Mechanical properties of tissue conditioners. Part I: Theoretical considerations, behavioral characteristics, and tensile properties J. A. McCarthy, Sorthwestern
B.D.Sc., F.R.A.C.D.S.,
University
M.S.,* and J. B. Moser, M.S., Ph.D.**
Dental School, Chicago,
Ill.
T
he inflammation and accompanying edema of the soft tissues underlying ill-fitting dentures have been the concern of dentists for many years. Hall,’ in 1921, believed that this condition was caused by displacement of the tissues during impression making. It has been demonstrated, however, that such changes result primarily from ill-fitting or poorly constructed dentures,? and that this condition can return to normal if the patient does not wear the dentures for 48 to 72 hours.? The long-term effects of such ill-fitting dentures are believed to include patient discomfort and destruction of the supporting alveolar bone. ’ The use of a material designed to “recondition” such abused tissues and restore a normal healthy state was first reported in 1961.a Since its inception, this new class of dental materials has stimulated several clinical and laboratory studies, and several manufacturers have developed products that are designed to produce “tissue conditioning” of the soft tissues underlying artificial dentures. However, compared to other commonly used classes of dental materials, comparatively few research results on these materials have been published. To date, no standards have been adopted by the American Dental Association for the guidance of dentists or manufacturers. Aside from use as a tissue conditioner, or, more aptly, a reconditioner of abused tissues, this class of dental materials has also been advocated as useful for the development of functional impressions prior to further treatment+” and as useful expedients in immediate denture construction, cleft palate speech From a thesis presented as partial fulfillment .Mastcr of Science degree in Prosthodontics Univenity *Associate ‘*.4ssociate
Dental Professor, Professor,
0022-.7913/78/0140-0089U)0.90/0Q
School, Chicago, 111. Department of Removable Department
of Biological
1970 ‘l-he
of requirements for a at Northwestern Prosthodontics. Materials.
C. V. Mosb)
Co.
aids, immediate surgical splints, stents for hemophiliacs, and postsurgical periodontal packs.;, ’ The literature is concentrated, however, upon the two major uses of this material, i.e., as a conditioning material per se and as a means of generating and recording a functional impression. Whereas there is a general consensus as to the efficacy of these materials as tissue conditioners, the published reports suggest a controversy as to the rationale underlying their utilization as impression materials.
THEORETICAL
CONSIDERATIONS
The theoretical requirements of a material that is at once effective in tissue conditioning and in functional impressions must include: (1) The material should be neither directly nor indirectly toxic in the environment and manner in which it is employed. (2) The material should be easily managed, both in terms of manipulation and removal (3) The manipulative variables affecting its behavior should be known and readily controllable. Aside from these general prerequisites, there is also a need to understand the physical properties necessary for the effective use of a material in either situation. Tissue conditioners. The efficacy of these materials, when used as conditioners, derives from the fact that they flow readily throughout the denture when it is inserted in the mouth. Through close adaptation to the tissues, the functional stresses are equitably distributed, thereby allowing the tissues to return to a normal state. It has been further suggested that, as the tissues change topography, the material continues to flow and adapt, thereby maintaining its close approximation and an equitable load distribution.’ For a material to behave in this fashion, it should exhibit initial properties of even consistency plus an ability to flow readily. If the material is to be maintained within the denture confines while under THE JOURNAL
OF PRCW-HETIC
DENTISTRY
89
Mc(‘AR7
functional loading, the flow properties should bc modified to develop the more solid-state characteristics of plasticity. That is, the material should initially act like a viscous liquid to achieve initial adaptation but should then change to a solid plastic mass that will deform under functional load conditions. This plastic mass should maintain the deformed state. allowing for c0ntinuir.g adaptation !c; a changing soft-tipsue topography. Functional impression materials. .4s the name implies, such a rnaterial should register the mean (modal) shape of the soft tissues while they are bring loaded under functional conditions. When a denture is either under function (mastication. speec.h, or pamfunction) or in the resting state. lhr forces exerted upon the tissues will vary in magnitude and direction. Roth duration and patterns of activity will modify the forcr parameters. Therefore the final shape, as registered by the impression material. should be a composite. or modal form, i.e.. an integration of the magnitude and direction of the forces and the duration and type of activity. For a material to behave in such a prescribed manner, it should at first flow readily to attain maximum adaptation and to redistribute its mass according to the force paramrtrrs, tirne function. and activity patterns, and then should achieve a modulus of elasticity and an elastic limit such as to resist further pcl-manent deformation when the denture is rcnwved (especially when uncicrcuts are presrnt) and during subsequent cast construction. Three further constraints are placed upon such a material: (1) It should he dimensionally stable after removal from the mouth. (2) It should he capable of accurate reproduction of detail. (3) It should he compatible with gypsum products. Mucll oi‘ the controversy that exists with respect to the suitabilit) of this class of materials for functional impressions relates to variations in interpretation of the requisite physical characteristics as described above. BEHAVIORAL
CHARACTERISTICS
I’hese materials arc usually supplied as a powder and liquid. According to the manlrfacturer’s directions, they are to be mixed in a liquid-powder ratio by volume. that varies from 1: 1.5 to I :‘2. ‘l‘he actual proportions are not considered critical,’ and in the literature supplied with their material, many manufacturers state that the liquid-powder ratio can br: varied to suit the clinician’s needs. The powder is basically poll;(ethyl mcthacrylate) or related copolymers of ethyl lvith methyl and/or iso-butyl methacrylatc.“’ ‘I‘he liquid is comprised of 90
HY :\\I)
MO‘+
K
essentially ethanol. aromatic ester plastlciyc.l\. Llrlcl flavoring agents,“’ :.’ The ethanol cont(tnl (It the. liquid varies from 6’;; to NE (bv volume; 111 thosr, The ester &iponent. \vhic,h materials analyzed.‘” volumetrically comprises the most of the remainder varies from product to product. These rstvrs act a\ plasticizers and art: considered to be ha>lcall>- dibllphthalafe OP b’Jty!-p~tha!a~e-~,~!t)‘!-~!~(.~Jtyl latr.:”
I.’
Initially, the powder-liquid combination l’orm~ :I free-running fluid which increases in Gscosity (IS thcethanol and plasticizer penetration occurs. ‘I‘hc, material becomes sufficiently viscous for inscrtiori into the mouth within 2 to :< minutes. and it rc.achc,\ its final gelation within I.5 to 20 minutrs.‘. I)uring the viscous phase the material exhibits non-Yewtonian flow characteristics, producing an increased coefficient of viscosity as the flow rate decreases.” ‘Fhe set gel at first demonstrates primarily plastic, characteristics strain-ratca dfqmthat are dent,‘!
t(
!I
As this material is primarily a solid/liquid solution.‘” the material should continue in its plastic phase, provided there is no environmental interaction. It has been demonstrated that intraorally these plastic properties are gradually 10s~ and the maccrial exhibits a more elastic nature.’ ’ ” This is due to a threefold sequence of events consisting of ( I ) ethanol loss, (2) water adsorption. and (3) loss of placticizcr. ‘I‘he ethanol toss commences immediately after the material is immersed in an aqueous medium and continues until most of the ethanol is depleted.’ ’ It is to be expected that a similar loss (not necessarily a~ the same rate) would occur if the material \verc exposed to the air. Simultancousiy, the poiymer itself adsorbs water from the environment. ThP degree of water adsorption is dependent upon the rhemical c.omposition of’ the polymer and the proportion of’ polymer in the gel. The net result of ethanol loss and water adsorption is usually a weight loss.” “I It has been assumed that such weight toss would result in a volumetric change that would nlitigatcagainst dimensional stability.“. “’ ‘This is not necessarily a valid assumption. It would he a relevant criterion if the material is to he used as a conventional impression materia!, Snt if’ the material is left in situ until an equilibrium has been established, the initial weight loss (with or without significant volumetric change) can he accommodated by the plasticity of the material. ‘I-he final weight loss, with an associated increase in hardness, varies from materiai to lnaterial anti is JULY
1978
VOLUME
JO
NUMBER
I
MtCHANICAI
PROPERTIES
OF TISSUE
CONDITIONERS.
PART
I
basically rrlated to the initial ethanol concentration. :\ liquid containing 2.5’7 ethanol can result in a weight loss of approximately 6% of the set material aftrsr 24 hours of immcrsion.“~ Rv doubling the powder-liquid ratio of the set material. however. it has been shown that weight loss can be halved while still 1naintainin.q a material that is apparently clinically workable.” :Z further weight stabilization has been achieved by the utilization of a low-ethanol liquid (8%) in combination with a powder of suitable molecular wcisht and particle size to develop acceptable manipulation characteristics. In this case the ethanol loss and water adsorption tend to balance one another.’ As the erhanol loss and water adsorption do not occur at the same rate (ethanol loss being initially faster). the physical properties of the set material should vay. Initially an increasing hardness occurs (as ethanol loss exceeds water adsorption), and then a degree of softening occurs as water adsorption increases. Hardening continues progressively as water adsorption reaches equilibrium while ethanol, and eventually the plasticizer, are continually leached into the saliva.“X Thus the salient intraoral behavioral characteristics of these materials may be described according to the flow chart in Fig. I. Unfortunately. the time dependency of the sequence described in the flow chart has not been fully established. This is particularly true in the transformation from the plastic to the elastic phases. Phases 1 to 3 are completed within I5 to 20 minutes, and the final phase occurs variously with different products. As determined clinically. this can occur from 1 week to 1 month. If the material is to be used parimarily to help heal abused and inflamed tissues. then the longer the material maintains itself within the plastic phase the better. If the material is to be used as an impression material. it is essential that the commencement and duration time of the elastic- phase be known for each material. \Vhen used as a conditioning material, the manufacturers and several authors recommend that the material be replaced approximately every 3 days.‘-” However, it would appear that this time lapse is based morr upon clinical expediency than controlled clinical and/or laboratory experimentation. In considering the capacity of these materials to act as functional impression agents. both die replication experiments and the study of the permanent deformation characteristics of several of these materials indicate that, within 7 hours of mixing, some are capable of accurate undercut reproduction and of THE
IOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 1. Flow chart of the sequential behavic>r,ll chardctrristics of tissue conditioners in the or,\1 rnvircrnment. the development of sufficient reduction in permanent set so as to compare favorably \vith clastomcric impression materials.” ”
EXPERIMENTAL
RATIONALE
During function. tissue-conditioning materials are constantly subjected to complex variable loads consisting of compressive. tensile. and shear components. These loads cannot as yet hc resol\.ed quantitatively into their pure components, although it could be anticipated that compression would be the predominant component. ‘Tissll~-conditioning materials. as all soft polymeric materials. \vill deform continuously without fracture under tomprcssive loading. When tested under standard conditions. increased compressive strength valur~ will be observed as a function of specimen distortions.‘” During compression loading the data analvsis is further complicated by frictional end effects impossihlc to eliminate in soft matc+als ‘I es1 results obtained under these conditions are meaningJess, 1” 28’ In polymers of strength adequate to allow valid testing in both tension and compression, elastic moduli were found to bc similar. ” ‘Tensilt. tests were chosen becausr of better known stress distribution characteristics. Stresses and strains remain uniaxial in tension over large load and deformation ranges. A further possible behavioral tlifferrnce clue to variations between intraoral and laboratory conditions ma): arise from a temperature dependence of the mechanical properties similar to thnt reported for other polymer systems. This elfcct is likely to bc slight in view of the small tempcraturr differential (16” C) betwt~c~n aginq (37” C) and [cht conditions (21”
B.D.Sc., F.R.A.C.D.S.,
University
M.S.,* and J. B. Moser, M.S., Ph.D.**
Dental School, Chicago,
Ill.
T
he inflammation and accompanying edema of the soft tissues underlying ill-fitting dentures have been the concern of dentists for many years. Hall,’ in 1921, believed that this condition was caused by displacement of the tissues during impression making. It has been demonstrated, however, that such changes result primarily from ill-fitting or poorly constructed dentures,? and that this condition can return to normal if the patient does not wear the dentures for 48 to 72 hours.? The long-term effects of such ill-fitting dentures are believed to include patient discomfort and destruction of the supporting alveolar bone. ’ The use of a material designed to “recondition” such abused tissues and restore a normal healthy state was first reported in 1961.a Since its inception, this new class of dental materials has stimulated several clinical and laboratory studies, and several manufacturers have developed products that are designed to produce “tissue conditioning” of the soft tissues underlying artificial dentures. However, compared to other commonly used classes of dental materials, comparatively few research results on these materials have been published. To date, no standards have been adopted by the American Dental Association for the guidance of dentists or manufacturers. Aside from use as a tissue conditioner, or, more aptly, a reconditioner of abused tissues, this class of dental materials has also been advocated as useful for the development of functional impressions prior to further treatment+” and as useful expedients in immediate denture construction, cleft palate speech From a thesis presented as partial fulfillment .Mastcr of Science degree in Prosthodontics Univenity *Associate ‘*.4ssociate
Dental Professor, Professor,
0022-.7913/78/0140-0089U)0.90/0Q
School, Chicago, 111. Department of Removable Department
of Biological
1970 ‘l-he
of requirements for a at Northwestern Prosthodontics. Materials.
C. V. Mosb)
Co.
aids, immediate surgical splints, stents for hemophiliacs, and postsurgical periodontal packs.;, ’ The literature is concentrated, however, upon the two major uses of this material, i.e., as a conditioning material per se and as a means of generating and recording a functional impression. Whereas there is a general consensus as to the efficacy of these materials as tissue conditioners, the published reports suggest a controversy as to the rationale underlying their utilization as impression materials.
THEORETICAL
CONSIDERATIONS
The theoretical requirements of a material that is at once effective in tissue conditioning and in functional impressions must include: (1) The material should be neither directly nor indirectly toxic in the environment and manner in which it is employed. (2) The material should be easily managed, both in terms of manipulation and removal (3) The manipulative variables affecting its behavior should be known and readily controllable. Aside from these general prerequisites, there is also a need to understand the physical properties necessary for the effective use of a material in either situation. Tissue conditioners. The efficacy of these materials, when used as conditioners, derives from the fact that they flow readily throughout the denture when it is inserted in the mouth. Through close adaptation to the tissues, the functional stresses are equitably distributed, thereby allowing the tissues to return to a normal state. It has been further suggested that, as the tissues change topography, the material continues to flow and adapt, thereby maintaining its close approximation and an equitable load distribution.’ For a material to behave in this fashion, it should exhibit initial properties of even consistency plus an ability to flow readily. If the material is to be maintained within the denture confines while under THE JOURNAL
OF PRCW-HETIC
DENTISTRY
89
Mc(‘AR7
functional loading, the flow properties should bc modified to develop the more solid-state characteristics of plasticity. That is, the material should initially act like a viscous liquid to achieve initial adaptation but should then change to a solid plastic mass that will deform under functional load conditions. This plastic mass should maintain the deformed state. allowing for c0ntinuir.g adaptation !c; a changing soft-tipsue topography. Functional impression materials. .4s the name implies, such a rnaterial should register the mean (modal) shape of the soft tissues while they are bring loaded under functional conditions. When a denture is either under function (mastication. speec.h, or pamfunction) or in the resting state. lhr forces exerted upon the tissues will vary in magnitude and direction. Roth duration and patterns of activity will modify the forcr parameters. Therefore the final shape, as registered by the impression material. should be a composite. or modal form, i.e.. an integration of the magnitude and direction of the forces and the duration and type of activity. For a material to behave in such a prescribed manner, it should at first flow readily to attain maximum adaptation and to redistribute its mass according to the force paramrtrrs, tirne function. and activity patterns, and then should achieve a modulus of elasticity and an elastic limit such as to resist further pcl-manent deformation when the denture is rcnwved (especially when uncicrcuts are presrnt) and during subsequent cast construction. Three further constraints are placed upon such a material: (1) It should he dimensionally stable after removal from the mouth. (2) It should he capable of accurate reproduction of detail. (3) It should he compatible with gypsum products. Mucll oi‘ the controversy that exists with respect to the suitabilit) of this class of materials for functional impressions relates to variations in interpretation of the requisite physical characteristics as described above. BEHAVIORAL
CHARACTERISTICS
I’hese materials arc usually supplied as a powder and liquid. According to the manlrfacturer’s directions, they are to be mixed in a liquid-powder ratio by volume. that varies from 1: 1.5 to I :‘2. ‘l‘he actual proportions are not considered critical,’ and in the literature supplied with their material, many manufacturers state that the liquid-powder ratio can br: varied to suit the clinician’s needs. The powder is basically poll;(ethyl mcthacrylate) or related copolymers of ethyl lvith methyl and/or iso-butyl methacrylatc.“’ ‘I‘he liquid is comprised of 90
HY :\\I)
MO‘+
K
essentially ethanol. aromatic ester plastlciyc.l\. Llrlcl flavoring agents,“’ :.’ The ethanol cont(tnl (It the. liquid varies from 6’;; to NE (bv volume; 111 thosr, The ester &iponent. \vhic,h materials analyzed.‘” volumetrically comprises the most of the remainder varies from product to product. These rstvrs act a\ plasticizers and art: considered to be ha>lcall>- dibllphthalafe OP b’Jty!-p~tha!a~e-~,~!t)‘!-~!~(.~Jtyl latr.:”
I.’
Initially, the powder-liquid combination l’orm~ :I free-running fluid which increases in Gscosity (IS thcethanol and plasticizer penetration occurs. ‘I‘hc, material becomes sufficiently viscous for inscrtiori into the mouth within 2 to :< minutes. and it rc.achc,\ its final gelation within I.5 to 20 minutrs.‘. I)uring the viscous phase the material exhibits non-Yewtonian flow characteristics, producing an increased coefficient of viscosity as the flow rate decreases.” ‘Fhe set gel at first demonstrates primarily plastic, characteristics strain-ratca dfqmthat are dent,‘!
t(
!I
As this material is primarily a solid/liquid solution.‘” the material should continue in its plastic phase, provided there is no environmental interaction. It has been demonstrated that intraorally these plastic properties are gradually 10s~ and the maccrial exhibits a more elastic nature.’ ’ ” This is due to a threefold sequence of events consisting of ( I ) ethanol loss, (2) water adsorption. and (3) loss of placticizcr. ‘I‘he ethanol toss commences immediately after the material is immersed in an aqueous medium and continues until most of the ethanol is depleted.’ ’ It is to be expected that a similar loss (not necessarily a~ the same rate) would occur if the material \verc exposed to the air. Simultancousiy, the poiymer itself adsorbs water from the environment. ThP degree of water adsorption is dependent upon the rhemical c.omposition of’ the polymer and the proportion of’ polymer in the gel. The net result of ethanol loss and water adsorption is usually a weight loss.” “I It has been assumed that such weight toss would result in a volumetric change that would nlitigatcagainst dimensional stability.“. “’ ‘This is not necessarily a valid assumption. It would he a relevant criterion if the material is to he used as a conventional impression materia!, Snt if’ the material is left in situ until an equilibrium has been established, the initial weight loss (with or without significant volumetric change) can he accommodated by the plasticity of the material. ‘I-he final weight loss, with an associated increase in hardness, varies from materiai to lnaterial anti is JULY
1978
VOLUME
JO
NUMBER
I
MtCHANICAI
PROPERTIES
OF TISSUE
CONDITIONERS.
PART
I
basically rrlated to the initial ethanol concentration. :\ liquid containing 2.5’7 ethanol can result in a weight loss of approximately 6% of the set material aftrsr 24 hours of immcrsion.“~ Rv doubling the powder-liquid ratio of the set material. however. it has been shown that weight loss can be halved while still 1naintainin.q a material that is apparently clinically workable.” :Z further weight stabilization has been achieved by the utilization of a low-ethanol liquid (8%) in combination with a powder of suitable molecular wcisht and particle size to develop acceptable manipulation characteristics. In this case the ethanol loss and water adsorption tend to balance one another.’ As the erhanol loss and water adsorption do not occur at the same rate (ethanol loss being initially faster). the physical properties of the set material should vay. Initially an increasing hardness occurs (as ethanol loss exceeds water adsorption), and then a degree of softening occurs as water adsorption increases. Hardening continues progressively as water adsorption reaches equilibrium while ethanol, and eventually the plasticizer, are continually leached into the saliva.“X Thus the salient intraoral behavioral characteristics of these materials may be described according to the flow chart in Fig. I. Unfortunately. the time dependency of the sequence described in the flow chart has not been fully established. This is particularly true in the transformation from the plastic to the elastic phases. Phases 1 to 3 are completed within I5 to 20 minutes, and the final phase occurs variously with different products. As determined clinically. this can occur from 1 week to 1 month. If the material is to be used parimarily to help heal abused and inflamed tissues. then the longer the material maintains itself within the plastic phase the better. If the material is to be used as an impression material. it is essential that the commencement and duration time of the elastic- phase be known for each material. \Vhen used as a conditioning material, the manufacturers and several authors recommend that the material be replaced approximately every 3 days.‘-” However, it would appear that this time lapse is based morr upon clinical expediency than controlled clinical and/or laboratory experimentation. In considering the capacity of these materials to act as functional impression agents. both die replication experiments and the study of the permanent deformation characteristics of several of these materials indicate that, within 7 hours of mixing, some are capable of accurate undercut reproduction and of THE
IOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 1. Flow chart of the sequential behavic>r,ll chardctrristics of tissue conditioners in the or,\1 rnvircrnment. the development of sufficient reduction in permanent set so as to compare favorably \vith clastomcric impression materials.” ”
EXPERIMENTAL
RATIONALE
During function. tissue-conditioning materials are constantly subjected to complex variable loads consisting of compressive. tensile. and shear components. These loads cannot as yet hc resol\.ed quantitatively into their pure components, although it could be anticipated that compression would be the predominant component. ‘Tissll~-conditioning materials. as all soft polymeric materials. \vill deform continuously without fracture under tomprcssive loading. When tested under standard conditions. increased compressive strength valur~ will be observed as a function of specimen distortions.‘” During compression loading the data analvsis is further complicated by frictional end effects impossihlc to eliminate in soft matc+als ‘I es1 results obtained under these conditions are meaningJess, 1” 28’ In polymers of strength adequate to allow valid testing in both tension and compression, elastic moduli were found to bc similar. ” ‘Tensilt. tests were chosen becausr of better known stress distribution characteristics. Stresses and strains remain uniaxial in tension over large load and deformation ranges. A further possible behavioral tlifferrnce clue to variations between intraoral and laboratory conditions ma): arise from a temperature dependence of the mechanical properties similar to thnt reported for other polymer systems. This elfcct is likely to bc slight in view of the small tempcraturr differential (16” C) betwt~c~n aginq (37” C) and [cht conditions (21”
