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Prosthodontics

Authors: Dr. Anthony Kevin Fernandes, Dr. Faizan Ahmed Khan

INTRODUCTION

Prosthodontics is greatly concerned with the prevention and treatment of Chronic Soreness from dentures and with the preservation of the supporting structures. the problem of managing patients with congenital or acquired anatomic abnormalities, systemic deficiencies, detrimental psychologic factors(bruxism), faulty prosthesis, or a combination of these problems with hard rigid polymers was once are very difficult, if not impossible to solve. With the advent of elastomer polymers the management of these problems has been greatly enhanced. The softness and flexibility that these materials have, by virtue of their physical and chemical composition affords an opportunity to protect the supporting tissues from functional and parafunctional occlusal stresses.1

The most expedient and yet the most effective method for treating abused basal tissues is for are patient to remove the dentures from the mouth for an extended period of time. For most patients however social and economic considerations preclude this simple but direct approach. The need to accomplish rehabilitation of abused tissues without the continuous removal of patient’s denture has led to the development and widespread acceptance of elastomeric polymers called tissue conditioning materials.2

Tissue conditioners are viscoelastic, i.e, they will flow under a prolonged applied load but are resilient under dynamic loads such as during mastication. It has been suggested that they will thus allow traumatized, inflamed and deformed mucosal issues to return to health, provided there are no gross errors in the existing dentures.3

Unfortunately, one of the disadvantages of tissue conditioners is that they gradually hardest with time and eventually. For this reason, regular replacement, sometimes as often as every 3 to 4 days, is advocated until the mucosal tissues are in optimum condition. The hardening of conditioners has been attributed to the leaching out of the plasticizer with the penetration into the saliva.4

Denture lining materials are of several types and are used for a variety of reasons. Occasionally, the fitting surface of an acrylic denture needs replacement in order to improve the fit of the denture. In this case, there are two options. Either the whole of the denture base can be replaced with fresh heat curing acrylic resin, or a lining of a self-curing resin may be applied to the fitting surface of the existing base. Some patients are unable to tolerate a ‘hard’denture base and must be provided with a ‘permanent’ soft cushion on the fitting surface of the denture.5 The materials which satisfy the various requirements listed above can be classified into three groups: (1) Hard reline materials;(2) Tissue conditioners;(3) Soft lining materials.

Tissue conditioners are soft denture liners which may be applied to the fitting surface of a denture. They are used to provide a temporary cushion which prevents masticatory loads from being transferred to the underlying hard and soft tissues. These materials should undergo a degree of plastic flow for 24–36 hours after mixing to allow for soft tissue changes once ‘trauma’ has been removed and to capture the shape of the supporting tissues in function as opposed to a static or unloaded relationship.

Tissue conditioners have several applications. For example, when the soft tissues have become traumatized due to wearing an ill-fitting denture the dentist would like the tissue to recover before recording impressions for new dentures.6 Tissue conditioners are often applied to the dentures of patients who have undergone surgery. This reduces pain and helps prevent traumatization of the wound.

A layer of tissue conditioner in the fitting surface of the denture enables a functional impression to be obtained over a period of a few days

HISTORY

Twitchell in 1869 used the earliest soft lining material recorded (soft rubber). Lamav. W. Harris in 19537 carried out a progress report on immediate permanent Reliners. He said that the earlier reported by Skinner and Pomes

and Beall and Caul indicated that the first immediate liners were satisfactory at the onset, but within a few months started wraping It was Lytle who in 19578 demonstrated that alveolar bone is resorbed beneath restricted areas of excessive denture pressure and that new bone forms in these same areas when a new denture is made to fit conditioned mucosa. In 19589 the study by chase was inspired by Lytle’s thinking. However, the addition to this concept was that the force or pressure of denture and the movements of the mandible are used continuously and progressively to condition the oral mucosa

Travagilini, Gibbons, Craig 196010 stated that resilient liners for dentures are used with the hope of minimizing pressure and reducing trauma to the supporting tissues without sacrificing contact

W. W. Crase in 196111 reported that :

  1. Conditioning of the mucosa by the continuous stress of force and motion transmitted to the basal seat tissues though occluding dentures dentures by a resilient flowing treatment material.
  2. Use of the completed treatment impression as a final dynamic impression. This action is called dynamic adaptive stress.

J.E. Robinson in 196312 made use of a medical grade silicon of permanent resiliency as a denture base material. C.R. Means in 196414 did a study on the use of home refiners in denturess. These materials are produced as wax and guaze pads, as soft plastic sheets to be adapted to the denture on the tissue surface and then trimmed as a plastic capable of flowing to confirm to the shape of the arch when applied to the tissue surface of the denture, or as a powder and monomer

Craig concluded that:

  1. Home reliner materials cannot replace treatment by dentist in solving prosthetic problems :
  2. All denture patients should be placed on regular semi-annual recall for examination and evaluation. This would prevent patient from doing irreversible damage to the health of their oral structures by use of home refiners.
  3. Patients must understand the necessity and value of this recall service.

In a study in 198115, authors analyzed 15 soft lining materials; 5 silicone rubber, 7 soft acrylic compounds and 3 experimental materials to determine their water sorption, water solubility, viscoelastic properties and bonding to poly methyl methacrylate in distilled water at 37°C. They observed that in silicone rubber materials, Molloplast-B and Per-Fit had low water absorption; the behavior of heat curing soft acrylic materials was dependant on the balance between the loss of plasticizer by solution and the absorption of water and the experimental animal (Hydrocryl`s Soft Liner) absorbed a large amount of water and reached equilibrium quickly.

In another study in 198816, authors conducted an evaluation of different types of resilient soft liners and tissue conditioners which included acrylic based resilient soft liners, silicone resilient soft liners, and other alternative soft polymers like vinyl chloride and vinyl acetate polymers and copolymers, polyurethane soft liner and tissue conditioners and discussed their properties. It was observed that heat cured material is somewhat superior in its properties like water sorption and solubility than the self curing type.

In this study in 199417 the cushioning effect of soft denture liners was evaluated with the use of a free drop test with an accelerometer. The materials tested included SuperSoft, Kurepeet-Dough, Molteno Soft, and Molloplast-B brands. All materials were found to reduce the impact force when compared with denture base resin. A 2.4 mm layer of soft denture material demonstrated good shock absorption. The Molloplast-B and Molteno Soft materials showed excellent shock absorption. When the soft denture liner was stored in distilled water for 180 days, the damping effect recorded for all materials tested was increased. The aging of all materials also affected the cushioning effect.

COMPOSITION AND CHARACTER

Tissue conditioners are composed of polyethy 1 methacrylate and an aromatic ester- ethylalcohol mixture. These materials when mixed form a gets ethyl alcohol having great affinity for the polymer. Optimum properties are obtained when small proportions of alcohol used and are reasonable gelling rate is obtained that minimizes distortions under masticating. Because of their continuous flow and viscosity, these materials have to be used within the hard- denture base. Several compositions are commercially available that exhibit different flow characteristics; others require the addition of plasticing liquids to improve their flow. It is important to know these characteristics, in selecting the right material for use in a given clinical situation. These materials are intend to be used for relatively brief period (days).

Although a large number of studies have been conducted to investigate the physical and chemical properties of this class of dental materials, little information is available relating to the polymers themselves. Many of these materials intended for medium to short use are composed of Poly (ethyI methacrylate ) or co – polymers of poly (ethyl/ methyI methacrylate). Such matherials are blended to undergo gelation in the mouth when mixed with, typically, between 80 and 95% platicizer and a balance of ethyi alcohol.

ADJUNCT AS A DETERMINANT OF POTENTIAL BENEFITS FROM A TREATMENT MODALITY

Frequently patients have very well constructed dentures but cannot wear them comfortably because of chromic soreness. The use of a tissue conditioner help in determining whether a resilient liner could remedy the problem and allow the wear the dentures comfortably. Having the patient wear the tissue conditioner in the denture for approximately 8 to 10 weeks is sufficient time to determine the potential benefits from the use of resilient liner.

INDICATIONS FOR USE FOR RESILIENT LINERS
  1. Thin, non-resilient mucosal coverage
  2. Poor ridge morphology where the mental nerve is at the crest of the ridge
  3. Persistent denture sore mouth
  4. Acquired or congenital oral defects
  5. Xerostomia
  6. The need to promote mucosal healing
  7. In cases of bilateral undercuts
  8. Irregular foundation
  9. Single denture opposing natural teeth
  10. Mandibular distal extension base partial dentures
CLASSIFICATION
A.
  • 1. Short- term soft liners( tissue conditioners.)
  • 2.Long- term soft liners (Boucher)
B.
  • Heat-polymerized acrylic resin
  • Autopolymerized acrylic resin
  • Heat polymerized silicone
  • Autopolymerized silicone
  • Treatment liners ( tissue conditioners) (IJP 1990
SHORT-TERM SOFT LINERS (TISSUE CONDITIONERS)

Tissue conditioners remain soft for a limited period (days to weeks). For adequate cushioning, tissue conditioners should be replaced with afresh mix every 2 to 3 days. This procedure is continued till full recovery of tissues.

INDICATIONS
  • Treatment and conditioning of abused/ irritated denture supporting tissues prior to impression making for new dentures
  • For provisional adjunctive/ diagnostic purposes such as recovery of vertical dimension of occlusion and correcting occlusion of old prosthesis
  • Temporary relining of immediate dentures/ immediate surgical splints
  • Relining cleft palate speech aids
  • Tissue- conditioning during implant healing
  • Functional impression materials
Powder-
  • Polymer, polyethylmethacrylate(PEMA) or its copolymers
  • Liquid –Mixture of
  • Ethyl alcohol(solvent)
  • Aromatic ester- dibutyl phthalate(plasticizer)
MECHANISM OF ACTION
  • Tissue conditioners show combination of both viscous fluid and elastic solid behavior
  • Viscous behavior allows adaptation gel to inflamed/ irritated mucosa,improving the fit of denture
  • During chewing, the material demonstrate a time dependent elastic behavior that allows it to recover initial deformation, absorbing impact forces and cushioning the underlying tissues.
  • Short-term soft liners used for functional impressions differ from those used for tissue conditioning
  • Functional impression material should display good flow but with minimal elastic recovery, and exhibit adequate dimensional stability in terms of weight change , water sorption, and solubility
  • Casts should be poured immediately after removal from patient’s mouth.
DISADVANTAGES
  • Temporary nature of tissue conditioners stems from the fact that both the alcohol and Plasticizer leach out and are partially replaced with water
  • Material thus hardens within a considerably short time
  • Material becomes vulnerable to surface deterioration, contamination and fouling by microorganisms.
TECHNIQUE

To use the tissue conditioner material as an adjunct in conditioning traumatized, hypertrophic and displaced denture bearing tissues, the following technique is described.

Prerequisites for the use of this material are as follows:

That the dentures have adequate coverage of the bearing area, a good centric relation, adequate occlusal vertical dimension and no gross interferences in eccentric jaw positions; or that the dentures could have the above mentioned prerequisites incorporated with minimal adjustments. These are of paramount importance, since most of the discomfort from dentures can be attributed to poor occlusion.

PREPARATION OF THE DENTURE

From the denture base (fig 1), all undercuts and some area immediately on the ridge to a depth of 1 mm or more is removed. The borders or flangets and the hard palatal area in the maxillary denture is retained as vertical stops in seating or placing it on the ridge. If the borders are not well defined, modeling compound is used inside the denture and in occlusion, to provide a tripod reference to relate the denture when placing it back in the mouth with the conditioning material in it. Wherever the denture base is short, it should be extended using activated acrylic resin to provide support for the soft material, the important thing to remember is that the dentures should be provided with room for the conditioning material that is sufficient to allow the displaced and traumatized tissue over to a normal state.

Fig. 1
PREPARATION AND PLACEMENT OF THE TISSUE CONDITIONER IN THE MOUTH

The material used is a three component system; the polymer (powder), the monomer (liquid) and a liquid plasticizer (“Flow Control”). The ratio of use of any of these components may vary depending upon its use. For conditioning tissues, a ratio of 1 ¼ parts of polymer to 1 part of monomer is recommended with the addition of approximately ½ cc of the plasticizer (flow control). (fig. 2)

 
Fig. 2
 

The latter should be added to the liquid monomer and mixed prior to mixing it with the polymer. This differs from what the manufacturer recommends. However this modification has been found to prevent the material from getting rough and hard after four to five days in use. It makes the material soft, smoother, glossy and it prolongs its durability in use to approximately six to eight weeks without getting hard and rough.

Mix in a glass jar by slowly adding the power to the liquid and stirring continuously until the desired amount of it is incorporated in the mixture. The material will thicken by virtue of its own reaction. While the material is still creamy and running pour it into the denture.( fig.2) Make certain that the entire the denture base is covered.(fig.3,fig 4)

 
Fig. 3
 
Fig. 4
 

Where the material ceases to flow readily, insert the denture in the patient’s mouth. Slowly but firmly carry the denture to place. Use the opposing dentition as a guide to centric relation. Hold the dentures in this position at the desired occlusal vertical dimension for three minutes.(fig.5)

 
Fig. 5
 

Following this, instruct the patient to move his or her lips and cheeks to border mould the material. The excess that might be loose in the mouth is removed. By now the material will have set sufficiently that the denture can be removed and the excess material that has come out over the labial and buccal aspects can be removed or trimmed away. This can be done with a sharp knife, scalpel or scissors.(fig.6)

 
Fig. 6
 

Inspect the denture for pressure areas in which the pink part of the denture base will show through. Pressure areas should be relieved and small amounts of the material added and the denture then returned to the mouth for contouring. Once the criterion of even thickness of 1 mm or better of conditioning material is satisfied, cover the sharp cut edges as well as the material surfaces with a small amount of the “Flow Control” to allow the conditioner to continue to flow and contour itself as the tissues recover. Also this will allow the sharp edges to be rounded and become smoother and glossy as the patient functions with the denture.

Return the dentures to the mouth to check for comfort and instruct the patient in care of the denture. He or she should be told return the following day for inspection and correction of pressure areas and this procedure will have to be repeated every three or four days until the traumatized and initiated tissue have fully recovered. The patient should not eat hard to chew foods the first eight hours following the application of the material, since this may have a tendency to squeeze the conditioner out of shade, destroying what has been previously accomplished.

If maxillary and mandibular arches are both involved in the treatment, one should be treated at a time. Preferably, use the arch with the most stable denture as a guide in positioning the one being treated. Do not attempt to polish the conditioning material when used for this purpose.

Modifications of the technique according to the use of tissue conditioners

Uses

Power/liquid Ratio

Use “Flow Control”

Hardness share A

Time in use

Adjunct in tissue conditioning

1.25/1

½ cc in the liquid coat the set material with it

5

3 to 4 days

Temporary obturators

1.5/1

½ cc in the liquid; coat the set material with it

15-20*

7 to 10 days

Stabilize base plates and surgical splints

1.75/1

Not used in base plates; coat surgical splints with it

20-25*

1 to 10 days

Adjunct in the impression making procedure

1.5/1

Not used in base plates; coat surgical splints with it.

20-25*

1 to 10 days

Adjunct as diagnostic tool for resilient liner

1.5/1

½ cc in liquid coat the set material with it.

15-20*

4 to 8 weeks


Will harden With time
RESILIENT LINERS

The second group of elastomer polymers consists of the resilient liners. These materials are heat processed to the hard resin denture base and are used with the definitive prosthesis. Investigators have reported that the length of wear of these materials have ranged from six month to five years. Depending on the type of material used (Velum rubber, vinyl or acrylic resin and silicones). Much controversy exists as to whether these materials are temporary or permanent. Because currently available materials do not meet all of the essential requirements for the ideal material, they are generally considered to be temporary expedients for the immediate solution of specific denture problems. Nonetheless, they are excellent adjuncts in the prevention of the supporting tissues. Resilient materials that serve for more than two year can be classified as essentially adequate for this purpose.


COMPOSITION AND CHARACTER

The materials used for resilient liners have been velum rubber, vinyl and vinyl acrylic polymers acrylic polymers and silicone elastomers. The vinyl and acrylic polymers are made resilient by the addition of oily or alcohol-type plasticizers, or by copolymerization with the monomer units. More recently, the hydrophilic polymer has been developed.

USE OF RESILIENT LINERS
  • Ridge Atrophy Or Resorption
  • Surgery Contraindicated
  • The Bruxer (Broxomania)
  • Relif Areas
  • Restoration Of Congenital Or Acquired Oral Defect
  • Xerostomia
  • Edentulous Arch Opposing Natural Dentition
HOME RELINERS

Over the counter reliners or home reliners though not available in India, but available in the west cause more harm than good. This is because the customer believes the material can be used indefinitely, understanding only that new applications or changes must be made periodically. Because the customer has had no professional training and very likely, inadequate professional instruction in regard to his dentures and their maintenance, he is not expected to know of the changes occurring in his oral tissues. Home reliner materials either induce or, certainly, perpetuate pathologic changes in the oral tissues. The affected tissues are the mucosa of the denture bearing areas and the supporting alveolar bone. Each additional application of the home reliner requires more material. This indicates bone resorption which creates more space between the tissue surface of the denture and the residual alveolar ridge.

When the home reliner material is applied to the denture and the denture is placed in the mouth, there is noticeable immediately an increase in the vertical dimension of occlusion with a concomitant decrease in the interocclusal

distance. With insufficient interocclusal distance, the soft tissues are traumatized and resorption and deformation of the underlying bone occurs.

For these reasons the dental profession must be more emphatic in its instructions to denture patients. Patients must be instructed to return to the dentist for adjustments or corrections instead of using home reliner materials applied by an untrained individual.

Some specific controls regarding the sale of home reliner materials to the general public should be instituted by the appropriate federal agency..

CONCLUSION

The application of elastomer polymers in the prevention and treatment of chronic tissue irritation from denture is an excellent alternative to the use of hard polymer resins and it is useful for preserving the health of the remaining denture supporting tissue. Wider applications will be found in the future once the present short-coming of the available materials are overcome, whether by improving these materials or by developing new ones.

REFERENCES
  1. W.W. Chase Journal of Prosthetic Dentistry, 11:804-815,1961
  2. Eisenring Re. Belge Stomatol, 52:97-119,1955
  3. Gole, Smith, Plein Journal of Prosthetic Dentistry,50:466-472,1983
  4. J.B. Gonzalez and W.R. Laney Journal of Prosthetic Dentistry, 18: 438- 444.1966
  5. Graham, Jones, Sutow J.Dent. Res, 70(5):870-873,1991
  6. Hall J.A.Dent 8:919,1951
  7. Le Mar W Marris Journal of Prosthetic Dentistry, 3:178-180, 1953
  8. Ostlund Acta Odotol Scand, 16: 1-23, 1957
  9. Lytle Journal of Prosthetic Dentistry, 7:27-42, 1958
  10. Travalgini, Gibbons, Craig Journal of Prosthetic Dentistry 1960, 10: 664-672,
  11. W. W. Crase J Journal of Prosthetic Dentistry 1961, 11, 675-681
  12. J.E.Robinson Journal of Prosthetic Dentistry, 13: 669-675, 1963
  13. D.R. Means Journal of Prosthetic Dentistry, 14: 623-634,1964
  14. Bruno Tassarotti Journal of Prosthetic Dentistry, 28: 13-18, 1972
  15. Amin WM, Fletcher AM, Ritchie GM: The nature of interface between polymethyl methacrylate denture base materials and soft lining materials. J Dent 1981; 9: 336-346.
  16. Brown D: Resilient soft liners and tissue conditioners: Br Dent J
  17. Shock-absorbing behavior of four processed soft denture liners Original Research Article. The Journal of Prosthetic Dentistry, Volume 72, Issue 6, December 1994, Pages 599-605 .Fumiaki Kawano, Masayuki Kon, Andrew Koran, Naoyuki Matsumoto