Journalof Oral Rehabilitation, 1989, Yohime 16, pages 461-413 • ^̂ - v ,,:•'•_.

A laboratory and clinical evaluation of three dentalluting cements

p. J. KNIBBS and A.W. G. WALLS Department of Operative Dentistry,The Dental School, University of Newcastle upon Tyne, Newcastle upon Tyne,U.K.

SummaryThe loss of material from specimetis of three luting cements was measured aftercotititiuous erosion cycling in the laboratory. The glass ionomer luting cementshowed significantly less material loss than the zinc polycarboxylate and zinc phos-phate luting cements.

Two hundred and fifty restorations cemented with one of the three materialswere studied clinically for marginal integrity and retention over 3-5 years. The datawere tested using survival analysis. Zinc phosphate cement gave the best clinicalperformance. Possible explanations for the poor correlation between the findings inthe laboratory and clinical study are discussed.

IntroductionRestorations that are constructed outside the patient's mouth and are subsequentlycemented permanently in the mouth, require a luting agent to aid retention and toinfill small spaces between the restoration and the prepared tooth.

A clinical evaluation of luting cements is not easy because much of the material ishidden by the seated restoration. However, there is inevitably an exposed cementline at the restoration margin. The cement must be able to resist dissolution orerosion at this site. Breakdown of the cement matrix at the margin may lead to failureof the restoration as a result of the initiation of secondary caries in the marginaldefect or, with continued dissolution of the cement luting agent, by loss of therestoration.

The loss of material from luting cements by dissolution and erosion has beenstudied in a number of ways. IVIesu (1982) devised a method whereby luting cementswere tested in thin films for degradation that could be visualized and measured.Another method was reported by Beech & Bandyopadhyay (1982). They subjectedspecimens to a continuous jet of water or lactic acid solution. More recently, amodification of this jet test to make it more applicable to luting cements has beenreported (Gulibivala, Setchell & Davies, 1988). An alternative test using intermit-tent erosion cycling has also been described and was thought to be especiallyappropriate to the clinical situation because the apparatus allowed for turbulence tosimulate the combined in vivo effects of dissolution and abrasion (Walls, JVIcCabe &Murray, 1985). There have also been in vivo studies that utilize intra-oral devices tohold specimens in the mouth for various times, and a number of materials have been

Correspondenee: Mr P. J. Knibbs, Department of Operative Dentistry, The Dental Sehool, FramlingtonPlaee, Newcastle upon Tyne NE2 4BW, U.K.

467

468 P. J. Knibbs and A. W. G. Walls

studied (Osborne et al, 1978; Pluim et al, 1984; Ibbetson, Setchell & Amy, 1985).Many cements are available to the dental profession; three in common use are

zinc phosphate, zinc polycarboxylate and glass polyalkenoate. The properties ofthese materials have been reviewed (Smith, 1983). Zinc phosphate has been inwidespread use for many years, zinc polycarboxylate cement was first described bySmith (1968) and a glass polyalkenoate suitable for use as a luting material was firstdescribed in 1977 by Wilson et al

Recently, the technology has been developed to vacuum-dry the liquid compo-nents of some cements and to incorporate all the active constituents into a powderthat may be blended with water or a solution of tartaric acid to initiate cementformation (McLean, Wilson & Prosser, 1984). Zinc polycarboxylate and glasspolyalkenoate luting cements have been produced in this way and have received afavourable assessment in clinical usage by general dental practitioners (Knibbs,Plant & Shovelton, 1986).

The purpose of this study was to evaluate the susceptibility to erosion of a zincphosphate cement, a powder/water zinc polycarboxylate cement and a powder/waterglass polyalkenoate luting cement, and to determine whether the results explainsurvival data on the chnical performance of these materials when used as lutingagents for full coronal restorations.

Materials and methodsLaboratory investigationThe intermittent erosion cychng test has been fully described (Walls et al., 1985).The materials used in this study are shown in Table 1. The zinc phosphate (ZP) wasproportioned using a 2-ml disposable plastic syringe and an electronic balanceaccurate to within 0-001 g, to give the recommended powder/hquid ratio for cemen-tation. The zinc polycarboxylate (PC) and glass polyalkenoate (GI) cements weredispensed using the manufacturer's scoops and dropper bottles and were mixed asrecommended for room temperature.

Twenty-four specimens of each cement were prepared by placing the mixedmaterial into wells cut in squares of Perspex, covering with a cellulose acetate matrixand conditioning at 37°C and 100% relative humidity under a 1kg load. To investig-ate whether the time after setting influenced the susceptibility to erosion, 12 speci-

Table 1. The materials studied

Cementtype

Zincphosphate(ZP)

Zincpolycarb-oxylate (PC)

Glasspolyalk-enoate (GI)

Brand

ZincCement

Poly FPlus

Aquacem

Manufacturer

DeTrey Division,Dentsply

DeTrey Division,Dentsply

DeTrey Division,Dentsply

Batch no.

Powder*

FB2986/04(L)Not known (C)

FC10386/05(L)BM1882/12(C)

FE3986/05(L)DA1683/1(C)

Liquid

FB8886/06

Powder/liquidratio (g/ml)

0-20-0-25:0-l

5:1

3:1

' L, laboratory study; C, clinical study.

Evaluatiort of lutirtg cements 469

mens of each cement were tested after 15 min and 12 specimens of each were testedafter lh. -

The specimens were lapped flat on a rotary pregrinder using 800 grit carborun-dum paper with continuous water irrigation, before intermittent immersion in 0-1 Msodium lactate/lactic acid buffer at pH 4-0 and distilled water at pH 5-5. A profilo-meter was used to assess the loss of material.

Clinical irtvestigationThe cHnical performance of the materials was assessed by reviewing single-unit fullcoronal restorations placed in the mouths of patients attending a university dentalpractice. Teeth were prepared by a single operator (PJK), the clinical technique forpreparation and impression-taking being standardized. The restorations were allfabricated by the same commercial laboratory. The restorations were tried prior tocementation and only those with good marginal integrity and retention werecemented. The type of cement to be used for each restoration was chosen at random.The cements were proportioned and mixed according to the manufacturer's instruc-tions. A record was made of the patient's age, the tooth restored, the tooth vitality,the type of crown and whether the tooth had a previous full coronal restoration.

The restorations were examined at recall appointments for marginal defects witha dental probe that was blunted to give a tip size of approximately 0-4 mm. Therestoration was scored as marginally deficient if the probe penetrated a defect whenpassed in either direction across the margin. Loss of the restoration or obviouslooseness was scored as a retentive failure.

The clinical evaluation continued over a 3-5-year period until the operator leftthe university dental practice and further follow-up was not practicable.

The data was tested using survival analysis as described by Peto and co-workers(Petoef fl/., 1977).

ResultsThe mean depths of erosion of the three cements after 15 min and 1 h are given inTable 2. The data were tested using the Student r-test. All the cements weresignificantly less susceptible to erosion after lh than after 15 min (P<0-0001). TheGI cement was far less susceptible to erosion than the other two cements for both the15-min and 1-h specimens (P<0-0001). There was no significant difference after15min between the depth of loss for the ZP and PC cements.

Table 2. Mean depth loss (um) for the three cements tested 15 minor 1 h after mixing by intermittent erosion cycling

Cement

ZPPCGI

Mean depthloss

75-2574-5011-36

15 min

s.d.

3-194-452-95

Mean depthloss

38-8156-302-32

Time

l h

s.d.

1-483-201-19

Differences between all values were significant to P<0-0001, exceptfor those joined by the vertical bar. The difference between thesevalues was not significant.

470 P. J. Knibbs and A. W. G. Walls

Table 3. The restorations in the elinieal studygrouped aeeording to eement type and failuresobserved over the 42-month study period.

Restoration

PJCFVCPBCTotal

ZP

373928

104

PC

27282580

Cement

GI

34112166

Total

987874

250

Porcelain jaeket crown (PJC)Full veneer erown (FVC)Porcelain bonded erown (PBC)

Two hundred and fifty full coronal restorations were studied over a period of42months. Ninety-eight restorations were porcelain jacket crowns, 78 were full-veneer gold crowns and the remaining 74 restorations were porcelain bonded toprecious metal crowns.

Fewer full-veneer gold crowns were cemented with GI than with the other twocements. However, because the survival analysis of the cemented restorationsshowed no significant differences between any of the variables except for that ofcement type, this imbalance in the sample is of no significance. Table 3 shows thenumber of restorations in each cement group. The survival curve with regard tomarginal integrity for the three cements is shown in Fig. 1. There was no significantdifference between the GI or PC cement groups, but the ZP group showed signifi-cantly fewer marginal deficiencies. A large proportion of all the crowns surviving to3-5 years had probable marginal defects.

The survival curve with regard to retentive failure for the three cements is shownin Fig. 2. The analysis showed significant differences between the cement groups.Visual inspection of this survival curve shows a ranking of performance as follows:

•3

o

otx.o

0-5-

' - • i :

— Zinc phosphate

Zinc polycarboxylate

••• Glass polyalkenoate

10 20—I—30 40

Age of restoration (months)

Fig. 1. Survival ehart showing the proportion of restorations with marginal defects plotted against time foreach cement group.

Evaluation of luting cements All

C

•= 0-5Ho

oD.O — Zinc phosphate

— Zinc polycarboxylate

— Glass polyalkenoate

0 10 20 30 40

Age of restoration (months)

Fig. 2. Survival chart showing the proportion of restorations lost or loose plotted against time for eachcement group.

DiscussionThe results of the laboratory investigation correlate well with other studies on therelative solubilities of dental cements. In a study using this method of intermittenterosion cycling, a similar ranking of cements, namely GI<ZP<PC, was found,although the PC cement in that study showed far greater loss of material than thepowder/water cement tested in the present study (Walls et al., 1985). It is possiblethat the increased molecular weight of the vacuum-dried polyacrylic acid in thenewer formulations leads to this improved performance.

A recent study using the jet test with modified apparatus to simulate the clinicalsituation for luting cements also showed the GI to be least susceptible to loss ofmaterial, although the results obtained at pH 2-5 showed the PC performing betterthan the ZP (Gulibivala et al., 1988). The results of these in vitro studies generallycompare well with in vivo studies that have assessed the loss of material fromspecimen holders cut in cast restorations (Osborne et al, 1978), removable pros-theses (Pluim et al., 1984) or holders supported in orthodontic brackets (Ibbetson etal., 1985). Such general agreement adds support to the relevance of the findings of invitro studies. It is therefore difficult to understand the results of the chnical survivalanalysis which give a performance ranking of ZP>PC^GI. The GI cement, whichwas so resistant to erosion in vitro, gave the poorest clinical performance.

There were no differences in the survival analysis of any other variables, such astype of crown or previous restorations, suggesting that of the factors recorded onlycement type was important. There are no certain explanations for these findings.However, a number of factors can be discussed.

The size of the marginal gap between restoration and tooth may influence thedegree of degradation of the luting cement. Mesu (1982) demonstrated a markedreduction in lute degradation when gap sizes were reduced from 100//m to 20/<m,although his analysis showed that other factors such as pH, molarity and cement typeinteracted to affect the overall degradation. The GI and PC cements were moreinfluenced by gap size than was ZP.

472 P. J. Knibbs and A. W. G. Walls

Alkumnu et al. (1988) showed the mean gap size around porcelain jacket crownsto be 73-S2jum for the labial margin and in excess of lOO^m for the lingual margin.The restorations in our clinical series would be likely to have gaps of this magnitude,and certainly in excess of 40jum, which was the size of the probe tip used to testmarginal integrity prior to cementation and at later review. Gap size and cement typemay have interacted to give the clinical results that were unexpected in the light ofthe in vitro data, but the full explanation is probably more complex.

Moisture contamination after the set of the material was previously thought todeleteriously affect GI cements, and a varnish was recommended to protect therecently set material. This study showed that the 1-h specimens were significantlyless susceptible to erosion than the 15-min specimens, confirming that time is animportant factor. However, this does not explain the clinical findings because theresults of the 15-min erosion testing in this study showed the GI to be significantlyless susceptible to erosion than the other two cements and would thus be expected toperform well. Gulibivala and co-workers (1988) tested the GI cement in their jet testapparatus using short curing times and early exposure to moisture and found noadverse change in disintegration even under these severe conditions. Therefore itseems unlikely that post-setting moisture contamination in vivo would explain themarginal disintegration.

The laboratory investigation was carried out on later batches of the materialsthan those used in the clinical study. Batch variation is a possible explanation.

Long-term chnical exposure may produce results differing from those observedin the in vivo erosion studies. The in vivo studies varied in duration from 8 to 26weeks (Osborne etal. 1978; Pluim etal., 1984; Ibbetson etal., 1985). In the survivalanalysis of this study over these short time periods there were no differences inperformance between the cement types. This suggests that such in vivo erosionstudies should be continued for considerably longer time periods because the degra-dation process in vivo may not necessarily be expressed in a time constant assuggested in vitro by Mesu (1982).

Other factors, such as the effects of plaque metabolism, tooth brushing andfatigue, may be responsible for the clinical performance differing from laboratoryperformance. This problem of relating laboratory data to the clinical situation is wellknown and the results of this study serve to demonstrate the importance of long-termclinical testing of dental materials to supplement laboratory and short-term in vivotesting.

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Evaluation of luting cements 473

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Manuscript accepted 20 May 1988