156
J. Dent. 1992;
Crown behaviour
20:
156-I
62
during cementation
P. R. Wilson School of Dental Science, University
of Melbourne,
Australia
ABSTRACT Complete seating of restorations during cementation is hindered by the need for excess cement to be expressed. It has been shown that postcementation seating is usually worse than precementation seating. Techniques have been developed to minimize this problem, including venting. die spacing and limiting cement placement.
and these techniques are reviewed. The behaviour of the crown during cementation consequences of crown cementation are discussed. KEY WORDS: J. Dent.
1992;
Crown, 20:
Seating, 156-l
Cementation,
62 (Received
Review
2 July
1991;
Correspondence should be addressed to: Mr P. R. Wilson, 71 1 Elizabeth Street, Melbourne, VIC 3000, Australia.
accepted
Cementation should be a procedure which allows full seating of a crown with no damage to the tooth or restoration. Cements are used to retain restorations and occupy marginal voids. The use of cements presents problems of flow as the cement film should ideally disappear at the margin of a completely seated crown, with excess having been expressed via the marginal opening or a vent. Incomplete seating ofcrowns can occur during cementation (Jorgensen, 1960a; Fusayama et al., 1963: Bassett, 1966; Abelson, 1980; Grajower et al.. 1985). Different strategies have been developed to allow placement of appropriate amounts of cement and escape of excess. The purpose of this article is to discuss the problems of seating full crowns and to examine techniques designed to aid crown seating.
FILM THICKNESS
The physical behaviour of luting cements has been investigated under several different testing regimens, with the American Dental Association (ADA) specification (last revised in 1978) being the most widely used. The ADA test consists of plates (2 cm* in area) separated ‘by a portion of the standard mix’ and approximated with a force of 14.7 N until set, and then the film thickness is measured. The accepted standard of maximum film thickness of 25 pm determined by this method has been @ 1992 Butterworth-Heinemann 0300-5712/92/030156-07
Ltd.
28 November
School of Dental Science,
INTRODUCTION
CEMENT
is considered, and
1991)
University of Melbourne,
criticized by Jorgensen and Petersen (1963), who consider the ADA method to be a measure of viscosity. Oilo and Evje (1986) found that the film thickness attained in a simulated crown-die situation with complete occlusal venting was lower than that found using the ADA test. They considered that actual cementation involves the application of more shear force to the cement film than the ADA method. It has been demonstrated that the use of tapered surfaces of 2 cm* area gave a significantly lower cement film thickness than two flat plates (Jorgensen and Petersen, 1963; Windeler, 1979). 0ilo and Evje (1986) also showed that the length of time allowed to elapse between mixing the cement and starting the test had a profound effect on final film thickness. Windeler (1979) reported that the temperature at the time of mixing had an effect on final film thickness. presumably by altering the rate of viscosity increase, and commented that the geometry of crown preparations led to a magnification of occlusal seating discrepancy if the axial film thickness was increased. Jorgensen (1960b) postulated that the two-phase nature (particle and liquid) of zinc phosphate cement can have significant effects on crown seating and showed that the particulate matter in the cement aggregated into lumps up to 100 pm in diameter. He considered that the more viscous lumps delayed the approximation of crown and tooth. and as the lumps deformed and spread laterally. they acted as a filter to the liquid component of the cement. It is clear that the film thickness of cements can be markedly less than the size of these particle aggregates, as
Wilson:
Grajower et al. (1985) found a minimum film thickness of 4 urn in sectioned crown-die complexes cemented with zinc phosphate cement. The aggregates must therefore deform appreciably during cementation and hence delay seating. Another problem encountered is that the outlet area for the expressed cement diminishes as the crown seats, hence further impeding cement flow and decreasing the exit pathway.
FILM THICKNESS
157
showed an initially slow rise in viscosity, with a subsequent rapid increase. He suggested that the glass-ionomer setting profile was more ideal. Kyrios et al. (1989) found that glass-ionomer cements had an effective working time of up to 240 s in simulated cementation. Cements that have a prolonged low viscosity phase should have benefits in crown seating, as Wilson et al. (1990) have shown that low viscosity fluids allowed crowns to seat significantly faster than high viscosity fluids.
AND RETENTION
Jorgensen and Esbensen (1968) showed that there was no association between film thickness and retention for cemented crowns. Increased film thickness was created by lack of seating of the test crowns by a defined axial amount (from 20 to 140 urn). Eames et al. (1978) found that die-spaced crowns (with 25 pm of relief) were 25 per cent more retentive than unrelieved crowns. Hembree and Cooper (1979) determined the retentive abilities of different cements on spaced and non-spaced crowns and inlays. The mean values of retention rose for all combinations with spacing, but was statistically significant only for inlays cemented with EBA cement. Vermilyea et al. (1983) found that the retention of gold copings cemented to natural teeth with zinc phosphate cement was decreased when the coping was prepared on a die coated with 4050 urn of die-spacer. The retention of copings did not decrease when spacing was used together with polycarboxylate or modified zinc oxide eugenol cements. and was of a similar level to that of zinc phosphate cement used with spacing. The effects of increased cement film thickness on retention remain indeterminate. The balance of evidence suggests that die spacing has little effect on retention.
VISCOSITY
Crown cementation
OF CEMENTS
In an extensive review of the properties and future of cements. Smith (1983) consistently referred to their viscosity as an important factor in their handling characteristics. Polycarboxylate cement viscosity was determined by Combe and Greener (1975) who found it to be pseudo-plastic, in that the apparent viscosity decreased with shear rate. Kay (1984) demonstrated that setting zinc phosphate cement was dilatant. as the apparent viscosity increased with shear rate, and that the viscosity of the cement was exponentially time dependent. He calculated, using a theoretical model, that the initial viscosity of the cementing medium had a major effect on the seating of the crown. Jorgensen (1960a) showed that an increase in the powder-liquid ratio (and therefore viscosity) led to an increased film thickness in an experimental crown-die model, and also that the rate of viscosity increase was temperature dependent (Jorgensen and Petersen, 1963). In a determination of the viscosity of setting cements, Cook (1984) found that the viscosity of zinc phosphate and polycarboxylate cements rose steadily during setting, whilst glass-ionomer (polyalkenoate) and silicate cements
CEMENT
EXPOSURE
AT THE MARGIN
The consequence of a marginal discrepancy caused by poor seating is increased cement exposure and increased occlusal height. McLean and von Fraunhofer (1971) measured the in viva film thickness of restorations and demonstrated that the marginal gap could range from 10 to 160 urn. They cited clinical success in a 5-year unpublished study ofover 1000 restorations with marginal discrepancies of a similar order, and remarked that gaps of less than 80 urn were impossible to detect with an explorer. It is clear that a space will exist between the casting and the tooth at the margin before and after cementation. Behrend (1984) considered that ‘at best a band of rough cement will be present’ when a crown is cemented. Pilo et al. (1988) commented that there appeared to be little clinical correlation between marginal opening and cement loss. The effect of marginal design on the width of the marginal discrepancy is relevant to exposure of cement. Hunter and Hunter(1990) defined the term ‘seating’as the vertical (axial) discrepancy at the external margin, and ‘sealing’ as the discrepancy observed at right angles to the two opposing surfaces. A shoulder design reflects completely the lack of seating of the crown, whilst the bevel margin effect. referred to by Rosner (1963). would decrease the marginal gap (improve the ‘seal’) between the crown and the tooth. provided that the restoration seated fully. In a comprehensive study of the effects of finishing line form on the final seating of crowns. Gavelis et al. (1981) demonstrated that the cemented crown-die complex had cement films at the margin ranging from 3 1 to 105 pm. The lowest sealing (31 pm) occurred with a feather-edge margin, but with a seating of 163 l.trn. Shoulder preparations had a sealing of 67 urn and the lowest seating of 85 pm. Hunter and Hunter (1990). in an extensive review of crown margin design, concluded horizontal crown margins (shoulders) may provide the best method of minimizing seating discrepancies and maximizing gingival health. Moore et al. (1985) considered sealing in an analysis of crown seating. concluding that the postcementation gap was greater than would be expected given the lack of seating due to the cement presence. Although open to criticism regarding the controls used. this was the first paper to describe the use of sectioned dies to determine the film thickness of cements in which it was speculated that crown distortion may be approaching detectable levels.
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FORCES USED TO CEMENT
CROWNS
The force used for crown seating experiments has varied greatly, from ‘finger pressure’ (Moore et al., 1985) to 440 N in the servo-hydraulic tests of Kay (1984). Jorgensen and Petersen (1963) showed that there was an improvement in the final seating of cemented crowns as the force was increased from 0.5 kg (5 N) to 10 kg (98 N), but that the increase in seating was very small beyond 5 kg (49 N). Eames et al. (1978) noted that there was a significant difference in crown seating in the absence of cement when the force of seating varied from ‘finger pressure’ to ‘biting pressure’. with a rebound effect of between 75 and 150 urn after removal of ‘biting pressure’. Following internal etching of the crown. the rebound dropped to approximately 30 urn. This suggested that internal etching of crowns led to a decrease in the amount of residual distortion of the crown present on seating. but this has not been explicitly shown. The rebound phenomenon did not occur if force was kept on the crown as the cement set, implying that there were residual stresses within the crown tooth and cement on setting. It is not clear what effects long-term stress would have on the physical structure of set cement and crown materials, and their resulting lifespan. Grajoweretal. (1985) showed that an increase in seating force from 2 kg (20 N) to 10 kg (98 N) led to an increase in seating of uncemented crowns by an average of 63 urn. They commented that there were interferences between the crown and the die and that the increased force caused furrowing of the tooth. Repeated placements of the crowns gave slightly improved seating, as a result of the dentine being scored by internal crown imperfections. These furrows could be seen by retention of dye after tooth cleaning. It was noted that the internal surface of the crowns was not regular, resulting in tooth-metal contact occurring in isolated areas. The probability of crown and tooth contact and the effect of seating force indicate that for experimental crown seating studies the force of seating must be defined. The effect of die-tooth damage must be eliminated either by making a new die for each seat (Gavelis er al., 1981) or by using a wear-resistant die (Wilson et al., 1990).
PRESSURE MEASUREMENTS CEMENTATION
IN
There are few data on the magnitude and time changes of pressures in the cement film during cementation. Data are limited to a qualitative study by Hoard et al. (1978). and restricted quantitative investigations by Kay (1984). Hoard et al. (1978) mounted three strain gauges (occlusally, cervically and at the cuspal tip) inside a metal die, and measured their response under different experimental conditions. In their initial analysis of uncemented crown seating. they found that there was no significant strain gauge activity when a force of 49 N was applied to the crown-die complex. This was interpreted to mean that
there was no significant occlusal or axial contact, although it would be more accurate to say that there was no strain gauge deformation during seating, rather than no crown-die contact. Hoard et al. (1978) also measured the pressure at the cement-die interface with internally mounted strain gauges and concluded that the less viscous cements (zinc oxide-eugenol compared with zinc phosphate) produced less pressure within the cement film. and less residual pressure. It was noted that the most consistent place for pressure generation was at the cervical aspect of the die, where the crown might be undergoing diametral expansion. The lack of activity in internally placed strain gauges does not preclude distortion of the crown. It has been shown by 0ilo (1978) that zinc phosphate cement shrinks by 0.5 per cent on setting, with the production of’slits’ (linear cracks) within the cement film. This occurrence could mitigate for residual stress within the crown or tooth. The magnitude of this contraction in the cement film in cemented crowns must be very small. as with a cement thickness of 20 IJ-mthe shrinkage would be only 100 nm. Kay (1984) found that pressures of up to 11 MPa were generated when crowns were pressed onto dies with forces of 440 N. He investigated the pressure changes during simulated cementation with non-setting fluids (mineral oil and toothpaste) and found that these liquids gave the most consistent readings of an inconsistent experimental system. In general. the viscosity ofthe material did not affect the pressure generation for a given force programme. but was a major factor in the final seating discrepancy. The use of a non-setting fluid model in experimental systems would seem to be indicated initially to remove the effect of time on the viscosity profile of the cementing medium. As the powder and liquid nature of most cementing media may also have an effect on the behaviour of the cement film. removal of this factor would further simplify experimental models.
CONSEQUENCES GENERATION
OF PRESSURE
Kay (1984) discussed four possible scenarios regarding the effects of pressure generation during cementation and the interaction with tooth tissue, using the tube Poiseuille equation as described by Skalak and Chein (1981) and substituting the values for dentine permeability found by Pashley et al. (1983). In the worst case. with smear layer removed and dentine etched, significant pressure gradients towards the pulp would occur and mass transport of the least viscous (and most reactive) constituents of the cement would be possible. He concluded that the calculated pressure gradients from cementation could be enough to precipitate pulpal necrosis. It should be noted that the forces used to seat the crowns (440 N) were nearly an order of magnitude greater than those found to be generated in the oral environment for crown cementation (90 N: Grieve, 1969). The conclusions
Wilson:
of Kay (1984) may be overstated, when consideration is given to the force (and therefore pressure) applied clinically. The consequences of prolonged mechanical deformation of the tooth structure caused by crown cementation are not clear. There could be the potential for piezoelectric effects. as it has been shown that whale dentine has the potential for piezoelectric activity. but to a much lower level than bone (Marino and Gross, 1989). Resorption of dental tissues initiated by such electrical potentials is not a clinically recognized problem.
DEFORMATION
OF CROWNS
It has been shown by Bridger and Nicholls (1981) that the metal substructure in a metal-ceramic prosthesis distorts with the application of the porcelain. and that the deformation is released elastically if the porcelain is dissolved from the framework with hydrofluoric acid. It has been demonstrated that marginal distortion of a metal-ceramic crown is associated with the application of porcelain (Shillingburg ef al.. 1973: Ishawata et al.. 1977: Faucher and Nicholls. 1980) and with degassing (Dederich et al.. 1984). These papers demonstrated that the porcelain to metal bond may not be stress free. and that the crowns could be measurably distorted. DeHoff and Anusavice (1981) calculated the effect of the mismatch of porcelain-metal coefficients of thermal expansion on the marginal opening of metal-ceramic crowns could be up to 21 urn. Anusaviceet al. ( 1985). using a mathematical model of an upper central incisor. calculated that the effect of loading a metal-ceramic crown palatally with a force of 200 N produced a maximum strain of 0.014 per cent. well below the accepted fracture strain level of0.l per cent (Jones, 1983) for dental porcelain. These analyses assume that there was force transmission via the cement bond as the set cement was rigid. This would not be possible during cementation. and the crown may be liable to greater deformation. Wilson et al. (1990) measured the cervical deformation and found strain levels ofgold crowns during cementation under some conditions close to fracture strains for porcelain. Jorgensen (1960a) observed that incompletely seated crowns were frequently tilted, and remained in that position once the cement had set. The different film thicknesses and perhaps crown-tooth contact in the tilted crown may lead to localized strains in the crown higher than the average values calculated by Wilson et al. (1990). Van Nortwick and Gettleman (1981) showed that there was a highly significant correlation between the completeness of crown seating and lack of tilting in the seated crown. Measures designed to improve seating should result in less tilting of the crown.
FAILURE MODES
IN PORCELAIN
SYSTEMS
Warpeha and Goodkind (1976) stated that unpredictable clinical failures could occur in metal-ceramic bonding
Crown
cementation
159
systems. and described three of the most frequent modes of failure as cleavage at the porcelain to metal interface. fracture through the opaque or body porcelain and surface crazing. The unpredictable manner of failure suggests that several factors may be important for the longevity of metal-ceramic restorations. Porcelain is vulnerable to tensile stresses. and diametral distortion during cementation may be enough to precipitate fracture in a ceramic or metal-ceramic crown if the section is insufficiently stiff. It is possible that stresses generated in cementation could contribute to immediate or delayed clinical failure. Occasionally, the porcelain of metal-ceramic crowns fractures during cementation. This phenomenon may be due to the substructure flexing and straining the porcelain superstructure past 0.1 per cent strain (Jones, 1983). Abbate et al. (1989) found that several ceramic crowns fractured during cementation with a force of 50 N. Previously. direct contact of the crown with the tooth had been assumed to be the major cause of fracture (Chiche and Pinault, 1988) rather than flexure from hydraulic deformation. Davis et al. (1989) found that disclosing the tit of crowns with a tit-checking solution revealed crownto-die contacts and that removal ofthese contacts led to an improved seating. and a diminished marginal gap. The effect on crown deformation and therefore possible fracture was not shown. but could be expected to be beneficial. Strategies to avoid intracementation fracture should be designed both to allow a passive tit of the crown on the tooth. and to minimize the effects of force transmission via the cement.
VENTING It has been shown repeatedly that more complete seating of crowns may be achieved by allowing cement trapped during crown cementation to be released through an occlusal perforation (Kaufman et al.. 1961: Bassett. 1966; Cooperetal.. 1971: Van Nortwick and Gettleman, 1981). It has been suggested that venting may also lead to a lower pressure within the cement film during cementation (Kay. 1984). The siting ofvent holes varies according to the tooth being crowned. Molars and premolars can allow for an occlusal perforation. but incisors must have an axial vent. usually placed lingually. Brose et al. (1984) found that the placing of a lingual vent hole increased the seating discrepancy of castings compared to unvented controls. to a level that represented a clinically open margin. Dykemaet al. (1986) reported that many dentists obtain satisfactory results without venting of any kind. Repair of the perforation leads to a weak link in the crown shell, and complicates the placement procedure. The suggestion from the work of Brose et al. (1984) that lingual vents are ineffective indicates the need for an occlusal perforation and repair of the vent.
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DIE SPACING Various techniques can be used to provide space for the final cement film, including grinding or etching the crown internally (Hollenback, 1928; Basset, 1966). or producing an oversize die by application of a die spacer (Fusayama et al., 1964; Eames et al.. 1978). The amount of die enlargement has been recommended to be from 20 to 40 urn (Campagni et al.. 1982). Oliva et al. (1988) showed that a 40 urn thickness of die spacer was obtained with four coats of a proprietary product (Trutit: George Taub, Jersey City. NJ, USA). Campbell (1990) found that one coat of spacer designed for Dicer cast ceramic (Dentsply Int.. York, PA, USA) produced a layer 38 urn thick, which was measured as equivalent to four layers of Tru-Fit. He noted that the Dicer spacer was thinned over the line angles of the preparation, and that this may hinder cement outflow. Grajower et al. (1989) found that when a deliberately undersized casting was produced, four layers of Tru-Fit reduced the average seating discrepancy from 649 to 29 urn. with mean axial wall cement thickness from 20 to 60 urn with no apparent trend related to thickness of spacer. The thickness of axial die-spacer for optimum seating was 70 urn, as 100 urn of die-spacer did not result in improved seating. This work suggests that the thickness of die-spacer used should allow approximately 40 urn of axial space for cement when the crown is fully seated. Incomplete coverage of the axial walls of the preparation with die-spacer resulted in crown seating discrepancies similar to those where spacer was absent, and was therefore not recommended. An alternative approach to providing increased cement escape space all around the crown with die spacing is to provide specific pathways for cement flow from the occlusal surface. Leland-Webb et al. (1983) found that provision of four grooves in the axial walls of the preparation significantly reduced the seating discrepancy of cemented crowns from 200 urn to around 50 urn. Occlusal grooving of the preparation alone or added to axial grooves had no advantage. This finding supports the argument that axial cement flow is the major problem in seating the full crown.
one-fortieth that of gold, it cannot be assumed that the test crowns behaved in a similar way to the more rigid crowns used clinically. Placement of appropriate amounts of cement as described decreased the need for venting. Oliveira et al. (1979) found that vibration of crowns during cementation before static loading improved the seating of crowns and inlays by on average 21 urn. Vibration directed perpendicular to the axial walls of crowns as described by Van Nortwick and Gettleman (1981) did not improve the seating of cast restorations, contrary to the expected film thinning caused by vibration as reported by Koyano et al. (1978). Vibration techniques must be viewed with some caution as Olin (1990) has shown that prolonged ultrasonic treatment of recently cemented crowns reduced retention by approximately one-half.
CONCLUSIONS Strategies for improving the seating of restorations during cementation are directed towards aiding the flow of cement from the internal surface of the crown. When using zinc phosphate cement, die spacing sufficient to leave 40 urn of axial space in the tooth-crown assembly appears appropriate to produce even and complete seating before cement setting. Clear evidence for the optimum thickness of die-spacer with respect to seating and retention needs to be produced. If considerable die spacing is used, what are the consequences in terms of crown position radially on the tooth’? Is the use of powder and liquid cement systems problematical for the thin film thicknesses used in dentistry’? Is continued force application after seating deleterious in creating residual stresses within the set crown-cementtooth complex? Much remains to be resolved about the cementation process and the optimal procedure for gaining the best prognosis for the restored tooth.
Acknowledgements I would like to thank Dr A. J. Hunter and Dr M. J. Tyas for their detailed comments on the manuscript.
References
PLACEMENT
TECHNIQUES
The crown has traditionally been tilled with cement and then placed upon the tooth, and an axial force used to seat the crown. Techniques have been developed of painting cement inside a crown with tine brushes (Ishikiriamietal., 1981) which lead to more complete seating. Assif et al. (1987) examined placement strategies for zinc phosphate cement by using clear acrylic crowns and a standardized cementation procedure. and concluded that it was most effective to paint the axial walls of the tooth preparation and the marginal area of the crown. The use of acrylic crowns allowed visualization of the cement flow, but as the modulus of elasticity and acrylic is approximately
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Vermilyea S. G.. Kuffler M. J. and Hupet E. F. (1983) The effects of die relief agent on the retention of full coverage castings. J. Prosthrt. Dent. 50, 207-21 I. Warpeha W. S. and Goodkind R. J. (1976) Design and technique variables affecting fracture resistance of metal/ ceramic restorations. J. Prosthet Dent 35, 291-298. Wilson P. R.. Goodkind R. J.. DeLong R. et al. (1990) Deformation of crowns during cementation. J. Prosthet. Dent. 64, 601-609. Windeler A. S. (1979) Powder enrichment effects on film thickness of zinc phosphate cement. J. Prosthet. Dent. 42,
Rosner D.( 1963) Function. placement and reproduction of bevels for gold castings. J. Prosthet. Dent. 13, 1160-l 165. Shillingburg H. T.. Hobo S. and Fisher D. W. (1973) Preparation design and margin distortion in porcelain fused to metal restorations. .I. Prosthrt. Dent. 29, 276-281. Skalak R. and Chein S. (1981) Capillary flow: history. experiments and theory. Biorhrology 18, 307-330. Smith D. C. (1983) Dental cements: current status and future prospects. Dent. C/in. North Am. 6, 763-792. Van Nortwick N. T. and Gettleman L. (1981) Effect of internal relief. vibration and venting on the vertical seating of cemented crowns. J. Prosthet. Dent 45, 395-399.
Forthcoming Original
Research
299-304.
Articles Reports
Cardiopulmonary performance following changes tration of intravenous diazemuls P. Gallon. G. M. Walton and J. P. Rood
in position
A preliminary investigation into the effect of increased dimension on mandibular movement during speech C. A. Burnett and T. J. Cliffbrd Assessment of elderly people on entry to residential arrangements G. Hoad-Reddick
and the adminis-
occlusal
vertical
homes and continuing
care
Predictions of restoration deterioration P. J. Smales, D. A. Webster aid P. 1. Lepparci A denture base resin with low water absorption M. J. Barsby The finishing of amalgam restorations: C. J. Collins and R. W. Bryant A technique for sections c. c. Yol.lngsoll
three-dimensional
Dentine bonding agents-characteristic depth J. F. McCabe and S. Rushy Dental biomechanics N. E. Waters
and the dental
a three-year
microleakage
bond strength
curriculum
clinical
study
assessment
as a function
using
tooth
of dentine
J. Dent. 1992;
Crown behaviour
20:
156-I
62
during cementation
P. R. Wilson School of Dental Science, University
of Melbourne,
Australia
ABSTRACT Complete seating of restorations during cementation is hindered by the need for excess cement to be expressed. It has been shown that postcementation seating is usually worse than precementation seating. Techniques have been developed to minimize this problem, including venting. die spacing and limiting cement placement.
and these techniques are reviewed. The behaviour of the crown during cementation consequences of crown cementation are discussed. KEY WORDS: J. Dent.
1992;
Crown, 20:
Seating, 156-l
Cementation,
62 (Received
Review
2 July
1991;
Correspondence should be addressed to: Mr P. R. Wilson, 71 1 Elizabeth Street, Melbourne, VIC 3000, Australia.
accepted
Cementation should be a procedure which allows full seating of a crown with no damage to the tooth or restoration. Cements are used to retain restorations and occupy marginal voids. The use of cements presents problems of flow as the cement film should ideally disappear at the margin of a completely seated crown, with excess having been expressed via the marginal opening or a vent. Incomplete seating ofcrowns can occur during cementation (Jorgensen, 1960a; Fusayama et al., 1963: Bassett, 1966; Abelson, 1980; Grajower et al.. 1985). Different strategies have been developed to allow placement of appropriate amounts of cement and escape of excess. The purpose of this article is to discuss the problems of seating full crowns and to examine techniques designed to aid crown seating.
FILM THICKNESS
The physical behaviour of luting cements has been investigated under several different testing regimens, with the American Dental Association (ADA) specification (last revised in 1978) being the most widely used. The ADA test consists of plates (2 cm* in area) separated ‘by a portion of the standard mix’ and approximated with a force of 14.7 N until set, and then the film thickness is measured. The accepted standard of maximum film thickness of 25 pm determined by this method has been @ 1992 Butterworth-Heinemann 0300-5712/92/030156-07
Ltd.
28 November
School of Dental Science,
INTRODUCTION
CEMENT
is considered, and
1991)
University of Melbourne,
criticized by Jorgensen and Petersen (1963), who consider the ADA method to be a measure of viscosity. Oilo and Evje (1986) found that the film thickness attained in a simulated crown-die situation with complete occlusal venting was lower than that found using the ADA test. They considered that actual cementation involves the application of more shear force to the cement film than the ADA method. It has been demonstrated that the use of tapered surfaces of 2 cm* area gave a significantly lower cement film thickness than two flat plates (Jorgensen and Petersen, 1963; Windeler, 1979). 0ilo and Evje (1986) also showed that the length of time allowed to elapse between mixing the cement and starting the test had a profound effect on final film thickness. Windeler (1979) reported that the temperature at the time of mixing had an effect on final film thickness. presumably by altering the rate of viscosity increase, and commented that the geometry of crown preparations led to a magnification of occlusal seating discrepancy if the axial film thickness was increased. Jorgensen (1960b) postulated that the two-phase nature (particle and liquid) of zinc phosphate cement can have significant effects on crown seating and showed that the particulate matter in the cement aggregated into lumps up to 100 pm in diameter. He considered that the more viscous lumps delayed the approximation of crown and tooth. and as the lumps deformed and spread laterally. they acted as a filter to the liquid component of the cement. It is clear that the film thickness of cements can be markedly less than the size of these particle aggregates, as
Wilson:
Grajower et al. (1985) found a minimum film thickness of 4 urn in sectioned crown-die complexes cemented with zinc phosphate cement. The aggregates must therefore deform appreciably during cementation and hence delay seating. Another problem encountered is that the outlet area for the expressed cement diminishes as the crown seats, hence further impeding cement flow and decreasing the exit pathway.
FILM THICKNESS
157
showed an initially slow rise in viscosity, with a subsequent rapid increase. He suggested that the glass-ionomer setting profile was more ideal. Kyrios et al. (1989) found that glass-ionomer cements had an effective working time of up to 240 s in simulated cementation. Cements that have a prolonged low viscosity phase should have benefits in crown seating, as Wilson et al. (1990) have shown that low viscosity fluids allowed crowns to seat significantly faster than high viscosity fluids.
AND RETENTION
Jorgensen and Esbensen (1968) showed that there was no association between film thickness and retention for cemented crowns. Increased film thickness was created by lack of seating of the test crowns by a defined axial amount (from 20 to 140 urn). Eames et al. (1978) found that die-spaced crowns (with 25 pm of relief) were 25 per cent more retentive than unrelieved crowns. Hembree and Cooper (1979) determined the retentive abilities of different cements on spaced and non-spaced crowns and inlays. The mean values of retention rose for all combinations with spacing, but was statistically significant only for inlays cemented with EBA cement. Vermilyea et al. (1983) found that the retention of gold copings cemented to natural teeth with zinc phosphate cement was decreased when the coping was prepared on a die coated with 4050 urn of die-spacer. The retention of copings did not decrease when spacing was used together with polycarboxylate or modified zinc oxide eugenol cements. and was of a similar level to that of zinc phosphate cement used with spacing. The effects of increased cement film thickness on retention remain indeterminate. The balance of evidence suggests that die spacing has little effect on retention.
VISCOSITY
Crown cementation
OF CEMENTS
In an extensive review of the properties and future of cements. Smith (1983) consistently referred to their viscosity as an important factor in their handling characteristics. Polycarboxylate cement viscosity was determined by Combe and Greener (1975) who found it to be pseudo-plastic, in that the apparent viscosity decreased with shear rate. Kay (1984) demonstrated that setting zinc phosphate cement was dilatant. as the apparent viscosity increased with shear rate, and that the viscosity of the cement was exponentially time dependent. He calculated, using a theoretical model, that the initial viscosity of the cementing medium had a major effect on the seating of the crown. Jorgensen (1960a) showed that an increase in the powder-liquid ratio (and therefore viscosity) led to an increased film thickness in an experimental crown-die model, and also that the rate of viscosity increase was temperature dependent (Jorgensen and Petersen, 1963). In a determination of the viscosity of setting cements, Cook (1984) found that the viscosity of zinc phosphate and polycarboxylate cements rose steadily during setting, whilst glass-ionomer (polyalkenoate) and silicate cements
CEMENT
EXPOSURE
AT THE MARGIN
The consequence of a marginal discrepancy caused by poor seating is increased cement exposure and increased occlusal height. McLean and von Fraunhofer (1971) measured the in viva film thickness of restorations and demonstrated that the marginal gap could range from 10 to 160 urn. They cited clinical success in a 5-year unpublished study ofover 1000 restorations with marginal discrepancies of a similar order, and remarked that gaps of less than 80 urn were impossible to detect with an explorer. It is clear that a space will exist between the casting and the tooth at the margin before and after cementation. Behrend (1984) considered that ‘at best a band of rough cement will be present’ when a crown is cemented. Pilo et al. (1988) commented that there appeared to be little clinical correlation between marginal opening and cement loss. The effect of marginal design on the width of the marginal discrepancy is relevant to exposure of cement. Hunter and Hunter(1990) defined the term ‘seating’as the vertical (axial) discrepancy at the external margin, and ‘sealing’ as the discrepancy observed at right angles to the two opposing surfaces. A shoulder design reflects completely the lack of seating of the crown, whilst the bevel margin effect. referred to by Rosner (1963). would decrease the marginal gap (improve the ‘seal’) between the crown and the tooth. provided that the restoration seated fully. In a comprehensive study of the effects of finishing line form on the final seating of crowns. Gavelis et al. (1981) demonstrated that the cemented crown-die complex had cement films at the margin ranging from 3 1 to 105 pm. The lowest sealing (31 pm) occurred with a feather-edge margin, but with a seating of 163 l.trn. Shoulder preparations had a sealing of 67 urn and the lowest seating of 85 pm. Hunter and Hunter (1990). in an extensive review of crown margin design, concluded horizontal crown margins (shoulders) may provide the best method of minimizing seating discrepancies and maximizing gingival health. Moore et al. (1985) considered sealing in an analysis of crown seating. concluding that the postcementation gap was greater than would be expected given the lack of seating due to the cement presence. Although open to criticism regarding the controls used. this was the first paper to describe the use of sectioned dies to determine the film thickness of cements in which it was speculated that crown distortion may be approaching detectable levels.
158
J. Dent. 1992; 20: No. 3
FORCES USED TO CEMENT
CROWNS
The force used for crown seating experiments has varied greatly, from ‘finger pressure’ (Moore et al., 1985) to 440 N in the servo-hydraulic tests of Kay (1984). Jorgensen and Petersen (1963) showed that there was an improvement in the final seating of cemented crowns as the force was increased from 0.5 kg (5 N) to 10 kg (98 N), but that the increase in seating was very small beyond 5 kg (49 N). Eames et al. (1978) noted that there was a significant difference in crown seating in the absence of cement when the force of seating varied from ‘finger pressure’ to ‘biting pressure’. with a rebound effect of between 75 and 150 urn after removal of ‘biting pressure’. Following internal etching of the crown. the rebound dropped to approximately 30 urn. This suggested that internal etching of crowns led to a decrease in the amount of residual distortion of the crown present on seating. but this has not been explicitly shown. The rebound phenomenon did not occur if force was kept on the crown as the cement set, implying that there were residual stresses within the crown tooth and cement on setting. It is not clear what effects long-term stress would have on the physical structure of set cement and crown materials, and their resulting lifespan. Grajoweretal. (1985) showed that an increase in seating force from 2 kg (20 N) to 10 kg (98 N) led to an increase in seating of uncemented crowns by an average of 63 urn. They commented that there were interferences between the crown and the die and that the increased force caused furrowing of the tooth. Repeated placements of the crowns gave slightly improved seating, as a result of the dentine being scored by internal crown imperfections. These furrows could be seen by retention of dye after tooth cleaning. It was noted that the internal surface of the crowns was not regular, resulting in tooth-metal contact occurring in isolated areas. The probability of crown and tooth contact and the effect of seating force indicate that for experimental crown seating studies the force of seating must be defined. The effect of die-tooth damage must be eliminated either by making a new die for each seat (Gavelis er al., 1981) or by using a wear-resistant die (Wilson et al., 1990).
PRESSURE MEASUREMENTS CEMENTATION
IN
There are few data on the magnitude and time changes of pressures in the cement film during cementation. Data are limited to a qualitative study by Hoard et al. (1978). and restricted quantitative investigations by Kay (1984). Hoard et al. (1978) mounted three strain gauges (occlusally, cervically and at the cuspal tip) inside a metal die, and measured their response under different experimental conditions. In their initial analysis of uncemented crown seating. they found that there was no significant strain gauge activity when a force of 49 N was applied to the crown-die complex. This was interpreted to mean that
there was no significant occlusal or axial contact, although it would be more accurate to say that there was no strain gauge deformation during seating, rather than no crown-die contact. Hoard et al. (1978) also measured the pressure at the cement-die interface with internally mounted strain gauges and concluded that the less viscous cements (zinc oxide-eugenol compared with zinc phosphate) produced less pressure within the cement film. and less residual pressure. It was noted that the most consistent place for pressure generation was at the cervical aspect of the die, where the crown might be undergoing diametral expansion. The lack of activity in internally placed strain gauges does not preclude distortion of the crown. It has been shown by 0ilo (1978) that zinc phosphate cement shrinks by 0.5 per cent on setting, with the production of’slits’ (linear cracks) within the cement film. This occurrence could mitigate for residual stress within the crown or tooth. The magnitude of this contraction in the cement film in cemented crowns must be very small. as with a cement thickness of 20 IJ-mthe shrinkage would be only 100 nm. Kay (1984) found that pressures of up to 11 MPa were generated when crowns were pressed onto dies with forces of 440 N. He investigated the pressure changes during simulated cementation with non-setting fluids (mineral oil and toothpaste) and found that these liquids gave the most consistent readings of an inconsistent experimental system. In general. the viscosity ofthe material did not affect the pressure generation for a given force programme. but was a major factor in the final seating discrepancy. The use of a non-setting fluid model in experimental systems would seem to be indicated initially to remove the effect of time on the viscosity profile of the cementing medium. As the powder and liquid nature of most cementing media may also have an effect on the behaviour of the cement film. removal of this factor would further simplify experimental models.
CONSEQUENCES GENERATION
OF PRESSURE
Kay (1984) discussed four possible scenarios regarding the effects of pressure generation during cementation and the interaction with tooth tissue, using the tube Poiseuille equation as described by Skalak and Chein (1981) and substituting the values for dentine permeability found by Pashley et al. (1983). In the worst case. with smear layer removed and dentine etched, significant pressure gradients towards the pulp would occur and mass transport of the least viscous (and most reactive) constituents of the cement would be possible. He concluded that the calculated pressure gradients from cementation could be enough to precipitate pulpal necrosis. It should be noted that the forces used to seat the crowns (440 N) were nearly an order of magnitude greater than those found to be generated in the oral environment for crown cementation (90 N: Grieve, 1969). The conclusions
Wilson:
of Kay (1984) may be overstated, when consideration is given to the force (and therefore pressure) applied clinically. The consequences of prolonged mechanical deformation of the tooth structure caused by crown cementation are not clear. There could be the potential for piezoelectric effects. as it has been shown that whale dentine has the potential for piezoelectric activity. but to a much lower level than bone (Marino and Gross, 1989). Resorption of dental tissues initiated by such electrical potentials is not a clinically recognized problem.
DEFORMATION
OF CROWNS
It has been shown by Bridger and Nicholls (1981) that the metal substructure in a metal-ceramic prosthesis distorts with the application of the porcelain. and that the deformation is released elastically if the porcelain is dissolved from the framework with hydrofluoric acid. It has been demonstrated that marginal distortion of a metal-ceramic crown is associated with the application of porcelain (Shillingburg ef al.. 1973: Ishawata et al.. 1977: Faucher and Nicholls. 1980) and with degassing (Dederich et al.. 1984). These papers demonstrated that the porcelain to metal bond may not be stress free. and that the crowns could be measurably distorted. DeHoff and Anusavice (1981) calculated the effect of the mismatch of porcelain-metal coefficients of thermal expansion on the marginal opening of metal-ceramic crowns could be up to 21 urn. Anusaviceet al. ( 1985). using a mathematical model of an upper central incisor. calculated that the effect of loading a metal-ceramic crown palatally with a force of 200 N produced a maximum strain of 0.014 per cent. well below the accepted fracture strain level of0.l per cent (Jones, 1983) for dental porcelain. These analyses assume that there was force transmission via the cement bond as the set cement was rigid. This would not be possible during cementation. and the crown may be liable to greater deformation. Wilson et al. (1990) measured the cervical deformation and found strain levels ofgold crowns during cementation under some conditions close to fracture strains for porcelain. Jorgensen (1960a) observed that incompletely seated crowns were frequently tilted, and remained in that position once the cement had set. The different film thicknesses and perhaps crown-tooth contact in the tilted crown may lead to localized strains in the crown higher than the average values calculated by Wilson et al. (1990). Van Nortwick and Gettleman (1981) showed that there was a highly significant correlation between the completeness of crown seating and lack of tilting in the seated crown. Measures designed to improve seating should result in less tilting of the crown.
FAILURE MODES
IN PORCELAIN
SYSTEMS
Warpeha and Goodkind (1976) stated that unpredictable clinical failures could occur in metal-ceramic bonding
Crown
cementation
159
systems. and described three of the most frequent modes of failure as cleavage at the porcelain to metal interface. fracture through the opaque or body porcelain and surface crazing. The unpredictable manner of failure suggests that several factors may be important for the longevity of metal-ceramic restorations. Porcelain is vulnerable to tensile stresses. and diametral distortion during cementation may be enough to precipitate fracture in a ceramic or metal-ceramic crown if the section is insufficiently stiff. It is possible that stresses generated in cementation could contribute to immediate or delayed clinical failure. Occasionally, the porcelain of metal-ceramic crowns fractures during cementation. This phenomenon may be due to the substructure flexing and straining the porcelain superstructure past 0.1 per cent strain (Jones, 1983). Abbate et al. (1989) found that several ceramic crowns fractured during cementation with a force of 50 N. Previously. direct contact of the crown with the tooth had been assumed to be the major cause of fracture (Chiche and Pinault, 1988) rather than flexure from hydraulic deformation. Davis et al. (1989) found that disclosing the tit of crowns with a tit-checking solution revealed crownto-die contacts and that removal ofthese contacts led to an improved seating. and a diminished marginal gap. The effect on crown deformation and therefore possible fracture was not shown. but could be expected to be beneficial. Strategies to avoid intracementation fracture should be designed both to allow a passive tit of the crown on the tooth. and to minimize the effects of force transmission via the cement.
VENTING It has been shown repeatedly that more complete seating of crowns may be achieved by allowing cement trapped during crown cementation to be released through an occlusal perforation (Kaufman et al.. 1961: Bassett. 1966; Cooperetal.. 1971: Van Nortwick and Gettleman, 1981). It has been suggested that venting may also lead to a lower pressure within the cement film during cementation (Kay. 1984). The siting ofvent holes varies according to the tooth being crowned. Molars and premolars can allow for an occlusal perforation. but incisors must have an axial vent. usually placed lingually. Brose et al. (1984) found that the placing of a lingual vent hole increased the seating discrepancy of castings compared to unvented controls. to a level that represented a clinically open margin. Dykemaet al. (1986) reported that many dentists obtain satisfactory results without venting of any kind. Repair of the perforation leads to a weak link in the crown shell, and complicates the placement procedure. The suggestion from the work of Brose et al. (1984) that lingual vents are ineffective indicates the need for an occlusal perforation and repair of the vent.
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J. Dent. 1992;
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DIE SPACING Various techniques can be used to provide space for the final cement film, including grinding or etching the crown internally (Hollenback, 1928; Basset, 1966). or producing an oversize die by application of a die spacer (Fusayama et al., 1964; Eames et al.. 1978). The amount of die enlargement has been recommended to be from 20 to 40 urn (Campagni et al.. 1982). Oliva et al. (1988) showed that a 40 urn thickness of die spacer was obtained with four coats of a proprietary product (Trutit: George Taub, Jersey City. NJ, USA). Campbell (1990) found that one coat of spacer designed for Dicer cast ceramic (Dentsply Int.. York, PA, USA) produced a layer 38 urn thick, which was measured as equivalent to four layers of Tru-Fit. He noted that the Dicer spacer was thinned over the line angles of the preparation, and that this may hinder cement outflow. Grajower et al. (1989) found that when a deliberately undersized casting was produced, four layers of Tru-Fit reduced the average seating discrepancy from 649 to 29 urn. with mean axial wall cement thickness from 20 to 60 urn with no apparent trend related to thickness of spacer. The thickness of axial die-spacer for optimum seating was 70 urn, as 100 urn of die-spacer did not result in improved seating. This work suggests that the thickness of die-spacer used should allow approximately 40 urn of axial space for cement when the crown is fully seated. Incomplete coverage of the axial walls of the preparation with die-spacer resulted in crown seating discrepancies similar to those where spacer was absent, and was therefore not recommended. An alternative approach to providing increased cement escape space all around the crown with die spacing is to provide specific pathways for cement flow from the occlusal surface. Leland-Webb et al. (1983) found that provision of four grooves in the axial walls of the preparation significantly reduced the seating discrepancy of cemented crowns from 200 urn to around 50 urn. Occlusal grooving of the preparation alone or added to axial grooves had no advantage. This finding supports the argument that axial cement flow is the major problem in seating the full crown.
one-fortieth that of gold, it cannot be assumed that the test crowns behaved in a similar way to the more rigid crowns used clinically. Placement of appropriate amounts of cement as described decreased the need for venting. Oliveira et al. (1979) found that vibration of crowns during cementation before static loading improved the seating of crowns and inlays by on average 21 urn. Vibration directed perpendicular to the axial walls of crowns as described by Van Nortwick and Gettleman (1981) did not improve the seating of cast restorations, contrary to the expected film thinning caused by vibration as reported by Koyano et al. (1978). Vibration techniques must be viewed with some caution as Olin (1990) has shown that prolonged ultrasonic treatment of recently cemented crowns reduced retention by approximately one-half.
CONCLUSIONS Strategies for improving the seating of restorations during cementation are directed towards aiding the flow of cement from the internal surface of the crown. When using zinc phosphate cement, die spacing sufficient to leave 40 urn of axial space in the tooth-crown assembly appears appropriate to produce even and complete seating before cement setting. Clear evidence for the optimum thickness of die-spacer with respect to seating and retention needs to be produced. If considerable die spacing is used, what are the consequences in terms of crown position radially on the tooth’? Is the use of powder and liquid cement systems problematical for the thin film thicknesses used in dentistry’? Is continued force application after seating deleterious in creating residual stresses within the set crown-cementtooth complex? Much remains to be resolved about the cementation process and the optimal procedure for gaining the best prognosis for the restored tooth.
Acknowledgements I would like to thank Dr A. J. Hunter and Dr M. J. Tyas for their detailed comments on the manuscript.
References
PLACEMENT
TECHNIQUES
The crown has traditionally been tilled with cement and then placed upon the tooth, and an axial force used to seat the crown. Techniques have been developed of painting cement inside a crown with tine brushes (Ishikiriamietal., 1981) which lead to more complete seating. Assif et al. (1987) examined placement strategies for zinc phosphate cement by using clear acrylic crowns and a standardized cementation procedure. and concluded that it was most effective to paint the axial walls of the tooth preparation and the marginal area of the crown. The use of acrylic crowns allowed visualization of the cement flow, but as the modulus of elasticity and acrylic is approximately
Abbate M. F.. Tjan A. H. L. and Fox W. M. (1989) Comparison of the marginal tit of various ceramic crown systems. J. Prosthrt. Dent. 61, 527-530. Abelson J. (1980) Cementation of cast complete crown retainers. J. Prosthet. Dent. 11, 533-536. American Dental Association (1978) J. Am. Dent. Assoc. 57, 121-123. Anusavice K. J.. Hojjatie B. and DeHoff P. (1985) Influence of metal thickness on stress distribution in metal ceramic crowns. J. Dent. Res. 64, (abstr. 641). Assif D.. Rimer Y. and Aviv 1. (1987) The flow of zinc phosphate cement under a full-coverage restoration and its effect on marginal adaptation according to the location of cement application. Qtrintessrr~cr ht. 18, 765-774. Bassett R. W. (1966) Solving the problems of cementing the full veneer cast gold crown. J. Prosthrt. Dent. 16, 740-747.
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Behrend D. A. (1984) Crown margins and gingival health. Ann. R. Amt. Co//. Dent. Surg. 8, 138-145. Bridger D. V. and Nicholls J. 1. (1981) Distortion of ceramometal fixed partial dentures during the firing cycle. J. Pros&V. Dent. 45, 507-514. Brose M. 0.. Woelfel J. B.. Rieger M. R. et al. (1984) Internal vents for posterior complete crowns. J. Prosthet. Dent. 51, 755-760. Campagni W. V., Preston J. D. and Reisbeck M. H. (1982) Measurement of paint on die spacers for casting relief. J. Prosthrt. Dent. 47, 606-611. Campbell S. D. (1990) Comparison of conventional paint-on die spacers and those used with the all-ceramic system. J Prosthrt. Dent. 63, 151-155. Chiche G. J. and Pinault A. (1988) Essentials in Dental Ceramics. An Artistic Approach. Chicago. Yearbook Medical Publishers. p. 120. Combe E. C. and Greener E. H. (1975) The rheology of zinc polycarboxylate cements. J. Dent. Res. 54, 79. Cook W. D. (1984) Rheological studies of the setting of waterbased cements. Amt. Dent. J. 29, 44-49. Cooper T. M.. Christensen G. J.. Laswell H. R. et al. (1971) Effect of venting on cast gold full crowns. J. Prosthet. Dent. 26, 621-626. Davis S. H.. Kelly J. R. and Campbell S. D. (1989) Use of an elastomeric material to improve the occlusal seat and marginal seal of cast restorations. J. Prosthet Dent. 62, 288-291. Dederich D. N.. Svare C. W. and Turner K. A. (1981) The effect of repeated firings on the margins of non-precious ceramometals. J. Prosrhrt. Dent. 51, 628-630. DeHoff P. H. and Anusavice K. J. (1984) Effect of metal design on marginal distortion of metal crowns. ./. Dent. Res. 63, 1327-1331. Dykema R. W.. Goodacre C. J. and Phillips R. W. (1986) Johnston 2 Modern Practice in Fixed Prosthodon tics. Philadelphia, W. B. Saunders. Eames B. W.. O-Neal S. J., Monteiro J. et al. (1978) Techniques to improve the seating of castings. J. Am. Dent. Assoc. 96, 432-437. Faucher R. R. and Nicholls J. I. (1980) Distortion related to margin design in porcelain fused to metal restorations. J. Prosthet. Dent. 43, 149-155. Fusayama T.. Ide K.. Kurosa A. et :rJ. (1963) Cement thickness between cast restorations and preparation walls. J. Prosthet. Dent. 13, 354-364. Fusayama T.. Ide K. and Hosada H. (1964) Relief of resistance of cement of full cast gold crowns. J. Prosthrt. Drnt. 14, 95-106. Gavelis J. R.. Morency J. D.. Riley E. D. et al. (1981) The effect of various finishing line preparations on the marginal seal and the occlusal seat of full crown preparations. J. Prosthet. Dent. 45, 138-145. Grajower R.. Lewenstein I. and Zeltser C. (1985) The effective minimum thickness of zinc phosphate cement for luted non-precious crowns. J. Oral Rrhahil. 12, 235-245. Grajower R.. Zuberi Y. and Lewenstein I. (1989) Improving the tit of crowns with die spacers. J. Prosthrt. Dent. 61, 555-563. Grieve A. R. (1969) A study of dental cements. Br. Dent. J. 127, 405-410. Hembree J. H. and Cooper E. W. (1979) Effect of die relief on retention of cast crowns and inlays. Operative Dent. 4, 104-107. Hoard R. J.. Caputo A. A.. Contino R. M. et al. (1978) Intracoronal pressure during crown cementation. J. Prosthrt. Dent. 40, 520-525. Hollenback G. M. (1928) A practical contribution to the standardization of casting technique. J. AIII. Dent. Assoc. 10, 5-9.
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Forthcoming Original
Research
299-304.
Articles Reports
Cardiopulmonary performance following changes tration of intravenous diazemuls P. Gallon. G. M. Walton and J. P. Rood
in position
A preliminary investigation into the effect of increased dimension on mandibular movement during speech C. A. Burnett and T. J. Cliffbrd Assessment of elderly people on entry to residential arrangements G. Hoad-Reddick
and the adminis-
occlusal
vertical
homes and continuing
care
Predictions of restoration deterioration P. J. Smales, D. A. Webster aid P. 1. Lepparci A denture base resin with low water absorption M. J. Barsby The finishing of amalgam restorations: C. J. Collins and R. W. Bryant A technique for sections c. c. Yol.lngsoll
three-dimensional
Dentine bonding agents-characteristic depth J. F. McCabe and S. Rushy Dental biomechanics N. E. Waters
and the dental
a three-year
microleakage
bond strength
curriculum
clinical
study
assessment
as a function
using
tooth
of dentine
