Polymerisation shrinkage with bulkfill composites

Raymond L Bertolotti, DDS, PhD, reviews bulkfill composite materials.

In my opinion, bulkfill composite never made sense. Composite is not white amalgam.

In all composites, polymerisation shrinkage is the enemy. So, what about composite that has low shrinkage? Shrinkage is shrinkage, and bacteria which get in the resulting gaps are very small. Typical gaps are way larger than bacteria.

For assured success, the tooth bond to composite interface must be sealed, with no gap. Low shrinkage in composite is usually accompanied by high modulus, effectively negating any benefit of the reduced shrinkage. The resulting high shrinkage stress tends to break the bond to the tooth, leading to a gap or white line (fractured enamel) when on enamel.

To overcome polymerisation shrinkage issues, there is a major development. That is initiating polymerisation at the bond, effectively causing the polymerisation shrinkage of the bulkfill to be directed toward the tooth. The result is no gap between the composite filling and the tooth.

Naturally, you might ask how this is possible after all these years. To make a long story very short, the concept was published many years ago (Bertolotti, 1991), but the genius of Dr Sean Chen devised new polymerisation chemistry to greatly enhance the concept. Polymerisation is initiated at the composite to bonding agent interface and creates a cascade of polymerisation with shrinkage which then progresses into the composite, effectively pulling composite toward the bond.

There is independent confirmation of the concept from two highly respected researchers: Professor Alireza Sadr at the University of Washington who uses optical coherence tomography (Bakhsh et al, 2011) and Professor Nathan Lawson at the University of Alabama who uses more conventional microscopic techniques.

Here are some of professor Sadr’s images: Two very popular bulkfills, SDR is shown first, Sonicfill next, then the revolutionary material, Bulk EZ. You can easily see the gap on the pulpal floor in the first two, but the Bulk EZ shows no gap, in this severe C-factor class I filling. Say goodbye to post-op bite pressure sensitivity and to recurrent caries at the gingival margin. Also, you will see remarkably stain-free margins and absence of white lines (Figures 1a, 1b and 1c).

Figure 1a: Surefill SDR

Figure 1b: Sonicfill

Figure 1c: Bulk EZ

These images are screen shots taken from a video which you can view: http://depts.washington.edu/b4t/jdr-appendix/.

This video is amazing. Video D is Bulk EZ and you can see in slow motion shrinkage occurring to form a gap. Just compare the shrinkage of these bulkfills and you will better understand the lack of gap formation in Bulk EZ. (Due to the configuration of the OCT probe, the video images are upside down.) As you can see, ‘bulkfills work very well until you turn the curing light on. Exception, Bulk EZ.’

This discussion reminds me of what professor Junji Tagami said to me last year: ‘Bulkfills work well when incrementally placed’.

Of special interest is Clearfil Photo Core in Professor Sadr’s video. It is not gap free, but close and clearly better than the bulkfills other than Bulk EZ. Clearfil Photo Core is a light cure core material that might not be expected to work well, but in personal clinical experience, it does. The manufacturer says it will light cure 5 mm depth, but that is not correct. In an ISO 4049 test, we measured over 9 mm. No wonder it has been so successful for all these years since we began using it in 1986. Further confirming the above results, from professor Lawson at the University of Alabama (Figure 2).

Figure 2: Percentage of cervical margin with microleakage (Figure courtesy of professor Nathaniel Lawson)

Note that Bulk EZ was the only material which had no microleakage at the cervical margin. The other bulkfill product tested is 3M Filtek BF.

Here is a nice clinical comparison of Bulk EZ and a flowable composite placed in contact with enamel, courtesy of Mike Nelson DDS. Mike placed an occlusal-buccal composite restoration, but underfilled the buccal surface inside the matrix. After removing the matrix, he injected some light curing flowable to fill the void. After light curing, you can see a big margin difference (Figure 3).

Figure 3: Bulk EZ comparison with traditional composite

There is no white line adjacent to the Bulk EZ like there is around the flowable. This is great proof of both no gap and stress reduction, as evidenced by lack of white line with Bulk EZ (Figure 4).

Figure 4: There is no white line adjacent to the Bulk EZ

In a microscope, the white line at the flowable composite to enamel junction is easily seen.  It is a zone of microfracture of the enamel, caused by polymerisation stress. With Bulk EZ, there is no white line due to lower polymerisation stress, insufficient to fracture the enamel.  

So how does this all affect the critical gingival margin on a class II composite? Here is a comparison of Bulk EZ and Sonicfill in our laboratory test. Testing was done by thermal cycling in water, using a stain to mark leakage. 

Sonicfill

The leakage that we can easily see after thermal cycling is quite remarkable. The gingival margin was on phosphoric acid etched enamel. Kerr’s instructions say that any bonding  agent can can used with Sonicfill, so we used Optibond Solo Plus, Kerr’s bestseller (Figure 5).

Figure 5: Sonicfill phosphoric etch and rinse

Bulk EZ

The result for Bulk EZ is remarkably different. No leakage is evident. Danville’s instructions say that any methacrylate bonding agent may be used. We used Danville’s Prelude One and it worked very well (Figure 6).

Figure 6: Contact polymerisation

The Bulk EZ instructions for use give the option to place a layer of normal light curing composite over the Bulk EZ when used in posterior restorations. That recommendation was made since initially it was not known how well the Bulk EZ would resist wear. Without an added layer of composite, it stays polished well and the margin integrity (not easily tested in the lab) can’t be beat. Clinical experience has shown that small fillings need not be covered, but larger ones, especially those replacing cusps, should be covered with a more wear-resistant composite.

Figure 7: Bulk EZ after three years and eight months

Figure seven shows a nice clinical result provided by Mike Nelson DDS. 

It is clear that Bulk EZ is a game changer. I very highly recommend this product.

For more information

Professor Ray Bertolotti will present two full days of CPD entitled ‘All you need to know about adhesive dentistry’ in Ireland, in the Schull Harbour Hotel, West Cork, on 19-20 April 2023. He will be supported by Professor Edward Lynch, who is listed in the Top 100 Doctors in Dentistry Worldwide and is also the recipient of the 2022 Distinguished Scientist Award from the International Association for Dental Research in Pharmacology, Therapeutics and Toxicology.

To apply for this limited attendance course please email edward.lynch@hotmail.com.