Effect of tubule orientation in the cavity wall on the seal of dental filling materials: an in vitro...

Effect of tubule orientation in the cavity wall on the seal of

dental filling materials: an in vitro study

M . - K . W U a , A . J . D E G E E b & P . R . W E S S E L I N K a

a Department of Cariology, Endodontology, Pedodontology and b Dental Materials Science, Academic Centre for DentistryAmsterdam (ACTA), Amsterdam, the Netherlands

Summary

Dentinal tubules are oriented perpendicularly to the

root canal walls but parallel to the lateral walls of

class I occlusal preparation. It was hypothesized that

the contact surface area of the material may depend

on the tubule orientation in the cavity wall to which

the material is applied, and that the difference in

contact surface may affect the seal provided by the

filling material. Standard central lumens, 2.6 mm in

diameter and 3 mm high, were machined in human

crown or root specimens. After removal of the smear

layer with a conditioner, the specimens in each

experimental group, consisting of 20 crown and 20

root specimens, were filled with amalgam, Fuji II glass

ionomer (with or without varnish), or gutta-percha

with Ketac-Endo root canal sealer. A modified fluid

transport model was used to test the leakage along

the fillings. Selected specimens were then split

longitudinally and observed in a scanning electron

microscope. The micrographs showed that all the test

materials were pressed into the dentinal tubules. The

contact surface of the material was calculated to be at

least 45% larger in root specimens than in crown

specimens, depending on the depth of the tubular

penetration of the test material. The leakage results

showed that all the test materials leaked less in root

specimens than in crown specimens (P � 0:0000 for

amalgam, P � 0:0374 for Fuji II with varnish,

P � 0:0088 for Fuji II without varnish, P � 0:002 for

gutta-percha with sealer). It was concluded that the

tubule orientation in the cavity wall may influence the

seal provided by certain dental filling materials.

Keywords: dentinal tubules, filling materials,

leakage.

Introduction

Microorganisms are considered essential for the

development of caries and inflammation in the pulpal and

periapical tissues (Orland et al. 1954, Orland et al. 1955,

Kakehashi et al. 1965, BraÈnnstroÈm et al. 1971, Goldberg

et al. 1981, Cox et al. 1987). It is generally agreed that

the tight seal provided by all types of dental restorations,

including definitive and temporary dental restorations,

root canal fillings and root-end fillings, is of great clinical

importance (Seltzer & Bender 1965, Harty et al. 1970,

BystroÈm & Sundqvist 1981). Basically, the seal of a filling

depends on the closeness of the contact between filling

material and wall surface. The surface of the lateral walls

of a class I occlusal cavity, to which the dentinal tubules

run parallel, has a substantially lower number of dentinal

tubule openings than the surface of the root canal walls,

to which the tubules are oriented perpendicularly

(Cagidiaco & Ferrari 1995). The less porous wall surface

of a class I preparation may reduce the closeness of the

contact between the filling material and the wall surface,

resulting in more leakage than the more porous wall

surface in a root canal or a root end preparation.

In the present study, different types of dental filling

materials were used to obturate human tooth crown and

root specimens. The purpose was to compare the seal of

these materials when brought into contact with cavity

walls where the tubules are oriented in different

directions, i.e. tubules parallel to the cavity wall (crown)

versus tubules perpendicular to the cavity wall (root). In

addition, the contact between the cavity walls and the

material was examined by scanning electron microscopy.

Materials and methods

Preparation and filling of the crown and rootspecimens

A total of 100 crowns were removed from human

mandibular molars just above the enamel-cementum

326 q 1998 Blackwell Science Ltd

International Endodontic Journal (1998) 31, 326±332

Correspondence: Dr M.-K.Wu, Department of Cariology, Endodontology,

Pedodontology, ACTA, Louwesweg 1, NL-1066 EA Amsterdam, the

Netherlands (fax: �31 20 6692881; e-mail: [email protected]).

junction, by means of a low-speed saw (Isomet 11±1180;

Buehler Ltd, Evanston, IL, USA). Using a hollow diamond

bur (internal diameter 8 mm), a crown cylinder was

prepared from each crown (Fig. 1). A central lumen,

2.6 mm in diameter, was machined in each crown

specimen. The lowest point of the enamel edge of the

lumen was marked to indicate the level to which the

lumen was to be filled. The bottom (root side) of the

specimen was ground off until the distance between the

bottom and that mark was 3 mm.

A total of 100 root sections, each 3 mm high, were cut

from human central incisors at the enamel-cementum

junctions (Fig. 1). The central pulp lumen of each root

specimen was machined to a diameter of 2.6 mm. The outer

surface of the root specimens was modified using a stone bur

in a low-speed handpiece. The aim was to obtain a smooth,

cylindrical surface, which provides a better fit with the

plastic tube of the fluid transport device (Fig. 2).

The 100 crown and 100 root specimens were divided into

four experimental groups consisting of 20 crown and 20

root specimens, and two control groups consisting of 10

crown and 10 root specimens. The crown and root

specimens of each of the experimental groups were filled

with one of the following materials: (i) Tytin amalgam

(Batch: 41048; Kerr, Romulus, MI, USA), (ii) Fuji II glass

ionomer cement (Batch: powder 940111B, liquid

940110 A; GC Corporation, Tokyo, Japan) with Fuji varnish

(Fuji II, varnished group), (iii) Fuji II without varnish (Fuji

II, nonvarnished group) or (iv) gutta-percha (GP) cylinder

and Ketac-Endo sealer (Batch: 009 16053; Espe GmbH,

Seefeld, Germany). The varnish was applied on the Fuji II

surface as recommended by the manufacturer for the first

group of Fuji II, to prevent the material from drying out. The

second (extra) group with Fuji II without varnish was

included in this study as a control on the first group of Fuji II

since varnish could itself contribute to the sealing.

A GP cylinder was obtained by first cutting off the

needle and the tapered part of the GP- filled plastic

cannula (Ultrafil; Hygenic, Akron, OH, USA) using a low-

speed saw and then pressing the cold GP cylinder out of

the cannula, using the Ultrafil injection syringe. The

standard GP cylinder of the GP-filled cannula, 2.5 mm in

diameter, was cut into pieces 3 mm long.

Fig. 1 Schematic representation of the preparation of a crown

specimen from a human mandibular molar and a root specimen from

a human upper central incisor.

Fig. 2 Fluid transport device for leakage determination.

Effect of wall surface on seal 327

q 1998 Blackwell Science Ltd, International Endodontic Journal, 31, 326±332

The test materials were mixed according to the recom-

mendations of the manufacturers. The crown or root

specimen was placed on a glass plate and filled with

amalgam or Fuji II. The amalgam was condensed in three

equal increments, by means of a 1.5 mm-diameter

condenser. Using a scale, the condensation force was

verified at approximately 6.8 N (Basker & Wilson 1971).

Fuji II was condensed using an Ash 6. To prevent the

glass ionomer from drying out, the material was coated

with a thin layer of Fuji varnish for the specimens in the

varnished group; the specimens in the nonvarnished

group were placed in a condition of 100% humidity

immediately after filling. In obturating the specimens with

the root canal filling materials, the outer surface of the GP

cylinder and the walls of the central lumen were first

coated with Ketac-Endo sealer. The GP cylinder was then

gently pressed into the central lumen. Since the diameter

of the lumen was 2.6 mm and that of the GP cylinder

2.5 mm, the thickness of the sealer layer was approxi-

mately 0.05 mm.

The 10 crown and 10 root specimens which served as

the positive controls were obturated with Tubli-Seal root

canal sealer (Batch: base 41±004, accelerator 41005;

Kerr Manufacturing Co., Romulus, MI, USA), which in a

previous study had resulted in severe leakage (Wu et al.

1994). The 10 crown and 10 root specimens which

served as the negative controls were obturated with 2.5-

mm-diameter GP cylinders and Ketac-Endo sealer (Batch:

009 16053; Espe GmbH).

To harden the filling materials, all the obturated

specimens were kept at 378C and 100% humidity for

48 h.

Before filling, GC dentin conditioner (GC Corporation,

Tokyo, Japan) was used to remove the smear layer from

the crown and root specimens which were to be filled

with Fuji II. For the other crown or root specimens, 40%

citric acid was used to remove the smear layer.

Surface seal of the specimens

Before filling, the bottom surface of all the crown

specimens, with the exception of the 10 in the negative

control group, was coated with two layers of nail

varnish to prevent leakage along the dentinal tubules.

After filling and setting of the materials, the lateral

surface of all the experimental and positive control

specimens, the lateral and bottom surfaces of the 10

crown specimens in the negative control group, and the

entire surface of the 10 root specimens in the negative

control group were coated with two layers of nail

varnish.

Leakage determination

The leakage test was performed using a modified fluid

transport model described by Wu et al. (1993). The

crown or root specimen was connected to a plastic tube

as shown in Fig. 2. This connection was closed tightly,

using pieces of stainless steel wire. The plastic tube on

either side of the specimen was filled with deionized

water. A standard glass capillary was connected to the

plastic tube at the outlet side of the specimen. Using the

syringe, water was sucked back approximately 3 mm,

into the open end of the glass capillary. In this way, an

air bubble was created in the capillary. The whole set-

up was then placed in a water bath (208C), and, using

the syringe, the air bubble was adjusted to a suitable

position within the capillary. Finally, a head-space

pressure of 20 kPa (0.2 atm) from the coronal side was

applied to force the water through the voids along the

filling, thus displacing the air bubble in the capillary

tube. The volume of the fluid transport was measured

by observing the movement of the air bubble; the displa-

cement of the air bubble was recorded as the fluid

transport result (F), which was expressed in mL per day.

The fluid transport results were statistically analysed

using Mann±Whitney or Kruskal±Wallis tests.

Observation in SEM

After the leakage was determined, two root specimens

were randomly selected from each experimental group

for further observation in the scanning electron

microscope. For this part of the investigation one

additional crown specimen and one additional root

specimen were prepared; these were etched for smear

layer removal, but were not obturated. Two grooves

were made in a diametral direction along the long

axis of the specimens, using a fissure bur in a turbine

handpiece. The specimens were then split longitudin-

ally into two halves with a chisel. Using an

elastomeric impression material (Extrude; Kerr)

impressions were made of each half and cast in

epoxyresin (Araldite; Ciba-Geigy, Maastricht, the

Netherlands). The casts were gold sputtered and

examined in a scanning electron microscope (XL20;

Philips, Eindhoven, the Netherlands).

In order to estimate the contact surface of the

different cavity walls after smear layer removal, SEM

micrographs (magnification: �2000) of the respective

surfaces were used in the calculations. This magni-

fication resulted in a total surface of

60� 47 � 2820 mm2.


328 M.-K. Wu et al.

Results

In the 20 negative controls no fluid transport was

recorded, whilst all the 20 positive controls displayed

severe leakage (F > 20 mL per day). The results of the ex-

perimental groups are shown in Table 1. All the test

materials showed more leakage in the crown than in the

root specimens (P � 0:0000 for amalgam, P � 0:0374 for

Fuji II varnish group, P � 0:0088 for Fuji II nonvarnish

group, P � 0:002 for GP with Ketac-Endo sealer). In both

crown and root specimens, Fuji II leaked less than the

other materials (P � 0:0000). No significant difference

was found between the varnished and nonvarnished Fuji

II groups (P � 0:4294 for crown specimens, P � 1:0000

for root specimens). In crown specimens, no significant

difference was found between amalgam and GP with

Ketac-Endo sealer (P � 0:1907); in root specimens,

amalgam leaked less than GP with Ketac-Endo sealer

(P � 0:0458).

In the scanning electron microscope, there were

marked differences between the crown and root specimens

after smear layer removal. At the root wall surface, many

dentinal tubule openings were seen, whilst at the crown

wall surface the dentinal tubules were parallel to the wall.

Micrographs of root specimens which had been filled

showed that both amalgam and Ketac-Endo sealer cement

had penetrated into the dentinal tubules (Figs 3 and 4),

and that there were many semicylindrical cement tags

protruding from the Fuji II material surface (Fig. 5).

Figure 6 is a schematical representation of the SEM

micrographs (at �2000) of a lateral wall in a crown and

a root canal wall; this representation was used in the

calculation of the contact surfaces. The lateral wall in the

crown contained three semicylindrical tubules 47 mm

long, with a diameter of 4 mm. The root canal wall

contained 35 reverse-trunk cone-like tubule openings,

which had a diameter of 5mm at the orifice and 3 mm

Fig. 3 Tags of Ketac-Endo sealer appeared in the dentinal tubules

(�2000).

Table 1 Leakage of a number of dental filling materials, in crown versus in root

Number of specimens

Groups Fb� 0 0 < F � 10 10 < F � 20 F > 20 Total

Amalgam in crown 0 1 1 18 20

in root 11 4 3 2 20

Fuji II, in crown 16 4 0 0 20

varnished in root 20 0 0 0 20

Fuji II, in crown 14 5 1 0 20

nonvarnished in root 20 0 0 0 20

GP� sealera in crown 1 3 1 15 20

in root 4 8 3 5 20

a Gutta-percha cylinder with Ketac-Endo sealer applied in a thin layer.b F in mL per day.

Fig. 4 Tags of amalgam dislodged from their dentinal tubules

(�200) .



lower down. The total contact surface of the root canal

was calculated to a depth of 5mm.

The contact surface per mm2 of the lateral wall in the

crown was 0.1 mm2 (10%) larger than a 1 mm2 flat

surface, whereas the contact surface of 1 mm2 root canal

wall, including the surface of the dentin tubule walls to a

depth of 5mm, was 0.6 mm2 (60%) larger than a 1 mm2

flat surface and 0.5 mm2 (45%) larger than the surface of

a 1 mm2 lateral wall in the crown.

Discussion

A number of devices designed to measure microleakage of

dental fillings have been described in the literature. The

marginal leakage of amalgam fillings in ceramic discs was

studied by Mahler & Nelson (1984, 1994) using an air

pressure device. However, the drying effect of compressed air

passing along a restoration has no clinical relevance (Taylor

& Lynch 1992) and is known to have a detrimental effect on

glass ionomer cements (Hotta et al. 1992).

Pashley et al. (1983) developed a model system which

determined leakage in class I restorations by fluid

transport under air pressure (Derkson et al. 1986). In this

model the fluid transport is measured by the movement of

an air bubble in a fluid-filled capillary tube, which is

placed between a pressurized reservoir and the inlet side

of the test specimen. Although this set-up is workable for

relatively short-term measurements, a slight leakage from

one of the pressurized connections in the device during

long-term experiments increasingly interferes with the

leakage measurement. To deal with this problem Wu et al.

(1993) modified the device in such a way that the

pressure was confined to the inlet connection to the

specimen. The outlet connections with the glass capillary

containing the air bubble were not under pressure

(Fig. 2), which eliminated any influence on the position of

the air bubble. This modification made the device useful

for long-term measurements on specimens with low fluid

transport fluxes, as in the case of long root canal fillings

(Wu et al. 1993, Georgopoulou et al. 1995). In the

present study it was necessary to carry out long-term

experiments under conditions of low pressure (0.2 atm), as

there was a danger that excessive pressure would damage

the filling in the test specimen. The negative controls

showed that the air bubbles remained stable over a long

period of time, indicating that small displacements of the

air bubbles measured for the experimental specimens were

reliable.

The results in Table 1 show that the various test

materials displayed differing degrees of leakage, and that

they leaked more as crown fillings than as root fillings.

The higher leakage in the crown specimens was not due

to water passing through the dentinal tubules running

parallel to the cavity walls, because none of the 10 crown

specimens in the negative control group showed any

leakage. Figs 3 and 4 show that both Ketac-Endo and

amalgam were pressed into the tubules, forming relatively

long tags. In the case of Fuji II, which formed shorter tags

Fig. 5 Many short tags protruded from the surface of the Fuji II

material surface (�200) .

Fig. 6. A schematic representation of the different wall surfaces. Lateral wall in a crown cavity with half-cylinder-like tubules, 4 mm in diameter

and 47mm long. Root canal wall with reverse-trunk cone-like tubule openings, R � 5 mm, r � 3 mm, D � 5mm.


330 M.-K. Wu et al.

(Fig. 5), it was calculated that the contact surface was ap-

proximately 45% larger than that of the lateral walls in

the crown preparations. Amalgam and Ketac-Endo sealer

had even larger contact surfaces, since they penetrated

more deeply into the tubules. In the case of the adhesive

glass ionomer cements, a larger contact surface benefits

adhesion, whilst the use of amalgam, a nonadhesive

material, may provide a tighter mechanical interlock

(Fig. 4). The hypothesis that a larger contact surface

increases the seal is supported by Jodaikin & Austin

(1981) and Pashley & Depew (1986), who found that the

leakage along amalgam fillings was reduced when the

smear layer was removed from the cavity walls.

The differences in the degree of penetration into the

tubules recorded for the three materials are clearly related

to the viscosity of the mixed pastes. Fuji II has the highest

viscosity. The lower viscosity of Ketac-Endo made possible

a deeper penetration. However, as the film thickness of

this material is 22 mm (Wu et al. 1997) and the tubule

orifices are only 3 mm in diameter (Pashley et al. 1995),

most of the filler particles were probably left behind, and

only the polyalkenoic acid was actually able to enter the

tubules together with the smaller glass particles, inducing

the setting reaction observed (Wilson & McLean 1988).

Also in the amalgam mix, most particles are larger than

the dentinal tubules (Craig 1993), so that only the

mercury and the smallest alloy particles, which brought

about the setting, entered the tubules.

The high leakage of Tytin amalgam when it was used

in crowns (Table 1) is probably caused by the spherical

shape of the alloy particles; in previous studies spherical

alloys were found to have a high propensity for microleak-

age (Fayyad & Ball 1984, Ben-Amar et al. 1987, Kim

et al. 1992, Mahler & Nelson 1994). The sealing ability is

expected to improve over time, due to corrosion

(Liberman et al. 1989, Ben-Amar et al. 1995).

The two groups Fuji II varnished and nonvarnished

showed similar leakage data (Table 1). This indicates that

the specimens with varnish, which was applied on the

glass ionomer surface as recommended by the manufac-

turer to prevent it from drying out, were comparable to

the nonvarnished specimens, which were placed in a

condition of 100% humidity immediately after filling. Fuji

II provided the best seal of all the materials, in both

crown and root specimens. It may be that the seal is

determined solely by the adhesive properties of the glass

ionomer, and is not influenced by other mechanisms,

such as swelling through water sorption. As shown by

Feilzer et al. (1995), these materials do not swell as a

result of contact with water. This would also explain the

relatively high leakage values for the glass ionomer Ketac-

Endo, which adhered to dentine on one side, but could be

pulled away by setting shrinkage stresses from the gutta-

percha side, to which it had no adhesion. Because of the

lack of water swelling, the loss of interfacial integrity

resulted in high leakage values. However, the fact that

higher leakage was recorded for the crown specimens

than for the root specimens indicates that the adhesion to

dentine may be affected by the smaller contact surface in

the crown specimens.

Placing GP with Ketac-Endo in a tooth crown has no

clinical relevance. However, the purpose of this investi-

gation was to study the effect which the tubule

orientation in cavity walls has on the sealing of these

three materials.

Conclusion

Tubules with an orientation perpendicular to the cavity

walls, as in root canals, provide a better sealing for

certain filling materials than tubules parallel to the

cavity walls, as in the case of lateral walls in class I

occlusal cavities.

Acknowledgements

The authors wish to thank Ms B. Fasting for correcting

the manuscript for English and Mr A. Werner for his

technical assistance.

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332 M.-K. Wu et al.

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