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Pros and cons of hard and resilient denture base materials
General use of metal or resilient denture base liners is not recommended. Results obtained with these materials are varied and unpredictable. The needs of most edentulous patients can be met using conventional resin denture bases.
Pros and cons of hard and resilient denture base materials Dewey H. Bell, Jr., DDS Frederick J. Finnegan, DDS John E. W ard, DDS, R ich m o n d , Va
Com plete dentures serve as adequate substitutes in m ost patients who have lost their natural teeth. Yet there are a significant num ber of patients w hose dental needs are not met by conventional denture service, and these problem denture pa tients need special attention, care, and treat ment. A variety of instrum ents, techniques, and tooth forms aid the problem denture patient. Among the m aterials suggested, special denture base materials such as gold or chromium -cobalt or soft denture base liners such as silicon rubber and the soft acrylics have received much atten tion. T he metal bases or metal liners are hard, rigid, nonresilient, and heavy. T he soft denture liners are resilient, flexible (to a degree), and light. Al though there are other differences, these physi cal properties are extrem e opposites. Is it better to use a hard liner or a soft liner to provide needed retention, stability, com fort, pa tient acceptance, and preservation of the den ture supporting structures? Can a hard liner or a soft liner be used to solve the same type of clini
cal problem ? C an a hard or a soft denture liner contribute significantly tow ard solving denture problem s beyond what might be achieved with m ore conventional denture bases? T hese are som e questions that need answering.
M etal bases
■ G old bases: In 1951, S chultz1 reported on the use of gold-lined denture bases to achieve tissue com patibility, accuracy, and dim ensional stabil ity. In 1957, F ab er2 quoted Skinner3 and P eyton4 to point out the dim ensional instability of acrylic resins and suggested the use o f cast metal bases for m andibular dentures. H e listed the advan tages of the cast metal base as greater accuracy, less warpage than the acrylic resin during pro cessing, greater strength, less breakage, greater therm al conductivity, less porosity and hence less plaque retention, and less deform ation of the base in function. F ab er also stated that there JAD A , V ol. 94, M a rc h 1977 ■ 511
would be less tissue change if a metal base is used. H e said that plaque retention as a result of porosity and the poor therm al conductivity of the acrylic resin bases contributed to ridge re sorption. G rünew ald5 said that the m ost im portant fea tures o f gold-base m andibular dentures are weight and bracing, but added that close tissue adaptation and tissue tolerance also are im port ant. H e calculated the weight of the teeth and bone lost from extraction and resorption to be about 30 pennyw eight (dwt) for the average size mandible. F rom this calculation, Grünewald con cluded that the ideal weight for a gold base is 16 dwt. U sing this weight, the finished denture with porcelain teeth would about equal the weight of the lost structures. G rünew ald pointed out that m ost m andibular dentures weigh less than half as m uch as the structures they are replacing, and that this reduced weight might contribute to im proper m uscle function and reduction in normal rest vertical dimension. L ang,6 in 1974, reported on the use of gold in the construction of mandibular denture bases for 42 patients. T he degree of ridge resorption of the patients treated was not identified. Some disadvantages of gold bases are addi tional cost and difficulty in refitting. G rünew ald5 stated that gold bases are contraindicated in pa tients with noticeable loss of muscle tone and alveolar ridge structure, cancer phobia, and pa tients who feel that conventional denture bases are too heavy.
Fig 1 ■ C h ro m iu m -c o b a lt d e n tu re base lin e r w e ig h e d 12 d w t and 0.08 g rain .
Fig 2 ■ C h ro m iu m -c o b a lt d e n tu re base (b o rd e rs in c lu d e d
in
m etal ca sting ). F in al d e n tu re u s in g p o rc e la in te e th w e ig h e d 25
■ C hrom ium -cobalt alloy bases: T he chromium -cobalt alloys have at least one notable advan tage over gold as a denture base liner: greatly reduced cost. T oday, a chromium -cobalt base fabricated by a com m ercial dental laboratory costs about an eighth of that of a gold base of 16 dwt. T he chrom ium -cobalt alloys are consider ably lighter, harder, and stronger (more rigid) than gold. T hese properties give chromium-co balt alloys considerable advantage over gold in the fabrication o f metal bases for maxillary den tures, and these alloys are not too light to pro vide weight for a mandibular denture base. W here noticeable resorption has occurred, the additional thickness necessary to make a 10- to 12-dwt base is not a particular problem (Fig 1). If you have the courage to include the borders in the casting, additional weight is obtained with ease. The com pleted denture shown in Figure 2 weighed alm ost 26 dwt. T he prim ary disadvan512 ■ JAD A, V o l. 94, M a rch 1977
d w t and 17 g rain s.
tage of chrom ium -cobalt alloy bases is that they are difficult to adjust. H ow ever, with the proper equipm ent and a little extra time, these bases can be satisfactorily adjusted. ■ Alum inum bases: T he successful use of the aluminum base has been rep o rted .7,8 They are light and easy to adjust. T he cost for an aluminum base at a commercial dental laboratory is about the sam e as that for a chrom ium -cobalt base. O ther than additional cost, the disadvantages of aluminum bases are discoloration and pitting, which Lundquist8 believes are the result of expo sure to commercial denture cleaners. The metal loses its bright shiny color and becomes dull gray. The aluminum bases are so light that they cannot be used to substantially increase the weight o f the com pleted denture.
■ Discussion o f metal bases: There are some obvious contraindications for the use of metal bases. For example, you would not choose a metal denture base for an immediate denture procedure, or in any situation where a signifi cant, rapid change in the denture-bearing struc tures was anticipated. Metal bases that add sig nificant weight to the finished denture should not be used for patients who complain about the weight of conventional denture bases. There is one firm indication for the use of met al base dentures, but this indication comes “ af ter the fact,” that is, after repeated fracture or breakage of a conventional base. We have often observed that maxillary dentures fracture from use and that mandibular dentures fracture from abuse. The material we prefer for maxillary met al bases is chromium-cobalt because of its rela tive lightness, strength, low cost, and durability. Provisions must be made for ample palatal relief in the metal casting. Gold is the preferred metal for a mandibular denture base because it is dura ble, relatively easy to adjust, and its weight is sometimes desirable. When the cost of the gold base precludes its use, we have used chromiumcobalt because of its superior strength. Neces sary adjustments can be accomplished satisfac torily and without undue delays if a diamond wheel-shaped stone and a high-speed handpiece are used. The altered surfaces are polished with an abrasive rubber wheel. Other indications for metal base dentures are less clear, but they, too, generally come “ after the fact,” after patients have had disappointing denture experiences. But sometimes these pa tients experience the same degree of disappoint ment with metal bases, which have been found to have some of the same inadequacies as con ventional dentures. We also have seen metal bases used to solve denture problems that were probably caused by inadequate conventional dentures. This is not a valid basis for drawing any conclusions about metal bases, and successes gained in these in stances should not be credited to the material. We believe that most edentulous patients can be successfully treated using conventional denture bases. Thus, if a patient’s disappointing den ture experience is a result of an inadequate pros thesis, the patient should be provided with an adequate conventional denture. But a metal base can be used if the patient is willing to accept the additional cost, and if the dentist is willing to ac cept the inconveniencies of adjusting and main taining it.
■ Advantages o f metal bases: Generally, the advantages claimed for using various metal den ture bases are similar: greater strength to prevent or minimize breakage and prevent warpage and deformation of the base with use, greater accur acy because they overcome the dimensional changes that occur during the polymerization of conventional denture resins, better tissue toler ance because they are less porous, and addition al weight for increased stability. (Increased weight pertains to the mandibular denture only, and aluminum is excluded.) We believe that some of the advantages claimed for use of metal base dentures are valid: they are stronger, more rig id, and less likely to break or fracture than con ventional denture bases. Metal bases are less porous, generally heavier and more accurate, and there is less deformation of the base during function. But we question the implications that these advantages will provide increased reten tion, better stability and tissue tolerance, im proved comfort and function, and preservation of the denture foundation. Not all patients derive these benefits and, thus, the results are unpre dictable. For this reason, and because of the ad ditional cost of metal bases (particularly gold) and the increased difficulty in adjusting and re fitting, we think these bases should be used with discretion. As one of their advantages, metal bases are said to provide better tissue tolerance than con ventional denture bases. However, if tissue tol erance is a problem, most often it is related to poor oral hygiene, continuous wearing of the denture, occlusal faults that result in improper seating of the base, fungal infections, grossly ill-fitting bases, inadequate saliva for lubrication, systemic disorders, or a poor general health in dex. We find that if a satisfactory level of tissue tolerance cannot be achieved by correcting or improving these conditions, the problem is not solved by using a metal base denture. Another advantage claimed for metal bases is that they have better thermal conductivity. This advantage is considerably diminished when the metal base is encased in a bulk of acrylic res in. As yet another advantage of metal bases, less deformation during function is claimed. How ever, we believe that some deformation of the base may be advantageous, considering the way mandibular dentures move during mastication. This is purely speculative. Resin denture bases have been reported to have as much as 8V2 times as much deformation during function as metal Bell— Finnegan—Ward: DENTURE BASE MATERIALS ■ 513
denture bases.9 Does this deform ation, in part, account for the degree of success attributed to them ? Still, with all other apparent advantages of m etal base dentures, there has to be some ex planation as to why their use fails to provide m ore predictable, favorable results. Although our casting techniques for metal bases are quite sophisticated and the bases are accurate, the use of metal bases does not ensure im proved denture retention, stability, com fort, and function. P art of the explanation might be the nature of the denture foundation. C ast metal denture bases have an accuracy potential that is not reflected by the denture foundation. The shape or configuration of the denture foundation has been reported to change during the day; its shape is not co nstant.10 The changes that occur may minimize the effectiveness of an accurate m etal base. T he following illustrates the potential change in the denture foundation under com plete den tures. A p atien t’s dentures w ere rem oved for 48 hours prior to rebasing her m axillary denture. T he denture was in the lab for 24 hours, and the degree of retention was zero when it was insert ed. T he tissue contact as registered with a pres sure indicator paste is shown in Figure 3. We cannot believe that the denture base was that in accurate. Using a pressure-sensitive paste and a steel burr for adjusting acrylic resin, the tissue surface was reshaped to a point of gaining reten tion. This patient has been observed for seven years, and the tissue response has been rem ark able considering the circum stances under which the denture was inserted. A lthough it is not desirable for the oral muco-
F ig 4 ■ A d a p ta tio n o f pala ta l m u c o s a to d e n tu re base w ith la rg e re lie f area.
sa to have to adapt to the denture base, it can and it does (Fig 4). M ost patients who rem ove their dentures at night experience some looseness when the dentures are first inserted the next morning. Within 5 to 15 m inutes, the dentures are snug again. T he soft tissues conform to the pressures of denture bases and they can be in jured if the pressures are great. W hen these pres sures are substantial and sustained for a long period of time, even bony changes have been re p o rted .11
R esilien t d e n tu re liners
One o f the realities in conventional com plete denture therapy is the “ sandwiching effect” of the resilient m ucoperiosteum of the denturebearing area betw een two nonresilient substanc es, the supporting bone and the tissue surface of the denture base. ■ R e s ilie n t lin e rs a n d c o m fo r t a n d o r a l h e a lth :
Fig 3 ■ P re ssure in d ic a to r paste re co rd o f d e n tu re c o n ta c t a t in s e rtio n a fte r rebasing.
514 ■ JADA, V ol. 94, M arch 1977
With few exceptions, most denture patients will experience im proved comfort and oral health with use o f one of the many tissue treatm ent m aterials available. H ow ever, when it becom es necessary to convert to one of the more durable and serviceable resilient denture base liners, pa tient discom fort and mucosal irritation often recur. Resilient denture base liners and tissue treat ment m aterials have different characteristics. T he tissue treatm ent m aterials retain their resil iency over days or w eeks, whereas denture base liners retain their resiliency over months or years. W hen tissue treatm ent material is being used therapeutically, it is changed every few
days. In effect, the denture receives a new base every few days; thus, it is thoroughly cleansed and its resiliency is renewed. This may be one reason for the superior clinical response ob tained with the tissue treatment material. A second noticeable difference between these two materials is the greater degree of resiliency of the tissue treatment materials initially, and when the material is mixed using a greater amount of liquid to gain additional softness. Another reason for superior clinical response with the tissue treatment materials may be a re sult of the difference in surface texture. The sur face texture of some of the resilient denture lin ers is coarse compared with that of most tissue treatment materials. Another factor affecting the surface of resilient denture base liners is the surface texture that remains after attempts to adjust the denture liners. We have found no satis factory method of producing a well-polished sur face to the adjusted resilient denture base liners of the silicone rubber type. The difference in the thickness of the resilient layer may account for different clinical respons es. When using tissue treatment materials, space is provided for a generous layer of the resilient material. Since the treatment period is generally short, we are not as concerned with denture frac ture. However, the latter problem must be con sidered in the construction of the final denture, and this often restricts the thickness of the resil ient liner. Finally, patients generally are selective about their foods during treatment with tissue-conditioning materials. Once the treatment is com pleted and the new dentures are inserted, pa tients are eager to test their functional capabili ties. The sudden increased functional demands on the supporting mucosa may account for some disappointing results. ■ Resilient liners and xerostomia: The surface texture of some of the resilient liners seems to generate increased frictional resistance causing irritation of the oral mucosa. We have noted this problem in several instances when resilient liners were used in the mandibular denture for patients with moderate to severe xerostomia. The prob lem can be minimized using a conventional resin denture base with a polished tissue surface. The surface texture of some of the resilient liners is not as smooth as that of a well-processed resin denture base, particularly if the latter is carefully polished.
■ Resilient liners and bony undercuts: Resilient denture base liners have been used to spring over undercuts in the denture-bearing area to provide increased retention or to avoid surgical removal of the undercut. Use of the resilient liners in this situation can work well, but it is not without its problems. When the undercut is significant, it is advisable that the denture border be composed of the resilient material. A single bony undercut, unless it is severe, poses no special problem in denture placement; the denture can be placed using a rotational path of placement. It is the op posing bony undercuts that cause problems in denture placement. If these are small, they usu ally can be managed with conventional denture bases, using a specific path of placement and moderate adjustments on the denture borders. If, because of their size, the undercuts cannot be managed this way, surgical reduction will pro vide a much more desirable end result than the use of a resilient liner. If surgical reduction of the undercut would reduce the denture-bearing area significantly, or if surgery is risky, the use of a resilient liner is preferred. Resilient liners also are suggested in instances of low tissue tolerances that are not caused by local factors such as ill-fitting dentures, faulty occlusion, or poor oral denture hygiene. These conditions usually can be managed. But low tis sue tolerances resulting from systemic diseases, metabolic disturbances, endocrine imbalances, and senility are more difficult to manage. In these instances, the mucoperiosteum undergoes var ious degrees of atrophy and degeneration. These tissues often are friable, sensitive, and easily abused, and the use of a resilient liner may pro vide a measure of comfort for only some patients.
Other denture bases
■ Silica-coated denture bases: It is claimed that silica-coated denture bases have increased wet tability and have been reported to increase re tention.12,13 At Virginia Commonwealth Uni versity, over 300 mandibular denture bases were vapor coated* with a layer of silica. This was done after patients had worn the dentures two to three weeks after completion of the postinser tion adjustments. Not one patient detected increased denture Bell— Finnegan—W ard: DENTURE BASE MATERIALS ■ 515
retention after the dentures had been silica coat ed. ■ R e s ilie n t h y d ro p h ilic lin ers: Hydrophilic den
ture base liners may provide increased reten tion. Since the material readily absorbs mois ture, it is said to act like an adhesive without being an adhesive. H ow ever, three clinical eval uations of resilient hydrophilic liners showed disappointing results in increased denture reten tio n .14'16 In our opinion, their use for this pur pose is unreliable.
Factors affecting denture stability
■ Irr e g u la r itie s o f m u c o s a : T he trem endous am ount of denture base m ovem ent shown in som e studies is intriguing.17' 19 These findings have led us to believe that m inute accuracy of the m andibular denture base is not always neces sary, nor desirable. When there has been notice able loss o f the alveolar ridge and the oral m u cosa presents many minute folds and irregulari ties, the resulting im pression and subsequent denture base presents many irregularities. With the assum ption that the m andibular denture base will be in constant motion during function, these irregularities have been carefully removed and the tissue surface has been given a sm ooth, high polish; this has led to amazingly good results (Fig 5). T he rationale for this highly irregular procedure is to present a sm ooth surface to the m ucosa as the denture base shifts over it during function. F o r years we have advocated this pro cedure in reverse. We provide a sm ooth, round, highly polished surface for the denture borders in those areas where we anticipate the tissues to be moving against the denture border. A polished tissue surface is advocated in those instances w here past experience has dem onstrated notice able m andibular instability, or where it is bound to occur. ■ M a s tic a to r y h a b its : H ow patients chew with
their dentures is another factor that influences denture stability. M ost patients chew on one side and some switch from one side to the other, which invites denture instability. T he degree of instability can be readily dem onstrated if the pa tient sim ulates chewing on one side, using a small cotton roll (Fig 6). Sim ultaneous, bilateral mas516 ■ JADA, V o l. 94, M a rch 1977
F ig 5 ■ T o p , p o lis h e d tis s u e s u rfa c e o f m a n d ib u la r d e n tu re b ase s to Im p ro v e s o ft tis s u e re s po n se to p o lis h e d d e n tu re base s ix years p o s tin s e rtio n . B o tto m , n in e years a fte r in s e rtio n .
F ig 6 ■ D egree o f in s ta b ility can be d e m o n s tra te d to p a tie n t u s in g a sm a ll c o tto n ro ll. W h e n th e p a tie n t b ite s on o n e side, th e d e n tu re s “ ju m p ” to g e th e r o n o th e r side.
tication provides the best opportunity for m an dibular stability during m astication, and this, too, can be dem onstrated with cotton rolls. Pa tients, particularly those with noticeable alveolar resorption, should practice bilateral m astication until they have m astered it. A patient’s ability to m aster the chewing tech nique can be tested. Place two small, wet pieces of cotton roll on the patient’s tongue (Fig 7, left).
F ig 7 ■ Left, tw o sm a ll, w et p ie ce s o f c o tto n ro ll a re p la c e d on th e p a tie n t's to n g u e . R ig h t, c o tto n ro lls are used to d e m o n s tra te to p a tie n t d e g re e o f d e n tu re s ta b ility a tta in e d w ith b ila te ra l fu n c tio n . D iffic u lty p a tie n t h as in p o s itio n in g c o tto n ro lls in d i ca te s w h a t he m ay e n c o u n te r later.
T he difficulty the patient has in positioning the pieces of cotton to sim ulate bilateral mastication (Fig 7, right) will be an indication of the degree of difficulty he may encounter later. ■ M uscular coordination: N eurom uscular con trol, patient adaptation, or “ denture know how ” also is related to denture stability. It has been noted that patients with this kind of control get along rem arkably well with alm ost any kind of denture base. M ost of us have seen patients w ear ing a m andibular denture that has been broken into two or more pieces, and a few patients can even chew with them . We have seen patients wearing a maxillary denture with a hole in it the size of a dime. We have seen others wearing the typical “ ridge runner” type of m andibular den ture and enjoying adequate function and excel lent mucosal health. Perhaps we should learn m ore about neurom uscular control.
Summary Some of the pros and cons of hard and resilient denture base liners have been discussed. Within the present stage of m aterials developm ent, the scope of current biological knowledge, and the bounds of clinical experience, the general use of hard and soft m aterials as denture base liners is not justified. W here there appears to be an in dication for their use, the results obtained are varied and unpredictable. T he needs o f most edentulous patients can be met using conven tional resin denture bases com bined with sound prosthodontic principles and practices. T he many shortcom ings of the available resil ient denture base liners often create more prob lems than they solve. They need to be regular ly examined and replaced often. Although resil
ient liners may increase patient com fort and tis sue tolerance, their use does not always pro vide these benefits. T he problem is finding the right resilient material. Although it can be predicted th at metal base dentures will prevent or minimize repeated breakage or fracture of the denture base, other indications for their use are less clear. T here is often a psychological advantage in providing special treatm ent such as a special kind of denture base that enhances patient ac ceptance. This kind of patient response com pli cates evaluation of the use of unconventional denture bases. T he choice of a denture base m aterial should be based on patient needs, clinical findings, ex perience, and clinical judgm ent, rather than on hopes and frustrations. H ow ever, the limitations of our knowledge and the biologic and anatom ic shortcom ings in a specific patient will occasion ally force a decision as “ the only way to go. ” Additional research is needed in m aterials de velopm ent and clinical testing before the indica tions for these special kinds o f denture bases are clear, and their possible benefits can be more accurately predicted.
P re sen ted b e fo re th e A m e ric a n D ental A s s o c ia tio n a t th e ADA -F e d e ra tio n D e n ta ire In te rn a tio n a le m e e tin g in C h ic a g o , O c to b e r 1975. Dr. B e ll is p ro fe s s o r and c h a irm a n o f re m o v a b le p ro s th o d o n tlc s at th e M e dica l C o lle g e o f V irg in ia , S c h o o l o f D e n tis try , R ic h m o n d , V a 23298. Dr. F in n e g a n is a s s o c ia te c lin ic a l p ro fe s s o r and Dr. W ard is a s s is ta n t p ro fe s s o r a t th e M e d ic a l C o lle g e o f V irg in ia , S c h o o l o f D e n tistry. A d d re s s re q u e s ts fo r re p rin ts to Dr. B e ll. ♦D u ra b o n d process. D u ra lliu m Dlv., J. F. J e le n k o & C o., C h i cago, 60607. 1. S c h u ltz , A.W . C o m fo rt and c h e w in g e ffic ie n c y in d e n tu re s . J P ro s th e t D e n t 1 :38 J a n -M a rc h 1951. B e ll— F in n e g a n — W a rd : DENTURE BASE M A TE R IA LS ■ 517
2. Faber, B.L. Low er cast metal base denture. J Prosthet Dent 7:51 Jan 1957. 3. S kinn er, E.W. A crylic denture base m aterial: their physical properties and m anipulation, J Prosthet D ent 1:161 Jan-M arch 1951.
12. O'B rien, W.J., and Ryge, G. W ettability of poly- methyl m ethacrylate treated with silicon tetrachloride. J Prosthet D ent 15:671 July-Aug 1965. 13. B oucher, L. J., and others. T h e effects of a microlayer of silica on the retention of m andibular com plete dentures. J Pros
4. Peyton, F.A. Physical and clinical characteristics of syn thetic resins used in dentistry. JADA 30:1179 Aug 1943.
thet Dent 19:581 June 1968. 14. Bla'hova', Z., and N eum ann, M. R etention of com pleted
5. G rünew ald, A.H. Gold base lower dentures. J Prosthet Dent 14:432 M ay-June 1964.
dentures lined with soft-curing resins. J Prosthet D ent 25:371 April 1971.
6. Lang, B.R. Th e use of gold in construction of m andibular d entu re bases. J Prosthet Dent 32:398 O ct 1974.
15. Bell, Jr., D.H. Clinical evaluation of a resilient denture liner. J Prosthet Dent 23:394 April 1970.
7. C am pb ell, D.D. The cast alum inum base denture. JADA 23: 1264, 1936.
16. Battersby, B.J.; Gehl, D.H.; and O ’Brien, W.J. Effect of an elastic lining of th e retention of dentures. J Prosthet Dent 20:498
8. Lundquist, D.O. Alum inum alloy as a denture-base m aterial. J Prosthet Dent 13:102 Jan-Feb 1963.
D ec 1968.
9. R egli, C.P., and Kydd, W.L. Prelim inary study of the lateral deform ation of m etal base dentures in relation to plastic base dentures. J Prosthet D ent 3:326 May 1953. 10. Stephens, A.P.; Cox, C.M.; and Sharry, J.J. Diurnal varia tion in palatal tissue thickness. J Prosthet Dent 16:661 July-Aug 1966. 11. Lytle, R.B. C om plete denture construction based on a study of th e de form ation of the underlying soft tissues. J Prosthet Dent 9:53 9 July-Aug 1959.
518 ■ JADA, Vol. 94, March 1977
17. Sm ith, D.E., and others. M obility of artificial dentures d u r ing com m unication. J Prosthet D ent 13:839 S ept-O ct 1963. 18. W oelfel, J.B.; Hickey, J.C.; and Allison, M .L. Effects of pos terior tooth form on jaw and denture m ovement. J Prosthet D ent 12:922 S ept-O ct 1962. 19. Ardran, G .M ., Kemp, F.H. and M unz, F.R.: Observations on th e behavior of full low er dentures. An X -ray cinem atographic study with special reference to the perspextray im pression m eth od. Dent Pract D ent Rec 7:180 M arch 1957.
Pros and cons of hard and resilient denture base materials Dewey H. Bell, Jr., DDS Frederick J. Finnegan, DDS John E. W ard, DDS, R ich m o n d , Va
Com plete dentures serve as adequate substitutes in m ost patients who have lost their natural teeth. Yet there are a significant num ber of patients w hose dental needs are not met by conventional denture service, and these problem denture pa tients need special attention, care, and treat ment. A variety of instrum ents, techniques, and tooth forms aid the problem denture patient. Among the m aterials suggested, special denture base materials such as gold or chromium -cobalt or soft denture base liners such as silicon rubber and the soft acrylics have received much atten tion. T he metal bases or metal liners are hard, rigid, nonresilient, and heavy. T he soft denture liners are resilient, flexible (to a degree), and light. Al though there are other differences, these physi cal properties are extrem e opposites. Is it better to use a hard liner or a soft liner to provide needed retention, stability, com fort, pa tient acceptance, and preservation of the den ture supporting structures? Can a hard liner or a soft liner be used to solve the same type of clini
cal problem ? C an a hard or a soft denture liner contribute significantly tow ard solving denture problem s beyond what might be achieved with m ore conventional denture bases? T hese are som e questions that need answering.
M etal bases
■ G old bases: In 1951, S chultz1 reported on the use of gold-lined denture bases to achieve tissue com patibility, accuracy, and dim ensional stabil ity. In 1957, F ab er2 quoted Skinner3 and P eyton4 to point out the dim ensional instability of acrylic resins and suggested the use o f cast metal bases for m andibular dentures. H e listed the advan tages of the cast metal base as greater accuracy, less warpage than the acrylic resin during pro cessing, greater strength, less breakage, greater therm al conductivity, less porosity and hence less plaque retention, and less deform ation of the base in function. F ab er also stated that there JAD A , V ol. 94, M a rc h 1977 ■ 511
would be less tissue change if a metal base is used. H e said that plaque retention as a result of porosity and the poor therm al conductivity of the acrylic resin bases contributed to ridge re sorption. G rünew ald5 said that the m ost im portant fea tures o f gold-base m andibular dentures are weight and bracing, but added that close tissue adaptation and tissue tolerance also are im port ant. H e calculated the weight of the teeth and bone lost from extraction and resorption to be about 30 pennyw eight (dwt) for the average size mandible. F rom this calculation, Grünewald con cluded that the ideal weight for a gold base is 16 dwt. U sing this weight, the finished denture with porcelain teeth would about equal the weight of the lost structures. G rünew ald pointed out that m ost m andibular dentures weigh less than half as m uch as the structures they are replacing, and that this reduced weight might contribute to im proper m uscle function and reduction in normal rest vertical dimension. L ang,6 in 1974, reported on the use of gold in the construction of mandibular denture bases for 42 patients. T he degree of ridge resorption of the patients treated was not identified. Some disadvantages of gold bases are addi tional cost and difficulty in refitting. G rünew ald5 stated that gold bases are contraindicated in pa tients with noticeable loss of muscle tone and alveolar ridge structure, cancer phobia, and pa tients who feel that conventional denture bases are too heavy.
Fig 1 ■ C h ro m iu m -c o b a lt d e n tu re base lin e r w e ig h e d 12 d w t and 0.08 g rain .
Fig 2 ■ C h ro m iu m -c o b a lt d e n tu re base (b o rd e rs in c lu d e d
in
m etal ca sting ). F in al d e n tu re u s in g p o rc e la in te e th w e ig h e d 25
■ C hrom ium -cobalt alloy bases: T he chromium -cobalt alloys have at least one notable advan tage over gold as a denture base liner: greatly reduced cost. T oday, a chromium -cobalt base fabricated by a com m ercial dental laboratory costs about an eighth of that of a gold base of 16 dwt. T he chrom ium -cobalt alloys are consider ably lighter, harder, and stronger (more rigid) than gold. T hese properties give chromium-co balt alloys considerable advantage over gold in the fabrication o f metal bases for maxillary den tures, and these alloys are not too light to pro vide weight for a mandibular denture base. W here noticeable resorption has occurred, the additional thickness necessary to make a 10- to 12-dwt base is not a particular problem (Fig 1). If you have the courage to include the borders in the casting, additional weight is obtained with ease. The com pleted denture shown in Figure 2 weighed alm ost 26 dwt. T he prim ary disadvan512 ■ JAD A, V o l. 94, M a rch 1977
d w t and 17 g rain s.
tage of chrom ium -cobalt alloy bases is that they are difficult to adjust. H ow ever, with the proper equipm ent and a little extra time, these bases can be satisfactorily adjusted. ■ Alum inum bases: T he successful use of the aluminum base has been rep o rted .7,8 They are light and easy to adjust. T he cost for an aluminum base at a commercial dental laboratory is about the sam e as that for a chrom ium -cobalt base. O ther than additional cost, the disadvantages of aluminum bases are discoloration and pitting, which Lundquist8 believes are the result of expo sure to commercial denture cleaners. The metal loses its bright shiny color and becomes dull gray. The aluminum bases are so light that they cannot be used to substantially increase the weight o f the com pleted denture.
■ Discussion o f metal bases: There are some obvious contraindications for the use of metal bases. For example, you would not choose a metal denture base for an immediate denture procedure, or in any situation where a signifi cant, rapid change in the denture-bearing struc tures was anticipated. Metal bases that add sig nificant weight to the finished denture should not be used for patients who complain about the weight of conventional denture bases. There is one firm indication for the use of met al base dentures, but this indication comes “ af ter the fact,” that is, after repeated fracture or breakage of a conventional base. We have often observed that maxillary dentures fracture from use and that mandibular dentures fracture from abuse. The material we prefer for maxillary met al bases is chromium-cobalt because of its rela tive lightness, strength, low cost, and durability. Provisions must be made for ample palatal relief in the metal casting. Gold is the preferred metal for a mandibular denture base because it is dura ble, relatively easy to adjust, and its weight is sometimes desirable. When the cost of the gold base precludes its use, we have used chromiumcobalt because of its superior strength. Neces sary adjustments can be accomplished satisfac torily and without undue delays if a diamond wheel-shaped stone and a high-speed handpiece are used. The altered surfaces are polished with an abrasive rubber wheel. Other indications for metal base dentures are less clear, but they, too, generally come “ after the fact,” after patients have had disappointing denture experiences. But sometimes these pa tients experience the same degree of disappoint ment with metal bases, which have been found to have some of the same inadequacies as con ventional dentures. We also have seen metal bases used to solve denture problems that were probably caused by inadequate conventional dentures. This is not a valid basis for drawing any conclusions about metal bases, and successes gained in these in stances should not be credited to the material. We believe that most edentulous patients can be successfully treated using conventional denture bases. Thus, if a patient’s disappointing den ture experience is a result of an inadequate pros thesis, the patient should be provided with an adequate conventional denture. But a metal base can be used if the patient is willing to accept the additional cost, and if the dentist is willing to ac cept the inconveniencies of adjusting and main taining it.
■ Advantages o f metal bases: Generally, the advantages claimed for using various metal den ture bases are similar: greater strength to prevent or minimize breakage and prevent warpage and deformation of the base with use, greater accur acy because they overcome the dimensional changes that occur during the polymerization of conventional denture resins, better tissue toler ance because they are less porous, and addition al weight for increased stability. (Increased weight pertains to the mandibular denture only, and aluminum is excluded.) We believe that some of the advantages claimed for use of metal base dentures are valid: they are stronger, more rig id, and less likely to break or fracture than con ventional denture bases. Metal bases are less porous, generally heavier and more accurate, and there is less deformation of the base during function. But we question the implications that these advantages will provide increased reten tion, better stability and tissue tolerance, im proved comfort and function, and preservation of the denture foundation. Not all patients derive these benefits and, thus, the results are unpre dictable. For this reason, and because of the ad ditional cost of metal bases (particularly gold) and the increased difficulty in adjusting and re fitting, we think these bases should be used with discretion. As one of their advantages, metal bases are said to provide better tissue tolerance than con ventional denture bases. However, if tissue tol erance is a problem, most often it is related to poor oral hygiene, continuous wearing of the denture, occlusal faults that result in improper seating of the base, fungal infections, grossly ill-fitting bases, inadequate saliva for lubrication, systemic disorders, or a poor general health in dex. We find that if a satisfactory level of tissue tolerance cannot be achieved by correcting or improving these conditions, the problem is not solved by using a metal base denture. Another advantage claimed for metal bases is that they have better thermal conductivity. This advantage is considerably diminished when the metal base is encased in a bulk of acrylic res in. As yet another advantage of metal bases, less deformation during function is claimed. How ever, we believe that some deformation of the base may be advantageous, considering the way mandibular dentures move during mastication. This is purely speculative. Resin denture bases have been reported to have as much as 8V2 times as much deformation during function as metal Bell— Finnegan—Ward: DENTURE BASE MATERIALS ■ 513
denture bases.9 Does this deform ation, in part, account for the degree of success attributed to them ? Still, with all other apparent advantages of m etal base dentures, there has to be some ex planation as to why their use fails to provide m ore predictable, favorable results. Although our casting techniques for metal bases are quite sophisticated and the bases are accurate, the use of metal bases does not ensure im proved denture retention, stability, com fort, and function. P art of the explanation might be the nature of the denture foundation. C ast metal denture bases have an accuracy potential that is not reflected by the denture foundation. The shape or configuration of the denture foundation has been reported to change during the day; its shape is not co nstant.10 The changes that occur may minimize the effectiveness of an accurate m etal base. T he following illustrates the potential change in the denture foundation under com plete den tures. A p atien t’s dentures w ere rem oved for 48 hours prior to rebasing her m axillary denture. T he denture was in the lab for 24 hours, and the degree of retention was zero when it was insert ed. T he tissue contact as registered with a pres sure indicator paste is shown in Figure 3. We cannot believe that the denture base was that in accurate. Using a pressure-sensitive paste and a steel burr for adjusting acrylic resin, the tissue surface was reshaped to a point of gaining reten tion. This patient has been observed for seven years, and the tissue response has been rem ark able considering the circum stances under which the denture was inserted. A lthough it is not desirable for the oral muco-
F ig 4 ■ A d a p ta tio n o f pala ta l m u c o s a to d e n tu re base w ith la rg e re lie f area.
sa to have to adapt to the denture base, it can and it does (Fig 4). M ost patients who rem ove their dentures at night experience some looseness when the dentures are first inserted the next morning. Within 5 to 15 m inutes, the dentures are snug again. T he soft tissues conform to the pressures of denture bases and they can be in jured if the pressures are great. W hen these pres sures are substantial and sustained for a long period of time, even bony changes have been re p o rted .11
R esilien t d e n tu re liners
One o f the realities in conventional com plete denture therapy is the “ sandwiching effect” of the resilient m ucoperiosteum of the denturebearing area betw een two nonresilient substanc es, the supporting bone and the tissue surface of the denture base. ■ R e s ilie n t lin e rs a n d c o m fo r t a n d o r a l h e a lth :
Fig 3 ■ P re ssure in d ic a to r paste re co rd o f d e n tu re c o n ta c t a t in s e rtio n a fte r rebasing.
514 ■ JADA, V ol. 94, M arch 1977
With few exceptions, most denture patients will experience im proved comfort and oral health with use o f one of the many tissue treatm ent m aterials available. H ow ever, when it becom es necessary to convert to one of the more durable and serviceable resilient denture base liners, pa tient discom fort and mucosal irritation often recur. Resilient denture base liners and tissue treat ment m aterials have different characteristics. T he tissue treatm ent m aterials retain their resil iency over days or w eeks, whereas denture base liners retain their resiliency over months or years. W hen tissue treatm ent material is being used therapeutically, it is changed every few
days. In effect, the denture receives a new base every few days; thus, it is thoroughly cleansed and its resiliency is renewed. This may be one reason for the superior clinical response ob tained with the tissue treatment material. A second noticeable difference between these two materials is the greater degree of resiliency of the tissue treatment materials initially, and when the material is mixed using a greater amount of liquid to gain additional softness. Another reason for superior clinical response with the tissue treatment materials may be a re sult of the difference in surface texture. The sur face texture of some of the resilient denture lin ers is coarse compared with that of most tissue treatment materials. Another factor affecting the surface of resilient denture base liners is the surface texture that remains after attempts to adjust the denture liners. We have found no satis factory method of producing a well-polished sur face to the adjusted resilient denture base liners of the silicone rubber type. The difference in the thickness of the resilient layer may account for different clinical respons es. When using tissue treatment materials, space is provided for a generous layer of the resilient material. Since the treatment period is generally short, we are not as concerned with denture frac ture. However, the latter problem must be con sidered in the construction of the final denture, and this often restricts the thickness of the resil ient liner. Finally, patients generally are selective about their foods during treatment with tissue-conditioning materials. Once the treatment is com pleted and the new dentures are inserted, pa tients are eager to test their functional capabili ties. The sudden increased functional demands on the supporting mucosa may account for some disappointing results. ■ Resilient liners and xerostomia: The surface texture of some of the resilient liners seems to generate increased frictional resistance causing irritation of the oral mucosa. We have noted this problem in several instances when resilient liners were used in the mandibular denture for patients with moderate to severe xerostomia. The prob lem can be minimized using a conventional resin denture base with a polished tissue surface. The surface texture of some of the resilient liners is not as smooth as that of a well-processed resin denture base, particularly if the latter is carefully polished.
■ Resilient liners and bony undercuts: Resilient denture base liners have been used to spring over undercuts in the denture-bearing area to provide increased retention or to avoid surgical removal of the undercut. Use of the resilient liners in this situation can work well, but it is not without its problems. When the undercut is significant, it is advisable that the denture border be composed of the resilient material. A single bony undercut, unless it is severe, poses no special problem in denture placement; the denture can be placed using a rotational path of placement. It is the op posing bony undercuts that cause problems in denture placement. If these are small, they usu ally can be managed with conventional denture bases, using a specific path of placement and moderate adjustments on the denture borders. If, because of their size, the undercuts cannot be managed this way, surgical reduction will pro vide a much more desirable end result than the use of a resilient liner. If surgical reduction of the undercut would reduce the denture-bearing area significantly, or if surgery is risky, the use of a resilient liner is preferred. Resilient liners also are suggested in instances of low tissue tolerances that are not caused by local factors such as ill-fitting dentures, faulty occlusion, or poor oral denture hygiene. These conditions usually can be managed. But low tis sue tolerances resulting from systemic diseases, metabolic disturbances, endocrine imbalances, and senility are more difficult to manage. In these instances, the mucoperiosteum undergoes var ious degrees of atrophy and degeneration. These tissues often are friable, sensitive, and easily abused, and the use of a resilient liner may pro vide a measure of comfort for only some patients.
Other denture bases
■ Silica-coated denture bases: It is claimed that silica-coated denture bases have increased wet tability and have been reported to increase re tention.12,13 At Virginia Commonwealth Uni versity, over 300 mandibular denture bases were vapor coated* with a layer of silica. This was done after patients had worn the dentures two to three weeks after completion of the postinser tion adjustments. Not one patient detected increased denture Bell— Finnegan—W ard: DENTURE BASE MATERIALS ■ 515
retention after the dentures had been silica coat ed. ■ R e s ilie n t h y d ro p h ilic lin ers: Hydrophilic den
ture base liners may provide increased reten tion. Since the material readily absorbs mois ture, it is said to act like an adhesive without being an adhesive. H ow ever, three clinical eval uations of resilient hydrophilic liners showed disappointing results in increased denture reten tio n .14'16 In our opinion, their use for this pur pose is unreliable.
Factors affecting denture stability
■ Irr e g u la r itie s o f m u c o s a : T he trem endous am ount of denture base m ovem ent shown in som e studies is intriguing.17' 19 These findings have led us to believe that m inute accuracy of the m andibular denture base is not always neces sary, nor desirable. When there has been notice able loss o f the alveolar ridge and the oral m u cosa presents many minute folds and irregulari ties, the resulting im pression and subsequent denture base presents many irregularities. With the assum ption that the m andibular denture base will be in constant motion during function, these irregularities have been carefully removed and the tissue surface has been given a sm ooth, high polish; this has led to amazingly good results (Fig 5). T he rationale for this highly irregular procedure is to present a sm ooth surface to the m ucosa as the denture base shifts over it during function. F o r years we have advocated this pro cedure in reverse. We provide a sm ooth, round, highly polished surface for the denture borders in those areas where we anticipate the tissues to be moving against the denture border. A polished tissue surface is advocated in those instances w here past experience has dem onstrated notice able m andibular instability, or where it is bound to occur. ■ M a s tic a to r y h a b its : H ow patients chew with
their dentures is another factor that influences denture stability. M ost patients chew on one side and some switch from one side to the other, which invites denture instability. T he degree of instability can be readily dem onstrated if the pa tient sim ulates chewing on one side, using a small cotton roll (Fig 6). Sim ultaneous, bilateral mas516 ■ JADA, V o l. 94, M a rch 1977
F ig 5 ■ T o p , p o lis h e d tis s u e s u rfa c e o f m a n d ib u la r d e n tu re b ase s to Im p ro v e s o ft tis s u e re s po n se to p o lis h e d d e n tu re base s ix years p o s tin s e rtio n . B o tto m , n in e years a fte r in s e rtio n .
F ig 6 ■ D egree o f in s ta b ility can be d e m o n s tra te d to p a tie n t u s in g a sm a ll c o tto n ro ll. W h e n th e p a tie n t b ite s on o n e side, th e d e n tu re s “ ju m p ” to g e th e r o n o th e r side.
tication provides the best opportunity for m an dibular stability during m astication, and this, too, can be dem onstrated with cotton rolls. Pa tients, particularly those with noticeable alveolar resorption, should practice bilateral m astication until they have m astered it. A patient’s ability to m aster the chewing tech nique can be tested. Place two small, wet pieces of cotton roll on the patient’s tongue (Fig 7, left).
F ig 7 ■ Left, tw o sm a ll, w et p ie ce s o f c o tto n ro ll a re p la c e d on th e p a tie n t's to n g u e . R ig h t, c o tto n ro lls are used to d e m o n s tra te to p a tie n t d e g re e o f d e n tu re s ta b ility a tta in e d w ith b ila te ra l fu n c tio n . D iffic u lty p a tie n t h as in p o s itio n in g c o tto n ro lls in d i ca te s w h a t he m ay e n c o u n te r later.
T he difficulty the patient has in positioning the pieces of cotton to sim ulate bilateral mastication (Fig 7, right) will be an indication of the degree of difficulty he may encounter later. ■ M uscular coordination: N eurom uscular con trol, patient adaptation, or “ denture know how ” also is related to denture stability. It has been noted that patients with this kind of control get along rem arkably well with alm ost any kind of denture base. M ost of us have seen patients w ear ing a m andibular denture that has been broken into two or more pieces, and a few patients can even chew with them . We have seen patients wearing a maxillary denture with a hole in it the size of a dime. We have seen others wearing the typical “ ridge runner” type of m andibular den ture and enjoying adequate function and excel lent mucosal health. Perhaps we should learn m ore about neurom uscular control.
Summary Some of the pros and cons of hard and resilient denture base liners have been discussed. Within the present stage of m aterials developm ent, the scope of current biological knowledge, and the bounds of clinical experience, the general use of hard and soft m aterials as denture base liners is not justified. W here there appears to be an in dication for their use, the results obtained are varied and unpredictable. T he needs o f most edentulous patients can be met using conven tional resin denture bases com bined with sound prosthodontic principles and practices. T he many shortcom ings of the available resil ient denture base liners often create more prob lems than they solve. They need to be regular ly examined and replaced often. Although resil
ient liners may increase patient com fort and tis sue tolerance, their use does not always pro vide these benefits. T he problem is finding the right resilient material. Although it can be predicted th at metal base dentures will prevent or minimize repeated breakage or fracture of the denture base, other indications for their use are less clear. T here is often a psychological advantage in providing special treatm ent such as a special kind of denture base that enhances patient ac ceptance. This kind of patient response com pli cates evaluation of the use of unconventional denture bases. T he choice of a denture base m aterial should be based on patient needs, clinical findings, ex perience, and clinical judgm ent, rather than on hopes and frustrations. H ow ever, the limitations of our knowledge and the biologic and anatom ic shortcom ings in a specific patient will occasion ally force a decision as “ the only way to go. ” Additional research is needed in m aterials de velopm ent and clinical testing before the indica tions for these special kinds o f denture bases are clear, and their possible benefits can be more accurately predicted.
P re sen ted b e fo re th e A m e ric a n D ental A s s o c ia tio n a t th e ADA -F e d e ra tio n D e n ta ire In te rn a tio n a le m e e tin g in C h ic a g o , O c to b e r 1975. Dr. B e ll is p ro fe s s o r and c h a irm a n o f re m o v a b le p ro s th o d o n tlc s at th e M e dica l C o lle g e o f V irg in ia , S c h o o l o f D e n tis try , R ic h m o n d , V a 23298. Dr. F in n e g a n is a s s o c ia te c lin ic a l p ro fe s s o r and Dr. W ard is a s s is ta n t p ro fe s s o r a t th e M e d ic a l C o lle g e o f V irg in ia , S c h o o l o f D e n tistry. A d d re s s re q u e s ts fo r re p rin ts to Dr. B e ll. ♦D u ra b o n d process. D u ra lliu m Dlv., J. F. J e le n k o & C o., C h i cago, 60607. 1. S c h u ltz , A.W . C o m fo rt and c h e w in g e ffic ie n c y in d e n tu re s . J P ro s th e t D e n t 1 :38 J a n -M a rc h 1951. B e ll— F in n e g a n — W a rd : DENTURE BASE M A TE R IA LS ■ 517
2. Faber, B.L. Low er cast metal base denture. J Prosthet Dent 7:51 Jan 1957. 3. S kinn er, E.W. A crylic denture base m aterial: their physical properties and m anipulation, J Prosthet D ent 1:161 Jan-M arch 1951.
12. O'B rien, W.J., and Ryge, G. W ettability of poly- methyl m ethacrylate treated with silicon tetrachloride. J Prosthet D ent 15:671 July-Aug 1965. 13. B oucher, L. J., and others. T h e effects of a microlayer of silica on the retention of m andibular com plete dentures. J Pros
4. Peyton, F.A. Physical and clinical characteristics of syn thetic resins used in dentistry. JADA 30:1179 Aug 1943.
thet Dent 19:581 June 1968. 14. Bla'hova', Z., and N eum ann, M. R etention of com pleted
5. G rünew ald, A.H. Gold base lower dentures. J Prosthet Dent 14:432 M ay-June 1964.
dentures lined with soft-curing resins. J Prosthet D ent 25:371 April 1971.
6. Lang, B.R. Th e use of gold in construction of m andibular d entu re bases. J Prosthet Dent 32:398 O ct 1974.
15. Bell, Jr., D.H. Clinical evaluation of a resilient denture liner. J Prosthet Dent 23:394 April 1970.
7. C am pb ell, D.D. The cast alum inum base denture. JADA 23: 1264, 1936.
16. Battersby, B.J.; Gehl, D.H.; and O ’Brien, W.J. Effect of an elastic lining of th e retention of dentures. J Prosthet Dent 20:498
8. Lundquist, D.O. Alum inum alloy as a denture-base m aterial. J Prosthet Dent 13:102 Jan-Feb 1963.
D ec 1968.
9. R egli, C.P., and Kydd, W.L. Prelim inary study of the lateral deform ation of m etal base dentures in relation to plastic base dentures. J Prosthet D ent 3:326 May 1953. 10. Stephens, A.P.; Cox, C.M.; and Sharry, J.J. Diurnal varia tion in palatal tissue thickness. J Prosthet Dent 16:661 July-Aug 1966. 11. Lytle, R.B. C om plete denture construction based on a study of th e de form ation of the underlying soft tissues. J Prosthet Dent 9:53 9 July-Aug 1959.
518 ■ JADA, Vol. 94, March 1977
17. Sm ith, D.E., and others. M obility of artificial dentures d u r ing com m unication. J Prosthet D ent 13:839 S ept-O ct 1963. 18. W oelfel, J.B.; Hickey, J.C.; and Allison, M .L. Effects of pos terior tooth form on jaw and denture m ovement. J Prosthet D ent 12:922 S ept-O ct 1962. 19. Ardran, G .M ., Kemp, F.H. and M unz, F.R.: Observations on th e behavior of full low er dentures. An X -ray cinem atographic study with special reference to the perspextray im pression m eth od. Dent Pract D ent Rec 7:180 M arch 1957.