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Category Archives: Dental Materials

Nacera Ant Stained

Staining Zirconia and its Impact on Strength

Photo: Nacera Zirconia (Nacera.US), ceraMotion® (Dentaurum), by Carlo Paoletti, Odt

 

For dental purposes, the consistency of zirconia and its strength have made it a reliable and highly popular material for fixed prosthetics. Fortunately, we’re able to mask the high value nature of sintered zirconia with coloring liquids to make it more esthetic. However, some have begun to ask if the penetrating stains will weaken the structure of zirconia, and if so, how much and in which ways.

Background

It has been reported that stains for ceramics rely on a multitude of metal ions for colorations. Theil and Stephan explained the following color sources in a patent application: Iron (Fe) for brown, erbium (Er) for light violet, neodynium (Nd) for light pink, cerium (Ce) for cream or orange, terbium (Tb) only for light orange, manganese (Mn) for black, and praseodymium (Pr) for dark yellow. Unfortunately, coloring ions have the potential to become impurities in zirconia that can adversely affect its properties. Some studies have reported a decrease in flexural strength, while others have reported no effects. The different results on strength, however, might be due not on whether or not stain has been applied, but instead, its concentration, as reported by Sutter et al. An alternative type of esthetic stain, reported by Holand et al in 2012, avoids metal ions by adding metal oxides to zirconia powder prior to its pressing. Unfortunately, oxides have proven to be sometimes unreliable when used for darker shades, and can lead to surface pitting.

The Research

Testing conducted at Kagoshima and Aichi Gajuin Universities provided mixed answers to how coloring affects zirconia strength. Researchers at the Universities tested four zirconia brands: “P-NANOZR (ceria-stabilized TZP and alumina nanocomposite (30 vol.% alumina)) [20]. Cercon, ZENOSTAR, and Zirkonzahn Prettau are Y-TZPs.” Test samples were made by dipping the zirconia into “six kinds of coloring liquid for 30 min at room temperature and dried in air. The immersed plates were sintered for 2 h at 1350 ºC for Cercon, at 1450 ºC for P-NANOZR and ZENOSTAR, and at 1600 ºC for Zirkonzahn Prettau…Three-point flexural strengths were determined at a span length of 16 mm. Fracture toughness was determined by an indentation method in terms of Palmqvist cracks.”

Results

Test results showed flexural strength and fracture toughness were largely unaffected by the test stains, the only exception being stains with Er ions. There were no unusual differences in x-ray diffraction patterns “with and without coloring except those for Er,” which was associated with a phase change to cubic zirconia, while the other samples remained as tetragonal zirconia. For example, a SEM of ZENOSTAR stained with Er showed a shift to cubic on its surface. Also noticed was that “the concentrations of Er and Nd increased in the large grains and that of Zr decreased.” The investigators concluded that the surface structure of these particular samples were likely cubic, which does not undergo phase toughening in the presence of micro fractures.

The content of Fe2O3 and CoO ions in the sintered zirconia after firing was too small to be detected by x-ray diffraction. Only a small amount of these ions was required for the coloration of zirconia. According to analytical results of the coloring liquids’ baking powders, the three kinds of ivory liquid mainly consist of Fe ions and a small number of Cr ions, the two violet liquids mainly consist of Co ions and small amounts of Ca, Y, Mn, and P.

Discussion

The final results showed through X-ray diffraction that a phase shift to cubic structures was associated with stains containing Er. “The formation of the cubic phase resulted in a reduction of the flexural strength and fracture toughness. It is well known that cubic zirconia is weaker than tetragonal zirconia, because, as shown by Sutter et al, the stable cubic zirconia is impossible to be strengthened by the stress- induced transformation.”

According the scholars from Japan, “It is known that both Er and Nd act as stabilizers for cubic zirconia, creating a large strain in the crystal lattice due to their substitutions because the ionic radii,” facts substantiated by Katamura and his team in 1995. The findings indicate that stains “containing Er and/or Nd should be avoided. Furthermore, coloring with Fe and Co showed no remarkable property changes, indicating little reaction with zirconia and the formation of each oxide at grain boundaries. In other words, coloration with Fe and Co ions does not appear to affect the crystalline phase or mechanical properties of the final product, as previously found by Denry and Kelly.”

We question the results found in dental laboratories. What if zirconia is exposed to these coloring liquids for far less than 30 minutes? Regardless, there have been more reports of fractured cubic zirconia in the mouth than tetragonal zirconia. However, it is our believe that this problem is more related to mismanagement of the material than the type of coloring liquids.

Molar 3-10-16 W-O B

Facts about Zirconia Degradation and Strength

Full contour zirconia, like the above, is increasingly replacing lithium disilicate as the restoration of choice, even in anteriors. However, according to many dental lab owners, zirconia preparation guidelines provided by manufacturers lost during clinical procedures can place even zirconia at risk of failure. How important is meeting reduction criteria and margin design? In some ways a lot, while in other ways, not as much. Even researchers don’t always agree.

Most often, margins with a chamfer or a rounded shoulder are suggested, with occlusal reduction of 1.5 mm and axial reduction of at least 1.0 mm. But what are the tolerances when we don’t meet these guidelines?

A study by Beuer et al on zirconia axial thickness of 0.4 mm, showed preparation design differences led to significant variations in fracture strength. Vult von Steyern reported that load to fracture for zirconia three unit bridges was much higher with shoulder margins than when a deep chamfer was used. According to some, the data is less clear for single unit monolithic zirconia crowns.

According to Kobayashi et al, “A limiting factor could be the aging of Y-TZP, due to its potential sensitivity to Low Temperature Degradation (LTD).”  When Y-TZP is subject to low temperature degradation (LTD) in the presence of water it undergoes a phase change from tetragonal to a weaker monoclinic structure. This can decrease strength and alter the surface.

A study was conducted to determine the combined effects of margin design and LTD. Three designs were tested with .8 mm occlusal clearance: a shoulderless margin for control was stored dry, a .4 mm chamfer underwent 5,000 TC and mechanically loaded (1,200,000 at 50N), and a .8 chamfer that experienced LTD from 3 hours of autoclave to simulate 10 years at body temperature.  Full zirconia crowns with identical contours and uniform 50 µm cement layer thickness were designed for each preparation.

All crowns were subjected to airborne-particle abrasion with 50 µm aluminum oxide using 0.4 MPa pressure, cleaned with steam and 70% alcohol, then cemented to metal dies with RMGI (KetacCem, 3M ESPE) under a static load of 50 N for 10 minutes.

Preparation form Control group Thermocycling 5-55, 5000 cycles, 60 s per cycle Chewing simulation Autoclave 137, 2 bar for 3 h Chewing simulation
Shoulderless 5712 (758) 5487 (310) 4799 (500)
Slight chamfer 4703 (787) 4613 (626) 4527 (596)
Chamfer 5090 (741) 5138 (328) 3414 (457)

For the above: Chart Link 

The results showed margin design and LTD affect flexural strength. However, although some research has indicated that .5 mm occlusal thickness is adequate, the amount of tooth reduction was substantially less than manufacturer recommendations of 1 mm axially and 1.5 mm occlusally. Minimal reduction can have a negative effect.

In the above study, the shoulderless margin showed highest fracture loads. However, this margin design has been reported to be detrimental to gingival health, since feather style margins always terminate at a point of zero reduction, making them over-contoured. It is advisable, based on the study, to use a small chamfer instead of a feather edge margin.

Not all zirconia is the same. Some have impurities, internal voids, or inconsistent compaction. The affects of LTD will depend on which full contour zirconia is used, prep design, and material management in the lab and chair side. LTD can cause functional wear, and/or lead to micro fractures that may or may not close. This can be especially problematic for weakened zirconia crowns placed in high occlusal stress areas that are made too thin or drilled/adjusted after sintering.

The best thing we can do for patients when using zirconia, is to make sure we follow manufacturer guidelines, refrain from using lower strength “anterior” products in posterior regions, and use smooth diamonds with copious water when adjusting zirconia surfaces.

cemented-vs-screw-retained-crowns

Limitations to Hybridized Implant Restorations

Dentists restoring implants always want successful outcomes. One area of constant concern is the potential for peri implantitis that occasionally leads to implant failure. To help avoid this, many dentists have migrated to screw retained crowns and bridges, eliminating possible complications from cement. However, screw retained crowns also present risks for future complications. The answer might be a blending of the two.

Implant related cement sepsis is a known cause for peri implantitis. However, Korsch found, in 2014, this to be more cement type related that previously thought.  Another complication for cemented abutment crowns is abutment screw loosening. Screw retained implant crowns eliminate complications from cement related risks. However, a problematic lightly cemented implant crown can be removed and repaired or temporarily replaced with an easily fashioned temporary crown holding its position. A problematic screw retained crown is far more difficult and expensive to repair or replace, and its space more complex and time consuming to temporize. This has led some to rely on hybridized screw retained crowns that are cemented and cleaned extra orally with a prefabricated, lab-placed screw access hole.

It is important to understand materials’ strengths and weaknesses before deciding upon a new application, such as a hybrid screw retained implant restoration. In the past, implant crowns have been primarily made from porcelain fused to metal (PFMs). In recent years, there has been a move away from PFMs to cleaner and more esthetic all-ceramic crowns made from lithium disilcate or zirconia. Some dentists have shown a preference for lithium disilicate in esthetically critical cases. However, little is known about the long-term performance of this material as an implant crown with a screw access hole.

Research by Biskri in 2013, noted the brittleness, low elasticity, and unidirectional crystals of lithium disilicate. But the material has also been widely reported to be more fatigue resistant than feldspathic porcelain. Despite its benefits over traditional porcelain, research by Dhima in 2014 showed far more predictable strength when lithium disilicate is at least 1.5 mm thick, occlusally.

Lassle, in his 2015 master’s thesis, described testing the viability of hybridized lithium disilicate screw retained crowns affixed to Nobel conical, 5.5 stock abutments with a 1.5 mm collar. The crowns were digitally designed, mandibular first pre molars, with 2 mm of occlusal thickness and axial walls ranging from .5 mm gingivally, to 1.5 mm near the occlusal table. Occlusal access holes were created in #1 prior to glazing in the “blue” state, #2 after glazing, both using copious amounts of water for cooling. The control had no access hole. The crowns were silanated and cemented with RelyX™ Unicem (3M Espe), and allowed to set 24 hours prior to testing. A control group followed the same protocol, but without an access hole.

 

Results

implant loosen stats

 

It is clear from the results that placement of a screw access hole in lithium disilicate leads to a significant decrease in load bearing strength. According to Lassle’s findings, lithium disilicate would be contraindicated for this purpose.

Despite our potential for bias in selecting screw retained, cemented, or hybridized screw retained, some researchers believe there is inconclusive evidence of clinical significance between them, as reported by Sherif in 2014. Cement retained implant crowns are less expensive, seat passively, and are easier to work with. Screw retained implant crowns eliminate possible cement related complications, and offer retrievability after screw loosening. A deciding factor for the third option should be the material to be used.

The research conducted by Lassle was revealing. However, we should keep in mind that people don’t chew with a constantly increasing pressure of .1 mm per minute against a 3 mm steal ball. Yes, lithium disilicate is definitely weekend by a central fossa hole, as evidenced by the early fractures along the central groove. But that doesn’t mean they will always fail, clinically. However, if we are looking for greatest certainty when using hybridized screw retained implant crowns, zirconia would be a surer bet, according to testing by Hussien et al, in 2016, showing zirconia to be over 3 times stronger than lithium disilicate.

Nacera hi res logo

Is this the next Generation Zirconia: Results, Part 2

This may look like an ad, but it really isn’t. We are all simply very excited about a company that promised both strength and esthetics in zirconia, and delivered what they promised. So, read on, and if you agree, you’ll have a new tool to help your practice, your lab, and your patients. And one more thing. The company, Nacera US, is the only company helping private practitioners and dental labs become more competitive. To us, that is important.

A few months ago, a new zirconia from Germany came our way for testing called, Nacera. This is an update to an article we published last month on the experience of two lab members. We pointed out that Nacera claimed to be a higher grade of 1400 MPa zirconia purity with more translucency and improved esthetics. We determined after a few months of testing that the esthetics was better than any 1400 MPa full contour zirconia we had seen, with the potential to rival lithium disilicate, but stronger. In the photo below, you can see the unusual vitality that this full strength zirconia provides.

Roberto U Molar


There are two molars in this photo, one a finished full contour zirconia, the other, a newly sintered full contour zirconia. All samples in the photo are Nacera zirconia. The finished Nacera molar, provided by Roberto Rossi, MDT, was only lightly stained.

OPT-In lab members working with Nacera have commented on the improved margin accuracy and how true the shades are. That is not the case for all zirconia brands. Another aspect of Nacera zirconia is purity. Some brands have impurities that form pits on the surface during finishing, and also risk areas of weakness, internally. Other brands are naturally weaker and unpredictable because of faster and less thorough compression during compaction. We know better about Nacera.

To see a short video about Nacera manufacturing, click the below. When it ends, click it off or it will continue to other topics.

Nacera Video

This month, we visited the Nacera factory in Dortmund, Germany. The parent company, Doceram, is actually an industrial engineering company that routinely designs parts to meet sub micron tolerances. They then manufacture parts in zirconia  and test them to make certain those same tolerances have been met. This same precision approach is applied to dental zirconia, with constant testing and measuring. In fact, they test each zirconia batch, individually, and then measure each separate individual zirconia disc prior to packaging, printing the measurements on the label. Nothing is left to chance.

Visit Nacera US at Chicago Lab Day, Booth L-22

For information on Free Hands-on and Lectures, visit

Nacera US

The one word that kept surfacing during our visit was, Certainty. Dentists can now offer patients both strength and esthetics without paying more. However, if the need arises, a Certified Nacera Lab can also deliver the highest level esthetics when only veneered zirconia will meet a patient’s needs.

Ant Germano Rossi

Germano Rossi produced these outstanding results by veneering Nacera zirconia copings  with ceraMotion® (Dentaurum). CeraMotion® is used as a very thin, colored paste with a built-in glaze. When applied, it stays in place and finishes with only one bake. The thickness of the veneered surfaces are 1 mm or less.

Nacera US is a dental company. Dedicated to patients, dentists, and dental labs, Nacera US offers “best products” suitable for all dentists and dental labs who care about dentistry, regardless of the markets and patient budgets they serve.

We tested it, in fact, our lab members and their dentists are testing it daily. They, also, have come to the conclusion that What’s Inside Matters. That’s why OPT-In is proud to be a Nacera partner.

If you would like to try Nacera for your patients, contact us for a Certified Nacera Lab referral that fits your practice. Each was trained in November in how to consistently obtain the very most from Nacera. Having Certainty is always good. Having Nacera Inside takes Certainty to a higher level.

Almost forgot…the pink in the bottom photo, that too is Nacera!

Visit Nacera US for more information

Contact OPT-In for a Nacera Certified Lab

3 sample banner

OPT-In Tests a New Zirconia: The Results

A few months ago a new zirconia from Germany came our way for testing. It claimed to be a higher grade of 1400 MPa purity with more translucency and improved esthetics. Sound too good to be true? We thought so, however, after a few months of testing, we became believers.

Doceram Medical provided a couple of our labs with various samples of Nacera® zirconia, including Pearl 1 (white), and various value shades capable of reproducing 16 vita classic shades. After making the necessary adjustments to sintering and glazing times and temperatures, the results were in, all exceptionally positive. Nacera® zirconia was kinder to milling burs (lasted up to 15% longer), milled more precisely and smoothly, had less shrinkage variations (each lot/batch has its own shrinkage factor), and was more translucent than other high strength zirconia.

OPT-In member, Dennis King, CDT, has used the full line of Nacera® zirconia, including the multi shade pucks that come in A, B, C, and D shades. He has successfully provided several large full mouth cases of full contour Nacera® restorations that included anteriors, one case for a dentist’s wife. Dennis noticed that Nacera® mills far easier than e.max® and other zirconia, and that his burs have been lasting longer. He also stated that fit is consistently better due to more predictable shrinkage and smoother milling. With his successes fully documented, other OPT-In lab members have also commented positively on the same points.

Top photo of Nacera® Pearl 1.5 (A2), with  ceraMotion®, is courtesy of Master ceramist, Roberto Rossi, MDT, who has echoed Dennis’ comments. In a month of testing several different pucks and ceramMotion® (Dentaurum), Roberto noticed more translucency without sacrificing higher strength, better margins with less effort, and predictably better esthetics when compared to competing products. Of particular note, single-step stain & glaze ceraMotion®, distributed by Nacera US, not only improved esthetics, it saved time by requiring only a single stain & glaze bake.

Doceram Medical is establishing the Nacera® brand through a new US subsidiary, Nacera US. OPT-In members are excited about working with Nacera US through a special pricing program, exclusive for OPT-In members. Additionally, the Dental Lab Group, in Staten Island, will provide special Nacera®  outsource pricing [only] for those members without milling machines, as well as for custom milled titanium abutments.

Nacera US will be at booth L-22, Lab Day, Chicago, and provide continuous seminars in a private suite. Guests at the suite will enjoy food and beverages, and be able to test stain & glaze the new ceramMotion®  on samples of Nacera® zirconia. Roberto Rossi, and others, will be guest lecturers and guiding hands-on experiences. Nacera US has also promised important announcements in February, including the launching of their new website. For inquiries and more information about the entire line of Nacera® products, visit: Doceram Medical. For immediate questions, or to register for courses in Chicago, please call Nacera US at, 215-345-5283.

Zirconia, Cleaning Contaminates: What Works?

As most of us know, zirconia crowns and bridges are healthy, esthetic, and nearly indestructible. However, in certain conditions, long term cement retention can be unpredictable. This is especially true if the internal surfaces have been exposed to saliva. Unfortunately, there is not a lot of information about the best ways to eliminate contamination. Below, we try to bring more clarity to this topic.

Aladag, et al conducted research to determine the effect of saliva on the cementing surface of untreated zirconia. They reported that different cleaning methods, water spray, sodium hypochlorite, or Ivoclean® produced few improvements to bond strengths. The first point to note is that in this investigation, the tested zirconia surface was not air abraded.

Tunc’s team obtained different results from Aladag’s. Their research included Ivoclean, phosphoric acid, alcohol , water rinsing, steam cleaned, and air abrasion (after) saliva contamination. Their findings proved air abrasion to be best followed closely by Ivoclean. But that doesn’t answer the question about saliva contamination after zirconia has been air abraded.

Wille’s prosthodontic team from Germany tested the affect saliva has on zirconia surfaces that have been air abraded prior to contamination. Their testing included silicone disclosing agents, GC Fit Checker white or GC Fit Checker II for checking the fit of zirconia copings. Uncontaminated zirconia surfaces used as controls were compared to contaminated copings cleaned with “…water spray or ultrasonically in 99% isopropanol or using a newly developed cleaning paste (Ivoclean® from Ivoclar) .” Their findings showed that using a secondary application of isopropanol increased reduction of carbon residuals on the zirconia surfaces, more so than did an additional application of Ivoclean. However, none of the cleaning agents or additional applications removed all the silicone contaminants. This could be due to air abrasion increasing retention of the contaminants.

Feitosa’s group also looked at saliva contamination, but included the effects of aging on resin bond strengths. Their investigation included “one hundred and eighty zirconia specimens sandblasted with 50 μm aluminum oxide particles, immersed in saliva for one minute (with the exception of the control group, [C]), and divided into groups according to the cleansing method, as follows: water rinse (W); 37% phosphoric acid gel (PA); cleaning paste (ie, Ivoclean®) containing mainly zirconium oxide (IC); and 70% isopropanol (AL).” Resin SBS was evaluated “…after 24 hours, 5000 thermal cycles (TC), or 150 days of water storage.” The results”…showed that PA < AL and W < IC and C. SBS ranged from 10.4 to 21.9 MPa (24 hours), from 6.4 to 14.8 MPa (TC), and from 2.9 to 7.0 MPa (150 days). Failure analysis revealed a greater percentage of mixed failures for the majority of the specimens and a smaller percentage of adhesive failures at the ceramic-resin cement interface,” and that Ivoclean® was able to sufficiently clean saliva contaminated zirconia surfaces to maintain acceptable long term bond strengths.

Kim, et al’s research, in 2015, also examined saliva contamination of air abraded zirconia surfaces. Their testing included cleaning with “…water-spray rinsing (WS), additional air abrasion (AA), and cleaning with four solutions (Ivoclean® [IC]; 1.0 wt% sodium dodecyl sulfate [SDS], 1.0 wt% hydrogen peroxide [HP], and 1.0 wt% sodium hypochlorite [SHC].” A sample with no contamination (NC) was used as the control. Each zirconia sample was bonded to resin with Panavia F 2.0 prior to aging with 5000 thermocycles. Their results showed “…groups NC, AA, IC, and SHC had hydrophilic surfaces. Groups IC and SHC showed statistically similar bond strengths to groups NC and AA (P>.05), but not groups SDS and HP (P<.05). For groups WS, SDS, and HP, blister-like bubble formations were observed on the surfaces under SEM. Test results reiterated the cleaning effectiveness of Ivoclean®, and also supported the use of sodium 1.0 wt% hypochlorite.

Assessing Implant Torque Device Reliability

Does Steam Sterilization affect Accuracy of Spring- style Implant Torque Devices?

When accounting for complications over a 5 year span, Mashid et al reported that “screw loosening has been stated as the most common complication in implant” dentistry. One leading cause could be torque driver reliability.

Torque drivers include “toggle-type or friction-style,” and “beam-type or spring-style.” Many believe spring-style offer more accuracy even though accuracy becomes suspect after steam sterilization. To test the effects of steam sterilization, Mashid’s team tested 5 each from Nobel Biocare, Straumann [ITI, photo at top], and Biomet 3i [3i]). The “peak torque” of each was measured before and after steam sterilization.

Materials and Methods

All devices were tested ten times at 35 Ncm prior to steam sterilization. Sterilization of each was managed according to manufacturer directions: “Nobel Biocare devices should be dismantled for disinfection, cleaning and drying and then the parts should be assembled before sterilization. In the ITI group, dismantling of devices is proposed. Each component should be disinfected, cleaned, and dried, but sterilized separately. For 3i samples, dismantling of the device is not proposed and disinfection of the outer surface is the only protocol to be considered.” The recommended protocols and sterilizations were repeated 100 times at 134°C for 18 min, and then measured 10 times for torque accuracy.

Results

Before steam sterilization, all the tested devices stayed within 10% of their targeted torque values. After 100 sterilization cycles, there were no significant difference in the Nobel Biocare and ITI devices. There was, however, an increase of error values in the 3i group, which showed more than a 10% difference with a maximum difference of 14% in 17% of torque measurements. The authors also reported that 3i torque devices had developed “corrosion of the spring in the handle” that may have contributed to its inaccuracy.

Different research by Santos et al, tested Biomet 3i, Nobel Biocare, Straumann, and Conexao at 20 Ncm and reported 62.5% were within 10% of the tested value. However, when tested at a 32 Ncm target, “only 33.3% of all values from each manufacturer were considered accurate,” with ITI being the most consistent in accuracy for both values. During the Santos testing, each torque device had been in clinical use for less than 2 years. With the amount of use and “sterilization protocols” unknown, their findings might better reflect the real world of clinical dentistry than would a highly controlled, laboratory testing protocol. The reports reinforce the need for manufacturer recalibration with frequency matched to use.

implant screws

Preventing Implant Screw Loosening

Screw Loosening, a Frequent Problem?

Implant screw loosening is a major concern for both dentists and dental technicians. Unfortunately, the literature indicates it is a frequent problem, with loosening rates as high as 12.5%. Today, there are new ways to decrease its frequency.

Forces keeping screws tight include the “friction between the threads, between the head of the screw and the abutment, and between the implant and the abutment. The force that clamps two screw-tightened components together is called the preload and it depends on the composition of the materials, the texture of their surface and their degree of lubrication.”

Some screws have special surface treatments that “reduce the friction coefficient” and increase the preload to keep screws tighter, longer. One example is TorqTite® (Nobel Biocare Holding AG, Balsberg, Kloten, Switzerland), which uses a “diamond-like, carbon” lubricant.

Using the two types of screws described above, Saliba et al, tested the amount of torque required for screw removal that would simulate how and why an entire implant system would loosen, clinically.

Materials and Methods

Testing was performed on 20 Neodent titanium implants (Osteointegráveis, Curitiba, Paraná, Brazil), with 4.1 platforms, similar to the original Branemark design. Their hexagon bases were removed so abutments could be “rotated on the implant platform during the loosening of the fixation screw.”

Ten relied on an abutment held to the implant with non treated M2.0 titanium screws (Neodent). A different set of ten screws were covered with TorqTite® (Nobel Biocare). “The abutment was attached with a screw, first using a hexagonal 1.2 mm digital wrench (Neodent), followed by tightening with a prosthetic ratchet torque wrench (Neodent) to a torque of 32 Ncm.” Then, the screws were unthreaded to record the highest torque values “required to completely loosen the abutment.”

Results

Titanium screws covered with solid lubricants performed better than plain titanium screws in maintaining the prosthetic implant-abutment junction. The results showed that their unscrewing torque value was higher than the torque applied during seating.

Implant screw chart

Which Screw to Use

Which type of screw, titanium or gold, is another variable worth exploring. In trying to better address loosening, Farina, et al compared gold and titanium screws. Testing for loosening values after simulated mastication included 20 dentures with eight different groups representing passive and vertical misfits with gold or titanium screws. Their results were based on “(1) 6 months of use, torque loosening, re tightening, another 6 months of use, and loosening torque; and (2) 1 year of use followed by loosening torque.”

Their research findings concluded, “After 6 months and another 6 months of clinical use simulation, titanium screws showed higher loosening torque values than did gold screws for the same fit level (P <.05). After 1 year of clinical use simulation, titanium and gold screws in passively fit dentures showed higher loosening torque values than they did in misfit dentures (P <.05). The titanium screws presented a decrease in the loosening torque after 1 year in misfit dentures.”

Above chart and image at top of page:

Biomechanical considerations for the screw of implant prosthesis: A literature review; J Korean Acad Prosthodont. 2010 Jan;48(1):61-68. Korean; Authors, So-Min Im, et al

Zr polishing charts

Zirconia, Eliminating Abrasiveness

Zirconia: Polishing and aging, does it destroy enamel?

We know zirconia crowns and bridges are nearly indestructible. But, what is the best way to polish this very hard material so it won’t be highly destructive?

As it turns out, a highly polished zirconia surface, over time, is less abrasive than emax, empress, and feldspathic porcelain. According to research by Grumser, et al, the other materials degrade over time and cause enamel wear, as well as wear to their own surfaces: “Polished zirconia surfaces showed lowest wear for material and antagonist. Wear mechanism of common ceramics was characterized by abrasive wear.”

Burgess also looked into aging of zirconia. In his paper, Enamel Wear Opposing Polished and Aged Zirconia, he explained using artificial aging of zirconia and simulated mastication to determine age related breakdown. They reported: “All zirconia specimens showed less material and opposing enamel wear than the enamel to enamel control or veneering porcelain specimens. The micrographs of the veneering ceramic showed sharp fractured edges and fragments of wear debris. Zirconia may be considered a wear-friendly material for restorations opposing enamel, even after simulated aging.”

Hmaidouch’s research comparing polished zirconia to polished porcelain was the most definitive when considering the method of polishing, as seen in the charts, above:

Methodology

“Polishing of the specimens: the ground surfaces were polished with a handpiece for 15 s and with 2 N pressure. Polishing was performed using an NTI polishing kit (HP 802104; P341, P3401, P34001): coarse at 15 000 r·min−1, medium at 10 000 r·min−1 and fine at 5 000 r·min−1

No statistically significant differences were found between the roughness of coarse-polished specimens and roughness of fine-ground specimens for both non-veneered (P=0.54) and veneered zirconia (P=0.99), but through medium and fine polishing, the roughness was significantly reduced in both groups.

The ground surfaces of FZ using the coarse red diamond instrument showed grooves that were reduced after using the following two grinding instruments (medium and fine) and after using the coarse polisher.

Porcelain surfaces ground with coarse diamond burs show ridges and grooves; moreover, many voids appear due to incomplete condensation. After polishing, the condensation defects remain but have been smoothed and slightly rounded; the voids from the porosities appear shallower. Traces of ridges and grooves did not disappear after polishing was completed; however, the polished side contained pitted areas with numerous surface irregularities.

Lower roughness of non-veneered zirconia specimens than that of veneered zirconia (VFZ) specimens was observed after each treatment procedure. This difference can be explained by the different compositions. SEM investigation of the treated surfaces showed that defects on the veneered surfaces caused by grinding were deeper than those on the zirconia surfaces, which led to the higher roughness values.

The roughness of the VFZ surfaces was not significantly reduced after either fine grinding or coarse polishing, which can be explained by the deep defects (grooves) caused by coarse grinding. These defects could not be completely flattened or removed, thereby explaining the higher roughness of VFZ specimens compared with the fine-ground and coarse-polished FZ specimens, which did not acquire deep defects after coarse grinding because of their higher strength.”

The key to long-term, safe use of FCZ is, a highly polished surface. This can be obtained with a course polishing wheel. In this case, the NTI polishing kit was used successfully.

Zr chart abrasive

Zirconia Abrasiveness: Is it or Isn’t it?

Full contour zirconia (FCZ) is very strong, very impervious, and, very misunderstood when it comes to abrasiveness and cementation. This article discusses the facts about abrasiveness.

The short answer about FCZ abrasiveness is, it depends. Research published by Hmaidouch, et al, International Journal of Oral Science (2014) 6, 241–246 shows that a rough zirconia surface is highly abrasive, a glazed rough surface is highly abrasive, but a highly polished zirconia surface is less abrasive than other ceramics. In fact, research by Tambra, et al, In vitro wear of human enamel opposing YTZP zirconia has shown highly polished zirconia can be less abrasive to enamel than Type IV gold.

Many dentists believe that veneering zirconia copings with feldspathic porcelain is kinder to opposing tooth structures. However, both the above authors have shown that not to be the case. Feldspathic porcelain is known to chip and etch in the mouth, making it the worst material with respect to opposing [enamel] wear. However, in the real world, the extent to which feldpathic porcelain degrades and abrades is more complicated. For example, in the presence of Coca Cola, three different feldspathic porcelains had three different results, increasing enamel wear from 12-74%, depending on the material type, The effect of clinical polishing protocols on ceramic surface texture and wear rate of opposing enamel: a laboratory study. In his, 2013, doctoral thesis, Zaninovich, explained that surface texture and roughness are more important than hardness when considering abrading enamel, and that in acid environments, both enamel and porcelain can degrade to more abrasive surfaces. However, the structure of the porcelain, occlusion quality, and occlusal habits, all contribute to determining the extent of degradation.

A shiny and smooth FCZ doesn’t mean a non abrasive surface will endure a short lived glaze. However, creating a highly polished surface is both time consuming and expensive. For those reasons, clinicians should check with their dental labs to learn if their FCZ needs further polishing.

The below chart, presented by Hmaidouch, illustrates the importance of knowing the differences when managing FCZ.

  Ground Polished
Group Ceramic Glazed Coarse Medium Fine Coarse Medium Fine
1 Y-TZP only glazed 4.3±1.9 12.6±4.4 5.5±2.1 3.8±1.0 3.4±0.8 2.9±0.6 2.4±0.5
2 Y-TZP/VM9 veneer 7.2±2.7 24.1±4.5 15.0±2.4 13.5±2.6 12.2±2.5 10.2±2.0 8.1±2.0

 

We can see that glazed, alone, is not nearly as smooth as finely polished. We can also see the difference between zirconia veneered with [only one] feldspathic porcelain and FCZ. Less clear is, the effect that different polishing methods and surfaces will have on surface texture.

From a different perspective, Stawarczyk, et al, measured the enamel loss of three types of surface-treated zirconia and a base alloy, using a chewing simulator. They reported, top of page, that the polished zirconia showed a lower wear rate on enamel antagonists, as well as, within the material itself.

The short answer to the question about FCZ abrasiveness is summarized by Miyazaki as: “A smooth surface of zirconia can be obtained with adequate polishing, because the microstructure of zirconia is fine and homogeneous. Highly polished zirconia shows the least wear of antagonist among various dental materials…Therefore, surface finishing and polishing procedure of zirconia full-contoured restorations was critical for obtaining clinical success.” Current status of zirconia restoration.