Properties of composite systems and composite types
When choosing a composite system, material characteristics such as handling, sculptability, and viscosities affect the final restorative outcome, dictate specific manipulation techniques required, and influence delivery format.Among others, time from delivery to contouring and brushing, fracture and wear resistance, and polymerization shrinkage rates are important aspects of composite systems to consider before undertaking direct restorative procedures.With new state-of-the-art composites, however, colour stability is less of a concern, since nearly all systems demonstrate predictable and balanced colour stability throughout treatment and post-procedure.
In the past five years, dental professionals have witnessed major improvements in composite materials in terms of their particle sizes.Micro- and nano-hybrids produce enhanced luster and polish, while microfills remain unmatched for polishing ease and longevity when exposed to varying degrees of pH levels (i.e., low pH, high acidity, and brushing). Although polishability is not critical to composite selection, it is important.
As operators, dentists and technicians control a limited number of material characteristics and final restorative outcomes.With the ability to control only environmental aspects, finishing and polishing, manipulation, and proper polymerization, there is very little dentists can do to ensure the predictability of resins.Also, today’s resins demonstrate a range of 0.9% to 1.5% volumetric shrinkage, which supports esthetically pleasing results without pulling away and disrupting the hydrodynamics of the tubuli.7 This low shrinkage rate reduces the likelihood of postoperative sensitivity, marginal leakage, and creates better margins in restorations.
When building the incisal edge, Class IV incisal build-ups, and restoring worn dentition, composites that withstand the rigors of occlusion and mastication are required.The composite material must resist the abrasion that occurs from bruxing tendencies and brushing.9 Therefore, wear resistance is a necessary characteristic of direct composites in order for restorations to maintain surface texture and anatomy (Figs 3a & 3b).
Fig 3a & 3b Wear and fracture resistance are necessary characteristics of direct composites in stress-bearing areas.
Composite resins must also provide high polishability to mimic the gloss of natural enamel.10 Although composite restorations may exhibit a great final polish when the patient leaves the office, they should sustain that gloss over time.
Although most of today’s composites demonstrate excellent colour stability, other factors contribute to the lifespan of a direct restoration’s color.11 Polymerization and polishing can affect colour stability, as can a patient’s dietary and other habits.Typically, however, 10 to 20 years of colour stability can be expected with current composite systems.
Available composite materials vary based on filler particle size and shape, and there are many options from which to choose. Although each encompasses different characteristics, material selection will be determined based on the area in which the restoration is planned and the specifics of the case.
Microfills, conventional or reinforced, provide high sculptability and excellent wear resistance.With these composites, dentists can expect high polishability and very good color stability that typically lasts more than 20 years.Fracture toughness, however, is lower than with some other materials, and microfills should not be used over the incisal edge or to build up the incisal edge, as it will eventually break.13 Reinforced microfills do provide higher fracture resistance and, depending on the case, may be used in high stress-bearing areas.
Unlike microfills, a key benefit of hybrid composites is fracture toughness or resistance.Color stability is considered very good and sculptability is fair, depending on the specific system used.Wear resistance and polishability of hybrids, however, are not as good as other materials because hybrids present an average of larger particles, which are responsible for pitting of the finished surface, and they tend to be harder to polish than microfill materials.
Micro-Hybrids and Nano-Hybrids
Micro-hybrids are hybrids with a greater content of submicron particles. They demonstrate improved handling and polishability compared to conventional hybrids.Nano-hybrids are the state-of-the-art in the hybrid category and combine fracture toughness, sculptability, improved wear resistance, and color stability. Although the increased content of nano-particles does, in fact, produce a better polish, microfills still remain unchallenged with respect to long-term gloss.
Although there are few nano-filled products available, particle size and shape are the most important characteristics of these composites, as a configuration with spherical particles ultimately enables the best polishability. 13 Due to their smaller particle size, nano-fills exhibit very good fracture and wear resistance, along with good sculptability.These composites also demonstrate color stability.
It is important to remember that not all systems exhibit the same properties and that there is no truly perfect material.12 Ultimately, it is up to the dentist to maximize the best properties in each system and manipulate them to create the comprehensive, customized shades that suit the restoration.