Wear of Experimental Dental Materials: You Can’t Win Them All

Dental materials scientists are on a constant quest to make improvements to restoratives that increase their durability and clinical performance. Sometimes the improvements work out, sometimes they don’t. In either case, reliable testing methods are essential, such as the OHSU Oral Wear Simulator by Proto-tech Research of Portland, Oregon.

In 2013 Xie et al premiered “A High-Strength Cement System for Improved Dental Restoratives” in the Journal of Biomedical Science and Engineering, where improvements to the mechanical properties of glass ionomer cement were achieved though incorporation of a four-arm star polymer synthesized through the ATRP technique. Three experimental GIC formulations having star-shaped, hyperbranched, or star-hyperbranched polymers demonstrated significant improvements over Fuji IILC in compressive strength, compressive modulus, diametral tensile strength, flexural strength, fracture toughness and Knoop hardness. The cements were also subject to 70,000 cycles of wear in the Oral Wear Simulator, which applies 20N of load (here through a ceramic indentor) during its abrasive sliding phase, followed by a 90N pulse to produce an attrition wear region. The differences observed were dramatic. While the abrasive and attrition wear values correlated surprising well with the mechanical property results, a 50% improvement in compressive strength, for example, resulted in a 90% reduction in abrasion and attrition wear. Attrition was significantly higher than abrasion in all of these brittle materials, and the correlation between abrasive wear and flexural strength was a surprising r = 0.99. The only poor correlation to abrasive or attrition wear was by fracture toughness, r = 0.75, according to the reader’s calculation. So while the mechanical properties bode well for this experimental class of materials, the wear values generated by the Proto-tech machine declare it to be a home run. Significant difference between the materials were found due to the generally low deviations in the wear results.

A few years later Fareed and Stamboulis tested “Nanoclay-Reinforced Glass-Ionomer Cements…” (Dentistry Journal, 2017) in two wear simulators, hoping to find an improvement to cements by the addition of 1, 2, or 4 wt% of montmorillonite clay, as it had been found to make “remarkable” improvements to cement in a single, limited study. In this study, Vicker’s hardness measurements of the materials found no significant effect of the nanoclay additive at any level. Nonetheless, the researchers seemed disappointed that only limited improvement in wear depth was found in a reciprocating wear machine and only the material with 1% wt of nanoclay. The other simulator, the OHSU Oral Wear Simulator, produced results that were even more disappointing to the researchers. Significant decreases in wear resistance over the control material were observed at every level of nanoclay addition in both abrasion and attrition, thanks in part to deviations in the low (10%) range. While no correlation between wear and hardness was observed here, the hoped-for improvement in wear resistance was not manifested. This could have been not a surprise, as their associates Dowling and Fleming had sought improved wear resistance by the addition of 0.5 – 2.5 wt% native and organically-modified nanoclay to a commercial GI cement (2007). Neither additive evinced reduced wear resistance, and only the organically-modified nanoclay succeeded in not increasing the attrition wear. So tests performed using the oral wear simulator ten years apart produced the same trends: an inability to strengthen GI cement through nanoclay additives. However these authors decided to place the blame on the testing method employed, claiming that the OHSU system had demonstrated a 56% variation in abrasive wear depth and a dismal 78% variation in attrition wear in a 2006 study by Heintze. However, this Heintze study employed a simulator by Willytec, which has high deviations due to its uncontrolled loading method. The results Fareed et al reported with the OHSU machine had variations that average 11.6% over all materials, so the OHSU/Proto-tech machine is not to blame for the failure of the anticipated trend to appear.

So the quest for improved dental restoratives continues. To Xie and company, we hope their formulations led to improved dental products. And to Fareed et al, we wish better luck next time.

A 12 station version of the OHSU Oral Wear Simulator was supplied by Proto-tech to 3M, Inc in 2010, where it has provided vital data for numerous in-house studies.

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Influence of halogen irradiance seen in short- and long-term wear of resin-based composite materials.

G S Bhamra and G J P Fleming had a straight-forward goal: to show the effect of degree of conversion on wear resistance of four commercial dental composites (2009). They prepared specimen using 8 levels of light curing distance (and hence level of conversion) To raise the stakes they analyzed the specimens at four intervals during a wear marathon that lasted for 500,000 cycles. Removing and replacing specimens can add an error component, but the wear simulator they used, a four-station OHSU Oral Wear Simulator by Proto-tech produced reasonable results over the entire range of specimens. The effect of diminished light curing was surprisingly not strong. Only the mean depth was effected by the level of cure at the early cycling phase and mean wear volume was uneffected. In the later cycling phase wear volume was heightened in poorly-cured composite though the increase in wear depth was not significant. The relative depth to volume describes the shape of the wear interface. Heightened depth indicates a more V-shaped facet, while heightened volume for a given depth results from a broader wear interface. So the wear of under-cured materials in the early cycling here seems to have a more v-shaped facet, while later wear had a broader interface. This suggests that the wear antagonist is less flattened by the under-cured materials, as if the elutable components are lubricating the interface during the wear interaction.

It was very ambitious of Bhamra and Fleming to demonstrate the effect of cure on wear resistance, and at so many cure levels. Condon and Ferracane (1997) attempted to demonstrate this connection in a similar wear simulator. While the effects of filler level and silanation level in experimental composites were evinced in this study, the effect of cure on the abrasion and attrition wear was negligible. Baffled the team cast about for an answer, eventually finding that artificial aging in ethanol resulted in the increased wear that was expected from undercured composite. Seemingly the presence of the uncured components prevented higher wear rates in the under-cured composite, as Bhamra and Fleming observed. More recently Alkhudhairy (2017) tested four bulk-fil composites in an abrasive wear tester and saw no positive relation between level of cure and wear resistance, and actually somewhat better wear resistance among some poorly cured materials.

In their study 20N of abrasion force and 90N of attrition force were applied through a steatite ceramic ball. The researchers digitized the entire surface and recorded the total volume and maximum depth, since curing effects were the main concern here. The Oral Wear Simulator also distinguishes between abrasion wear and attrition wear on each specimen depending on the calculations done by the profilometer.

The progression of wear that was illuminated in this 2009 study was worthwhile. Despite their goal being thwarted somewhat by the complex nature of polymer composite wear, the long term results found a ranking for the materials and demonstrated the robust ability of this testing method.

Wear of dental composite effected by filler features, says Turssi, Ferracane and Vogel.

Using the OHSU Oral Wear Simulator, this team found significant differences in wear resistance among experimental composites with varied particulate shape. (Filler features and their effects on wear and degree of conversion in particulate dental resin composites, Biomaterials, 2005 Aug.) Spherical and irregular shaped filler of a range of sizes were added to typical dental composite resins and exposed to 50,000 cycles in the dual-mode tester, which applies a 20N sliding wear regimen to produce abrasion, followed by a 90N pulse to produce attrition wear. The twelve experimental formulations produced wear in the abrasive range of roughly 20 to 120 microns, falling into 6 different groups. The attrition wear ranged from roughly 30 to 140 microns and fell into 5 groups. The OHSU simulator, now offered by Proto-tech Research, successfully demonstrated the effect of varying filler type on wear, a relation that had defied demonstration through wear simulation to date. The study found a significant advantage among trimodal mixtures of particles over mono- or bi-modal particulates. This is a level of sensitivity that had not been matched by a wear simulator at that point in 2005, and has not been bested since. While the study would have benefited from SEMs of the wear surfaces of the novel composites, the authors provided important insights into the factors that effect wear of dental composite.

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Wear of dental materials: Clinical significance and laboratory wear simulation methods —A review, reviewed

In 2018, the third issue of Dental Materials Journal featured an article by S Heinze, FX Reichl and R Hickel that summarized the issues surrounding the simulating of wear in the oral environment, and proposed a set of standards for meaningful wear studies. The four benchmarks suggested include a variable force level, a sliding component to the wear action, recirculating liquid and computer control. The authors then endorsed three simulator systems that were developed commercially based on their compliance with these benchmarks. They expressed reservations about systems that were developed at institutions, such as the Proto-tech Oral Wear Simulator, which was developed by Dr Jack Ferracane and his team at OHSU. However, during the same year the OHSU/Proto-tech was cited as an example of the “very few laboratory tests” that “predict clinical behavior.” (Kelly, 2018)

The Proto-tech system does have adjustable force levels within the range described in the Heinze et al review. It also provides sliding action, which is further enhanced by a “lifting-off” phase during each cycle. This interruption of contact allows for the entrapment of fresh slurry during each cycle. Systems which maintain contact between the antagonist and the target are producing a “scrubbing” action which is not clinically realistic. This action traps debris particles which further participate in the wear regimen. The authors recommended the frequent replacement of the liquid phase in their third criteria, likely with hopes of flushing these trapped particles from the system. The Proto-tech system does not recirculate the fluid because its “lift-off” phase during every cycle expunges the wear debris. It is designed to produce 3-body wear for which a slurry is proscribed that might not perform well in a recirculating scenario. Given the microscopic amount of wear debris dissolved in the 5ml of slurry that the wear chambers allow, it has never been considered essential to flush the debris out, and there has been no evidence of participation of abrasive wear debris in the Proto-tech system.

For the final criteria, a commendable call for force control through a feedback loop was made. Of course consistent loading contributes to consistent results. However the hardware required to meet this benchmark can be prohibitive. All of the computer-controlled systems that the authors cite are single-station devices. One would have to purchase four of them to match the through-put capability of the Proto-tech Oral Wear Simulator, for example. For this machine, load levels in the range of 40 to 65N can be irratic because of an interaction within the control circuit with the force-producing solenoids. However the load levels when set to the recommended 20N abrasion and 90N attrition have been remarkably consistent over time. Here at Proto-tech we recently performed calibration on three simulators that have produced thousands of wear specimens for research. The force levels were identical to the values recorded when the machines were first delivered almost 20 years ago. So we are confident that the Oral Wear Simulator provides repeatable loading without an expensive and cumbersome feedback control mechanism, as shown in the figure below.

At low speed the forces produced by the Oral Wear Simulator are noise free and very consistent, providing the 20N load force level for abrasion followed by a 90N attrition pulse.

At a higher speed the force made by the Oral Wear Simulator is still very clean and consistent, though the abrasion force region has slightly slower response time.

Hats off to Heinze et al for a review that summarized the relevant variables and discussed how they are being addressed in modern wear simulation research. It is refreshing to see Heinze stress the importance of controlled loading. In his Sept, 2006 article in Dental Materials titled “How to Qualify and Validate Wear Simulation Devices and Methods”, the Willlytec wear system (also offered by SD Mechatronic) was featured, which collides a specimen against a dead-weight-loaded antagonist in a very non-computer-controlled fashion. This attempt at controlled loading by using dead weights actually creates a complex dynamic loading scenario dominated by high frequency harmonics which introduce huge uncertainty in the load level. Heinze displayed these chaotic load profiles in a graph (2004), but justified the method because of the constancy of the results they produced. With so much uncertainty about the load levels, one could reasonably wonder about the relevance of the results. Are they producing high attrition in some load-sensitive materials and less in lower-stiffness materials, which are more impact resistant? This unrealistic loading method could raise more questions than it answers.

The Willytec/SD Mechatronic loading system produces erratic vibrations and uncertain peak load values. The loading curves as reported by Heinze and further by Rues (2011), rather than an ideal step-function, are dominated by a combination of frequencies represented in the figure that are the result of the uncontrolled transfer of inertia in the impact-dependent design of this test. The high frequency component is an inertial harmonic that results from the compressive waves generated in the elastic components of the specimen and counters specimen, holders and drive. It is modulated by the stick-slip envelope which likely is produced by the ricochet-action that occurs as the lateral motion transitions from stick frictional interaction to a slipping frictional interaction. The peak packet amplitude decreases with time in an exponential fashion typical of internal friction. The interference wedge observed in some interactions (represented by the blue curve) implies a feedback mechanism of a slightly reduced frequency, as if an unwanted reverberation is present.

Despite this poorly controlled and questionable loading scenario, the Willytec system has been often reported by Heintze and others, often without any mention of this force disparity, and inaccurately compared to the Proto-tech/OHSU system. The sliding action of the OHSU system includes an 8mm transit, enough to enable distinction between the wear produced in the 20N abrasion region and the 90N attrition. (These levels were erroneously reported by Heintze in 2004 as being 50N and 80N.) The Willytec method includes a minute 0.8 mm transverse motion and a nominal 50N load, so it is uncertain what type of wear is being produced here. The small sliding action could promote abrasion, though no third-body medium is employed, but the chaotic loading and small diameter antagonist might provide attrition wear, though no microcracking and abfraction has been reported with this method, as has for the OHSU system when testing fatigue-sensitive materials such as microfilled or flowable composites. While the wear produced by the OHSU system can be reliably differentiated into abrasion and attrition, the Willytec seems to produce an uncontrolled combination of abrasion and attrition with some two-body adhesive wear and maybe some ultrasonic galling effects thrown in.

The dental restoratives industry and research community can expect more accurate and reliable wear testing than has been depicted by the Willytec/SD Mechatronic and as adopted by Ivoclar Co.. The OHSU/Proto-tech system has been producing meaningful results for dozens of researchers on topics as diverse as marginal degradation, antagonist wear, filler effects, resin composition effects, light curing effects and so on, and has been applied extensively to dental ceramics research. It produced the most realistic ranking of materials (in two wear types) in a round-robin study reported by Heintze et al in 2006. The single wear mechanism-results of the other instruments did not rank the materials realistically, while the Proto-tech/OHSU method ranked the materials realistically in both abrasive and attrition wear. The biorealistic and consistent loading supplied by the OHSU/Proto-tech Wear Simulator explains its credible results. Its should be included in any list of suggested oral wear simulating methods.

Trends in dental ceramics wear reproduced by oral wear simulator.

Glassy dental ceramics were found to be more abrasive and more likely to wear than zirconia-based ceramics, according to a study by MR Kaizer et al in the Dental Materials May 2019 issue. Subjecting the specimens to 450k cycles in the wear simulator, supplied by Proto-tech, Portland, Ore based on research conducted at OHSU School of Dentistry, the same trends as observed in many clinical studies appeared. Stark differences in the microstructure of the wear patterns were observed, and cross sections of the wear patterns were examined for microcracking in this novel, well-executed dental ceramic wear research study by the group at NYU led by Dr. Yu Zhang.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6462421/

Polymerization Stress Measures

R Danso et al published “Development of an oxirane/acrylate interpenetrating polymer network (IPN) resin system” in Dental Materials, October, 2018. They found significant differences in polymerization stress among experimental materials using the Pro-test Polymerization Stress and Shrinkage Measuring System by Proto-tech. The same materials were assessed for shrinkage with the Accuvol video-based system. Find out more about the Proto-tech system at https://www.prototechresearch.com. 

Proto-tech Research Polymerization Stress Tester ranks materials that Accuvol could not distinguish.

New Friction Tester

Proto-tech has introduced a new bench top instrument for evaluating the frictional characteristics of opposing materials, especially ball-shaped dental materials opposing plate specimens of dental materials.

The system uses an ultra-low vibration stepper motor and precision positioning stage to move the sample at speeds up to 25mm/min.  The opposing surface is screw-mounted to a set of dead weights to exert up to 100N. A program allows easy command of the positioning motor and continual recording of the frictional force value sensed by the built-in load cell.

 

The specimen is mounted in a well that allows for liquid immersion. A micrometer allows for lateral positioning to make multiple parallel test passes. Contact Proto-tech at https://www.prototechresearch.com for more information.

LED Curedome Shines in Polymer Research

The LED Curedome has found a variety of uses in the dental polymer research labs.

The Curedome was used to produce inorganic fillers bonded to immobilized enzyme in a thesis by Indraneil Mukerjee in 2009.

http://udini.proquest.com/view/mesoporous-materials-for-dental-and-pqid:1887489331/

It appeared in a study of biodegradable polymer adhesives in a thesis by Andreas Mylonakis in 2008.

http://www.scribd.com/doc/83132078/Biodegradable-Polymer-Adhesives-Hybrids-and-Nano-Materials

Also in another study of immobilized enzymes in polymer adhesives in a thesis by Sudipto Das in 2011.

http://144.118.25.24/bitstream/1860/3460/1/Das_Sudipto.pdf

The intense and reliable illumination provided by the LED Curedome is a powerful tool in the modern photopolymer lab. The LED Curedome 23 has been updated with a sleek and tough 3D printed enclosure and a digital display of the intensity for repeatable curing.

Find out more about the LED Curedome 23 at Proto-tech’s website:  https://www.prototechresearch.com.

Human Enamel Wear

Here is a study of enamel wear using the OHSU wear simulator. This model was built by Proto-tech about 12 years ago and its still going strong.

Three-Body Wear of Enamel Against Full Crown Ceramics

Friday, March 18, 2011: 8 a.m. – 9:30 a.m.

Location: San Diego Convention Center
Presentation Type: Oral Session

J.A. SORENSEN¹, E.A. SULTAN², and P.N. SORENSEN¹, ¹Pacific Dental Institute, Portland, OR, ²Hamad Medical Corp, Doha, Qatar

 

Objectives: For full mouth rehabilitation in bruxers prosthodontists will often use a full gold crown for second molars. Based on their ceramic structure several ceramic systems potentially offer clinical advantages such as reduced wear against antagonist tooth structure. The purpose of this study was to measure the three-body in vitro enamel wear of several ceramic systems compared to enamel and gold controls.Materials & Methods: The OHSU Oral Wear Simulator 3-body wear machine produces both abrasion (20N) and attrition (70N). Flat plane specimens of ceramic core material, monolithic full-contour pressed ceramics, PFM ceramics and feldspathic porcelains, and cast gold alloy were fabricated and an enamel control. Ceramics were finished up to 600 grit paper and polished with diamond paste. Human enamel cusps were milled to a 10mm diameter and polished. Specimens (N=6) were cycled 50,000 times in a mildly abrasive slurry of (poppy seeds/PMMA beads). Wear of antagonist enamel cusps was measured with 2-D video analysis. Mean surface area of wear was calculated and statistical analyses performed.Results: Mean Enamel Cusp Wear [mm2](sd) for groups: 1) Lithium disilicate(Empress 2)= 1.32(0.38); 2)Gold/platinum (Aquarius, IvoclarVivadent)= 2.14(0.33); 3) Y-TZP zirconia (Lava, 3M Espe)= 2.22 (0.34); 4) Fluorapatite/Leucite (d.SIGN, IvoclarVivadent)= 2.79 (0.83); 5) Enamel, Bovine = 2.98 (0.83); 6)Leucite pressed(Empress)= 3.22 (0.54); 7) Feldspathic porcelain (Omega 900, Vita)= 4.64 (0.74). ANOVA and Newman-Keuls tests showed significant differences between groups (p<.001). Similar groups were (1,2,3), (2,3,4,5), (4,5,6), (7).

Conclusions: Within the limits of this in vitro study, several of the ceramics tested were equivalent to gold as a low wear antagonist fixed prosthodontic material.

Oral Wear Simulator Supersized

Proto-tech’s oral wear simulator is now available in a 12-station model.

The new machine creates the same clinically-relevant wear patterns that model abrasion and attrition wear on each specimen with triple the capacity of their original model.