Egyptian Dental Journal, 52, 81 : 87, January, 2006

THE EFFECT OF TOPICALLY APPLIED 1.23% ACIDULATED

PHOSPHATE FLUORIDE ON ZINC PHOSPHATE CEMENT

Sahar. A. Al-Zain *

ABSTRACT

Topically applied fluoride preparations play an important role in reduction of caries incidence. It

was found that the commonly used 1.23% acidulated phosphate fluoride (APF) gel has a deleterious

effect on different dental materials. The effect of the gel on zinc phosphate cement is still not

known. It is expected that 1.23% APF gel may affect the microstructure of the zinc phosphate cement.

Thirty Teflon rings were prepared. Zinc phosphate cement was mixed and loaded into the rings.

Excess cement material was removed and samples were stored in 20 ml of distilled water at 37°C for

24 hours. They were then equally divided into five groups. Samples of the control group were stored

in 20 ml of distilled water at 37°C for 24 hours. The other four groups received four times

application of 1.23% APF gel for 1, 4, 6 and 10 minutes at each application with an intermediate

washing and immersion in 20 ml of distilled water at 37°C for one hour between applications.

Samples were prepared for examination under the scanning electron microscope. Results showed

that as the application time of the gel is increased, more reaction products are formed on the surface of

the set zinc phosphate cement. These products mask and may protect the surface from erosion by the

acidic APF gel. However, more surface roughness is produced that may accumulate bacteria and affect

the gingival health.

INTRODUCTION :

Topical fluorides are capable of enhancing the

remineralization of partially decalcified tooth

structure. They also reduce dentinal sensitivity.

Three types of fluoride preparations have been ad-

vocated for professional applications: Acidulated

Phosphate Fluoride (APF), Stannous Fluoride

(SnF2) and Sodium Fluoride (NaF). The efficiency

of various topical fluoride preparations depends on

the local pH and concentration of fluoride. 1.23%

APF gels are used as topical fluoride treatments in

almost all dental offices because of their ease of

application and their clinical effectiveness. Different

application times of 1.23% APF gels (1, 4, 6 and

10 minutes) have been used. For adults at high risk

for caries development, office application of APF

gels at 6- month intervals or more frequently is

appropriate.1 However, one of the problems as-

sociated with the application of 1.23% APF gel is its

deleterious damaging effect on the different res-

torative materials like composite resins2'7, Glass

ionomer7'10 and porcelain

11"14. APF gels has the

ability to etch the inorganic substances incorporated

in those materials. Therefore, it is pos-

Assistant Professor and consultant, Department of Prosthetic Dental Sciences, College of Dentistry, King Saud University,

Riyadh, Saudi Arabia.

sible that the zinc phosphate cement film at mar-

gins of cast restorations is also adversely affected

by the acidic nature of the 1.23% APF gel.

The purpose of this study is to evaluate the ef-

fect of topical application of 1.23% APF gel on the

microstructure of the zinc phosphate cement at dif-

ferent application times using the Scanning Elec-

tron Microscope (SEM).

MATERIAL AND METHOD:

Thirty Teflon rings of 10mm internal diameter

and 2mm thickness were prepared. Each ring was

seated on a Mylar strip that was placed on a glass

slab. The powder of zinc phosphate cement (Fleck's

Cement, mizzy, INC. Cherry Hill, NJ) was weighed

on an electronic microbalance to insure proper

proportioning and consistency. Mixing was then

done at room temperature on a glass slab following

the manufacturer's directions and the cement was

loaded into the rings. They were again covered with

another Mylar strip and a glass slab to extrude excess

cement material and to create a uniform surface

texture. Excess cement material was then removed.

All samples were immersed in 20 ml of distilled

water at 37°C for 24 hours. The specimens were then

equally divided into 5 groups, each containing five

samples. Samples of the control group were stored

in 20 ml of distilled water for 24 hours. The other

samples were repeatedly exposed to 1.23%

acidulated phosphate fluoride (APF) gel (Butler,

Sunstar Inc, Chicago, IL. USA) for four different

application times (1, 4, 6 and 10 minutes). The

process of APF application was repeated for a total

of four times for each sample simulating two-years

exposure to prophylactic fluoride treatment (one

application/six months). Each application was

followed by washing with distilled water and sub-

sequent immersion in distilled water at 37°C for

one hour. After completion of A,PF application,

samples were then stored in 20 ml distilled water at

37°C for 24 hours. All samples were prepared, gold

coated and examined under a JEOL JSM-6360LV

(JEOL, Tokyo, Japan) scanning electron micro-

scope. The surface micromorphology of the sam-

ples were evaluated and compared.

RESULTS:

Figure 1 (a & b) shows the

micro-morphology of the samples of the control

group. The surface of the set zinc phosphate cement

(ZPC) matrix is more or less smooth with areas of

cracks and small voids. There is an evidence of

many un-reacted irregularly shaped particles on the

surface of the set ZPC. There is a wide variation

in the shape and size of the incompletely dissolved

particles of zinc oxide overlaying the set cement

matrix. These particles are characterized by their

smooth surfaces and sharp and well-defined edges.

There are Small voids and cracks on the surface of

the set cement matrix.

Samples that received four times 1-minute

application of APF gel have lost the fine and

smooth characteristics of the set ZPC matrix and

the surrounding unreacted particles (Fig 2-a & b).

The surface particles vary in size and shape. Most

of them have smoother and flatter surfaces than

those of the samples of the control group. At many

areas, the particles are joined together. The surface

of the cement is rough due to the formation of re-

action products on it of varying sizes and shapes.

They partially mask the underlying set cement sur-

face. Voids and cracks are still evident.

Figure 3 (a, b & c) shows the

micro-morphology of the samples that received four

times 4-minute application of APF gel. The surface

particles are flattened more than those of the

previous groups. At many areas, the unreacted

cement particles are found adhering to each other to

form bundles that are overlaying the surface of the

set cement. The bundles have a characteristic

feature at this group of being narrow at their middle

area and widely separated at both ends giving them

a fan-shaped appearance. They have pitted surfaces

with rough edges. There is an evidence of formation

of globules precipitating on the surface of the set ce-

ment that completely mask the surface of the set

cement.

82 Sahar. A. Al-Zain E.D.J. Vol. 52. No. 1

THE EFFECT OF TOPICALLY APPLIED 1.23% ACIDULATED PHOSPHATE

Figure 4 (a, b &c) shows the

micro-morphology of the samples that received

four times 6-minute application of APF gel. The

surface particles are flattened more than those in

the previous group. They are condensed together

and adhere to each other. The fan-shaped structures

are still evident, but are more packed together than

the previous group. However, the globules that are

formed on the surface of the set cement is similar

to those seen in samples of the

4-minute application of APF gel.

Samples that received four times 10-minute

application of APF gel shows further flattening and

smoothening of the surface particles by the acidic

APF gel (Fig 5- a, b & c). The fan-shaped struc-

tures are not evident. The appearance of globules

that completely mask the surface of the set cement is

similar those of globules that are formed in the 4-and

6-minute APF application groups.

Fig l-a&b: Micromorphology of a sample of a control

group. Unreacted irregularly shaped cement par-

ticles of different sizes Cracks are evident. (Mag-

nification a: xlOO, b: x200)

Fig 2-a&b: Micromorphology of a sample that received four

times 1-minute 1.23% APF gel. Surface particles

have smooth surfaces. The reaction products par-

tially cover the set cement surface (Magnification a:

xlOO, b:x500)

83 E.D.J. Vol. 52. No. 1

(A) (A)

(B) (B)

(C)

Fig 3-a,b &c: Micromorphology of a sample that received four

times 4-minute 1.23% APF gel. Most of the unreacted

cement particles form fan- shaped bundles that have

pitted surfaces and rough edges. Globular reaction

products completely mask the set cement.

(Magnification a: xlOO, b: x500, c: x500)

(C)

Fig 4-a,b &c: Micromorphology of a sample that received

four times 6-minute 1.23% APF gel. More un-

reacted particles arc condensed and closely adhere

to each other. More flattening of the surfaces par-

ticles is evident. Globular reaction products are similar

to those seen in fig 3. (Magnification a: xlOO, b:

x500, c: x500)

E.D.J. Vol. 52. No. 1 Sahar. A. Al-Zain 84

(A)

(B) (B)

THE EFFECT OF TOPICALLY APPLIED 1.23% ACIDULATED PHOSPHATE

Fig 5-a&b: Micromorphology of a sample that received four times 10-minute 1.23% APF gel. Surface particles are further flattened and

are smooth. Globular reaction products are similar to those seen in fig 3 and 4. (Magnification a: xlOO, b: x500)

DISCUSSION:

When the zinc phosphate powder is brought

into contact with the liquid to begin the cement

mixing, wetting occurs and a chemical reaction is

initiated. The surface of the alkaline powder is dis-

solved by the acidic liquid resulting in the forma-

tion of a relatively insoluble zinc phosphate as fol-

lows:

3 ZnO + 2 H3Po4 + H2O —> Zn3(Po4)2 . 4H2O

Only the surface layers of the zinc oxide par-

ticles react, leaving unconsumed cores of residual

zinc oxide particles bound together by the phos-

phate matrix.15"18 The set cement is essentially a

hydrated amorphous network.15'16-18 It is very porous.15

In this study, there are unreacted irregularly shaped

particles with smooth surfaces and well-defined

edges on the surface of the zinc phosphate cement of

the control group. Because of the nature of the

cement and also due to preparation for Scanning

Electron Microscope (SEM) examination where the

residual moisture is removed, shrinkage of the

cement occurs which eventually leads to formation of

cracks and small voids. Trapped air bubbles during

cement mixing may also contribute to the formation

of small voids in the set cement.

Some studies reported the significant adverse

effect of the APF on other restorative materials.2-14

The application time of the APF gel have been rec-

ommended by manufacturers to be 1 min. How-

ever, some of the studies used a 4-minute application

time.3'5-7'8'19'20 Other studies used a topical

application of APF gel for 6 minutes.2-9'21 A

10-minute application time was also used by some

researchers.11'14 El-Badrawy & McComb19 and Yip et

al22 found that the application of 1.23% APF gel to

composite and glass ionomer cements for 4 minutes

resulted in increased surface roughness and

erosion of composite resin and glass ionomer ce-

ments. The low pH value affects the cement by

etching and eroding its surface. The APF has a ten-

dency to affect the inorganic substances incorporated

in the restorative materials. Therefore, it may affect

the zinc phosphate cement by etching both the

unreacted cement particles and matrix. However,

the globular reaction products that are formed on

the surface of the cement may inhibit etching and

erosion of ZPC. They may act as a barrier that

prevents damage of the cement by the acidic effect

of 1.23% APF gel. In this study, the 1-minute APF

application resulted in formation of reaction products

that are partially covering the set cement surface.

However, when APF application time is increased

more than 1 minute the cement particles

completely mask the underlying set cement surface.

The appearance of the globular prod-

E.D.J. Vol. 52. No. 1 85

(B) (A)

ucts is similar in the groups where APF was ap-

plied for 4, 6 and 10 minutes. Therefore, APF ap-

plication for 4, 6 and 10 minutes have the ad-

vantage of protecting the surface of the ZPC from

erosion by the acidic effect of the APF. On the other

hand, they may contribute to increased surface

roughness of the cement due to the formation of re-

action products on the surface of the ZPC. It was

found that colonization of bacteria may occur as a

result of surface roughness of the cement that is

produced by the acidic effect of the APF gel. 23-24

In

this study, the roughness that are produced by the

formation of the globules may harbor bacteria and

bacterial products which may initiate gingival

inflammation due to proximity of the margin of the

crown restoration to the gingival tissues.

In this study, as the APF application time is

increased, the surfaces of the unreacted particles on

the surface of the ZPC become flatter and smoother.

Some particles are joined together by the acidic

effect of the gel. The characteristic feature of sam-

ples at 4-minute APF gel application is the close

adherence of particles forming fan-shaped bundles

due to the increased exposure time to the acidic gel.

When the application time of APF gel is increased

to 6 minutes, the fan-shaped appearance is still ev-

ident, but the particles are more condensed together.

They closely adhere to each other and are packed

more than those of the samples of 4-minute APF gel

application. The samples in this study were placed

in distilled water for 24 hours after each APF

application. Consequently, any loose cement particles

were probably dislodged.

CONCLUSIONS:

Professionally applied 1.23% APF gel was found

to resulted in formation of reaction products on the

surface of the ZPC. At the 1-minute APF application,

the reaction products partially mask the surface of the

set cement. Whereas at the other groups receiving

APF for 4, 6 and 10 minutes, the globular reaction

products completely mask the underlying set cement

surface. Thus, they have the

advantage of protecting the cement surface from

further damage by the acidic APF. Therefore, they

inhibit cement erosion by the acidic APF gel. In ad-

dition, the globular structures may act as a reservoir

of fluoride that is beneficial in the oral environment.

However, they may result in a rough surface of the

cement which may accumulate bacteria.

Consequently, it may start the inflammatory process

of the gingival tissues due to the closed positioning of

the margin of the crown restoration to the gingiva.

Further studies on the effect of topical ap-

plication of the other fluoride preparations on the

zinc phosphate cement is needed.

ACKNOWLEDGEMENT:

This study was funded by the Research Center

at College of Dentistry, King Saud University, Ri-

yadh, Saudi Arabia. The author acknowledge the

support of the Research Center.

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