The Effects of Chairside Adjustment on the Surface Roughness of Contemporary Dental Ceramics: A Narrative Review

ABSTRACT

Finishing and polishing dental ceramics after chairside adjustments is necessary to regain the original surface topography. Though highly subjective, visual and tactile assessment are the most practical ways to assess surface roughness clinically. Several manufacturers recommend polishing kits for specific ceramic materials that differ in efficacy. Proper polishing protocols require the use of sequential polishing instruments that reduce the size of the adjustment abrasions, improve aesthetics and reduce plaque accumulation. If polishing systems are interchanged, stepwise use of a zirconia polishing system may have a “universal” effect on different ceramic substrates. Adjusting zirconia restorations could result in phase transformation and potentially compromise flexural strength. The extent of this degradation is dependent on the yttria content of the zirconia. This may be reduced by appropriate finishing and polishing.

KEYWORDS:

• Polishing
• dental ceramics
• surface roughness
• zirconia
• phase transformation

Introduction

The popularity of ceramic restorations continues to increase because of their aesthetic qualities. Various dental ceramics have been available in the market: glass-based ceramics such as feldspathic porcelain, glass-based systems with crystalline fillers such as leucite-reinforced ceramics (IPS Empress and lithium disilicate-reinforced ceramics (IPS Emax), crystalline-based ceramics with glass fillers (In-Ceram SPINELL, In-Ceram Alumina and In-Ceram Zirconia), polycrystalline solids (Zirconia) among others.¹

Chairside adjustments to these restorations may be required during delivery appointments. Adjustments are typically performed on the occlusal surfaces of indirect restorations to eliminate working/nonworking interferences. Adjustments can also be made proximally to facilitate seating of indirect restorations that are over-contoured proximally or when proximal contacts are too tight. Finally, adjustments can be made to indirect restorations to correct contours for aesthetic or functional reasons. This process typically involves use of a diamond bur, which inevitably roughens the ceramic surface, followed by polishing to restore the original smoothness of the surface. The key to effectively performing the finishing and polishing process is to follow a step-wise procedure, methodically introducing finer scratches to the ceramic substrate while eliminating the deeper scratches from the previous step. Sequentially moving from coarse to fine grit instruments is recommended for effective polishing.¹ Finishing and polishing instruments include diamond burs of varying abrasiveness, soft and hard polymeric cups, points and wheels impregnated with abrasive particles as well as polishing pastes (Figure 1).²

Figure 1. The dialite LD finishing and Polishing Systems for Lithium Disilicate (Brasseler USA), containing diamond burs, polymeric cups, discs, and points.

The factors that may affect the effectiveness of these instruments include the difference in hardness between the abrasive and the substrate being finished and polished, the durability of the bonding material used to carry the abrasive, the hardness size and shape of the abrasive particles, the polishing speed, the force applied during polishing, the presence/absence of lubrication during the polishing process, and the structure of the substrate being finished. In relation to abrasive particle hardness, it is important that the relative hardness of the abrasive is higher than that of the substrate in order to remove particles from the substrate. Though increasing polishing speed and pressure may increase the effectiveness of the polishing process, this should be performed within limits to prevent excessive heat generation. Manufacturers of polishing instruments will typically recommend optimum polishing speeds. Use of a lubricant such as water is also recommended to act as a coolant.^3–6

This article will provide a review of the literature on the polishing of dental ceramics under the following subtopics:

• Assessment of Surface Roughness

• Rationale of Chairside Finishing and Polishing of Dental Ceramics

• The Efficacy of Various Polishing Kits on the Surface Roughness of Dental Ceramics

• Chairside Adjustment of Zirconia and Phase Transformation

• The Effect of Interchanging Polishing Kits Among Different Dental Ceramics

This article will elaborate on the following points:

1. Proper polishing protocols require the use of sequential polishing instruments that reduce the size of the adjustment abrasions, improve aesthetics and reduce plaque accumulation.

2. Adjustments to zirconia restorations may lead to phase transformation, which could compromise the flexural strength of these restorations.

3. In vitro research has shown that zirconia polishers can effectively polish different ceramic substrates.

Assessment of Surface Roughness

Surface roughness can be assessed in vitro or clinically. The in vitro assessments tend to be more objective and commonly use tools such as profilometers as well as the atomic force microscope (AFM). A more subjective in vitro assessment is the scanning electron microscope (SEM). For in vitro assessment of surface roughness, the test specimens are required to be flat, eliminating the use of anatomic samples. The profilometer measures the surface roughness via a stylus that may or may not contact the test surface, moving along a linear path for a specified distance (20–50 nm) and under a specific contact force. The AFM stylus, which is smaller and thus more accurate, moves in the x-, y-, and z-axes. While the AFM measures square-shaped areas, the profilometer moves linearly. It has been suggested that studies published with both AFM and profilometer readings are more reliable, as the two instruments are complimentary to each other.⁷ The SEM produces magnified images of samples by scanning them using a beam of electrons. These images are then described subjectively.

ISO 4287 describes several roughness parameters,⁸ however, the most reported parameters in dental research on the topic of surface roughness are the arithmetic mean roughness value (Ra) and the root mean square of the assessed profile (RMS or Rq). Ra is the arithmetic mean of the absolute values of the profile deviations from the mean line of roughness profile. The RMS is the standard deviation of the distribution of the surface heights distribution.⁷ Though Ra values are easy to measure and define in addition to providing adequate description of height variations, they are not sensitive to slight changes in profile. Statistically, RMS values are more valuable as they are more sensitive to slight changes in the profile. Because of the complexity of surface roughness measurement, it is recommended that more than one parameter is used to describe surface characteristics. The majority of the literature on surface roughness of dental restorative materials report Ra and/or RMS values.⁹

The clinical assessment of surface roughness is extremely subjective. Visual and tactile inspection are the most used methods of clinical assessment of roughness in daily practice. Visually, a ceramic surface that appears dull/matte may be rougher than a ceramic surface with a shiny/glossy appearance due to the interaction of the surface with the incident light sources. Gloss is a measure of light reflection from a surface. In general, gloss and surface roughness have a negative correlation. Therefore, visual assessment for gloss may be an effective way of estimating surface roughness clinically.¹⁰ Tactile assessment of a ceramic surface may also give clues as to its relative roughness. It is also important to note that the presence of saliva on a ceramic will affect the interaction of the surface with incident light and therefore affect the ability to visually assess roughness. For this reason, clinical assessment of ceramic roughness should ideally be completed on dry surfaces.

Figure 2: An image demonstrating the facial aspect of a fixed partial dentures with one-half of the first pre-molar adjusted and the other half adjusted, finished, and polished.

Figure 2. An image demonstrating the facial aspect of a fixed partial denture with one-half of the first pre-molar adjusted and the other half adjusted, finished, and polished.

Rationale of Chairside Finishing and Polishing of Dental Ceramics

An ideal ceramic restoration is aesthetic, strong, and biocompatible. Many of these features can be controlled during manufacturing procedures.¹¹ However, clinical adjustments are usually necessary as part of finishing processes. Depending on the type of ceramic and how finishing procedures are completed, surface roughness is induced.

Surface roughness negatively impacts strength,^11,12 diminishes optical properties,^10–14 and encourages wear of opposing dentition.^15–17 Rough.*^18,19 surfaces encourage biofilm accumulation.^15,20

Clinical adjustment of crowns induces microcracks; regardless of ceramic material, the size and depth of microcracks is associated with a significant decrease in flexural strength.²¹ Wear of opposing dentition may affect maintenance of the vertical dimension of occlusion and occlusal scheme, which may contribute to additional problems associated with occlusal wear (Figures 3–5).²²

Figure 3. A clinical image demonstrating wear of the natural anterior mandibular dentition opposing feldspathic porcelain crowns.

Figure 4. A clinical image demonstrating wear of the mandibular natural dentition opposing a maxillary complete denture with porcelain denture teeth.

Figure 5. A clinical image demonstrating wear facets on teeth nos. 22, 23, 27 and 28 opposing lithium disilicate crowns.

The simplest means of achieving a smooth surface is to reglaze (natural glaze) an adjusted restoration prior to luting²⁰; however, this is not always practical and may encourage crack formation associated with repeated firings.^10,22 Aesthetics can be predictably improved via polishing rather than glazing.²³ Additionally, an over-glazed layer can be removed by occlusal adjustment or after a short period of function.²² Different materials require different polishing methods to obtain a smooth surface.²⁴ Finishing and polishing processes can introduce heat and surface roughness with resultant microcrack formation. Crack formation occurs in all ceramics and tends to occur where tensile forces are greatest. Therefore, care must be taken to select the most appropriate finishing and polishing methods to ensure the optimal outcome for a ceramic restoration.

The Efficacy of Various Polishing Kits on the Surface Roughness of Dental Ceramics

Several chairside polishing systems for dental ceramics are available in the market. These systems vary in characteristics and durability²⁵ and are typically specific to the ceramic type as recommended by manufacturers. Several in vitro studies compare the efficacy of different polishing systems on their corresponding dental ceramic substrates after simulated chairside adjustments and polishing with varying results.^25–27 The addition of a fine polishing paste after completion of steps involving rubber,*²⁸ wheels and points has been shown to significantly decrease the surface roughness of dental ceramics comparable to glazed specimens.^29,30

Some recommended polishing systems by the authors include the OptraFine polishing kit (Ivoclar Vivadent AG), the Dialite Adjusting, Finishing and Polishing kit by Brasseler USA, specifically Dialite HP, Dialite ZR and Dialite LD, and the BruxZir (Glidewell Dental) zirconia adjustment and polishing kit. After polishing with OptraFine, a diamond paste (OptraFine HP Polishing Paste; Ivoclar Vivadent AG) is recommended to provide an optimum polish.

It is important to note that in vitro studies on ceramic polishing are performed on flat specimens, which allows for profilometer or SEM assessments of roughness. The complex contours of an indirect restoration may provide challenges in obtaining a polished surface simply due to access limitations and the location where polishing is required. The selection of an appropriate shape of polisher to reach the location of an adjusted restoration (cup, wheel, disc, point, etc.) may play a critical role in arriving at an adequately polished ceramic restoration surface.

Chairside Adjustment of Zirconia and Phase Transformation

Zirconia is intrinsically brittle and derives its mechanical strengths and weaknesses from tetragonal to monoclinic-phase transformations. These transformations result in volumetric expansion and changes in grain size and shape.³¹ Phase transformation occurs spontaneously over time but can be induced during sintering and cooling cycles as well as clinical surface treatments, such as air abrasion, clinical finishing with burs, and polishing. Phase transformation can be both detrimental and beneficial to the mechanical properties of zirconia. The benefit of phase transformation occurs when individual grains undergo resultant volumetric expansion, which arrests the propagation of microcracks in zirconia. This phenomenon is called transformation toughening. On the other hand, excessive-phase transformation will compromise the strength and fracture toughness of zirconia as the monoclinic phase is inferior to the tetragonal phase in this respect.^30–32 While the resultant surface compressive stresses can increase flexural strength, phase transformation also encourages low-temperature degradation. Low-temperature degradation is the aging of zirconia, which is characterized by the slow transformation of the tetragonal to monoclinic phase. This is associated with grain pull-out, microcracking, and strength degradation. This process can be accelerated by specific conditions such as humidity, stress, and high temperatures of 200–300 degrees Celsius.³¹ Chairside adjustment of zirconia can compromise strength if the adjustments are not followed by adequate polishing. Adjustments with fine diamond burs introduce surface defects and potentially some microcracks into the zirconia. The accompanying phase transformation will result in increased toughness due to the resistance of crack propagation. Benefits from this transformation toughening are negated by surface defect sizes and depth of phase transformation; polishing after coarse adjustments can decrease flaw size and reduce the depth of phase transformation.^33,34 The negative effects of zirconia adjustment on flexural strength appear to be worse in 5Y-PSZ, followed by 4Y-PSZ, and least of all in 3Y-PSZ due to an increase in cubic zirconia particles with the increase in yttria content. This is because cubic zirconia particles lack the ability to undergo transformation toughening.³⁵

The Effect of Interchanging Polishing Kits Among Different Dental Ceramics

As demonstrated in the preceding subtopics, there are a myriad of manufacturer recommended dental ceramic polishing kits. These proprietary polishing kits, with the exception of a few universal polishers, are typically recommended for specific ceramic substrates. This can lead to not only a large inventory in dental offices but can also result in uncertainty around which kits and polishing protocols are the most effective. A pioneer study that investigated the effect of interchanging polishing kits found that the tested zirconia polishers could be used interchangeably on feldspathic porcelain and zirconia substrates, and this did not apply to the other porcelain polishers tested. In fact, the zirconia polishers resulted in a smoother polish on the feldspathic porcelain samples than the feldspathic porcelain polishers.²⁵ A subsequent study expounded on this by including a lithium disilicate substrate and both lithium disilicate and universal polishing kits. Similar to the former study, this study demonstrated that the zirconia polishing system used resulted in the smoothest surface, regardless of the ceramic substrate. The difference in surface roughness was statistically significant for the AFM measurements.³⁶

Conclusion

Surface roughness can be measured objectively using several types of devices such as the atomic force microscope and the profilometer. Clinical assessment of surface roughness, however, remains subjective. Surface roughness on dental restorations has detrimental effects on both the opposing and adjacent dentition in addition to increasing plaque accumulation, which may compromise patient’s oral health. The authors have found that the sequential use of a zirconia polishing kit in addition to a fine grit polishing paste results in a smooth surface finish for zirconia, lithium disilicate, and feldspathic porcelain restorations.

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Karina Fiona Irusa

Karina Fiona Irusa is currently an Assistant Professor at Tufts University School of Dental Medicine, Department of Comprehensive Care. She graduated in 2014 from The University of Nairobi, Kenya with a Dental Doctorate Degree and from the University of North Carolina at Chapel Hill with a Master of Science in Operative Dentistry and Biomaterials in 2021.

James Tonogai

James Tonogai is a prosthodontist who completed his specialist training at the University of Toronto in 2021. He completed his DDS degree at Western University in 2015 and practiced for 3 years in Hamilton, Ontario before pursuing his interest in prosthodontics.

Stephanie Yeung

Stephanie Yeung, DDS is a prosthodontist practicing in Los Angeles and Seattle. She graduated from the University of Southern California with her Dental Doctorate Degree and her Prosthodontics certification.

Terence Donovan

Terry Donovan received his D.D.S. from the University of Alberta in 1967, and practiced full time in Regina, Saskatchewan for 13 years. He received his Certificate in Advanced Prosthodontics from the University of Southern California in 1981. He has held various academic positions at the University of Southern California and the University of North Carolina at Chapel Hill for over 40 years.