Why Do We Polish? Part Two

Why Do We Polish? Part Two

Thomas D. Larson, D.D.S., M.S.D.:

Editors’ Note: Dr. Thomas Larson continues to supply us with articles of clinical relevance. The Publications Committee feels his topics address clinical situations and choices we deal with on a daily basis. Most of us and our readers are busy private practitioners with limited time to read scientific studies. To have someone of Dr. Larson’s academic experience and credentials providing our journal his extensive yet concise reviews of the literature is of great value, and we have dedicated a portion of our clinical department to these articles. “I chose Northwest Dentistry,” said Dr. Larson, “because if I were to publish in national journals, many of my former students would never see the articles. They are the audience to which my academic career is devoted. They are the audience that I am trying to affect by publishing best practices.” 


Abstract

What does polishing oral hard tissues and restorations accomplish? The two parts of this review will describe the effect of polishing on various restorative materials and teeth; the development of biofilm and adherence of plaque to teeth and restorations; the effects of unpolished versus polished surfaces on gingival health and longevity of restoration; and techniques for polishing various restorative materials.


Degradation of amalgam margins has been studied in five-year clinical trails using five different alloys for 126 patients in 296 restorations. Ridit analysis reveals that the amalgam margin degradation was influenced by the location in the mouth, the position on the teeth, the handling of the material, and the operator. Differences in occlusal design of the preparation in this same study did little toalter the degradation of the margin. Polishing of occlusal margins principally removes flash left by carving. After 1.5 years, polishing of 150 Class I and II amalgams in 48 patients was found to have asignificant effect on marginal integrity, though a smaller effect than the alloy choice or patient. Polishing leaves the surface smoother than either burnishing or carved surfaces, and it removes the tin-mercury (gamma 2 phase) alloy on the surface left after burnishing. Polishing decreases the ability of the amalgam to tarnish.

Polished and unpolished amalgams were placed into a saline solution and subsequently measured by atomic absorption spectrophotometry. Unpolished amalgams release the greatest amounts of mercury and silver from the surface within the first three hours after trituration compared to polished amalgams. The rate of dissolution is low once the amalgam is set and after formation of a pellicle.41 Ahigh copper amalgam has different surface chemistry than in its bulk. On its surface, a high copper amalgam has a hydrocarbon deposit and adsorbed water covering the intermettalic gamma2 phase {Sn(6-8)} and tin oxide and mercury in a free state. The amount of mercury on the surface eventually attains a constant concentration involved in the gamma2 phase, leaving no free mercury.42 Amalgam restorations corrode in the mouth. The depth of the corrosion depends upon the type of amalgam, with high copper amalgam more resistant to corrosion, and polished amalgams also more resistant. Corrosion causes a loss of micro hardness, with corrosion depth of 50-400 microns after two months in a 85mM NaCl solution.43 Polished amalgam restorations after three years have been found to have similar marginal integrity compared to burnished and carved amalgams.

Polished restorations after three years have substantially superior surface texture and less likelihood of surface discoloration (tarnish or corrosion)44 (Figure 9). Polishing amalgam restorations also has been found to delay the decision to replace them.45,46



Figure 9. Gingival health adjacent to unpolished amalgam restorations are affected primarily by the contour and fi t and secondarily by the corrosion and plaque adherence. Polishing an amalgam restoration decreases plaque accumulation and corrosion.

Figure 10: Five-year-old composite veneers on the maxillary right canine, lateral, and central show a lack of luster and reasonable gingival health. The maxillary left central is a porcelain fused to metal crown. Figure 11: Five-year-old composite resin veneers. Compare gingival health on maxillary teeth to mandibular teeth. Gingival margins, while not infl amed, appear rolled on the maxillary teeth compared to a knife edge margin of the gingiva on the mandibular teeth. Figure 12: Glass ionomer restorations fi nished with Sof-Flex XT disks (3M-ESPE).

Composite Resin and Glass Ionomer Restorations

Polishing of composite restorations with prophylaxis pastes causes varying roughness depending upon the particle size of the polishing paste, and generally leaves the surfaces rougher than required for bacterial adhesion (0.2 micron).47,48 Methods of polishing do not seem to affect the wear resistance of polished nanofilled or mini-filled composites,49 and both seem to roughen with the use of normal toothpastes in the mouth.50,51  Tooth brushing with use of a dentifrice results in increased surface roughness on posterior resin composites, even when sealed with a surface penetrating sealant52 (Figures 10, 11).

Finishing techniques and storage medium (acids from pH 2.4, 3.7, and 5.1) significantly lower the biaxial flexural strength and micro hardness.53 Leaving a resin-rich outer layer on a composite, either by glazing or as left in contact with a Mylar strip, causes significant color change faster in resin-based composites.54Microfill, hybrid, and flowable resin composites in vitro show degradation over 60 days stored in various media (Coca-Cola, sugar cane spirit, coffee and artificial saliva, E 110 food dye, vinegar and erythrosine) with a significant increase of surface roughness and a decrease in micro hardness.55,56

The act of polishing composite resin, especially when polishing dry, causes the formation of an amorphous layer on the external surface that appears to improve gloss and smoothness.57,58 The amorphous layer is most likely a smear of the heated resin material. Polishing, especially dry, can create significant frictional heat, sufficiently above the glass transitional temperature of resins (120-160°C), to melt the resin.58  

Glass ionomer restorations show the smoothest surface when polished with a carbide bur followed by Sof-Flex XT disks (3M-ESPE), significantly better than polishing pastes, rubber polishers, or carbide burs alone (Figure 12).  The polishing pastes and rubber polishers seem to preferentially remove the polysalt and resin matrix of a glass ionomer, leaving larger particles exposed.59

Aluminum oxide polishing disks and abrasive impregnated disks produce the smoothest results on composites and glass ionomer materials.60,61

Porcelain Materials

There is considerable argument in the literature as to whether porcelain materials should always be left in a glazed state62 or if polishing after glazing is an acceptable procedure.63 Glazing porcelain imparts a very smooth, amorphous surface and offerssome significant advantage by filling in all of the porosity, cracks, and crazing created by contouring the porcelain with rotary instruments. This decreases the rate of wear on opposing teeth, as glazed porcelain does less damage to opposing teeth than unglazed porcelain.64 The surface free energy of the glazed surface, regardless of the type of porcelain, is high. This results in increased bacterial accumulation due to the significantly basic surface.65  When exposed to saliva, this surface bonds with the anionic groups of salivary proteins, orienting their cationic sites toward the bulk. This leads to selective adsorption of bacteria to these surfaces.66 Polishing after adjustment on a glazed porcelain surface can lead to a smooth surface that causes no more wear of the opposing teeth than a glazed porcelain surface.67

The smoother the surface of the porcelain, the stronger the material is in biaxial strength. Among porcelains, differences in biaxial strength can be attributed to stress concentration of an applied load due to the roughness of the surface caused by mechanical finishing or chemical action.68  Polishing porcelain can remove small surface imperfections that affect strength. How the polish is accomplished also can affect the strength of the porcelain. In a clinical simulation in vitro, porcelain was polished using different forces and polishing regimes. Increasing the speed of the polish from 10,000 rpm to 20,000 rpm significantly reduced the flexural strength of the ceramic material.69

Longevity of Restorations

Research in the literature does not directly link the polish of a material to its longevity. That is, a failure to polish cannot be directly attributed to its failure. We may be able to indirectly infer some relationships by the mode of failure among the different restorative materials and whether polishing would affect the areas of failure.

The failure of a restoration is a complex judgment with numerous reasons provided for restoration replacement. One of the most interesting findings is that the age of replaced restorations is shortest for the group of clinicians with the least experience and longest for the group of clinicians with ≥ 30 years experience.70 With greater clinical experience comes more certain knowledge — of when, for example, a marginal discrepancy is significant and requires replacement of the restoration — and the ability to rate the risk of doing nothing versus replacement.

Annual failure rates for posterior, stress-bearing restorations are reported in Table I. These values are from two separate studies, and the spread of failures leads to the conclusion that many factors can cause failure. In these two meta-reviews of clinical studies, the most common reasons for replacement were secondary caries, fracture, marginal discrepancies, wear, and postoperative sensitivity.71,72

Indirect restorations exhibit significantly greater longevity than direct restorations.72  Longevity has been reported in a different fashion, looking at survival rate, using data from one dentist in Belgium with samples of 722 amalgam restorations, 115 composite restorations, and 89 crowns placed in 428 adults. Twenty-eight percent of the restorations had failed in total due to fracture of the restoration (8%); secondary caries (6%); fracture of the cusp (5%). Failures occurred in premolar teeth (34%) more often than molars (27%); and occurred in 28% amalgams, 30% resins, and 24% of the crowns. The survival times are more than 14.6 years for crowns, 12.8 years for amalgam restorations, and 7.8 years for composite restorations.73

Larger direct restorations tend to last fewer years than smaller restorations.74,75,76  Premolar restorations tend to last longer than molar restorations.76,77 Finnish studies of longevity report on the age of restoration when they are being redone. Secondary caries is the most common reason for replacement, found in 36% of composite restorations, 52% of glass ionomer restorations, and 41% for amalgam restorations. Fracture of the restorations occurred in 23% of composite, 11% glass ionomer, and 22% of amalgam restorations. Lost restorations occurred in 16%. Median age for replacement was 15 years for amalgam, six years for composite, and seven years for glass ionomer.78,79

A survey of a single dentist’s practice (Dr. R.V. Tucker) evaluating high quality cast gold restorations up to 52 years of age showed survival rates of 97% at nine years, 90.3% at 20 years, 94.9% at 25 years, 96.9% at 39 years, and 94.1% for restorations >40 years old.80 On the other end of the spectrum, in a dental school population, gold restorations placed from 1963 to 1993 in 890 patients numbered 3,518 gold restorations. One hundred eleven of the restorations were lost. Ten-year survival rates were 76.1% for occlusal inlays; 88.3% for MO inlays; 83.4% for DO inlays; 87.5% for MOD inlays; and 86.1% for partial crowns. This averages to 85.7% for all restorations.81

Polishing of restorations may be inferred as limiting the formation of secondary marginal caries by improving marginal adaptation and reducing plaque accumulation on its surface. Polishing of amalgam specifically removes any flash left on the margin. Amalgam flash contributes to the formation of marginal discrepancies when it fractures, leaving a marginal gap. Polishing of amalgam restorations specifically can limit the corrosion and weakening of the amalgam restoration by corrosion and limiting the bulk fracture due to corrosion. Polishing of composite restorations also removes marginal flash, which may or may not be bonded.  Fewer problems occur when marginal flash extends beyond the tooth preparation of composites when bonded to the tooth.  Unbonded composite flash will discolor under the margin, and the edge will generally curl, leaving a white line while polishing. This type of flash can lead to marginal caries. This can lead to the decision to replace, because the marginal discoloration is inferred as caries (penetrating stain). Marginal discrepancies due to unfinished surfaces can ultimately lead to recurrence of caries or periodontal disease due to plaque adherence at the unfinished margin when in apposition to the gingival tissue. Marginal gaps in sites where hygiene is more easily accomplished tend not to become carious except in highly cariogenic environments. A failure to polish porcelain materials on the occlusal surface after in vivo adjustments can result in excessive wear of the opposing dentition.64,67Failure to polish axial surfaces of porcelain after adjustment can lead to accumulation of plaque. The reader may infer, therefore, that polishing positively affects restoration longevity, though what it adds to each restoration’s lifespan is unknown.  


*Adapted from Hickel R, Manhart J. J Adhes Dent. 2001 Spring;3(1):45-64. And Manhart J, Chen H, Hamm G, Hickel R. Oper Dent. 2004 Sep-Oct;29(5):481-508. This table may be read so that in ten years, on average, 30% of amalgam restorations will fail, and 70% will survive; whereas 14% of cast gold restorations would fail after ten years, and 86% would survive.

Techniques for Polishing Restorative Materials

From research previously quoted, it is recognized that each restorative material requires its own specific polishing regime due to the properties of the restorative material and the properties of the polishing materials being used. From an esthetic perspective, patients can recognize a difference in surface smoothness of between 0.25-0.50 microns. The maximum roughness of one-half micron is recommended to keep the surface smooth enough for patients to be unable to detect any roughness.82  Surface roughness and luster can affect the esthetic perception of a restoration. The luster of the restorative material is a measure of the light reflectance from the surface and is due not only to surface texture but particle size of the fillers used — e.g., in composites. Luster similar to adjacent teeth tends not to call attention to the restored surface if the texture and color are also matched.

Dentists use a variety of techniques while polishing, with no specific technique favored among operators. Some use continuous pressure; some use intermittent pressure with variable loads; some use a combination during the polishing time.83Jeffries has reviewed polishing materials and listed them as cited in Table II.84 He recommends the use of sequential abrasives. The gross reduction, contouring, and margination of the material are completed using a ≈100-50-micron particle size abrasive.  An intermediate abrasive to remove scratches from the previous abrasive using particle sizes ≥ 15-20 micron is used next. A very fine abrasive is used to produce luster, utilizing particle sizes < 20 to 0.3 micron. These abrasive-sized particles may be found on coated abrasive disks (SofLex-3M/ESPE, Super Snap-Shofu, Flexi Disc-Cosmedent, Moore Flex, and rotary diamonds); in bonded abrasive silicone and rubber polishing rotary instruments (Enhance-Dentsply Caulk, Blue Midget, Composite Points, PoGo-Dentsply, diamonds, and white stones); and as loose abrasives (aluminum oxide, diamond pastes, pumice, and SiO2). The SofLex system of aluminumoxide-coated abrasive disks comes with four particle sizes: As an example, coarse = 55 micron; medium = 40 micron; fine = 24 micron; and extra fine = 8 micron. Please see Table IV for a listing of particle sizes of different finishing materials.

In an exhaustive study that looked at load, speed, and time required to polish samples of amalgam, composite resin, and glass ionomer, Jones et al have reported the optimum ranges for each material to produce the smoothest surface.85 

The optimal finish producing the smoothest surface as measured by a profilometer with the various finishing disks using the optimal load, speed, and time is listed in Table III.  

Polishing can be accomplished using a circular motion as with burs and diamonds; a planar motion where the axis of rotation is perpendicular to the surface as in using finishing disks; or a reciprocating motion, as in the use of a finishing strip. Planar motion produces the smoothest surfaces for amalgam and composite resin restorations. Polishing dry produces the smoothest surface on amalgam and composites, but the temperature rise may be significant. Lubricating during polishing with water can significantly lower the temperature rise.87 Polishing intra-orally can increase the temperature of the pulp. In an in vitro study, the temperature in the pulp chamber of extracted teeth while polishing an amalgam increased more than 20 degrees Celsius (36 degrees F) within 30 seconds while polishing continuously and dry. Using a water coolant while polishing with intermittent pressure causes a decrease in temperature of four degrees Celsius (-7.2 degrees F).88 If polishing is to be done dry, a light intermittent pressure should be used together with air cooling to minimize overheating of the material and the tooth. 


*Adapted from Jones CS, Billington RW, Pearson GJ. J Oral Rehabil. 2005 Sep;32(9):686-92. In following Table II, read… the optimal load in polishing amalgam restorations starts out heavy, decreases by half, and becomes light on the final finish step. The speed starts out slower on the coarse disk and increases somewhat as the finishing proceeds through the other three disks. The same amount of time is spent polishing with each disk, except for a slight increase while polishing with the medium grit disk for amalgam.

*Adapted from Jones CS, Billington RW, Pearson GJ. J Oral Rehabil. 2005 Sep;32(9):686-92. **Standard deviation appears in parentheses. Note that the use of fine and extra fine finishing disks is not recommended for glass ionomer because it makes the surface rougher.

Amalgam Restoration Polishing Techniques

Waiting 24 hours before polishing amalgam restorations improves their final surface roughness.89 Different high copper amalgams using the same polishing techniques produce different results in surface smoothness, indicating that the surface roughness may be more dependent upon the particle size, shape, and distribution.90  But it may also indicate that different polishing protocols are required for each amalgam to achieve optimal smoothness.

Amalgams generally require margination with rotary finishing burs on the occlusal surface in order to adequately follow the occlusal anatomy. The smoothest surface for amalgam restorations seems to be imparted using bonded abrasive polishing points and cups.91It is recommended that they be used together with coated abrasive disks where appropriate in sequential order as suggested above. Interproximal surfaces can best be smoothed with interproximal finishing strips, and can produce a noticeable difference in surface smoothness.92

Composite Restorations Polishing Techniques

The ability to produce the smoothest surface of a composite material is dependent upon the filler size and dispersal technology used for the composite restorative product. Microfill composite resin (e.g.,A110-3M/ESPE, Heliomolar-Vivadent Ivoclar, ReEnamel-Cosmedent) with filler particles of 0.4 micron will generally polish to the smoothest surface. The nanomer composite resins and nanocluster composite resins (e.g., Filtec Supreme-3M/ESPE) can polish to similar smoothness but less luster than the microfill resin composites. The micro hybrid resin composites (e.g., Esthe-X-Dentsply Caulk, Point 4- Kerr, Vitalesence-Ultradent, Venus-Kulzer) will polish about as smooth as a hybrid resin composite, but with more luster. The hybrid resin composites (e.g., Z-250-3M/ESPE, Prisma TPH-Caulk) will polish significantly smoother than the glass ionomer, compomer, or resin modified glass ionomer restorations.

One polishing technique is unable to produce the same results on all types of composite resins and glass ionomers.63,93,94The filler used in the various composite resin products varies in hardness, and different polishing materials have different hardness. Fine and extra-fine diamond and carbide finishing burs are useful for polishing occlusal surfaces and concave areas. While the coated disks such as SofLex or Enhance seem to work quite well on many composites such as the hybrids and microhybrids, others types of composites such as the microfills polish best with silicone- or rubber-impregnated rotary devices. It is suggested that the type of composite should be matched to the specific polishing system recommended by the manufacturer.93 In all situations, avoid finishing dry so as to prevent overheating and leaving a smear layer of resin that will cause the restoration to change color faster than if polished wet.57,58

The finishing systems most adaptable to the most products seem to be the aluminum oxide polishing disks, which produce the smoothest surfaces on resin-modified glass ionomers, glass ionomers, hybrid resin composites, and microhybrid resin composites.60,95,96,97,98,99 Following a polish with aluminum oxide discs, micro-hybrid resin composites can improve in gloss with the use of an abrasive paste such as Prisma Gloss-Caulk (particle size 1 micron) or Prisma Gloss Extra Fine-Caulk (particle size 0.3 micron).49,60,100

Microfill and microhybrid resin composites can be polished equally well using a diamond-impregnated rubber or silicone polisher such as PoGo-Dentsply,101 Astropol-Vivadent/Ivoclar,102 or diamond-impregnated felt wheels such as Diafix-Mueller Dental103.

Table IV shows the particle sizes of some of the finishing systems available for composite and glass ionomer materials.84,104


*Adapted from Jones CS, Billington RW, Pearson GJ. Br Dent J. 2004 Jan 10;196(1):42-5; and from Gedik R, Hurmuzlu F, Coskun A, Bektas OO, Ozdemir AK. JADA 2005 Aug;136(8):1,106-12.

Ceramic Restorations Polishing Technique

Polishing ceramic restorations after glazing will lower its surface energy. Polishing with finishing diamond points followed by diamond gels produces the smoothest surface.105,106  More recent research indicates that use of bonded, abrasive, diamond impregnated silicone and rubber points, cups, and discs followed by a extra-fine diamond polishing paste can improve the finish of porcelains to equal or surpass that of glazed porcelain.107,108

Conclusions

Polishing restorations to a very smooth surface produces favorable results in improving gingival health, reducing plaque adherence by decreasing surface roughness and surface energy, improving esthetics (luster), and probably increasing longevity. Each material requires a specific polishing technique suitable for its particle size, distribution, and hardness. Polishing while lubricating with water or air to decrease temperature using intermittent pressure adjusted for the specific material will produce the smoothest surface with the least temperature rise that could be injurious to the restorative material and/or the tooth.




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*Dr. Larson is Associate Professor, Department of Restorative Sciences, Division of Operative Dentistry, University of Minnesota School of Dentistry, Minneapolis, Minnesota. Email is
larso004@umn.edu.