In Poland there is such a saying: „What the eye does not see, the heart does not care about”. So tell me – have You ever seen composite shrinkage?
We do talk about it a lot. There are lectures, scientific researches, we learn techniques how to work with composite materials to minimalize it. But can we, as clinicians, see what is it all about? How strong it really is? And does it exist at all? In this short article i would like to show You simple experiment to show clinically existence and true strength of composite polymerisation shrinkage.
For the purpose of the clinical experiment, we’ll use glass anesthesia ampules. They’re top side open, we’ll treat as class I cavity.
Polymerization shrinkage is a very simple concept, yet it has highly important implications on the long-term stability of a composite resin. As a result of the polymerization process, the liquid or resin is converted into a solid, which results in a density change that reduces the overall volume.(1)
But this is just a number, information on how much volume is reduced during solidifying of composite material. It does not imply any clinical information. What we should care about is polymerization shrinkage stress – stress is a force that contracting material is generating on the hybrid layer and tooth tissue. Polymerization shrinkage stress of resin-based materials has been related to several unwanted clinical consequences, such as enamel crack propagation, cusp deflection, marginal and internal gaps, and decreased bond strength, shrinkage stress has been associated with post-operative sensitivity and marginal stain.(2)
This is why it important to know how different materials influence our clinical work strategy.
Fig.1
To prepare glass ampules we prepare them for the adhesive procedure. The first step was to clean the inside of the cavity – sandblasting was used to remove the protective film from the glass and enhance the surface. Then adhesive procedure as following: 60 seconds hydrofluoric acid and universal adhesive with silane (ScotchBond Universal, 3M)
Fig.2
The first material we tested was a flowable composite. The entire cavity was filled with a single layer and then cured with a lamp. Flowable resin-based composites are conventional composites with the filler loading reduced to 37%-53% (volume) compared to 50%-70% (volume) for conventional mini-filled hybrids. This altered filler loading modifies the viscosity of these materials. But due to lower filler load, the polymerization shrinkage is higher. (3) After polymerization we could observe that polymerization shrinkage stress is sufficient to crack glass ampule.
Fig.3
During University we learn to use standard composite in small increments and to condense them with oblique layers – but do we really know why? In this test, we used standard composite in huge portions – filling the entire cavity and polymerizing it. We can clearly observe shrinkage – there are places where adhesion to walls has been broke due to shrinkage. Bur most important clinically is information on how much stress polymerization generates.
This force was sufficient to break the glass ampule. This is something to think about during our clinical procedures – if glass could be broken when we place too much composite in a big cavity, how would our teeth cusps react to such forces? That’s why working with small composite increments is so important.
Fig.4
We can see shrinkage also in this 4 times speed up video – shrinkage is happening after polymerization, even for few minutes – not only during the curing procedure. We can observe as adhesion to walls is slowly breaking. Clinically composite shrinkage can lead to marginal and internal gaps and decreased bond strength, shrinkage stress has been associated with post-operative sensitivity and marginal stain.(2)
Fig.5
Last few years more and more popularity and attention is given to bulk type materials. Composite resins, known as bulk-fill resins, with modifications in their chemical formulation and polymerization properties, have been developed to minimize or eliminate polymerization shrinkage, increasing the depth of polymerization as well as cytotoxicity. Bulk-fill resins with a 4–6 mm single increment have low shrinkage stress and a high degree of polymerization at this depth, due in particular to the increase in translucency and to the presence of polymerization modulators. (4,5,6)
In this test, same as for the previous we placed a bulk portion of composite to fill the entire cavity and we polymerized it. What we can see is – shrinkage is not visible, we don’t see that adhesion to walls was lost or any cracks on glass – thus shrinkage stress with such a big portion of the material is acceptable.
Summary
Knowledge about bulk type composites properties can lead to improvement to our daily clinical quality of life. If we can work with larger portions of the material, we can shorten our daily procedures. This simple experiment shows both that, composite shrinkage is real and can lead to real-life problems but using bulk type materials can minimalize those problems, especially in big class I cavities. Important though is the fact that working with large composite increments can lead to problems with proper condensation of those materials and the presence of voids – we should pay a lot of attention to properly condense it in a cavity.
Bibliography
1. Composite resin polymerization and relevant parameters S.R. Schricker, in Orthodontic Applications of Biomaterials, 2017
2. Polymerization shrinkage stress of composite resins and resin cements – What do we need to know? Carlos José SOARES André Luis FARIA-E-SILVA Monise de Paula RODRIGUES Andomar Bruno Fernandes VILELA Carmem Silvia PFEIFER Daranee TANTBIROJN Antheunis VERSLUIS Braz. oral. res. 31 (suppl 1) – Aug 2017
3. Flowable Resin Composites: A Systematic Review and Clinical Considerations Kusai Baroudi Jean C. Rodrigues, J Clin Diagn Res. 2015 Jun; 9(6): ZE18–ZE24.
4. Marovic D, Tauböck TT, Attin T, Panduric V, Tarle Z. Monomer conversion and shrinkage force kinetics of low-viscosity bulk-fill resin composites. Acta Odontol Scand. 2015 Aug; 73(6):474-80
5. Benetti AR, Havndrup-Pedersen C, Honoré D, Pedersen MK, Pallesen U. Bulk-fill resin composites: polymerization contraction, depth of cure, and gap formation. Oper Dent. 2015 Mar-Apr;40(2):190-200.
6. Physicomechanical characterization and biological evaluation of bulk-fill composite resin, Braz. oral. res. 32 2018, Armiliana Soares NASCIMENTO
