Monday, August 12, 2013
SELECTIVE GRINDING IN COMPLETE DENTURE
SELECTIVE GRINDING IN COMPLETE DENTURE
Occlusal harmony in complete denture is necessary if the dentures are to be comfortable, to function efficiently, and to preserve the supporting structures. It is difficult to see occlusal discrepancies intraorally with complete denture. The resiliency of the supporting soft tissues and displaceability of the tissues in varying degrees tend to disguise premature occlusal contacts. The tissues permit the dentures to shift; as a result, after the first interceptive occlusal contact the remaining teeth appear to make satisfactory contacts. The eye cannot be relied upon to observe occlusal discrepancies, and the patient cannot be depended upon to diagnosed occlusal faults. It is the responsibility of the dentist to find and correct these occlusal discrepancies and permit the patient to depart free of occlusal disharmony. Occlusal faults can be determined by obtaining and interocclusal record from the patient and remounting the dentures on an articulator. These faults can be corrected by careful selective grinding procedures. Remounting of the dentures on the articulators and selective sliding procedures should be carried out at the time of placement of the dentures.
Selective gliding is defined as the, “intentional alteration of the occlusal surfaces of the teeth to change their form.”
-- Glossary of the Prosthodontic terms 1999.
Teeth are altered by selective grinding to make simultaneous cusp tip to cusp tip contact on both sides of the arch when the jaws are in left or a right lateral position, balanced occlusion in a static eccentric position exists. When the mandible is in a straight protruded relation with the maxilla and the posterior teeth are altered to make cusp contacts at the same time to anterior teeth make incisal edges contact balanced occlusion protrusion exists.
REVIEW OF LITERATURES: -
Schuyler, Friedrich and Vaeghan in 1935 observed the disturbances in occlusal relationship and opening of the bite of full dentures made of acrylic resin, even when the flask was completely closed during processing.
Osborne and Taylor in 1941 have noted the disturbance and attributed it to over packing and the accompanying displacement of teeth in the mold. It was felt, however, that these changes were caused in part by the volumetric change of acrylic resin during polymerization. They used the following method to estimate the amount of change in occlusal relationship -
The top of the incisal guide pin was flush with the top of the articulator at the time the cases were waxed. The distance that the pin dropped in order to make contact with the incisal guide plane when the finished dentures were placed in centric position on the articulator gave a rough means of comparison of the amount of disturbance of occlusal relationship in each case.
Avoiding remounting the dentures on articulator and selective grinding leads to,
1) A deformation of underlying soft tissues,
2) Discomfort, and
3) Destruction of the underlying supporting bone. Later occlusal errors may be concealed and impossible to locate and correct because of distorted and swollen tissues.
CAUSES OF OCCLUSAL DISHARMONY: -
1) Incorrect registration of retruded contact position (RCP)
2) Irregularities in setting the teeth.
3) Tooth movement when flasking and packing
4) Incomplete flask closure.
1) Incorrect registration of retruded contact position (RCP)
This is probably the most common cause of error in the occlusion of finished dentures. During registration considerable care is taken to obtain a correct vertical dimension and the physiological fully retruded position of the mandible, but often, when brought together, the record blocks exert uneven pressure on their respective supporting alveolar ridges, and this condition passes unnoticed. On finishing the denture the teeth are found to occlude only in the area where the premature contact of the occlusal rims occurred the remainder of the teeth being slightly out of degree of contact. The degree of separation will be related the degree of premature contact occurring between the rims.
Another fault causing errors in the occlusion of the finished dentures results from slight movement of record blocks on the ridges during registration due to their imperfect fit and inadequate retention. All these errors can usually be lessened by using an accurately fitting acrylic base in preference to a shellac base which invariably warps slightly.
2) Irregularities in setting the teeth
When setting up teeth the technician is unlikely to produce a perfectly even contact in retruded, protruded and lateral occlusions. Some teeth will be in good occlusion while others will be slightly out of occlusion, thus producing areas of heavy pressure. This cannot happen when the teeth are held firmly in the final denture base material and results in premature tooth contacts in the occlusion and articulation of the finished dentures
3) Tooth movement when flasking and packing
Movement of the teeth may occur at the time of boiling out the wax trial base after the dentures have been flasked and if such teeth are not correctly repositioned they will cause occlusal irregularities. Repositioned they will cause occlusal are not correctly repositioned they will cause occlusal irregularities. When packing acrylic dough, teeth may be driven into the investing plaster, particularly when packing follows soon after flasking and the plaster has a low crushing strength. Raped closure of the flask in the bench press will add to the hazard. Injection moulding techniques for packing acrylic are an obvious improvement.
4) Incomplete flask closure
Such an occurrence not only causes an increase of vertical dimension because of the alteration in tooth / cast relationship but also result in derangement of the occlusion which usually necessitates the total remake of the denture.
Remounting dentures with check records --
It can be appreciated that even with care on the part of dentist and technician errors may occur which influence the finical occlusion and articulation of finished dentures. In some instances these errors may corrected by careful use of marking paper or tape at the chair side, but such correction is often proved false when check records are taken for confirmation. It is far satisfactory, and often less time-consuming clinically, to register the retruded contact position of the finished denture with check record, mount the finished dentures on an adjustable articulator and then refine the occlusion either at the chair side or in the laboratory.
OBJECTIVES OF CORRECTING OCCLUSAL DISHARMONY : -
The objectives as stated by Schuyler in 1935 are,
1) Maximum distribution of stress in centric maxillo-mandibular relation.
2) Retention of the maxillo-mandibular opening.
3) Harmony of guiding inclines, which distributes eccentric occlusal stresses.
4) Reduction of the incline of guiding tooth surfaces, that occlusal stresses may be more favorably applied to the supporting tissues.
5) Retention of sharpness of cutting cusps.
6) Increase in food exits.
7) Decrease in contact surfaces.
It must be remembered that the occlusion of the natural dentition and the occlusion of the complete denture may differ in many respects, to the advantages and convenience of the dentures. For instance, a denture is a unit of 14 teeth fastened rigidly together, while natural teeth are anchored independently of each other. The natural teeth must be considered individually for occlusion, while the 14 teeth can be treated as one whole unit. We can have balanced occlusion in complete denture with some teeth out of contact or some inclines out of contact. The dentures would be balanced as far as 3 – point contact is concerned if only the cuspids and second molars worked in harmony.
ELIMINATING OCCLUSAL ERRORS IN ANATOMIC TEETH: -
Articulating paper of minimum thickness is used for marking the actual contacts of the teeth. Paper is interposed between the teeth and marking are obtained by tapping the teeth together. This can be done on both sides at the same time.
Grinding is done with mounted chayes stones no. 16, 11, 05. The marking process and the grinding are repeated until practically all the teeth contact in Centric Occlusion. During this grinding procedure, the incisal pin is relieved of contacts on the incisal guidance table to allow for the slight reduction in Vertical Dimension that must necessarily take place. After centric deflective occlusal contacts have been removed the pin is placed in contact with the incisal table and is kept in contact through the remainder of the grinding procedure.
I] TYPES OF OCCLUSAL ERROR IN CENTRIC OCCLUSION AND THEIR CORRECTION: -
3 types of occlusal errors can exist in Centric Occlusion, and each can be corrected by specific grinding for that error.
1) Any pair of opposing teeth can be too long and hold the other teeth out of contact.
Correction: - The fossae of the teeth are deepened by grinding so the teeth will in effect, telescope into each other. The cusps are not shortened.
2) The upper and lower teeth can be too nearly end to end.
Correction: - For correction of this error grinding in such a way as to move the upper cusp inclines bucally and the lower cusp inclines lingually. In the process the central fossae are made broader, the lingual cusp of the upper tooth is made more narrow when it is ground from the lingual side, and the buccal cusp of the lower tooth is made more narrow when it is ground from the buccal side. The cusps are not shortened
3) The upper teeth can be too far buccal in relation to the lower teeth.
Correction: - The lingual cusp of the upper tooth is made more narrow by broadening the central fossa, and the buccal cusp of the lower teeth is moved buccally by broadening the central fossa.
In effect, the upper lingual cusps is moved lingually and the lower buccal cusp is moved buccaly so the tooth telescope into each other. Cusps are not shortened.
II] TYPES OF WORKING SIDE OCCLUSAL ERRORS AND THEIR CORRECTION: -
6 types of errors can exist in the occlusal contacts on the working side. Each of these will cause other teeth to be hold out of contact in working occlusion, and each requires selective grinding of specific cusp inclines for its elimination.
1) Both the upper buccal cusp and the lower lingual cusp are too long.
Correction: - The length of the cusps is reduced by grinding to change the incline extending from the central fossa to the cusp tip. The central fossa is not made deeper, but the upper buccal cusps and the lower lingual cusps are made shorter so the other teeth will touch in that position.
2) The buccal cusps make contact but the lingucal cusps do not.
Correction: - Buccal cusps of the upper teeth are ground from the central fossa to the cusps tip to shorten the cusp and change the lingual incline of the cusps so it will be less steep.
3) The lingual cusps make contact but the buccal cusps do not.
Correction: - The lower lingual cusps are shortened by changing the buccal incline of the lower lingual cusp so it is not as steep. Upper lingual cusp is not shortened and the central fossa is not made deeper.
4) Upper buccal or lingual cusps are mesial to their intercuspative positions. This error may occur along positions. This error may occur along with any of the 3 above listed.
Correction: - Grinding is done so the mesial inclines of the upper buccal cusps are moved distally when the cusps are narrowed and the distal inclines of the lower cusps are moved forward. The same cuspal inclination is maintained in this procedure.
5) Upper buccal or lingual cusps are distal to their inter cusping positions. This error may occur along with the bucco-lingual errors.
Correction: - Grinding is dome from the distal of the upper cusps and from the mesial of the lower cusps.
6) Teeth on the working side may not contact.
Cause of this error is excessive contact on the balancing side.
III] TYPE OF BALANCING - SIDE ERRORS AND THEIR CORRECTION: -
There are 2 types of balancing side errors –
1) Balancing – side contact is so heavy that the working side teeth are held out of contact
Correction: - Paths are ground through the buccal cusps of the lower teeth to reduce the incline of the part of the cusp that is preventing the teeth on the working side from contacting.
No grinding is done from the lingual cusps that may be involved in this contact.
2) There is no contact on the balancing side.
Correction: - Shorten the buccal cusps of the upper teeth and the lingual cusps of the lower teeth on the working side. In this process, the lingual inclines of the buccal cusps of upper teeth and buccal incline of lingual cusps of lower teeth are made less step. No grinding is done on central fossae.
ELIMINATING OCCLUSAL ERRORS IN NON ANATOMIC TEETH :-
An Interocclusal Centric Relation record is made in a bite registration material with the opposing teeth just out of contact. Dentures are mounted on articulators and the following procedures are undertaken.
1) After being detected by articulating paper between the teeth, gross premature contact in Centric Relation are removed by grinding. Same procedures are used to locate and remove all occlusal interferences lateral and protrussive movements. The grinding is done that appear to have been ripped or elongated in processing. In Centric Occlusion no grinding is done on the distobuccal portion of the lower second molar. All balancing- side grinding is done on the lingual position of the occlusal surfaces of the upper second molar.
2) Abrasive paste is placed on the teeth on the articulator. These teeth are milled when the upper member of the articulator moves in and out of protrusive and right and left lateral excursions. When the teeth slide smoothly through all excursions, the dentures are removed from the articulators and washed. Seldom is any correction necessary to attain a bilaterally balanced occlusion.
3) Spot grinding is done to correct any small discrepancies in Centric Relation that remain after the grinding with abrasive paste. The dentist adjust them after identifying the discrepancies with articulating paper – using a light tapping motion with the articulator and grinding the marks to ensure even occlusal contact in Centric Occlusion.
SUMMARY AND CONCLUSION :
Selective grinding in complete denture Prosthodontics is an important laboratory procedure which is carried out by remounting of the dentures after processing is completed. This remounting may either be laboratory remount or patient remount.
Inspite of carrying out each step in denture construction very carefully it is seen that in the end when the dentures are remounting there is an occlusal prematurities or interferences may be needed.
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2. Charles Heartwell: Syllabus of Complete Denture. U.S.A. 1992, ed 4; 394-406.
3. Merrell Swenson: Complete Dentures. U.S.A. 1959, Mosby Company,
ed 4: 273-294.
4. Holt J : Research on remounting procedures.
J Prosthet Dent 1977, 38:388.
5. Moore P : Indicate pastes - Their behavior and use.
J Prosthe Dent 1979, 41:258.