Telescopic crowns have several advantages, such as excellent fit and functionality, ease of cleaning, and favorable survival rates. They have been widely applied clinically.

INTRODUCTION

Telescopic crowns have several advantages, such as excellent fit and functionality, ease of cleaning, and favorable survival rates. They have been widely applied clinically.

because gold alloys are generally used in telescopic crowns, their clinical application is limited because of its contraindication for patients with metal allergies. Moreover, the cost of this treatment is easily affected by precious metal prices. However, zirconia is now gaining at­

tention as an alternative to other metals because it can be used in patients with metal allergies and is available at stable prices.

Furthermore, because zirconia crowns are prepared by machines and a CAD/CAM system, there is no need for the compli­

cated technical operations of lost­wax casting. Con­

ventional telescopic crowns made from metal re­

quire precise fitting checks and retentive force ad­

justments by an experienced dental technician.

However, a previous study indicated that with zirco­

nia telescopic crowns, a stable telescopic crown re­

tentive force can be easily achieved using the CAD/

CAM system.

Telescopic crown preparation with a

CAD/CAM system appears to improve work effi­

ciency and reduce technical errors. Generally, zir­

conia used in dentistry is yttria­stabilized tetragonal zirconia polycrystal (Y­TZP). This type of zirconia has a major disadvantage of deterioration at low temperatures.

Zirconia articular head fractures have been reported as a result of such deteriora­

tion at low temperatures in artificial joints.

In the in­

dustrial world, although ceria­stabilized tetragonal zirconia polycrystal (Ce­TZP) has the advantage of excellent heat resistance, a very high toughness value, and stability at low temperatures, as is the case with Y­TZP, Ce­TZP has not been widely used because of its inferior strength and hardness.

In the field of nanomaterial engineering, ceramics are widely designed with microstructure control to improve their properties. One such attempt is the creation of nanocomposites.

Creating nanocom­

posites involves controlling the structure and or­

ganization of materials by adding nano­sized sec­

ond phase particles into crystal grains and at the grain boundary, which then form the parent phase of the polycrystalline substance.

Nanocomposites can be used to improve various features of materi­

Influence of the surface roughness of zirconia on the coefficient of static friction and retentive force of telescopic crowns

Suguru Fujiki

, Katsunori Torii

, Masaki Sato

, Junko Tanaka

and Masahiro Tanaka

1

Graduate School of Dentistry (Fixed Prosthodontics and Occlusion),

Department of Fixed Prosthodontics and Occlusion, Osaka Dental University, 8-1 Kuzuhahanazono-cho, Hirakata-shi, Osaka 573-1121, Japan

We clarified the coefficient of static friction for two types of zirconia with different sur­

face roughness that are used for telescopic crowns. We examined the effect on retention that alumina blasting had on the primary and secondary crowns. The two surface catego­

ries were smooth and rough. We used two types of zirconia, Y­TZP and Ce­TZP/A. The coefficient of static friction for Y­TZP and Ce­TZP/A were 0.18 and 0.17 when both sur­

faces were smooth, 0.18 and 0.17 when one surface was smooth and the other was rough, and 0.31 and 0.38 when both surfaces were rough, respectively. The retentive force of telescopic Ce­TZP/A crowns on the primary and secondary crowns was in­

creased by alumina blasting. (J Osaka Dent Univ 2020 ; 54 : 51­60)

Key words : Telescopic crown ; Zirconia ; Coefficient of static friction ; Surface rough­

ness ; Retentive force

als and successfully add new features.

Ce­TZP/

Al

O

nanocomposites (Ce­TZP/A) were developed in this manner as a multi­functional harmonized ce­

ramic material possessing multiple excellent char­

acteristics. The superb heat stability and high levels of toughness of Ce­TZP are maintained while im­

proving the disadvantages of Ce­TZP such as low strength and hardness. Ce­TZP is considered a successfully developed material offering enhanced features compared with the widely used Y­TZP.

Ce­TZP/A does not deteriorate, like Y­TZP, in ar­

eas exposed to low temperatures and moisture such as the oral cavity, and offers high strength and toughness. It can be an optimal material for restorations that are commonly subjected to bend­

ing stress such as bridges and telescopic crowns.

Taper angle, surface force, and the coefficient of static friction of the material are involved in deter­

mining how telescopic crowns display retentive force.

We previously used Ce­TZP/A to determine the conditions needed to achieve optimal initial re­

tentive force in telescopic crowns in clinical set­

tings.

In addition, we examined the effect on reten­

tive force of the number of times such crowns were fitted and removed.

However, there are few re­

ports investigating the coefficient of static friction of zirconia. In this study, our goals were to clarify the coefficient of static friction of two types of zirconia (Y­TZP and Ce­TZP/A) with different surface rough­

ness, compare the results with gold alloy, and clar­

ify the effect of primary and secondary crown sur­

face roughnesses of Ce­TZP/A telescopic crowns on retentive force.

MATERIALS AND METHODS

Comparison of the coefficient of static friction of zirconia and gold alloy

The materials used in our experiments are shown in Table 1. Figure 1 shows the fixed and moving

test specimens. The size of the moving test speci­

men was 10×15×2.5 mm. The Y­TZP, Ce­TZP/A and the gold alloy weighed 2.31, 2.11 and 5.62 g, respectively. Moreover, the size of the fixed test specimen was 18×20×2.5 mm, and Y­TZP, Ce­

TZP/A and the gold alloy weighed 5.51, 5.04 and 13.64 g, respectively. The two levels of surface roughness set for the test specimens for each ma­

terial were 0.03 and 1.0 μm for the smooth and rough samples, respectively. The calculated aver­

age roughness was used for each surface rough­

ness type. One smooth and one rough fixed test specimen were fabricated for each material. Five specimens were fabricated for each smooth and rough moving test specimen. The fabrication of the test specimen was outsourced to Sarton Works (Kanagawa, Japan). The surface roughness on the completed test specimens was measured using an Surftest SV­2100 assessment­type surface rough­

ness measurement device (Mitutoyo, Kawasaki, Ja­

pan ; Table 2).

Figure 2 shows scanning electron microscope (SEM) images of the Y­TZP, Ce­TZP/A and gold al­

Table 1 Materials

Code Product name Manufacturer

Y­TZP Ce­TZP/A Gold alloy

KZR­CAD Zr T KZR­CAD NANOZR YP Gold Type 4

Yamakin, Osaka, Japan

〃

〃

Fig. 1 Fixed and moving test pieces.

Table 2 Surface roughness

Materials Smooth Ra Rough Ra

Y­TZP Ce­TZP/A Gold alloy

0.031 (0.002) 0.031 (0.001) 0.030 (0.002)

0.951 (0.027)

0.955 (0.025)

0.965 (0.028)

Mean (SD), n＝5.

loy test specimens. An S­4800 SEM (Hitachi, To­

kyo, Japan) was used for these observations. The samples were coated with platinum to give it elec­

tric conductivity because zirconia is nonconductive.

An E­1030 (Hitachi) was used for ion sputtering. A friction measurement jig was attached to an EZ­SX tensile testing machine (Shimadzu, Kyoto, Japan) to measure the coefficient of static friction. The fixed and moving test specimens were dried after undergoing ultrasonic cleaning in distilled water for 5 min. The moving test specimens were fixed with double­sided tape onto the bottom of a 200­g weight and then placed on the fixed test speci­

mens. The moving test specimens were pulled at a crosshead speed of 5 mm/min, and the maximum load when sliding was measured (Fig. 3). The measurements were made indoors at 23.0±1.4°C

and a humidity of 54.3%±10.0%.

The coefficient of static friction (μ) was calculated using the formula μ＝P/W, where P is the maxi­

mum load and W is the weight of each moving test specimen when the 200­g weight is applied. Each measurement was performed five times and the mean value was calculated to determine the repre­

sentative values.

The coefficient of static friction was randomly measured among the same materials, with the fol­

lowing combinations of surface roughness : smooth

­smooth, smooth­rough, and rough­rough (n＝5).

For statistical analysis, one­way analysis of vari­

ance was performed for each surface roughness combination using the materials as variables. The level of statistical significance was set at 1%.

Tukey’s multiple comparison test was performed when a statistically significant difference was ob­

served. IBM SPSS Statistics ver. 26 (IBM Japan, Tokyo, Japan) was used for statistical analyses.

Retentive force of Ce-TZP/A telescopic crowns with differing surface characteristics

KZR­CAD Nanozr Ce­TZP/A (Yamakin, Osaka, Ja­

pan) was used for the primary and secondary crowns of the telescopic crowns (Table 3). The pri­

mary crowns, into which the abutment had been in­

tegrated for use on a premolar, were truncated and

Fig. 2 Scanning electron microscopic images.

Fig. 3 Schematic diagram of friction coefficient measurement.

had a length, width, and height of 8, 6 and 6.5 mm, respectively. The angle was curved with the line angle joining the axial and occlusal surfaces at a radius of curvature of 0.65 mm. The border had a deep chamfer shape, with 1/2 taper angles set at 2° and 4° (Fig. 4). We designed the crowns using CATIA V 5 CAD software (Dassault Systems, Vélizy

­Villacoublay, France). Based on the crown design, we then used CAM 250 i CAM equipment (Pana­

sonic Dental, Osaka, Japan) to perform machining on the KZR­CAD Nanozr Ce­TZP/A (Yamakin) and sintered the crowns in an inFire HTC speed furnace (Sirona Dental Systems, New York, USA). After ra­

diational cooling, we used a Ceramaster Coarse silicon point (Shofu, Kyoto, Japan) to perform me­

chanical polishing. A Polirapid polisher (Polirapid Dr. Montemerlo, Singen, Germany) and a Zircon­

Brite polishing agent (Dental Ventures of America, Coronz, CA, USA) to perform mirror polishing to complete the crowns. A completed primary crown is shown in Fig. 5. The completed primary crowns were scanned with a D700 dental scanner (3 shape, Copenhagen, Denmark), and Dental System 2015 CAD software (3 shape) was used to design the secondary crowns. The primary­secondary crown space was set at 10 μm and the secondary crown thickness at 0.4 mm. We added knobs at two sites to enable fitting and removal of the secon­

dary crowns. As with the primary crowns, we used Ce­TZP/A to fabricate the secondary crowns, which were not polished. Figure 6 shows a completed secondary crown.

We fabricated five pairs each of the primary and secondary crowns with 1/2 taper angles of 2° and 4°. The combination of the characteristics of the pri­

mary and secondary crown surfaces were as fol­

lows : polished­non­polished (polished primary crown surface and non­polished secondary crown internal surface), polished­blasted (polished primary crown and sand blasted secondary crown internal surface), and blasted­blasted (blast processing of the primary crown and secondary crown internal surface). Blast processing of the specimens was performed with Alumina, which is an aluminum ox­

ide (Morita, Kyoto, Japan) with a mean particle di­

ameter of 50 μm using a Jet Blast II sandblaster

Table 3 Materials used

Code Product name Manufacturer

Ce­TZP/A KZR­CAD NANOZR Yamakin, Osaka, Japan

Fig. 4 Dimensions of primary crowns (mm).

Fig. 5 Completed primary crowns.

Fig. 6 Completed secondary crowns.

(Morita). The injection pressure was 0.3 MPa, the distance from the nozzle to the specimen surface was approximately 10 mm, and the spray time per unit surface was 13 sec/cm

.

Retentive force and measurement methods The retentive force was measured using an EZ­SX tensile testing machine (Shimadzu). Each test specimen underwent ultrasonic cleaning in distilled water for 5 min and then was dried before the measurement. After applying a loads of 25 N and 50 N for 5 seconds each, the specimens were verti­

cally pulled at a crosshead speed of 40 mm/min, and the maximum resistance observed during sec­

ondary crown withdrawal was considered to be the retentive force. We measured the retentive force three times and used the mean of these measure­

ments as the representative value. The measure­

ments were performed indoors, at 21.0±1.2°C, and a mean humidity of 43.3±8.0%. Figure 7 shows the retentive force measurements being performed.

The retentive force measurement conditions were an applied loads of 25 N and 50 N, and 1/2 taper angles of 2° and 4°, with the combinations of primary­secondary crown surface characteristics being polished­non­polished, polished­blasted, and blasted­blasted. As previously mentioned, a one­

way analysis of variance was performed with the combinations of surface characteristics set as the factors, and the level for statistical significance set at 5%. Tukey’s multiple comparison test was per­

formed when a statistically significant difference was observed. We used IBM SPSS Statistics ver.

26 (IBM Japan) for statistical analyses.

RESULTS

Comparison of zirconia and gold alloy static friction coefficients

The mean values for the coefficient of static friction for Y­TZP, Ce­TZP/A and gold alloy were : 0.18, 0.17 and 0.27 when the surface roughness was smooth­smooth (Fig. 8), 0.18, 0.17 and 0.33 when the surface roughness was smooth­rough (Fig. 9), and 0.31, 0.38 and 0.57 when the surface rough­

ness was rough­rough (Fig. 10), respectively. Al­

though no significant differences were observed be­

tween Y­TZP and Ce­TZP/A for any of the surface roughness combinations (p＞0.01), values were significantly smaller for Y­TZP and Ce­TZP/A than

Fig. 7 Measurement of retentive force.

Fig. 8 Static frictional coefficients (smooth­smooth).

Mean±SD, **p＜0.01, n＝5.

Fig. 9 Static frictional coefficients (smooth­rough).

n＝5

for gold alloy (p＜0.01).

Retentive force of Ce-TZP/A telescopic crowns with different surface characteristics

Figure 11 shows the retentive force with the applied load of 25 N and a 1/2 taper angle of 2°. Mean val- ues for retentive force increased from polished-non- polished (9.7 N), to polished-blasted (10.9 N), and blasted-blasted (12.3 N) surfaces. The mean reten- tive force for the applied load of 50 N (Fig. 12) also increased, from polished-non-polished (20.0 N), to polished-blasted (22.2 N), and then blasted-blasted surfaces (27.1 N). Significant differences were noted for all groups. Figure 13 shows the retentive force with the applied load of 25 N and a 1/2 taper angle of 4°. Mean values for retentive force in-

Fig. 10 Static frictional coefficients (rough-rough).

n＝5

Fig. 11 Retentive force with a load of 25 N at a half angle of 2°．

*p＜0.05, n＝5.

Fig. 12 Retentive force with a load of 50 N at a half angle of 2°．

n＝5

Fig. 13 Retentive force with a load of 25 N at a half angle of 4°．

n＝5

Fig. 14 Retentive force with a load of 50 N at a half angle of 4°．

n＝5

creased from polished-non-polished (4.4 N), to polished-blasted (6.1 N) and the blasted-blasted surfaces (9.3 N). The mean retentive force for the applied load of 50 N (Fig. 14) also increased from polished-non-polished (9.9 N) to polished-blasted (13.6 N) and then blasted-blasted (20.5 N). Signifi- cant differences were noted for all the groups.

DISCUSSION

In recent years, ceramics are more frequently used for crowns and bridges in clinical dentistry because of concerns among patients regarding esthetic fac- tors and an increase in the number of patients with metal allergies. Y-TZP, in which yttria is used as a stabilizer, is mainly used for bridges, and in the mo- lar region. These are situations are exposed to high loads.

This enables the fabrication of ceramic res- torations that are resistant to fractures, attrition, and abrasion. Presently, gold alloys are mainly used for telescopic crowns. However, they are associated with issuses such as high metal prices and patients with metal allergies. To avoid these problems, we propose the use of zirconia materials as alterna- tives to gold alloys in telescopic crowns. Taper an- gle, surface force, and the material coefficient of static friction affect the retentive force of telescopic crowns.

We focused on examining the coefficient of static friction in this experiment, which is thought to be affected by factors such as material composi- tion, specimen surface processing, surface rough- ness, and temperature.

For the clinical application of zirconia telescopic crowns, we used board- shaped specimens shaped like the primary and secondary crowns and altered the surface rough- ness to be able to measure the changes in the co- efficient of static friction. The surface force, which affects friction, was fixed, and the coefficient of static friction was calculated among the groups of the same materials to conduct relative evaluation of Y-TZP, Ce-TZP/A and gold alloys.

When the surface roughness was smooth- smooth, the mean coefficients of static friction of Y- TZP and Ce-TZP/A were 0.18 and 0.17, respec- tively, indicating no significant difference between the materials in terms of friction. The lack of differ-

ence in the coefficients of static friction is likely due to the fact that these were both zirconia-based ce- ramics with similar surface textures and strengths.

However, the mean coefficient of static friction for the gold alloy, which had a surface roughness of smooth-smooth, was 0.27 which was significantly greater than for both Y-TZP and Ce-TZP/A. Fric- tional force is composed of two main factors : ad- hesion bound by molecular interaction generated at the contact site (the adhesion theory

) and drag at the specimen surface protrusions (the roughness theory). The force required to break adhesion and move the solid body, the force of surface protru- sions exceeding the inclination when external force is applied, or the force applied on protrusion defor- mation or destruction are collectively referred to as

“frictional resistance.” The frictional resistance that arises when one attempts to move a stationary ob- ject is the maximum static frictional force. When a surface is viewed under a microscope, one can see surface protrusions ; the sites where these surface protrusions are in contact with one another are known as real contact points, and this area is much smaller than the apparent contact surface area. A large amount of stress is concentrated on the real contact area, causing plastic deformation ; such deformation is thought to greatly affect friction.

There are various types of ceramics, including those with ion-binding or covalent-binding proper- ties. Zirconia has relatively strong ion-binding prop- erties, making adhesion easier than that for ceram- ics with strong covalent-binding properties. How- ever, as this material is very hard and has little plastic formation, which is crucial for adhesion,

the true contact area where adhesion occurs is smaller for materials made with zirconia than for those made with metal. This suggests that the difference in the coefficient of static friction between metal and zirconia arises from differences in adhesion.

Therefore, the coefficients of static friction for Y- TZP and Ce-TZP/A, influenced by adhesion, were lower than those for the gold alloy specimens.

The mean coefficients of static friction when the

surface roughness (Ra value) was smooth-rough

for Y-TZP, Ce-TZP/A and gold alloy were 0.18,

0.17 and 0.33, respectively. Compared to when the Ra value was smooth-smooth, the two types of zir- conia materials exhibited little change in their mean coefficients of static friction. However, the mean co- efficient of static friction of the gold alloy increased from 0.27 to 0.33. This indicates that for the gold alloy, increased surface roughness resulted in a de- creased real contact area ; therefore, the amount of load per unit area increased causing plastic de- formation of the protrusion tips, which facilitated ad- hesion. Because it is more difficult to achieve adhe- sion with zirconia than with metal, it appears that while there is increased contact area pressure with zirconia, the small amount of plastic deformation it undergoes indicates that there is little effect on ad- hesion capability, resulting in no change in the co- efficients of static friction for Y-TZP and Ce-TZP/A.

The coefficients of static friction for Y-TZP, Ce- TZP/A and gold alloy when surface roughness was rough-rough were 0.31, 0.38 and 0.57, respec- tively ; each material type had high values. The roughness theory states that the rougher the sur- face, the larger is the coefficient value, which is consistent with the results found in our study. In light of the adhesion theory, a greater actual con- tact area should lead to larger values for coeffi- cients of friction. Based on all our results, it ap- pears that both theories are simultaneously in- volved, with larger Ra values associated with a greater influence of the roughness theory, and with smaller Ra values associated with a greater influ- ence of the adhesion theory. When the surface roughness was rough-rough, it appears that for all materials, there was an increase in the force re- quired for surface protrusions to overcome the incli- nation or the force applied with protrusion deforma- tion or destruction.

Furthermore, we fabricated specimens to investi- gate how the retentive force is influenced by the surface roughness of the primary and external crowns. Though various factors can affect retentive force, we only focused on taper. The 1/2 taper an- gle is generally set at 6° for conventional telescopic crowns made from gold alloy ; however, a prelimi- nary experiment on Ce-TZP/A telescopic crowns in-

dicated that no retentive force was observed at a 1/2 taper angle of 6°, and a stable retentive force was observed when the angle was set at 2° or 4°.

Therefore, we set the 1/2 taper angle at 2° or 4° in this study. The minimum possible thickness of crowns made from Ce-TZP/A is thought to be 0.3 mm.

Considering the risk of fractures, we set the thickness at 0.4 mm ; however, for the conven- ience of processing, the thickness of the undercut region under the two knobs is not limited. The space between the primary and secondary crown was set to 10 μm because a stable retentive force was observed with this value in previous studies.

Moreover, we investigated whether sand blasting was effective in adjusting the retentive force of completed Ce-TZP/A telescopic crowns. In clinical practice, there is a risk that blasting the outer sur- face of the primary crown could cause the crown to have a rough surface, and thus retain more plaque.

Therefore, we first measured the retentive force on polished primary crowns and non-polished secondary crowns (polished-non-polished) followed by polished primary crowns and alumina blasted secondary crowns (polished-blasted), and finally alumina blasted primary crowns and secondary crowns (blasted-blasted). The injection conditions for blasting were set as previously reported by Blatz et al. to achieve changes in surface charac- teristics.

An injection pressure of 0.3 MPa was used, distance from the nozzle to the specimen surface was approximately 10 mm, and the spray time per unit surface was 13 sec/cm

. The mean re- tentive force for a 1/2 taper angle of 2° when the applied load was 25 N increased from polished-non -polished (9.7 N), to polished-blasted (10.9 N), and blasted-blasted (12.3 N) surfaces.

The mean retentive force for an applied load of

50 N also increased with the same trend, with 20.0,

22.2 and 27.1 N recorded, respectively. The mean

values for retentive force increased similarly, with

4.4, 6.1 and 9.3 N recorded, respectively. The

mean retentive force for an applied load of 50 N

also increased in this order, at 9.9, 13.6 and 20.5

N, respectively. Little difference was observed be-

tween the coefficients of static friction measured in

the first experiment between the smooth-smooth and smooth-rough surfaces, whereas the values for rough-rough surfaces were larger than those for smooth-smooth and smooth-rough ones. Because no differences were observed in the coefficients of static friction between smooth-smooth and smooth- rough surfaces, it was expected that there would also be no differences in the retentive force be- tween polished-non-polished and polished-blasted surfaces. However, the retentive force increased, from polished-non-polished, to polished-blasted and then blasted-blasted surfaces, with the largest val- ues noted for blasted-blasted surfaces. This sug- gests that when the external crowns of telescopic crowns are subjected to an applied load, changes occur in the surface characteristics, resulting in a large real contact area between the primary and secondary crowns, and therefore a larger coeffi- cient of static friction and increased retentive force.

Although zirconia is lighter than gold alloy, it is very strong and offers excellent biocompatibility and abrasion resistance. Moreover, because it can be used on patients with metal allergies, we anticipate its clinical application as an alternative to gold al- loys for telescopic crowns. Furthermore, Ce-TZP/A, which has superior characteristics to Y-TZP,

could be used to fabricate even more reliable all- ceramic telescopic crowns. However, because zir- conia has a lower coefficient of static friction than gold alloy, the taper angle needs to be accordingly decreased. In clinical practice, alumina blasting of the inner surface of the secondary crown or the outer surface of the primary crown can be an effec- tive technique for improving a crown when the re- tentive force of a telescopic crown made from Ce- TZP/A has decreased or when it is desirable to in- crease the initial retentive force of the crown. How- ever, further investigations are necessary to deter- mine to what extent retentive force can be main- tained following alumina blasting.

CONCLUSIONS

When the coefficients of static friction were deter- mined for Y-TZP and Ce-TZP/A crowns with differ- ent surface textures, lower values were obtained

than were observed for gold alloy. Our results also demonstrated that, in clinical practice, alumina blasting of the inner surface of the secondary crown or the outer surface of the primary crown can be effective in improving retentive force, when the retentive force of a telescopic crown made from Ce-TZP/A has decreased, or when it is desirable to increase the initial retentive force of the crown.

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