Study finds new way to toughen composite restorations

Composites are expected to last around ten years. Breaks in the material and secondary caries often contribute to their failures. Polymerization stress contributes to these failures.

As a result, researchers want to find a way to reduce polymerization stress generation in composite materials. A new study published in Scientific Reports earlier this year indicates thioruethane-modified filler interfaces can do that.  

Polymerization stress is a common problem in composite restorations. For direct restorations, the composite material bonded to the tooth surface during polymerization shrinks. The shrinkage pulling on the bond creates shrinkage stress, which contributes to gaps at the margin.  

The marginal gaps allow bacteria and saliva into the area and start the clock ticking to restoration failure. If there was a way to reduce the polymerization stress caused by shrinkage of composite materials during curing, then it could have positive effects on the restoration’s longevity. 

In February of 2019, Scientific Reports published an Open Access article, “Toughening of Dental Composite with Thiourethane-Modified Filler Interfaces” that suggests adding thiourethane-silanized inorganic fillers to resin composites reduced stress by 35 percent in composite materials.ⁱ Also, these thiourethane-modified composites doubled the mechanical properties of the material.ⁱⁱ 

The following summarizes the study from who the researchers are to how they did it to what they discovered.  

Related reading: How you can make your composite restorations last longer

Who did the research? 

• Ana P. Fugolin, Department of Restorative Dentistry, Division of biomaterials and Biomechanics, Oregon Health & Science University, Portland, Oregon, USA 


• Daniel Sundfeld, UNINGA, Maringa University Center, IN Maringa PR, Brazil 


• Jack L. Ferracane, Department of Restorative Dentistry, Division of Biomaterials and biomechanics, Oregon Health & Science University, Portland, Oregon, USA 


• Carmem S. Pfeifer, Department of Restorative Dentistry, Division of Biomaterials and Biomechanics, Oregon Health & Science University, Portland, Oregon, USA 


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Why did they test the thiourethane-modified filler interfaces? 

Reducing composite polymerization shrinkage and the resulting stress would decrease one of the most significant problems with the material. The researchers cite several studies that reported adding a small amount of thiourethane oligomers (polymers with a low molecular weight that have fewer units that repeat and whole physical properties are associated with the length of that chain) to the matrix of resin composites reduced stress by 50 to 60 percent, as well as increased the resistance to fracture.ⁱⁱⁱ 

The only drawback was that if you add too much of the thiourethane oligomers, it makes the material thicker (or even too thick), which changes handling for dentists. Also, concerns exist that if you put in more than 20 wt% of thioruethanes, it could decrease the elastic modulus.^iv

To overcome these concerns, the researchers chose to silanize the surface of the inorganic fillers with thiourethanes and see what happens to the mechanical properties and the rate of change for the chemical process during curing. By attaching the thiourethanes to the filler particles rather than replacing them, researchers posited that the thioruerthanes’ presence should not affect the monomer matrix’s viscosity. 

The researchers had two hypotheses: 

1. The thiourethane-oligomer-silanized filler particles can distribute through the resin matrix without causing problems with polymerization and depth of cure. 


2. It will improve resistance to breaking and polymerization stress in the finished restoration. 


Continue to the next page to see how the tests were performed and what the researchers discovered.

How did the researchers test them? 

• They created the thiourethane-modified resins, and prepared the control groups.  


• For fracture toughness (KIC), the researchers used the single-edge notch beam method. They made samples in a split steel mold with a 2.5 mm notch in the middle and photoactivated for two minutes per side using a radiometer to ensure they had the proper irradiance (800mW/cm²).  They tested using the universal test machine.  


• For polymerization stress, researchers use a cantilever beam apparatus. The cylindrical samples were light-cured for 40 seconds (at 280 mW/cm²). Then, researchers recorded the data digitally for 10 minutes to calculate the shrinkage. 


• For degree of conversion, three specimens were laminated between glass slides, cured, and measured with near-infrared spectroscopy.  


• For polymerization kinetics, researchers observed samples for 300 seconds at two scans per spectrum with 4 cm^-1 resolution. 


• For depth of cure, specimens were made in silicon molds, photocured, and stored for seven days. After a week, they embedded them in slow-curing epoxy resin and cut them to measure the depth of cure.  


• For viscosity, they measured unpolymerized resin in a cone-plate rheometer.  


Related reading: The evolution of resin composites for direct restorations

What were the results? 

• Thermogravimetric analysis revealed that the thiourethane fillers had 48.5 percent more organic content than the control groups. 


• KIC increased with the thiourethane-silanized particles, except for one that had similar values to the control. 


• Polymerization stress decreased significantly with the thiourethane-modified composites, except two that had similar values to the control. 


• When compared to the controls, there was not a significant change to the depth of cure or an increase in the viscosity when researchers added thiourethane-silanized filler particles to the material. 


• Rate of polymerization was statistically different. At the onset of vitrification, the DC at Rp max (degree of conversion at a rate of polymerization) was 7 to 14 percent, and highest in the groups that had benzene as the isocyanate.  


• All materials had a depth of cure 3.5 mm at least, which was the full depth of the samples. The thiourethane-silanized filler groups had similar or higher degrees of conversions as the controls. 

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What did they learn? 

The past studies that had shown improved results for reducing polymerization stress and increasing fracture toughness using thiourethane oligomers had limitations. They couldn’t add too many oligomers or it would increase the viscosity too much, which would keep out the inorganic fillers needed to facilitate the mechanical properties of the composite restorations. The past studies suggested that silanization of the oligomer to the surface of the filler particle could overcome this limitation. The researchers for this study wanted to test it and see if they could harness the same benefits without suffering the drawbacks of adding the oligomers directly to the resin matrix.  

When researchers added thiourethane-silanized filler groups to the composite, there was as much as 35 percent less stress, and the mechanical property values doubled. Furthermore, this did not alter the viscosity of the material and did not require a complicated photoactivation process.  

Continue to page three to see what more needs to be learned.

What needs more research? 

One interesting finding from the research was something the results did not show. In previous studies, the addition of thiourethane to the resin matrix led to events that delayed gelation and vitrification. Specifically, the thiourethane-modified composites delay in the point of conversion where the “monomer mobility limitations hamper polymerization and, thus, allow higher final conversion and delayed modulus development, ultimately delaying and reducing stress development.”  

However, here, researchers silanized the thiourethane to the surface of the inorganic fillers, and the reaction rate was the equivalent to or higher than the maximum rate of polymerization as the controls. At the same time, the degree of conversion at the maximum rate of polymerization was either equal or significantly lower than the controls. They also did not see significant correlations between these factors and measurements of stress. Furthermore, they didn’t see the same monomer mobility limitations with the thiourethane-silanized filler composites because they did not change the viscosity of the material.  

In the discussion, researchers attribute these findings to the fact that thiourethane was less concentrated than it was in the composites observed in the previous studies. In other words, because thiourethane was not added directly to the matrix, it reduced the effects on these outcomes. Also, the researchers suggest that gelation and vitrification did not play a role in decreased stress observed, and it must be explained by something else. More research might reveal these explanations. 

Click here to read the entire study from Scientific Reports. 

Composites have wonderful qualities for direct restorations. However, they have always fallen short on longevity, at least when compared to materials like amalgam. One of the main culprits that leads to eventual failure is polymerization stress caused by shrinkage during curing, which pulls on the bonded surface. If there were a way to decrease that stress, composite restorations could last longer.  

Like all research, however, more needs to be done on this essential finding. However, this study shows promise for using thiourethane-modified-silanized fillers to reducing stress, and all the related consequences of it, in direct composite restorations.