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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.

IOS image

Intra Oral Scanners, How Good are They

Traditional impression materials have served dentists and their patients quite well for decades. Does it make sense to replace the goop with microchips? Is the new technology better? Will the added expense be more cost effective in managing the continued pressure on dental fees?

Manufacturers and distributors all believe intraoral scanners should be in every practice. Of course, that’s their job. But dentists who have adopted the technology are universally pleased, reporting better accuracy, faster seat times, and in some cases, lower lab fees. But, let’s be clear. Those are subjective opinions. Below are some of the facts about intra oral scanners and their advantages.

Accuracy

One way to measure the advantages is accuracy. Intraoral scanning is consistently better when it comes to accuracy than are impression materials. That is not because PVS and poly ether aren’t good materials. They are. It is because intraoral scans are more consistently accurate in quadrants by eliminating the variables inherent to traditional impressions. Using strict research protocol, good impression materials provide errors of about 35 microns. In contrast, Pradies et al, reported scan results in 2015, showing “an average error of 18 – 30 μm for a single tooth and less than 40 to 60 μm error measured over the restoration and neighboring teeth and pontic areas, up to 7 units.” They also found that for a 4 unit bridge, the length error was less than 100 microns. However, accuracy declines when scanning a full arch. Testing showed that mean error for a full arch was 100 – 140 μm, indicating a good measure of unreliability.

There are many intraoral scanners to choose from. Deciding on which one might be as simple as feel, newly updated advances, ease of use, or ancillary costs. However, based on research, accuracy shouldn’t be a differential in most cases. Researchers at the University of Ohio tested three intra-oral scanners, 3M LAVA True Definition, 3Shape Trios and Cadent iTero, found similar accuracy, and showed that “digital intra-oral scanner impressions can be used for fabricating accurate short-span screw retained implant supported fixed dental prosthesis with a misfit range of 12.40 to 90.20 m.”

Implant Applications

Dental implant restorative is a growth area for dentistry minimally impacted by dental insurance. Intraoral scanners have become increasingly useful for implant diagnosis as planning becomes more digitized. In Lee et al’s report from 2015, 36 patients had a single missing molar replaced with an implant. “Of the 36 patients, 6 required contact adjustments, 7 required occlusal adjustments, and 3 required a gingivectomy around the implant to completely seat the restoration. Chair time for adjustments did not exceed 15 minutes.” Clearly, IOS and accompanying implant related technology [in the lab setting] provide better accuracy than building cases on stone models, plaster mountings, and resin-based adjuncts.

Tracking and Managing Occlusion

Another, but rather unexplored advantage of intraoral scanning technology is tracking and managing patient occlusion. In 2014, Meireles et al, looked into occlusal wear tracking: “Eight extracted teeth were etched with acid for different times to produce wear and scanned with an intra-oral optical scanner. Computer vision algorithms were used for alignment and comparison among models…Results demonstrated that it is possible to directly detect sub millimeter differences in teeth surfaces with an automated method with results similar to those obtained by direct visual inspection.”

The ROI

One thing to keep in mind is, chair time is expensive. If IOS can cut chair time and the cost of traditional impression materials, the investment might be worthwhile. However, there is a balance to consider because not all situations easily lend themselves to the current state of the art. Some clinicians have acquired lasers to help control the challenges gingival tissue presents.

As with all things, when considering an intra oral scanner, we need to avoid biases, do our homework, and keep an open mind.

ceramir kit

A new Cement Perfect for Zirconia and More

Ceramir Crown & Bridge (Doxa) is a new, unique category of permanent cement that might be the best suited cement available for zirconia. Additionally, it has been shown to chemically form hydroxy apatite on the surface of tooth structure, integrating the cement to the tooth, photo below, hence the description, “bioactive.”

ceramir dentin integration

What is Ceramir C&B?

Ceramir C&B is a hybrid composition of Calcium Aluminate and glass ionomer, that when combined with its liquid, undergoes an acid-base reaction similar to hydraulic cements.

The incorporation of the Calcium Aluminate gives it several unique properties that separate it from conventional GIC’s. After 3-4 hours of setting time, Ceramir C&B changes its pH from a very mild acid to a base of pH of ~ 8.5. Mild, lasting alkalinity allows continuous formation of apatite when adjacent to phosphate containing solutions of dentin. Additionally, the alkalinity of its dense matrix fixes the glass ionomer structure to help prevent the ionomer glass from continuously leaking over time. Below, we see hardened hydroxy apatite in the surface of the cement.

ceramir 50 micron cbs

Alkalinity also helps with pulpal compatibility. Histological data shows that even at a minimum distance from the pulp, there is virtually no irritation resulting in inflammation.

Ceramir C&B relies on the mode of hydration by its key component, Calcium Aluminate, and the ionic bond of the GIC to bond to the tooth. That means, etching enamel or dentin and using a bonding agent isn’t needed. Moreover, its hydrophilic nature makes the material insensitive to oral fluids, while its alkalinity helps prevent bacterial growth and sensitivity.

According to Johanna Engstrand, et al, Ceramir C&B’s properties are unique. “Zinc phosphates are too acidic and do not contribute with Ca ions. Resin-based materials are not alkaline, and do not show extended ion leakage. Glass ionomers have an ion leakage but are acidic and cannot induce HA formation on its surface.”

Published by the American Ceramic Society, 2010

Author, Hermansonn, et al

“Ceramir C&B has a 2 minute working time, and a setting time of 4-5 minutes. Its film thickness is 15 microns. Its compressive strength at 24 hours is 160 MPa, similar to many resin-based cements, such as Rely x Unicem (157 MPa) but far greater than Rely x Luting’s (96 MPa). However, after one month, the bioactive nature of Ceramir C&B boosts its compressive strength to 200 MPa.”

Unlike RMGIs that expand as much as 4% and then contract, and resin cements that shrink as much as 4% then expand. Ceramir net change is essentially zero: “Expansion of Ceramir C&B is at most, 0.4%, and is due to the free growth of hydrated crystals associated with the formation of apatite. However, bulk expansion, measured as expansion pressure was recorded to be zero.”

Bacteria Resistant and virtually no Microleakage

Ceramir C&B is the only cement with the necessary components to form HA. Zinc phosphates are too acidic and have no free Ca ions. Resins aren’t alkaline and don’t provide ion leakage to the extent required to form HA. RMGI ion leakage is acidic and therefore, can’t initiate HA formation on dentin.

One advantage of Ceramir alkalinity is its antibacterial properties. Secondary caries caused by a wide variety of bacteria is a major concern for all. Jiang, in 2011, conducted a resazurin test on Rely x™ Unicem, Ketac™ Cem Aplicap™ (RMGI), Harvard zinc phosphate, and Ceramir® C&B cements for their antibacterial (S. mutans) properties. After different time periods, up to 10 days, Ceramir C&B with its calcium aluminate showed the strongest antibacterial properties, while the RMGI showed none. The other cements have only slight antibacterial properties. The strong showing of Ceramir is due to its initial pH 5.4 ending at pH 8.5.

Another important attribute of Ceramir C&B cement is a lack of microleakage. Because the process of forming HA creates a nearly insoluble barrier intimately integrated with dentin, Ceramir cement was reported in 2015 by Jefferies, et al, to have only minimal microleakage, and far less than the tested GI and RMGI: “There was no evidence of marginal gap occlusion for the three conventional control cements, whereas both bioactive, surface apatite-forming cements demonstrated occlusion of the artificial marginal gaps.”

Ceramir margins alone

The photos, left, described by Jefferies:

“Artificial margin gaps at 8 months of incubation in phosphate buffered saline: upper left photomicrograph is Fuji I (glass ionomer material), ∼110 μm gap; upper right is Rely X Luting Cement (resin-modified glass ionomer), ∼80–100 μm gap; lower left ProRoot MTA (calcium silicate/Portland cement-like hydraulic cement), ∼100 μm gap; and lower right Ceramir Crown & Bridge (calcium aluminate/glass ionomer cement), ∼300 μm gap. Cement material is positioned above the gap space, whereas dentin segment is below the artificial gap.”

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.

etched porcelain

All-ceramics and Self-adhesive Cements: Is it a Good Idea?

In spite of the inherent brittleness and limited flexural strength of silica-based ceramics, final adhesive cementation with composite resin increases the fracture resistance of the ceramic restoration and the abutment tooth.

Bonding to silicate-based ceramics is usually obtained by two simultaneous mechanisms: 1) micromechanical retention provided by acid-etching of the ceramic surface, and 2) chemical coupling by the application of a silane coupling agent.

During acid etching (above photo, left), the hydrofluoric (HF) acid reacts with the glassy matrix that contains silica, dissolving the surface to the depth of a few microns. This glassy matrix is selectively removed and the crystalline structure is exposed. The silane coupling agent presents bifunctional characteristics, promoting a chemical interaction between the silica in the glass phase of ceramics and the methacrylate groups of the resin through siloxane bonds.

It has been demonstrated that silane primers may confer a resistance to the degradation of the ceramic-resin bond exposed to moisture and intraoral thermal changes. Etching and silanization increase the surface energy and the wettability of the ceramic substrate.

RelyX™ Unicem can achieve high or comparable bond strength to other investigated cements without any pretreatment steps such as etching, priming or bonding. However, other studies observed higher shear bond strength values after etching with HF acid and silanized. In a study by Kumbuloglu et al. RelyX™ Unicem showed lower shear bond strengths than the other resin cements investigated when no pretreatment of the ceramic surface was performed. In the study by Reich et al., only the RelyX™ Unicem, in contrast to Variolink (Ivoclar, Vivadent) and Calibra (DeTrey Dentsply, Konstanz, Germany), was able to survive the whole thermocycling procedure in the case of no pretreatment. This indicates that besides mechanical interlocking, additional bonding mechanisms with RelyX™ Unicem to the ceramic surface are possible.

The specific phosphoric acid methacrylates have the ability to provide physical interactions with the ceramic surface and are able to provide strong hydrogen bonding with hydroxyl groups present on the ceramic surface.  An increase in the bond strength after pretreatment with hydrofluoric acid and silane was also observed. This is in agreement with the study by Piwowarczyk  who reported that, in comparison with 10 cements from different classes, only RelyX™ Unicem exhibited high shear bond strength after 14 days of water storage followed by thermal cycling. In the same study, it was reported that the light polymerization of the self-adhesive resin cements enhances shear-bond strength when compared to auto polymerization.

The above doesn’t take into account, self-adhesive bonding to dentin and enamel. That is a related, but a separate consideration.

 

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.