth is being used. Progressively, as vertical dimension is lost due to ridge resorption the lower tooth move forward in their relation to the upper tooth and a static centric relation is lost. For this reason a greater degree of anterior freedom is done to maintain favorable function.
THEORIES OPPOSING THE CONCEPT OF LONG CENTRIC.
Though the concept of long centric has long been used and proved, controversies regarding the character of occlusal position still exists.
- According to Frank V.Celenza, while the posture limit of the area is definable and justifiable the lateral borders are not. They are according to him rather arbitrarily selected equating postural positions with the anterior centric limit and the lateral translations with the width of the area. These are eccentric positions incorporated into an area of centric. Also since the mandibular pathways are all curvilinear and at an angle to the horizontal plane, it is mechanically impossible to construct an area of centric which will provide continuous contact throughout the area unless it is congruent with the path. This can only be accomplished with cusp tips against curved surfaces as in protrusive balance. (Fig 7).
2. Therefore it is not possible to mechanically construct an area of centric that would satisfy the definition. Uneven contacts and lack of contact must result within the area. The patient is not free to select any given time any position within the area becomes centric .This is indirectly and unknowingly provided by the operator.
The lack of balance thereby created is not sound from a physiological standpoint since there will be uneven loading of the mechanoreceptors. Long centric, however does appear to be clinically acceptable. This may be tribute to the adaptive capacity of the patient but it is not a physiological sanction of the concept.
The idea of a central zone or area is further complicated by the inability to mechanically coordinate the centric pathways. Where would the starting point for escape grooves and centric coordination be located and how would this be registered?
- The so called long centric concept of occlusion resulted from the premise that an area of freedom in centric permit unimpeded closure to the retruded contact position during swallowing and to the ICP during mastication. This concept, have been advocated by Pankey and Mann, Dawson, and Ramford and Ash.
However, telemetry studies by Glickman et al challenged the long centric concept by finding that both masticatory and swallowing occurs at or very near the ICP and that little, if any contact occurs in the retruded contact position during either of these functions.
However, subjects of Glickman et al use head rests during feeding and data collection, which might have restricted postural movements of the head and accounted for the low incidence of occlusal contact in retruded position of mandible.
- Glickman made one more statement saying that by:
- Reshaping the cusps of teeth to conform to the most retruded position, which is down and back approximately 0.5mm from terminal hinge, the vertical component on posterior teeth is removed. This component is required to guide and support the condyles, as they move from the most retruded to the most superior or terminal hinge relation (Fig 8 (a).)
- The result is instability and destruction of tooth form, which is termed long centric, wide centric etc (Fig. 8(b)).
- Glickman also stated that the liberties taken by flattening the vertical component from posterior teeth, the envelope of motion is misinterpreted (Fig8 c).
Advantages of the long centric or freedom in centric concept
Occlusal adjustment can be carried out according to the freedom in centric concept, with minimum removal of tooth substance without changes in vertical dimension and with reasonable assurance of future functional stability of the occlusion. Occlusal adjustment of a Gnathological concept of “point centric” on the path of the retrusive hinge axis closure would necessitate excessive grinding of the teeth and closure of vertical dimension.
2. Restorative dentistry utilizing the freedom in centric concept does not require pantographic tracings of jaw movements and does not necessitate elaborate tracings and reproduction of the Bennett shift thus allowing in dental restorations to be completed in a semi adjustable articulator of the Hanau or Dentatus type. If group function of posterior teeth in lateral excursion is deserved this may be achieved by functional path generation in a wax pattern.
Reconstruction following pantographic tracings with transfer of the border movement to the instruments like the Denar or Stuart articulator, direct functional movements to a border path which require extremely precise execution of both recording and wax carving in order not to challenge the posterior ligamentous guidance of the temporomandibular joints. Furthermore such an artificial path does not reproduce normal functional excursions which are anterior to the retrusive lateral border paths and have a protective tooth guidance against challenge of the ligaments in retrusive border movements as shown by Posselt. The common practice among Gnathologists therefore is to disocclude the posterior teeth with a cuspid rise after they laboriously have carved in a detailed cusp and group lateral function patterns to harmonize with retrusive tracings.
Normal group function cannot be reproduced accurately in a Hanau or Dentatus articulator. Thus for extensive mouth reconstruction, a cuspid guided occlusion with some
By far, the most important feature related to masticatory efficiency seems to be the contact areas in and close to centric occlusion. For neuromuscular and functional harmony, this simplified occlusal concept of freedom in centric and cuspid guidance appears to result in a physiologically acceptable, stable occlusion with a minimum need for dental procedures both with regard to number of procedures and difficulty.
Since dentistry primarily is a health service , the cosmetic aspects of dentistry should be limited to the aspect of dentistry, which is visible to the public and “jewellery “ effect related to the occlusal surfaces is meaningless, since it can be observed only by the dentist . There is absolutely no scientific evidence to indicate additional functional or neuromuscular benefit from meticulous occlusal carving of the gnathology type compared with reconstruction using the freedom in centric concept.
- The freedom in centric as simplified in flat occlusal bite planes has been shown to induce marked muscle relaxation in bruxers. Electromyographic and clinical studies have established that both with and without bite planes a flat area with freedom in centric in front of CR provides harmonious muscle activity and comfort, thus eliminating bruxism and other
traumatogenicinfluences on the structures supporting the dentition. Fig 9
In providing freedom in centric relation there are few things which need to be kept in mind and check in the patients’ mouth. These things are:
- In establishing the need for long centric in any given patients it is absolutely essential that all interferences to terminal hinge closure be eliminated. If centric relation interference is present, the path of closure will be dictated by the proprioceptors of the teeth, instead of the normal physiologic function of the muscles.
In absence of any CR interferences, it is seen that the difference between the centric closure and light closure from rests rarely exceed 0.5mm
It might be difficult to understand how such minute differences in the path of closure can be significant but it is just such minute that make the difference between just acceptability and complete predictable comfort. The dentist will only have to provide a needed long centric for one patient who has been “locked into” centric relation to get an idea of the usual reaction of patients to their new freedom.
- Another most important aspect of freedom to close slightly anterior to centric relation is that the vertical dimension of occlusion must be same from back and front of each long centric contact area.
There are sound reasons for maintaining the same vertical dimension of occlusion through this centric area.
A). If the vertical dimension is greater when the teeth contact in centric relation than when it is when teeth close at the front end of the long centric, the centric contacts would be an incline. Such incline contacts would provide a “slide” from CR into the most closed position.
Instead of having “long centric” there would actually be unstable contacts that would cause the mandible to deviate from the centric contacts.
b) If the vertical dimension is less, when the teeth contact in centric relation as compared to when they touch at the front end of the long centric area, light closure would direct the lower teeth against upper incisors instead of into stable contacts. If the patient requires this occlusion and does not get it built in freedom, the lower incisors may strike the lingual inclines of the upper incisors in a manner that has a tendency to wedge the upper teeth labially. It is probably this “wedging” effect that causes most of the instability of occlusions that have not been provided with
It might be argued that the wedging contacts would occur with such light pressure that they would not possibly cause any harm. Such reasoning would continue to point out that when firm muscular pressure is exerted the condyles would then be pulled into CR and at this point, the pressure would be properly directed by correct centric stops.
To understand how such light pressure on such minute interference can cause problems of comfort and stability, it is necessary to have an acute appreciation for the exquisite sensitivity of the proprioceptive mechanism.
When teeth are in the way of any functional border position, the muscles moving the mandible have two choices:
They can either move the mandible in a pattern of closure and function that avoids the interference or they can move the mandible in a pattern of erasure, to get rid of interference.
It is well established how the erasure mechanism results in bruxism or clenching, in order to grind away or move the offending interferences. What is not generally appreciated is how the bruxism pattern can be triggered by such delicate contact on tooth surfaces that are in the way of normal physiologic muscle function.
Generally the patient with bruxism with perfect occlusion is not checked closely enough clinically. The interference are usually there but they are so seemingly insignificant that they have been missed. It is essential to understand the bruxism mechanism to be able to appreciate the exquisite sensitivity of prioprioceptive mechanism to any interference to any functional position.
Interference to light closure to rest can then be understood in a better perspective. They act as trigger for bruxism and clenching. The patient subconscious to obtain the freedom of relaxed closure ends up as a forced protrusion with the lower jaw trying to push the upper forward to move them out of the way. Because of this forward thrust against the upper teeth, it is not uncommon for locked in occlusions to develop slides. If the centric holding contacts of the posterior teeth have been developed to permit no “long centric” .The pressure against the distal inclines of the upper teeth will have a tendency to move forward. This brings the upper mesial inclines onto interference and the slide results.
Not all patients require this occlusion. Their centric closure and their light closure when they are in postural position are identical. If such patients are given a long centric they will not use it .but it will not hurt either.
In fact there are no contraindications for providing the freedom that goes with long centric .problem occurs when we fail to realize that long centric starts with a perfectly harmonized centric relation and that all we are doing is providing patients with the freedom to close anterior to that point at the same vertical. They are not forced to use either position, but they are free to use both positions and any point in between.
Before any occlusal adjustment or oral rehabilitation is undertaken , these has to be demonstrable and for such procedures, as in order biological systems , the masticatory system has wide range of adaptive capacity which means that normal function and health can be maintained for indefinite time for less than ideal occlusion characteristics.
Dentistry has a sad record of iatrogenic disease following well meaning attempts to make the human occlusion conform to theoretical concepts which in many instances later have been proved to be wrong.
Therefore treatment should be instituted only when there are pathological states present related to the teeth; the periodontium, the temporomandibular joint,or the neuromuscular system of occlusion or when reconstruction of functional capacity is indicated
A complete relaxation of the patients jaw muscles is absolutely necessary in order to determine centric relation. If relaxation cannot be achieved by manipulation of the mandible in a relaxed environment or gained by use of medication, the patient should be placed on a bite plane to eliminate faculty guidance from the teeth and thus induce jaw relaxation.
The bite plane may have to be adjusted several times to centric relation with freedom in centric and cuspid rise. No occlusal adjustment or oral reconstruction should be attempted until complete jaw relaxation has been achieved.
Providing “long centric “when occlusion is to be restored
If all posterior teeth of either arch are to be restored, an excellent opportunity is presented to see the difference, if there is one, between a firm closure into centric relation and a light closure from the postural rest position.
By preparing the entire upper or lower posterior teeth, the possibility of any prioprioceptive influence from them is eliminated. Since the prepared teeth have been reduced occlusally and cannot contact the teeth in the opposing arch, they cannot certainly interfere with any pattern of closure.
With all such chance of any posterior interference eliminated it is then rather simple. When needed, to correct any inclines on the anterior teeth that cause a deviation from terminal hinge closure.
By manipulating the mandible to make sure it does not deviate off its terminal axis, the interferences are marked and reshape by selective grinding to provide centric relation stops on as many anterior teeth as possible. Properly adjusted centric stops on anterior teeth should be stable enough that none of the teeth is jarred by firmly tapping the teeth together in a terminal hinge closure.
When this is accomplished, the muscles that move the mandible are free to close it in any manner that best suits them. Since there is no interferences terminal axis closure, the mandible is free to go there if the physiologic action of the muscles dictates.
If the muscles close the mandible into any position other than centric relation it is easily observed by checking with thin marking ribbon on the anterior teeth.
Consequently, this is the ideal time to determine whether the restorative patient requires a long centric and if so how much.
After the anterior CR stops have been perfected, the patient should sit up in a normal postural position.
The headrest should be removed and then the patient should lightly tap his teeth together from the relaxed jaw position. The thin ribbon should be interposed between teeth and the patient should repeat the light tapping.
The red marks made from this procedure will indicate on the lingual surfaces of the upper anterior teeth the first points that the lower teeth contact when the mandible is closed lightly by the unrestricted, unaided, physiologic action of the muscles when the patient is in a postural position.
To compare such a closure with the terminal axis closure of centric relation, the mouth should be held open to preserve the red marks while the patient is placed back into a supine position for marking centric relation with a darker colored ribbon.
If green ribbon is used to mark centric relation contacts over the red marks, it is simple to see there is a difference between a manipulated terminal hinge closure and the unmanipulated light closure from a postural rest position.
When extending the centric stops to include closure from postural rest position, the dentist should be sure never to grind on the green centric marks. A knife- edged inverted cone carborundum stone is practical to use for accurate grinding (Fig.12).
It is very important to check each tooth digitally for any jarring from the light rest closure. It is easy for mobile teeth to be moved rather than marked by the ribbon. Checking each tooth fro such movement while the patient taps is sometimes the only way such interferences are picked up. It is often necessary to hold mobile teeth in place with one finger while the patient taps in order to correctly mark them.
For those who are quite skeptical about the value of long centric following the preceding procedure can be quite enlightening.
It is quite convincing to see teeth jarred when the patient sits up and taps that were not jarred by tapping when the patient is lying back. It is also quite enlightening to see other patients who tap directly and precisely into a terminal hinge axis closure, in spite of the fact that before the interferences to centric relation were removed, their habitual closure was far from their centric closure.
(B) As the establishment of freedom of movement in centric occlusion has been recognized as an essential factor in the functionally generated path technique, it is first established in the incisal component of the upper anterior teeth. The contact of the lower anterior teeth is made upon a horizontal surface on the incisal guidance (the lingual surfaces of the upper anterior teeth) permitting slight eccentric mobility before the influences of the on an inclined plane of these surfaces become effective. The placing of stress upon a horizontal part of the occlusal surface rather than an inclined provides a favorable distribution of stress on the structures supporting the dentition.
When this eccentric freedom is established in the incisal guidance, the functionally generated path automatically establishes this same freedom of movement in the upper posterior occlusal forms. The horizontal surface on the incisal guide component automatically established slight horizontal surface in the depth of the central groove of the upper posterior teeth upon which the opposing cusp tips will function.
This slight eccentric freedom in the posterior occlusal contours is possible only when this freedom exists in the centric area of the incisal guidance. The incisal guidance is usually established on casts mounted in an articulating instrument (Hanau). Too often, the inclination of the incisal guide component of the articulating instrument defeats the effort to obtain the desired horizontal freedom in CO.
This freedom may be developed by waxing the desired inclines of the incisal guidance while the mandibular cast is protruded 0.5-1mm from centric relation. Then the cast is retruded to its original centric position with the incisal guide pin resting upon the incisal guide table which has been changed to a horizontal position. At this time, the lower anterior teeth will not contact the incisal guidance waxed on the upper anterior teeth. A horizontal ledge of wax is added to these wax patterns in order to make contact with the lower anterior teeth.
Since long it has been recognized the need for a horizontal surface on the incise guidance mechanism of articulating instruments upon which the incisal guide pin might rest and which would provide the desired eccentric freedom of movement in the horizontal plane before the pin contacts the established inclines. Such incisal guidance is now available. This area of horizontal freedom in the incisal guidance mechanism wills assure the desired freedom in the posterior occlusal contours, whether the functionally generated path technique or the articulating instrument itself is used in the final construction procedures.
Providing long centric in cusp form teeth
If porcelain teeth are being used, it has been suggested by Dr. Luzerene Jordan that after the posterior teeth have seen positioned in wax, they be individually removed and porcelain baked into the central fossae area of opposing cusp contact.
If posterior teeth of a plastic composition are being used, a cold cure resin can be placed in the central area of the posterior teeth of the completed dentures. This modification of the central fossae necessitates a slight opening of the vertical relation. An allowance should have been made for this to avoid infringing upon the freeway space.
Some dentists have advocated placing the horizontal area of freedom on the central fossae of only the lower teeth. This would have the lower buccal cusp slightly out of contact in centric. It would reduce functional stresses and by placing them slightly more to the lingual of the supporting ridges, it would increase denture stability.
SUMMARY AND CONCLUSION
The long centric concept is applicable t treatment of patients who need occlusal adjustment and / or oral rehabilitation.
This concept is supported both by scientific evidence and clinical experience. If restricting an occlusion only to centric relation is sometimes bad, restricting only to an acquired habitual closure is worse.
We have seen a temporomandibular joint syndrome that was directly caused by failure to provide along centric if centric relation was correct. Failure to provide needed long centric may lead to clenching and bruxism and a locked in feeling of mild discomfort, but in itself cannot cause a true joint pain – dysfunction syndrome.
On the other hand failure to provide access to centric relation not only can cause sever problems of discomfort, clenching and bruxism, but as already pointed out, it can cause pain and dysfunction of the muscles that move the mandible.
Because of the permissiveness of long centric, there is really no disadvantage of providing it. Since we are talking about a freedom of rarely more than 0.5mm, it does not create any problem for restoring the posterior occlusal form with good morphology.
If the patient has it and does not need it, he does not have to use it, applying the facts of long centric to complete denture occlusion gives the dentist a common starting point that he can repeat and prove. From this point, he can develop with the artificial teeth of his choice a comfortable and unrestrictive stable occlusion.
These are probably the reasons why Dr. L.D Pankey has been stressing for years” Any occlusion that is worthy of restoration, is worthy of “long centric”.
Click here to check pubmed article on Long Centric Occlusion.
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