An investigation of dental luting function of the marginal gap

cement

solubility

as a

Michael S. Jacobs, D.D.S., M.S.* and A. Stewart Windeler, D.D.S., M.Sc., Ph.D.** Sheppard Air Force Base, Wichita Falls, Tex.; and University of Texas Health Science Center, Dental School, San Antonio, Tex. The purpose of this study was to investigate the rate of type I zinc phosphate cement solubility as it relates to the degree of marginal opening. Standardized test samples were constructed that would simulate clinically relevant marginal gaps of 26, SO, ‘76, and 160 microns and their subsequent cement lines. The study was divided into two phases. Phase 1 evaluated the effects of simple diffusion on cement solubility in a static environment, whereas phase 2 investigated the effects of convective forces on cement dissolution in a dynamic environment. Both the phase 1 and phase 2 studies demonstrated no significant difference in the rate of cement dissolution for the 25, SO-, and 7S-micron test groups. The lSO-micron test groups for both studies, however, demonstrated an increase in the rate of cement dissolution. The results of the phase 1 and phase 2 studies should not be compared because different methodologies were used. (J PROSTHET DENT 1991;65:436-42.)

F

or SOyears zinc phosphateluting cementhasbeen widely usedfor cast restorations even though it has significant deficiencies.The most clinically relevant of these deficiencies is its solubility in oral fluids.l This dissolution may result in recurrent caries and loosening of the cast restoration. Becauseof their solubility, luting cements,in general,have been describedasthe “weak link” in restoring teeth with cast restorations.2 The rate of luting cement dissolution has been related empirically to the degreeof marginal opening. Thus, the larger the marginal gap and subsequentexposure of the dental luting cement to oral fluids, the more rapid is the .rate of cement dissolution.3 Fick’s first law of diffusion, however, would predict that the rate of cement dissolution due to diffusion is independent of the degreeof luting cement exposed.4The law alsostatesthat in a static environment the rate of cement dissolution is dependent on concentration gradients and the diffusion constant of the

The viewsexpressed hereinarethoseof the authorsand do not necessarily reflectthe policiesof the U.S. Air Forceor the Departmentof Defense. SecondPlace,John J. Sharry ProsthodonticResearchCompetition, AmericanCollegeof Prosthodontists, Baltimore,Md. *LieutenantColonel,U.S. Air Force,DC; St& Prosthodontist, RegionalHospital,SheppardAir ForceBase. **Professor,Divisionof Biomaterials,Departmentof Restorative Dentistry, University of TexasHealth ScienceCenter,Dental School. 10/l/23202 436

cement solute. Consequently, when the effect.9of abrasion are added, the rates of cement dissolution would not be expected to follow Fick’s first law of diffusion (Fig. 1). Cementsolubility hasbeeninvestigated by useof in vitro and in vivo methods.5-23Mes@ and Wolff et a1.21addressedluting cement solubility as it relates to clinically relevant marginal gapsand their subsequentcement line. Becausethe preciserelationship betweenmarginal gap size and cement dissolution is not known, this study investigated the rate of zinc phosphateluting cementsolubility as it relates to the size of the marginal gap.24

MATERIAL Fabrication samples

AND METHODS of the 25 pm control

test

Each test sampleconsistedof a thin measuredlayer of zinc phosphate cement interposed between two transparent quartz disks. The transparent quartz disks permitted cement dissolution to be visually monitored and photographed. To make the test samples,70 standard-size circular quartz disks with plane-parallel flat surface? were purchased (Quartz Scientific, Inc., Fairport Harbor, Ohio). The diskswere measuredwith a micrometer to ensurethat they satisfied the required specifications (25.4 mm in diameter and 6 mm thick). The disks were meticulously cleanedin an ultrasonic cleaner, washedin distilled water, and dried. The disks were then paired by placing them on top of one another, and an initial baselinemeasurementof their combined thickness was recorded. To ensure mea-

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A

~#=----j;

-

Diffusing

solute

B

F:li

c--

Dittuslng

Sdute

Fig.

1. Fick’s first law of diffusion.

surement in the samelocations before and after cementation, three different color pens were used to place vertical lines an equal distance from one another on the external edgesof the paired disks. Baseline measurementsat the three reference linesand the center of the paired diskswere made by using a micrometer. The recorded measurements were averaged and the mean of the four measurements represented the precementation thickness of the paired disks. The luting cement used in this study was type I zinc phosphatecement (Flecks’, Mizzy, Inc., Clifton Forge, Va.) A standardized mix of the luting cement wasprepared with 1.6gm of zinc plhosphatepowder and 0.6 ml of phosphoric acid liquid for leachtest sample. The powder and liquid usedto create the test sampleswasderived from the same lot number of zinc phosphate cement. The mixed zinc phosphate cement was placed between the two quartz disks and the disks were realigned according to the referencelines. The cemented quartz diskswere placed in a specially made alignment fixture designedto ensurethat the edgesof the quartz diskswerealigned in the samevertical plane. After 120secondsfrom the start of the mix, the quartz ‘diskswere centered under the vertical arm of a precision press and a load of 14.7 newtons (15 kilograms) wasapplied. After 15 minutes from the start of the mix, the quartz diskswere removed from the precision pressand examined to ensurethe continued alignment of the reference lines. After cementation, the four reference areaswere remeasured with the s,amemicrometer. The thickness of the cement interspersledbetweenthe quartz diskswascalculated

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Fig.

3. Group A (25 pm) predissolution test sample.

by subtracting the initial measurementof the disks from the postcementation measurement. The increase in the postcementation measurementcorrespondedwith the 25 pm film thickness of type I zinc phosphatecement. A 600grit sandpaperwas usedto create a flush external cement line in the samevertical plane asthe edgesof both quartz disks.A stainlesssteel clamp engraved with a group letter and number for identification purposeswasplaced around the test sampleto securethe cementeddisks (Fig. 2). The 25 pm group was designated group A and the samples within this group were numbered from 1 to 8 (Fig. 3). The 50pmgroup wasdesignatedasgroup B and alsonumbered from 1 to 8 (Fig. 4). The 75pm and 150pm test sampleswere designated as group C and group D, respectively, and numbered from 1 to 8. The prepared test sampleswere stored in a humidor.

Fabrication samples

of 60, 76, and 160 pm test

A method for manufacturing an increasing series of marginal gapswasdeveloped by placing circular stainless steel shimsbetween the quartz disks before cementation. The 50 wrn test samplewas created by inserting a 25 pm stainlesssteel shim between the quartz disks before its ce-

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Stainless

i Ouafiz

Sled

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Shim

Disk

I

5. Diagramatic representation of lateral view of test sample. Fig.

Fig. 4. Group B (50 pm) predissolution

test sample.

mentation. The shim was centrally positioned on one quartz disk, and the opposite quartz disk, which contained the zinc phosphate cement, was carefully placed on it. The test sample was positioned in the alignment fixture and placed in the precision press, which was loaded in the standard manner. After 15 minutes the disk was removed from the precision press, and postcementation measurements were made. The combination of the 25 pm shim and the 25 pm film thickness of zinc phosphate cement produced a 50 pm cement margin. The 75 and 150 pm test samples were created by use of the aforementioned technique; however, 50 pm and 125 Frn shims, respectively, were used (Fig. 5).

Test

solution

Previous studies have indicated that a dilute solution of lactic acid could be used as an appropriate in vitro test solution.13s ls A pilot study demonstrated that a test solution of 0.1 molar lactic acid could be used as the dissolving medium.

Phase

l-The

diffusion

study

A total of 32 test samples, consisting of eight 25,50, 75, and 150 pm test samples, were made for use in the diffusion phase of the investigation. A computer-generated random sequence was used to arrange the test samples on stainless steel rods over a clear plastic container, and the samples were completely immersed in 6 liters of the test

438

solution. The test solution and the test sampleswere covered to reduce evaporation and left undisturbed. The investigation wasstoppedwhen cement dissolution on any oneof the test sampleswaswithin 2 mm of its stainlesssteel shim.The sampleswereremovedfrom the test solution and photographed. Photography of the test sampleswas accomplishedin a standardized manner.The camerabody (Nikkon F-3), lens (Micro-Nikkor 105), and extension tube (PK-3 Nikkon) were mounted on a copystand and the lens was placed at maximum extension. Once the correct focal distance was obtained, the cameraremained stationary becausethe test samples were all of the same diameter. Panatomic-X (Kodak) film was used and the standardized film plane magnification wasx0.78. A Bright Box apparatus (Bartholomew Manufacturing Inc., Des Planes, Ill.) provided the background illumination. A 12-inch square piece of Plexiglassmaterial (Rohmand HaasCo., Philadelphia, Pa.) was used to support the test sample. The group letter and number of each sample was placed beneath it before it was photographed. The prints were processedin a standard manner and 8 X 10 inch enlargements were made (Fig. 6). A ZeissInteractive digital analysis system (Carl Zeiss, Inc., Thornwood, N.Y.) wasusedto measurethe areaof the remaining cementof eachtest samplefrom its photograph. A one-way analysis of variance (ANOVA) wasselectedas the statistical test of choicefor comparingthe areasof undissolvedcement amongthe four groups. After photographs were made of the test samples,the stainlesssteel clampswere removed from the test samples and the quartz diskswere carefully separated.With the use of a dissectingmicroscope,the cement core and the “halo” layers were separated from one another (Fig. 6). The cement collected from the two layers was individually examined by useof radiographic diffraction techniques.

Phase

a--The

dynamic

study

Four groups of test sampleswere made, measured,and identified in the samemanner as in phase1. Each group consisted of five test samples. The test samples were attached to the crossbarsof a specially designedmechanical apparatus. The crossbarsmoved in a vertical direction

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I. Phase 1 mean cement margins

Table

A B C

24.50 51.75 74.00 148.00

D N = 8 in each

Table

Standard deviation

Mean km)

Group

2.07 2.08 3.29 4.18

group.

Mean remaining cement areas of phase 1 test

II.

samples* Cement area (mm?

Group

A

Standard deviation

12,382 12,664 12,244 9,876

B C D N = 8 in each *As measured

group. from standard

609.51 460.90 316.85 475.37

photographic

enlargements.

III. Summary for one-way analysis of variance comparing average cement area remaining for each phase 1 test sample

Table

Source

DF

Group Error

3 28

ss 39!#00769.09 6372323.12

MS 13300256.36 227582.96

FValue

FProbability

58.44

0.001

of 70 mm. This movement was termed “one cycle” and 10 cycles were completed in 1 minute. The samples were placed in a plastic container that contained 10 liters of the lactic acid test solution. The test samples remained completely submerged throughout this phase of the investigation, which continued until cement dissolution of any one of the test samples was within 2 mm of its stainless steel shim. At the end of the test period the samples were removed from the test solution and photographed in the same manner as in phase 1..The region of the undissolved cement of each test sample was measured with the Zeiss Interactive digital analysis system. A one-way ANOVA was selected as the statistical test of choice for comparing the remaining cement area am,ong the groups.

RESULTS Phase l-The

diffusion

study

The diffusion study was terminated 30 days after the test samples were immersed in the test solution. The mean and standard deviation of the marginal openings of the test samples are listed in Table I. The phase 1 means and stan-

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Fig. 6. Postdissolution test sample (B-l) demonstrates centrally located cement core and outer “halo” layer. dard deviations for the remaining cement are listed in Table II. The results of ANOVA demonstrated a statistically significant difference among the four phase 1 groups (p < 0.001) (Table III). Duncan’s multiple comparison test, which was used to determine the differences between pairs of means, demonstrated that the region of the remaining cement in the group D test samples was statistically smaller than the cement remaining in groups A, B, and C (p < 0.05). Duncan’s multiple comparison test also demonstrated that there was no statistically significant difference between the areas of remaining cement in the group A, B, and C test samples (p < 0.05).

Phase

2-The

dynamic

study

The dynamic study was terminated after 7 days. The mean and standard deviation of the marginal openings of the test samples are listed in Table IV. The phase 2 means and standard deviations for the remaining cement are listed in Table V. The results of ANOVA demonstrated a statistically significant difference among the four phase 2 groups (p < 0.001) (Table VI). Duncan’s multiple comparison test demonstrated that the cement remaining in the group D test samples was statistically smaller than the cement remaining in groups A, B, and C 0, < 0.05). No statistically significant difference existed between groups A, B, and C (p < 0.05). The results of the phase 1 and phase

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Table

IV.

Phase 2 mean cement margins

Group

A

B C D

Mean (rm)

Standard deviation

24.8 53.2 73.2 149.6

3.11 2.48 3.27 6.65

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WINDELER

Table VI. Summary table for one-way analysis of variance comparing averagecement areasremaining for each phase2 test sample Source

DF

ss

MS

Group

3 16

28500110.15 5941136.80

9500036.71 371321.05

Error

FValue

FProbability

25.5644

0.001

N = 5 in each group.

Table

V. Mean remaining cement area of phase 2 test

samples* Group

A B C D

Cement area (mm*)



Standard deviation

17,314 17,232

167.27 503.25

16,956

496.80

14,428

978.37

N = 5 in each group. *As measured from standard photographic enlargements.

2 studies should not be comparedbecausedifferent methodologieswere used.

DISCUSSION The results from phase 1, the diffusion study, demonstrated that there wasno statistically significant difference in the rate of cement solubility between the 25,50, and 75 pm test samplesasdetermined by examining and measuring the photographic enlargementsmadeof eachof the test samples.Theseresults support Fick’s first law of diffusion because,as the law predicts, the rate of cement migration due to diffusion was independent of the size of the gap. Moreover, the driving forces involved in the dissolution of the cement were the diffusion constant of the cement solute and the concentration gradient. It should be noted, however, that Fick’s law applies only to dissolution that results from diffusion; therefore, cement exposedto abrasive forces or convective motions would not be subject to this law. The 150pm test samplesdemonstrateda slight, although statistically significant, increasein cementdissolution (Fig. 7). An examination of the photographs of the 150 Km test samplesrevealed that areasof the luting cement were separating from the cement core along the bottom of the vertically suspendedtest samples.This suggeststhat cement dissolution may not be solely dependent on diffusion and that erosionmay alsobe occurring becausethe gapbetween the quartz disks is relatively large. The decreasedstructural integrity of the luting cement causedby diffusion and the influence of gravitational forces may have contributed to the separation of the eroded cement from the cement core. 440

Phase2, the dynamic study, investigated the influence of a dynamic environment on the solubility of zinc phosphate cement. The 25, 50, and 75 pm groups demonstrated no statistically significant differences in cement dissolution. There was,however, evidencethat the mean dissolutionof the cement increasedslightly as the size of the marginal gapsincreasedfrom 25 to 75 pm. The 150 pm group demonstrated an increasedamount of cement dissolutionwhen comparedwith the smallermarginal gaps(Fig. 8). Because it is possiblethat the vertical movement of the test samples distributed the solutefrom the exposedcementand thereby causedthe cement to releasemore solute, it is possiblethat the increasedrate of cement dissolution demonstrated by these results wascausedby both diffusion and the forced convection of the solute. Thus, forced convection may have contributed to the increasedrate of cement dissolution. Becausethe 150pm gap waslarge enoughto permit small amountsof convection to occur within the gap, the effects of convection combined with diffusion may have significantly increasedthe dissolution rate. Each of the test samples in phase 1 evidenced two distinct layers of cement. The outer layer of the cement averaged0.5 mm to 1 mm in width and consistedof cement that was partially dissolved by the test solution. This “halo” layer encircled the remaining cement core (Fig. 6). To ascertain whether there was a difference in the chemical composition of the halo and the cement core, radiographic diffraction studies were performed on both of theselayers. The chemical compositionof the cement core and a standard mix of zinc phosphate cement were also compared by meansof radiographic diffraction. The matrix of zinc phosphatecementhasbeendescribed as an amorphous zinc aluminophosphate gel with unreacted zinc oxide powder particles averaging 2 to 8 pm in diameter.25The radiographic diffraction study revealed that zinc oxide wasabsentfrom the halo. This suggeststhat the unreacted zinc oxide particles may be amongthe first components of the set zinc phosphate cement to be displaced by dissolution.

CLINICAL

IMPLICATIONS

Cement lines are inevitable in fixed prosthodontics becausesomedegreeof marginal discrepancy will exist. This is significant becauseall of the commonly used luting cements dissolve in the oral environment. Ideally, the marginal discrepancy shouldrelate to the film thickness of the

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150001

25 Microns

50 Microns

75 Micmns

150 Microns

25 Mimns

50 Mkmns

75 Miaons

150 mfons

Fig. 7. Phase 1 diffusion study. Bar graph of mean areas of cement remai:ning after dissolution.

Fig. 8. Phase 2 dynamic study. Bar graph of mean areas of cement remaining after dissolution.

cement. Because type 1 zinc phosphate cement has a film thickness of 25 blrn, a marginal gap of 25 pm would probably be considered acceptable. It has been experimentally demonstrated that it is difficult to pre,cisely evaluate invisible marginal discrepancies.26-28 Christensen26 evaluated the ability of 10 experienced dentists to detect clinically acceptable margins. He reported that the mean acceptable gingival marginal discrepancy was 74 pm. Moreover, Dedmon27-28 demonstrated that when six experienced dentists evaluated the acceptablity of invisible marginal gaps, the mean acceptable marginal discrepancy was 104 km. The results of these studies suggest that marginal discrepancies significantly larger than the film thickness of the cement may be considered acceptable when visibility is limited. The phase 1 diffusion study demonstrated no statistically significant differences in the rate of cement dissolution for type I zinc phosphate cement when the width of the marginal gap was 25,50, and 75 pm. Similarly, the phase 2 dynamic study demonstrated no significant differences in cement dissolution when the marginal gap was 25,50, and 75 pm in width. It is possible that subgingival conditions are similar to thfe static conditions described in phase 1 of this study. If simple diffusion is the principal mechanism by which cement is lost through subgingival marginal openings, this study might explain why marginal openings greater than the film thickness of cement do not result in catastrophic failures more often. It is unlikely, however, that simple diffusion is the sole mechanism of cement loss in any part of th,e oral cavity. To the degree that it is not, marginal openings would be expected to affect the rate of cement dissolution. This study should in no way be interpreted as a suggestion by the authors that marginal openings are not important to the success of fixed restorations. Thus, it is recommended that every effort be made to optimize the marg:inal fit of the casting.

when the marginal opening were 25,50, and 75 pm in size. The 150 pm marginal gap, however, demonstrated a small but statistically significant increase in cement dissolution. The lack of statistically significant differences in cement solubility between the 25,50, and 75 pm test samples corresponds with the results anticipated by Fick’s first law of diffusion. Fick’s law maintains that the rate of dissolution due to diffusion is independent of the size of exposure. The variance found in the 150 pm test samples may have resulted from a combination of factors, including diffusion and erosion. The erosion was probably due to the larger size of the marginal gap and the impact of gravitational forces. The results of the phase 2 dynamic study suggest that the vertical movement of the test samples through the test solution created convection forces that contributed to the increased rate of cement dissolution. The 25, 50, and 75 pm test samples demonstrated no statistically significant difference in their rates of dissolution. The 150 pm test samples, however, demonstrated an increased rate of dissolution that was statistically significant. These results suggest that the 150 pm gap size was large enough to accommodate convection forces. The convective flow of solute and solvent within the gap may represent a mechanical effect that enhanced the erosion of the cement. The radiographic diffraction study of the “halo” layer of the luting cement revealed a lack of zinc oxide. This finding suggests that unreacted zinc oxide particles may be among the first components displaced from cement. Radiographic diffraction of the cement core, however, demonstrated a retention of the zinc oxide particles and the other chemical constituents present in a standard mix of zinc phosphate cement.

SUMMARY The phase 1 diffusion study demonstrated that the rate of luting cement solubility was not statistically different

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REFERENCES 1. Phillips RW. Science of dental materials. 9th ed. Philadelphia: WB Saunders Company, 1982:465. 2. Johnston JF, Phillips RW, Dykema RW. Modern practice in crown and bridge prosthodontics. 3rd ed. Philadelphia: WB Saunders Co, 1971:363. 3. Cooper TM, Christensen GC, Lame11 HR, Baxter R. Effect of venting on cast gold full crowns. J PROSTHET DENT 1971;26:621-5.

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4. Adams NK. Physical chemistry. 3rd ed. London: Oxford University Press, 19566323. 5. Norman RD, Swarm ML, Phillips RW. Studies on the solubility of certain dental materials. J Dent Res 1957;36:985-97. 6. Norman RD, Swartz ML, Phillips RW. Studies on film thickness, solubility, and marginal leakage of dental cements. J Dent Res 1963;42: 950-8. 7. Eichner K, Lautenschlager FP, Von Radnoth M. Investigation concerning the solubility of dental cements. J Dent Res 1968;47:280-5. 8. Wilson AD, Kent BE, Lewis BG. Zinc phosphate cements: chemical study of in vitro durability. J Dent Res 1970;49:1049-54. 9. Swartz ML, Sear C, Phillips RW. Solubility of cement as related to time exposure in water. J PROSTHET DENT 1971;26:501-5. 10. Wilson AD, Abel G, Lewis BG. The solubility and disintegration test for zinc phosphate dental cements. Br Dent J 1974;137:313-7. 11. Wilson AD. Specification test for the solubility and disintegration of dental cements: a critical evaluation meaning. J Dent Res 1976;55: 721-9. 12. Mesu FP. Degradation of luting cements measured in vitro. J Dent Res 1982;61:665-72. 13. Beech DR, Bandyopahyay S. A new laboratory method for evaluating the relative solubility and erosion of dental cements. J Oral Rehabil 1983;10:57-63. 14. Walls AWG, McCabe JF, Murray JJ. An erosion test for dental cements. J Dent Res 1985;64:1100-4. 15. Norman RD, Swartz ML, Phillips RW. A comparison of the intraoral disintegration of three dental cements. J Am Dent Assoc 1969;78:77782. 16. Richter WA, Ueno H, Phillips RW. Clinical evaluation of dental cement durability. J PROSTHKF DENT 1975;33:294-9. 17. Mitchem JC, Grunas DG. Clinical evaluation of cement solubility. J PROSTHET

DENT

18. Osborne JW, Swarm ML, Goodacre CJ, Phillips sessing the clinical solubility and disintegration

AND

WINDELER

RW. A method of asof luting cements. J

PROSTHET DENT 1978;40:413-7. 19. Mesu FP, Reedijk T. Degradation

20. 21.

22.

23. 24. 25. 26.

of luting cements measured in vitro and in viva. J Dent Res 1983;62:1236-40. Pluim LJ, Arends JH, Jongebloed WL, Stokroos I. Quantitative cement solubility experiments in viva. J Oral Rehabil 1984,11:1’71-9. Wolff MS, Spostol G, Rawhist, Osborne JW. In viva solubility of zinc phosphate cement: the effect of film thickness [Abstract]. J Dent Res 1985;64:316. Ibbetson RJ, Setcbeill DJ, Amy DJ. An alternative method for clinical evaluation of the disintegration of dental cements [Abstract]. J Dent Res 1985;64:672. Pluim LJ, Arends JH. The relation between salivary properties and in viva solubility of dental cements. Dent Mater 1987;3:13-8. Smith DC. Biocompatibility of dental materials. Vol II. 1st ed. Boca Raton: CRC Press, 1982:181. Servais GE, Carts L. Structure of zinc phosphate dental cement. J Dent Res 1971;50:613-20. Christensen GC. Marginal fit of gold inlay casting. J PROSTHET DENT 1966;16:298-303.

27. Dedmon HW. Disparity in expert opinions on size of acceptable margin openings. Oper Dent 1982;7:97-101. 28. Dedmon HW. Ability to evaluate nonvisible margins with explorer. Oper Dent 1985;10:6-11. Reprint

requests

to:

DR. MICHAEL S. JACOBS 3416 MT. VERNON WAY PLANO, TX 75023

1978;40:453-6.

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An investigation of dental luting cement solubility as a function of the marginal gap.

The purpose of this study was to investigate the rate of type I zinc phosphate cement solubility as it relates to the degree of marginal opening. Stan...
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