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Category Archives: Laboratory 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

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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.