In restorative care, there have long been continuous attempts to improve the marginal qualities of adhesive and composite restorations. In turn, clinicians would be able to reduce the incidence of marginal staining, gap formation, pulpal irritation, and bacterial infiltration that could demineralise surrounding healthy tissues, or even cause tooth cracks or fractures.[i]
Despite the progress of modern solutions, perfect margins can be difficult to achieve – and it is not for lack of trying. Clinicians need to understand the function of polymerisation in dental composite restorations, and what it means for the results they seek to attain.
With an improved knowledge of the chemical reactions that take place at the treatment site, dental professionals can choose composite solutions whose unique properties make their care more predictable.
Polymerisation problems
The chemical process of polymerisation involves the connection between molecular monomers, creating polymer chains. Many modern dental restorative composites feature a photo-activated system for polymerisation, which means this process is induced when exposed to light at a particular wave length or blue-coloured light.[ii] The depth of cure will vary between materials.
The polymerisation process sees double carbon links in monomers convert to single links in polymers, and the amount of change that occurs is called the degree of conversion (DOC).[iii] With a higher conversion, clinicians can observe an increase in the longevity, mechanical and physical properties of the restoration.iii
Polymers occupy less volume than monomers so, as this process occurs, a dental restoration undergoes polymerisation shrinkage.[iv] The distance between monomer chains is reduced, as weak van der Waals forces are converted into covalent bonds.iv With the shrinkage, the restoration seeks to minimise the space it takes up, which can create stresses on the existing adhesion to surrounding tooth structures, as well as the restoration and teeth themselves.
Herein lies the problem, as this occurs, the success of the treatment may be compromised. If the shrinkage stress is greater than the adhesive capabilities of the composite resin, margins may form. When the adhesion is greater, the stress is transferred throughout the surrounding tooth structures. Clinicians could observe marginal leakage, marginal staining, secondary caries formation, cuspal deflection, fracture of the tooth or restoration, and potential postoperative sensitivity.[v]
Impact on dentistry
Once the risks have been defined, clinicians need to understand the modern solutions that have been implemented to combat the problem on a molecular level.
One such example is the implementation of addition-fragmentation monomers (AFM) to a material. These allow a material to adapt the connections made during the polymerisation process and delay the gel-point, which is the moment at which a material develops a structure that can take on internal strain and loses its ability to flow.[vi]
We have established that during polymerisation, monomers combine to create single covalent bonds, which reduce the volume of the structure. Implementing AFM into a structure allows the polymer network to rearrange these connections without reducing cross link density or the final mechanical properties.[vii] Through this, the covalent bonds are formed, broken, and reformed once again before the composite solution settles in an optimal state. Studies into the use of AFM molecules have noted that by rearranging the polymer network, they delay the gel-point and reduce shrinkage stress,[viii] which would minimise the risk of restoration failure.
Bulk-fill
A clinician’s choice of approach may also affect the impact of shrinkage stress on a restoration, notably whether it is applied in bulk or through conventional placement.
The literature states that the use of bulk-fill composite resins in the posterior dentition reduces the level of shrinkage stress observed, and increases the fracture resistance of a restoration.[ix] In addition, clinicians can also benefit from a simplified treatment workflow, which reduces treatment time and benefits both clinician and patient.
Choosing the most effective composite resin solutions for each clinical need is key. A solution like the Filtek One Bulk Fill Restorative from Solventum, formerly 3M Health Care, offers an exclusive range of benefits to clinicians. With the implementation of innovative AFM monomers, clinicians can relieve stress during polymerisation, but the addition of aromatic urethane dimethacrylate (AUDMA) also helps to reduce the level of shrinkage and resultant stress that occurs. With up to 5mm depth of cure, restorations are completed in a one-step placement, yet aesthetics aren’t compromised owing to the exceptional TRUE nanotechnology which improves wear resistance and polish retention.
Shrinkage stress is a problem that all professionals must first understand in order to tackle. Through the appropriate choice of composite resin, the prevalence of all associated complications may be reduced, making for happier patients with more successful restorations.
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About Solventum
Solventum, enabling better, smarter, safer healthcare to improve lives. As a new company with a long legacy of creating breakthrough solutions for our customers’ toughest challenges, we pioneer game-changing innovations at the intersection of health, material and data science that change patients’ lives for the better while enabling healthcare professionals to perform at their best. Because people, and their wellbeing, are at the heart of every scientific advancement we pursue. We partner closely with the brightest minds in healthcare to ensure that every solution we create melds the latest technology with compassion and empathy. Because at Solventum, we never stop solving for you.
[i] Benetti, A. R., Peutzfeldt, A., Lussi, A., & Flury, S. (2014). Resin composites: Modulus of elasticity and marginal quality. Journal of dentistry, 42(9), 1185-1192.
[ii] Khan, A. A., Zafar, M. S., Ali A Ghubayri, A., AlMufareh, N. A., Binobaid, A., Eskandrani, R. M., & Al-Kheraif, A. A. (2022). Polymerisation of restorative dental composites: influence on physical, mechanical and chemical properties at various setting depths. Materials Technology, 37(12), 2056-2062.
[iii] Rajan, G., Raju, R., Jinachandran, S., Farrar, P., Xi, J., & Prusty, B. G. (2019). Polymerisation shrinkage profiling of dental composites using optical fibre sensing and their correlation with degree of conversion and curing rate. Scientific Reports, 9(1), 3162.
[iv] Soares, C. J., RODRIGUES, M. D. P., VILELA, A. B. F., PFEIFER, C. S., TANTBIROJN, D., & VERSLUIS, A. (2017). Polymerization shrinkage stress of composite resins and resin cements–What do we need to know?. Brazilian oral research, 31, e62.
[v] Venkatesh, A., Saatwika, L., Karthick, A., & Subbiya, A. (2020). A review on polymerization shrinkage of resin composites. Eur. J. Mol. Clin. Med, 7, 1245-1250.
[vi] Camargo, E. J. D., Moreschi, E., Baseggio, W., Cury, J. A., & Pascotto, R. C. (2009). Composite depth of cure using four polymerization techniques. Journal of Applied Oral Science, 17, 446-450.
[vii] Park, H. Y., Kloxin, C. J., Abuelyaman, A. S., Oxman, J. D., & Bowman, C. N. (2012). Novel dental restorative materials having low polymerization shrinkage stress via stress relaxation by addition-fragmentation chain transfer. Dental Materials, 28(11), 1113-1119.
[viii] Szebeni, D., Told, R., Kunsági-Máté, S., Szalma, J., Maróti, P., Böddi, K., & Lempel, E. (2024). Monomer elution and shrinkage stress analysis of addition-fragmentation chain-transfer-modified resin composites in relation to the curing protocol. Dental Materials.
[ix] Rosatto, C. M. P., Bicalho, A. A., Veríssimo, C., Bragança, G. F., Rodrigues, M. P., Tantbirojn, D., … & Soares, C. J. (2015). Mechanical properties, shrinkage stress, cuspal strain and fracture resistance of molars restored with bulk-fill composites and incremental filling technique. Journal of dentistry, 43(12), 1519-1528.
