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Low force cementation
Journal of Dentistry, Vol. 24, No. 4, pp. 269-213,1996 Copyright 0 1996Elsevier ScienceLtd. All rights reserved Printed in Great Britain
0300-5712(95)00074-7
0300-5712/96
$15.00 + 0.00
ELSEVIER
Low force cementation P. R. Wilson School of Dental Science,
University of Melbourne,
711 Elizabeth
Street, Melbourne,
V/C 3000, Australia
ABSTRACT
The marginaladaptationof full coveragerestorationsis adverselyaffected by the introduction of luting agentsof various minimumfilm thicknessesduring the cementationprocess.The increasein the marginal openingmay have long-term detrimental effects on the health of both pulpal and periodontal tissues.The purposeof this study wasto determinethe effects of varying seatingforces (2.5, 12.5,25 N), venting, and cement types on post-cementationmarginal elevation in cast crowns. Materials: A standardizedcement spaceof 40 /*rn wasprovided betweena machinedgold crown and a stainlesssteel die. An occlusalvent wasplaced that could be openedor closed.The post-cementation crown elevation was measured,following the use of two commercially available capsulateddental cements(Phosphacap,and Ketac-cemApplicap). Results: The results indicate that only the combination of Ketac-Cem Applicap and crown venting produced post-cementationcrown elevation of lessthan 20 pm when 12.5 N seatingforce was used. Higher forces (25 N) andventing were required for comparableseatingwhen usingPhosphacap(19 pm>. Conclusions: The amount of force required to allow maximum seating of cast crowns appearsto be cement specific,and is reduced by effective venting procedures.Copyright 0 1996Elsevier ScienceLtd. Objectiues:
KEY WORDS:
Cements,
J. Dent
24: 269-276
1996;
Cementation, (Received
Venting 23 May
1994;
reviewed
4 August
INTRODUCTION Crown cementation should result in a minimal marginal discrepancy and complete filling of the space between
the crown and the tooth with cement. To achieve this, excess cement is applied, and the seating force makes the cement flow. The seating force deforms the crown,‘, creates a pulpward pressure3, and will intrude the tooth into its socket. Low cementation forces should reduce all of these unwanted effects, but would only be acceptable if low marginal discrepancies were found after cementation. The provision of cement space has been shown to be effective in enhancing the seating of full crowns4-7. An alternative or additional technique is to allow the excess cement from between the crown and the tooth to escape via a perforation (‘vent’) on the occlusal surface of the crown. This exit route is much shorter than expression via the margin, and has been repeatedly shown to improve the seating of cemented crowns8,g. Venting also speeds the seat of crowns by a factor of
Correspondence should be addressed to: Dr P. R. Wilson, School of Dental Science, University of Melbourne, 711 Elizabeth Street, Melbourne, VIC 3000, Australia. Tel. 61 3 9341 0230; Fax 61 3 9341 0437. This paper contains material presented at the 5th Meeting of the International College of Prosthodontists, Burgenstock, Switzerland, 1993.
1994;
accepted
27 March
1995)
eight, which may allow a lower seating force, for the same seating rate as non-vented crowns2. The aim of both venting and increasing the cement space is to allow the seating force to be effective in causing excess cement to flow. Good seating of crowns can occur with a modest force (25 N)l”, and similar seating with a lower force (perhaps 2.5-12.5 N> may be possible by adding venting. This may allow the cementation of a bridge or splint to differentially mobile teeth, with the most mobile tooth receiving a vented crown. In turn, this would allow most of the seating force to be used in making the cement flow on the less mobile teeth. Therefore, the mobile tooth would bear less of the seating force, and be intruded less. The presence of the vent would still allow the crown to seat well on the tooth. The duration of force application may be important for the response of the tooth: short duration force application may have less of an intrusive effect than longer duration force application. Low force cementation may also allow cementation of multiple joined crowns with an achievable overall force. The forces recommended for cementation, when used for multiple joined crowns, may exceed the weight of the patient. Rapid seating would allow a reduction in the seating force, and this would make the clinical procedure less tiring for the clinician, and more comfortable for the patient.
270
J. Dent.
1996;
24:
No.
4
The use of venting may permit the use of cements with appropriate physical properties, but which would otherwise produce unacceptable seating discrepancies at low seating forces (such as zinc phosphate cement). However, the vent hole needs repair after cementation, and vent holes in all-ceramic crowns have been shown to weaken such crowns l1 . The objectives of this investigation were to determine whether the addition of venting into the cementation procedure would allow the use of a low seating force, and whether higher forces and venting would allow an acceptable seating discrepancy with zinc phosphate cement.
MATERIALS
force
AND METHODS
The apparatus used in this experiment is as described recently ‘, with some modifications. The apparatus allows uniaxial travel of different sized dies into a crown, and the separation of the crown and the die can be measured to the nearest 1 pm. The present experiments here required the application of a lower force than that of the loading carriage (17 N). A counterweight mechanism was added which could be adjusted to produce a seating force from 2.5 N to 17 N by alteration of the amount of lead shot in the counterbalance container (Fig. I). A gold crown (Mowrey, Par 7, St. Paul, MN, USA) was constructed to fit a stainless steel die made to the configuration of Gavelis et al.r2. The die was a truncated cone, with a base diameter of 8 mm, a height of 6 mm, and a side taper of 5” (included angle 10”). The die was one of a series of dies which were made undersized compared to the master die, which had been lapped into the machined internal surface of the crown. The die used here was smaller in height by 40 pm and in diameter by 80 pm, compared to the master die. Therefore, a space of 40 pm was present in the axial and occlusal regions when the crown was fully seated, with contact at the margin. The crown had a shoulder margin of 0.6 mm width, and had parallel external axial walls. A cross-sectional view of
Fig. 1. Diagram tion.
Seating
of loading
apparatus
with counterbalance
modifica-
Venl
i Fig, 2. Cross-section
diagram
of crown
and die.
the crown and die assembly as they are approximated, is illustrated in Fig. 2. The occlusal surface of the crown was perforated with a 1 mm tapped hole which could be open or sealed as required to permit or prevent venting. The external surface of the vent hole was connected to the atmosphere to prevent restriction of cement flow once expelled from the crown. The 40 pm undersize die was one of a set of dies, the largest one of which had been lapped into the crown, to produce no cement space. The crown and the largest die were placed together, and aligned in the testing machine. The crown was then hxed in position by being embedded in dental stone (Fuji Rock, GC Corp, Tokyo, Japan). The separation of the crown and die was measured with a Direct Current Differential Transformer (DCDT) (Hewlett Packard, Boston, MA, USA), which was capable of measuring in increments of 1 pm. The starting point for separation measurement was taken with the die inserted into the crown with no cement present, and the DCDT measured the vertical movement of the carriage to which the die was attached. The linear bearing assembly allowed movement only vertically, which precluded the possibility of tipping. Two capsulated cements were used: a zinc phosphate cement (Phosphacap; Ivoclar, Schaan, Liechtenstein), and a glass ionomer cement (Ketac-Cem Applicap; ESPE GmbH, Seefeld, Germany), mixed according to the manufacturers’ instructions. The contents of the capsule were injected into a disposable syringe. The syringe was used to place 0.15 ml of cement into the centre of the inverted crown for vented crowns and 0.07 ml for unvented crowns, and 30 s after mixing started, the carriage and die were released 2 mm from the full seat.
271
P. R. Wilson: Low force cementation
1000 -I w
Not vented Vented
800
L 200
2.5N
12.5N
25N
’
Phosphacap
2.5N
12.5N
Ketac-cem
7
I-
25N
Applicap
01-L
Fig. 3. Bar-chart of post-cementation seating elevation vs. cement type, venting and force applied.
0
i i0
6(
4b
1
Fig, 4. Copy of trace of seating of crown vs. time, with the data recording time indicated.
the vented crowns compared to 0.07 ml for the unvented crowns; otherwise there was incomplete cement coverage between the crown and the die due to loss of cement through the vent hole. The data were reviewed for order effects, but none was found. The 2.5 N seating force group had post-cementation elevations in excess of 0.5 mm, which would be irrelevant to clinical practice. An analysis of variance (CSS Statistica, Tulsa, OK, USA) was performed on the 12.5 N and 25 N data, and highly significant effects (all P < 0.001) due to cement type, seating force and venting were found.
RESULTS The results for the seating discrepancy are presented in Table I, and are plotted in Fig. 3. The trace shown in Fig. 4 was typical, in that the crown would quickly seat to a certain level, and then cease seating. There were some experiments with Ketac-Cem where the crown appeared to continue seating, but these were only in the cases where there was very wide separation of the crown and the die. There was no possibility of the crown seating to an acceptable level before the cement had set. It was necessary to use 0.15 ml of cement for
DISCUSSION Low force and cementation The combination of cement space, venting and KetacCem Applicap produced the lowest seating discrepancy
Table 1. Post cementation seating discrepancy ( pm fs.d.) 60 s after release Force Cement Phosphacap Ketac-cem
2.5 N
25 N
12.5 N
-
t
-
696 + 343 832 3~532
953 + 621
193-188
718+493
160
time (s)
The conditions were as follows: one of two cements was used, with three forces (2.5 N, 12.5 N and 25 N), one spacing (40 pm) and the presence or absence of venting. Each combination was performed 10 times, and the experimental order was arranged randomly. The seating discrepancy was recorded 60 s after seating commenced. Disassembly was started earlier if the crown appeared to have stopped seating, which facilitated cleaning.
Venting
sb
45+31
t
-
t
84+47
84k13
19+6
17k8
15+5
ilk3
272
J. Dent. 1996; 24: No. 4
(< 20 pm). This was achieved with a force of 12.5 N. Similar seating discrepancies have been achieved with 25 N and no venting lo . The forces recommended for cementation have ranged from 440 N13, maximal biting force14, to 50 N15. The consequences of high forces for the cementation of crowns have been discussed elsewhere’, but include increased crown deformation and increased pulpward pressure transmission16. The use of low seating forces (3 and 10 N) has been shown to decrease crown deformation significantly compared to 25 N seating force I7. A cementation strategy which allows good seating of crowns after cementation with the application of such low forces would also decrease intrusion of teeth into their sockets. The response of teeth to intrusive forces will be discussed later.
the marginal opening, but would not be reflected in clinical practice. Two possibilities could occur: crown tipping could occur, with binding and poor seatinglg, or a rocking motion, that could assist in seating”. It has been shown that increased gingival exudate is found when there is an increased width of cement exposed at the margin of a crown2r. In addition, there is a relatively larger loss of cement from wider marginal cement exposure than from narrow marginal cement exposure22. Clinically acceptable marginal discrepancies of uncemented castings have ranged from 34 to 114 pm, depending on the accessibility17. Cement exposure should therefore be kept as low as possible at the margin of the crown, and this would be facilitated by good seating during cementation.
Forces to cement joined crowns
Tooth movement
The cementation of multiple joined crowns, such as with a fixed bridge or a splint, requires that several crowns seat at the same rate, controlled by the amount of force applied to the prosthesis. The force distribution to individual teeth will depend on several factors: the size and form of the preparations, cementation variables such as cement space and venting, and displacement responses from individual teeth. It would be feasible to apply a force of 40 or 50 N to seat a prosthesis with four or five crowns, but if a force of 400 or 500 N is required, the seating force could exceed the weight of the patient. The rapidity of seating found when venting is used would also be useful for the multiple crown situation. The cement placement and seating procedure is extended in time with more crowns, and the cement may set to a point were it might hinder seating by the time that the prosthesis is ready for seating. In such a case, venting would allow rapid seating, and prevent delay into times when seating is extremely poor”.
The movement of a tooth in response to loading is both force- and time-dependent, with elastic and viscous components 23. In response to an intrusive load, there is an initial rapid intrusion, and then a slower intrusion, if the load is continued. One consequence for the tooth that is receiving a crown is that the force which seats the crown towards the tooth via the cement intrudes the tooth into the alveolus. If there is only one tooth being cemented, the intrusion of the tooth is unimportant, as the crown follows the tooth into the alveolus, and there is still an effective force to extrude the excess cement. The presence of a relatively mobile tooth among the abutments of a prosthesis may prevent the effective expression of cement from between that crown and tooth due to the mobile tooth intruding into the alveolus. The movement of the prosthesis would be controlled by the rate of extrusion of cement from between the less mobile abutments and the prosthesis. The delayed elastic recovery of the most mobile tooth which was intruded the most may not be able to express excess cement before the cement sets. It has been reported that the presence of a vent allows rapid seating of crowns with low forces (< 10 s, 10 N)24. Venting the crown for the most mobile tooth may therefore have two consequences: the tooth receives a lower proportion of the seating force, and the elastic recoil of the tooth from the socket can express the excess cement through the vent more easily than via the margin.
Seating elevation The lowest possible post-cementation marginal discrepancy is beneficial as the occlusion planned in the laboratory will be more nearly reflected in the cemented crown. Seating elevations of around 0.7 mm were found under the most unfavourable circumstances (2.5 N, + / - venting), and such an elevation would represent a significant increase in occlusal height. This would have consequences in the intercuspal position, and possibly in excursive movements. Subsequent adjustment would require extensive modification of the occlusal surface, and may lead to a perforation of the crown. A limitation of the testing protocol used here was that the crown was restrained to movement along its path of insertion. This facilitated the measurement of
Venting and specific cements There may be clinical situations where the higher modulus of elasticity of set zinc phosphate cement compared to glass polyalkenoate (ionomer) cement would be useful (e.g. high masticatory load). It would seem appropriate to use a vent when the use of zinc phosphate cement is indicated clinically, as a modest ce-
P. R. Wilson: Low force cementation
mentation force can then be used (2.5 N), and a low post-cementation seating discrepancy of less than 25 pm can be produced. The rapidity of seating when a vent is used (< 10 s>’ may also be indicated when the cement being used has a very short working time, as has been suggested for a
4-META based cement 25. The use of a resin composite based material may also offer a deft solution to the problems of restoring the vent hole, as it would be insoluble, and there should be an effective seal if the crown and cement bond together. The additional margin around the vent compared to the gingival margin of the crown which requires sealing is, however, relatively small, and problems associated with the long-term sealing of vent holes do not appear to be a serious clinical problem. The problems associated with vent holes are more related to the effect on the crown, with reduced strength in all-ceramic crowns when vented”.
CONCLUSIONS The addition of venting to the provision of cement space in the cementation procedure allows rapid and complete seating with Ketac-Cem Applicap, even when low forces (12.5 N) are used. Problems of intrusion of multiple differentially mobile teeth may be minimised by the use of a low force to seat the crowns, and the presence of venting on the mobile abutment may allow favourable force distribution. However, reduction of seating force to 2.5 N with or without venting produces unacceptable seating discrepancies with both cements examined. Venting and modest forces (25 N) may also allow the use of cements such as zinc phosphate, with better physical properties than glass ionomer (e.g. modulus of elasticity), with an acceptable post-cementation seating discrepancy. Venting may be useful for cements that have short working times, due to the rapidity of seating. Acknowledgements I would like to thank Mr A. Owen for his help in the modification of the apparatus, Mr P. Barnes for his technical assistance, and Associate Professor M. I. Tyas for his review of the manuscript.
References 1. Wilson PR. The effect of die spacing on crown deformation and seating time. Int J Prosthodont 1993; 6: 397-401. 2. Wilson PR, Goodkind RJ, Delong R and Sakaguchi R. Deformation of crowns during cementation. J Prosthet Dent 1990; 64: 601-609. 3. Wylie SG and Wilson PR. An Investigation into the
273
PressureTransmitted to the Pulp Chamber on Crown Cementation: A Laboratory Study. J Dent Res 1994;73: 1684-1689. 4. EamesWB, O’Neal SJ, Monteiro J, Miller C, Roan JD Jr and Cohen KS. Techniques to improve the seating of castings.J Am Dent Assoc 1978;96: 432-437. 5. FusayamaT, Ide K and HosodaH. Relief of resistanceof cement of full cast crowns. J Prosthet Dent 1964; 14: 95-106. 6. Grajower R, Zuberi Y and LewinsteinI. Improving the fit of crowns with die spacers.J ProsthetDent 1989; 61: 555-563. 7. Wang C-J, Millstein PL and Nathanson D. Effects of cement,cementspace,marginaldesign,seatingaid materials, and seatingforce on crown cementation. J Prosthet Dent 1992;67: 786-790. 8. Cooper T, Christensen G, Laswell H and Baxter R. Effect of venting on castgold full crowns.J ProsthetDent 1971;26: 621-626. 9. Van Nortwick W and Gettleman L. Effect of internal relief, vibration, and venting on the vertical seating of cementedcrowns.J ProsthetDent 1981;45: 395-399. 10. Wilson PR. CementFlow andForce Dissipationin Crown Cementation.PhD thesis,University of Melbourne, 1993. 11. Sallustio F, Waskwicz G and Billy E. The effect of venting on the strength of Dicer and Hi-Ceram crowns. Int J Prosthodont1992;5: 463-468. 12. Gavelis J, Morency J, Riley E and Sozio R. The effect of various finish line preparationson the marginalseal and occlusalseat of full crown preparations.J ProsthetDent 1981;45: 138-145. 13. Kay L. Studies of the flow phenomenaassociatedwith crown cementation. Master of Medical Science thesis, Harvard University, 1984. 14. ShepherdBG, McNamara DC and Henry PJ. Endurance of jaw elevator musclesduring cementation of a single molar crown. Aust Dent J 1991;36: 145-150. 15. Abbate MF, Tjan AHL and Fox WM. Comparisonof the marginal fit of various ceramic crown systems.J Prosthet Dent 1989;61: 527-531. 16. Wong R. Pulpward pressuretransmissionvariation with
different seating forces and cement space - a laboratory study. MDSc thesis,University of Melbourne, 1995. 17. ChristensenGJ. Marginal fit of gold inlay castings.J ProsthetDent 1966;16: 295-305. 18. Wilson PR. Crown seating with delayed placement of capsulatedcements.Aust Dent J 1994;39: 214-219. 19. Lange F. Experiments on cementation of crowns. Tanglaegebladet 1955;50: 181-196. 20. Gegauff AG and RosenstielSF. Reassessment of diespacerwith dynamicloading during cementation. J Prosthet Dent 1989;61: 655-658. 21. Felton DA, Kanoy BE, Bayne SC and Wirthman GP. Effect of in vivo crown margindiscrepancies on periodontal health. J ProsthetDent 1991;65: 357-364. 22. Jacobs MS and Windeler AS. An investigation of dental luting cementsolubility asa function of the marginalgap. J ProsthetDent 1991;65: 436-442. 23. Picton DCA. Tooth mobility - an update. Eur J Orthodont 1990;12: 109-115. 24. Vermilyea SG, Kuffler MJ and Huget EF. The effects of die relief agenton the retention of full coveragecastings. JProsthetDent 1983;50: 207-210. 25. Wu JC and Wilson PR. Optimal CementSpacefor Resin Luting Cements. Int J Prosthodont1994;7: 209-215.
0300-5712(95)00074-7
0300-5712/96
$15.00 + 0.00
ELSEVIER
Low force cementation P. R. Wilson School of Dental Science,
University of Melbourne,
711 Elizabeth
Street, Melbourne,
V/C 3000, Australia
ABSTRACT
The marginaladaptationof full coveragerestorationsis adverselyaffected by the introduction of luting agentsof various minimumfilm thicknessesduring the cementationprocess.The increasein the marginal openingmay have long-term detrimental effects on the health of both pulpal and periodontal tissues.The purposeof this study wasto determinethe effects of varying seatingforces (2.5, 12.5,25 N), venting, and cement types on post-cementationmarginal elevation in cast crowns. Materials: A standardizedcement spaceof 40 /*rn wasprovided betweena machinedgold crown and a stainlesssteel die. An occlusalvent wasplaced that could be openedor closed.The post-cementation crown elevation was measured,following the use of two commercially available capsulateddental cements(Phosphacap,and Ketac-cemApplicap). Results: The results indicate that only the combination of Ketac-Cem Applicap and crown venting produced post-cementationcrown elevation of lessthan 20 pm when 12.5 N seatingforce was used. Higher forces (25 N) andventing were required for comparableseatingwhen usingPhosphacap(19 pm>. Conclusions: The amount of force required to allow maximum seating of cast crowns appearsto be cement specific,and is reduced by effective venting procedures.Copyright 0 1996Elsevier ScienceLtd. Objectiues:
KEY WORDS:
Cements,
J. Dent
24: 269-276
1996;
Cementation, (Received
Venting 23 May
1994;
reviewed
4 August
INTRODUCTION Crown cementation should result in a minimal marginal discrepancy and complete filling of the space between
the crown and the tooth with cement. To achieve this, excess cement is applied, and the seating force makes the cement flow. The seating force deforms the crown,‘, creates a pulpward pressure3, and will intrude the tooth into its socket. Low cementation forces should reduce all of these unwanted effects, but would only be acceptable if low marginal discrepancies were found after cementation. The provision of cement space has been shown to be effective in enhancing the seating of full crowns4-7. An alternative or additional technique is to allow the excess cement from between the crown and the tooth to escape via a perforation (‘vent’) on the occlusal surface of the crown. This exit route is much shorter than expression via the margin, and has been repeatedly shown to improve the seating of cemented crowns8,g. Venting also speeds the seat of crowns by a factor of
Correspondence should be addressed to: Dr P. R. Wilson, School of Dental Science, University of Melbourne, 711 Elizabeth Street, Melbourne, VIC 3000, Australia. Tel. 61 3 9341 0230; Fax 61 3 9341 0437. This paper contains material presented at the 5th Meeting of the International College of Prosthodontists, Burgenstock, Switzerland, 1993.
1994;
accepted
27 March
1995)
eight, which may allow a lower seating force, for the same seating rate as non-vented crowns2. The aim of both venting and increasing the cement space is to allow the seating force to be effective in causing excess cement to flow. Good seating of crowns can occur with a modest force (25 N)l”, and similar seating with a lower force (perhaps 2.5-12.5 N> may be possible by adding venting. This may allow the cementation of a bridge or splint to differentially mobile teeth, with the most mobile tooth receiving a vented crown. In turn, this would allow most of the seating force to be used in making the cement flow on the less mobile teeth. Therefore, the mobile tooth would bear less of the seating force, and be intruded less. The presence of the vent would still allow the crown to seat well on the tooth. The duration of force application may be important for the response of the tooth: short duration force application may have less of an intrusive effect than longer duration force application. Low force cementation may also allow cementation of multiple joined crowns with an achievable overall force. The forces recommended for cementation, when used for multiple joined crowns, may exceed the weight of the patient. Rapid seating would allow a reduction in the seating force, and this would make the clinical procedure less tiring for the clinician, and more comfortable for the patient.
270
J. Dent.
1996;
24:
No.
4
The use of venting may permit the use of cements with appropriate physical properties, but which would otherwise produce unacceptable seating discrepancies at low seating forces (such as zinc phosphate cement). However, the vent hole needs repair after cementation, and vent holes in all-ceramic crowns have been shown to weaken such crowns l1 . The objectives of this investigation were to determine whether the addition of venting into the cementation procedure would allow the use of a low seating force, and whether higher forces and venting would allow an acceptable seating discrepancy with zinc phosphate cement.
MATERIALS
force
AND METHODS
The apparatus used in this experiment is as described recently ‘, with some modifications. The apparatus allows uniaxial travel of different sized dies into a crown, and the separation of the crown and the die can be measured to the nearest 1 pm. The present experiments here required the application of a lower force than that of the loading carriage (17 N). A counterweight mechanism was added which could be adjusted to produce a seating force from 2.5 N to 17 N by alteration of the amount of lead shot in the counterbalance container (Fig. I). A gold crown (Mowrey, Par 7, St. Paul, MN, USA) was constructed to fit a stainless steel die made to the configuration of Gavelis et al.r2. The die was a truncated cone, with a base diameter of 8 mm, a height of 6 mm, and a side taper of 5” (included angle 10”). The die was one of a series of dies which were made undersized compared to the master die, which had been lapped into the machined internal surface of the crown. The die used here was smaller in height by 40 pm and in diameter by 80 pm, compared to the master die. Therefore, a space of 40 pm was present in the axial and occlusal regions when the crown was fully seated, with contact at the margin. The crown had a shoulder margin of 0.6 mm width, and had parallel external axial walls. A cross-sectional view of
Fig. 1. Diagram tion.
Seating
of loading
apparatus
with counterbalance
modifica-
Venl
i Fig, 2. Cross-section
diagram
of crown
and die.
the crown and die assembly as they are approximated, is illustrated in Fig. 2. The occlusal surface of the crown was perforated with a 1 mm tapped hole which could be open or sealed as required to permit or prevent venting. The external surface of the vent hole was connected to the atmosphere to prevent restriction of cement flow once expelled from the crown. The 40 pm undersize die was one of a set of dies, the largest one of which had been lapped into the crown, to produce no cement space. The crown and the largest die were placed together, and aligned in the testing machine. The crown was then hxed in position by being embedded in dental stone (Fuji Rock, GC Corp, Tokyo, Japan). The separation of the crown and die was measured with a Direct Current Differential Transformer (DCDT) (Hewlett Packard, Boston, MA, USA), which was capable of measuring in increments of 1 pm. The starting point for separation measurement was taken with the die inserted into the crown with no cement present, and the DCDT measured the vertical movement of the carriage to which the die was attached. The linear bearing assembly allowed movement only vertically, which precluded the possibility of tipping. Two capsulated cements were used: a zinc phosphate cement (Phosphacap; Ivoclar, Schaan, Liechtenstein), and a glass ionomer cement (Ketac-Cem Applicap; ESPE GmbH, Seefeld, Germany), mixed according to the manufacturers’ instructions. The contents of the capsule were injected into a disposable syringe. The syringe was used to place 0.15 ml of cement into the centre of the inverted crown for vented crowns and 0.07 ml for unvented crowns, and 30 s after mixing started, the carriage and die were released 2 mm from the full seat.
271
P. R. Wilson: Low force cementation
1000 -I w
Not vented Vented
800
L 200
2.5N
12.5N
25N
’
Phosphacap
2.5N
12.5N
Ketac-cem
7
I-
25N
Applicap
01-L
Fig. 3. Bar-chart of post-cementation seating elevation vs. cement type, venting and force applied.
0
i i0
6(
4b
1
Fig, 4. Copy of trace of seating of crown vs. time, with the data recording time indicated.
the vented crowns compared to 0.07 ml for the unvented crowns; otherwise there was incomplete cement coverage between the crown and the die due to loss of cement through the vent hole. The data were reviewed for order effects, but none was found. The 2.5 N seating force group had post-cementation elevations in excess of 0.5 mm, which would be irrelevant to clinical practice. An analysis of variance (CSS Statistica, Tulsa, OK, USA) was performed on the 12.5 N and 25 N data, and highly significant effects (all P < 0.001) due to cement type, seating force and venting were found.
RESULTS The results for the seating discrepancy are presented in Table I, and are plotted in Fig. 3. The trace shown in Fig. 4 was typical, in that the crown would quickly seat to a certain level, and then cease seating. There were some experiments with Ketac-Cem where the crown appeared to continue seating, but these were only in the cases where there was very wide separation of the crown and the die. There was no possibility of the crown seating to an acceptable level before the cement had set. It was necessary to use 0.15 ml of cement for
DISCUSSION Low force and cementation The combination of cement space, venting and KetacCem Applicap produced the lowest seating discrepancy
Table 1. Post cementation seating discrepancy ( pm fs.d.) 60 s after release Force Cement Phosphacap Ketac-cem
2.5 N
25 N
12.5 N
-
t
-
696 + 343 832 3~532
953 + 621
193-188
718+493
160
time (s)
The conditions were as follows: one of two cements was used, with three forces (2.5 N, 12.5 N and 25 N), one spacing (40 pm) and the presence or absence of venting. Each combination was performed 10 times, and the experimental order was arranged randomly. The seating discrepancy was recorded 60 s after seating commenced. Disassembly was started earlier if the crown appeared to have stopped seating, which facilitated cleaning.
Venting
sb
45+31
t
-
t
84+47
84k13
19+6
17k8
15+5
ilk3
272
J. Dent. 1996; 24: No. 4
(< 20 pm). This was achieved with a force of 12.5 N. Similar seating discrepancies have been achieved with 25 N and no venting lo . The forces recommended for cementation have ranged from 440 N13, maximal biting force14, to 50 N15. The consequences of high forces for the cementation of crowns have been discussed elsewhere’, but include increased crown deformation and increased pulpward pressure transmission16. The use of low seating forces (3 and 10 N) has been shown to decrease crown deformation significantly compared to 25 N seating force I7. A cementation strategy which allows good seating of crowns after cementation with the application of such low forces would also decrease intrusion of teeth into their sockets. The response of teeth to intrusive forces will be discussed later.
the marginal opening, but would not be reflected in clinical practice. Two possibilities could occur: crown tipping could occur, with binding and poor seatinglg, or a rocking motion, that could assist in seating”. It has been shown that increased gingival exudate is found when there is an increased width of cement exposed at the margin of a crown2r. In addition, there is a relatively larger loss of cement from wider marginal cement exposure than from narrow marginal cement exposure22. Clinically acceptable marginal discrepancies of uncemented castings have ranged from 34 to 114 pm, depending on the accessibility17. Cement exposure should therefore be kept as low as possible at the margin of the crown, and this would be facilitated by good seating during cementation.
Forces to cement joined crowns
Tooth movement
The cementation of multiple joined crowns, such as with a fixed bridge or a splint, requires that several crowns seat at the same rate, controlled by the amount of force applied to the prosthesis. The force distribution to individual teeth will depend on several factors: the size and form of the preparations, cementation variables such as cement space and venting, and displacement responses from individual teeth. It would be feasible to apply a force of 40 or 50 N to seat a prosthesis with four or five crowns, but if a force of 400 or 500 N is required, the seating force could exceed the weight of the patient. The rapidity of seating found when venting is used would also be useful for the multiple crown situation. The cement placement and seating procedure is extended in time with more crowns, and the cement may set to a point were it might hinder seating by the time that the prosthesis is ready for seating. In such a case, venting would allow rapid seating, and prevent delay into times when seating is extremely poor”.
The movement of a tooth in response to loading is both force- and time-dependent, with elastic and viscous components 23. In response to an intrusive load, there is an initial rapid intrusion, and then a slower intrusion, if the load is continued. One consequence for the tooth that is receiving a crown is that the force which seats the crown towards the tooth via the cement intrudes the tooth into the alveolus. If there is only one tooth being cemented, the intrusion of the tooth is unimportant, as the crown follows the tooth into the alveolus, and there is still an effective force to extrude the excess cement. The presence of a relatively mobile tooth among the abutments of a prosthesis may prevent the effective expression of cement from between that crown and tooth due to the mobile tooth intruding into the alveolus. The movement of the prosthesis would be controlled by the rate of extrusion of cement from between the less mobile abutments and the prosthesis. The delayed elastic recovery of the most mobile tooth which was intruded the most may not be able to express excess cement before the cement sets. It has been reported that the presence of a vent allows rapid seating of crowns with low forces (< 10 s, 10 N)24. Venting the crown for the most mobile tooth may therefore have two consequences: the tooth receives a lower proportion of the seating force, and the elastic recoil of the tooth from the socket can express the excess cement through the vent more easily than via the margin.
Seating elevation The lowest possible post-cementation marginal discrepancy is beneficial as the occlusion planned in the laboratory will be more nearly reflected in the cemented crown. Seating elevations of around 0.7 mm were found under the most unfavourable circumstances (2.5 N, + / - venting), and such an elevation would represent a significant increase in occlusal height. This would have consequences in the intercuspal position, and possibly in excursive movements. Subsequent adjustment would require extensive modification of the occlusal surface, and may lead to a perforation of the crown. A limitation of the testing protocol used here was that the crown was restrained to movement along its path of insertion. This facilitated the measurement of
Venting and specific cements There may be clinical situations where the higher modulus of elasticity of set zinc phosphate cement compared to glass polyalkenoate (ionomer) cement would be useful (e.g. high masticatory load). It would seem appropriate to use a vent when the use of zinc phosphate cement is indicated clinically, as a modest ce-
P. R. Wilson: Low force cementation
mentation force can then be used (2.5 N), and a low post-cementation seating discrepancy of less than 25 pm can be produced. The rapidity of seating when a vent is used (< 10 s>’ may also be indicated when the cement being used has a very short working time, as has been suggested for a
4-META based cement 25. The use of a resin composite based material may also offer a deft solution to the problems of restoring the vent hole, as it would be insoluble, and there should be an effective seal if the crown and cement bond together. The additional margin around the vent compared to the gingival margin of the crown which requires sealing is, however, relatively small, and problems associated with the long-term sealing of vent holes do not appear to be a serious clinical problem. The problems associated with vent holes are more related to the effect on the crown, with reduced strength in all-ceramic crowns when vented”.
CONCLUSIONS The addition of venting to the provision of cement space in the cementation procedure allows rapid and complete seating with Ketac-Cem Applicap, even when low forces (12.5 N) are used. Problems of intrusion of multiple differentially mobile teeth may be minimised by the use of a low force to seat the crowns, and the presence of venting on the mobile abutment may allow favourable force distribution. However, reduction of seating force to 2.5 N with or without venting produces unacceptable seating discrepancies with both cements examined. Venting and modest forces (25 N) may also allow the use of cements such as zinc phosphate, with better physical properties than glass ionomer (e.g. modulus of elasticity), with an acceptable post-cementation seating discrepancy. Venting may be useful for cements that have short working times, due to the rapidity of seating. Acknowledgements I would like to thank Mr A. Owen for his help in the modification of the apparatus, Mr P. Barnes for his technical assistance, and Associate Professor M. I. Tyas for his review of the manuscript.
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different seating forces and cement space - a laboratory study. MDSc thesis,University of Melbourne, 1995. 17. ChristensenGJ. Marginal fit of gold inlay castings.J ProsthetDent 1966;16: 295-305. 18. Wilson PR. Crown seating with delayed placement of capsulatedcements.Aust Dent J 1994;39: 214-219. 19. Lange F. Experiments on cementation of crowns. Tanglaegebladet 1955;50: 181-196. 20. Gegauff AG and RosenstielSF. Reassessment of diespacerwith dynamicloading during cementation. J Prosthet Dent 1989;61: 655-658. 21. Felton DA, Kanoy BE, Bayne SC and Wirthman GP. Effect of in vivo crown margindiscrepancies on periodontal health. J ProsthetDent 1991;65: 357-364. 22. Jacobs MS and Windeler AS. An investigation of dental luting cementsolubility asa function of the marginalgap. J ProsthetDent 1991;65: 436-442. 23. Picton DCA. Tooth mobility - an update. Eur J Orthodont 1990;12: 109-115. 24. Vermilyea SG, Kuffler MJ and Huget EF. The effects of die relief agenton the retention of full coveragecastings. JProsthetDent 1983;50: 207-210. 25. Wu JC and Wilson PR. Optimal CementSpacefor Resin Luting Cements. Int J Prosthodont1994;7: 209-215.