Bioactive Dental Materials
Composition, properties, and indications for a new class of restorative materials
Steven R. Jefferies, MS, DDS, PhD
In the past 25 years, a new classification of dental restorative material has emerged. Bioactive cements, also termed bioactive chemical-bonded ceramics,1,2 have significantly improved the care we can give our patients in many clinical applications. Bioactive dental cements appeared with the introduction of ProRoot® MTA (DENTSPLY Tulsa Dental Specialties, www.tulsadentalspecialties.com) in the mid 1990s. Since that time, this category of dental materials has continued to evolve rapidly, and its bioactivity has created some confusion and controversy. The purpose of this article is to address basic questions and provide some clarity concerning this interesting class of restorative materials.
The terms bioactivity and bioactive material have recently emerged in both the dental literature and advertisements for new dental products. As when any new concept in dentistry emerges, the quest for understanding its relevance to clinical practices becomes critical.
The term bioactive material appears to have originated with Dr. Larry Hench as a result of his development of the calcium silicophosphate glass, Bioglass.3 His initial primary interest was the development of improved graft materials for orthopedic and bone reconstruction. Therefore, Hench’s research led to the discovery that Bioglass could chemically bond to bone. This property became the initial and longest-standing definition of a bioactive material.
However, the in vivo usage test needed to confirm that a material was bioactive by this definition was expensive and complicated. Based on the more straightforward test suggested by Kokubo and colleagues,4 a growing consensus has emerged regarding a more universal definition of bioactivity among materials scientists and clinician researchers. In the literature, the term bioactivity has the following definition when applied to dental materials: Bioactivity is the property of a biomaterial to form apatite-like material on its surface when immersed in a simulated body fluid (SBF) for a period of time. A standardized, internationally developed test is available from the International Standards Organization (ISO 23317:2007) that provides a specific protocol and evaluation standard to establish whether a material is, in fact, bioactive. This standard specifies that any material claimed to be bioactive must meet the endpoint of this international test—the ability to form measureable surface apatite by 28 days in a specific SBF containing inorganic phosphate. Based on this scientifically validated definition, the traditional formulations of glass ionomers, resin-modified glass ionomers (RMGIs), and fluoride-releasing composites would not be bioactive unless they demonstrated the formation of surface apatite in a SBF.5,6 Gandolfi and colleagues6 suggest terminology to differentiate the ion-releasing behavior seen in nonbioactive materials, such as glass ionomers or fluoride-releasing composites, by classifying such materials as biointeractive. This differentiates such materials from calcium silicate and calcium aluminate biomaterials, which the literature demonstrates are both biointeractive and bioactive (ie, apatite-forming).1,2,6
Defining bioactivity is an interesting proposition for the dental materials researcher, but the clinician needs more practical information to effectively select and utilize bioactive products in clinical practice. This includes how the properties of bioactive materials compare with those of other classes of dental materials and what they provide in terms of clinical efficacy; whether these materials are necessary in clinical practice, and if so, how and when they should be used; and if all resin formulations that contain bioactive components are equally effective.
Material Composition and Properties
Bioactive materials available commercially and utilized in clinical dentistry today fall primarily in one of two compositional classes: calcium silicates and calcium aluminates.1,2 Both materials, when their powder component is mixed with water, set with an acid-base chemical reaction and produce an alkaline pH after setting. The role of pH in bioactivity is potentially important, as all of the strictly water-based, calcium silicate/calcium aluminate cements significantly elevate the local pH level in solutions in which they are immersed. Accordingly, the most active of the bioactive materials demonstrate a high pH level in storage solutions. High pH levels (7.5 or higher) appear to stimulate more active and complete bioactivity.
Because these materials are primarily water-based, polysalt, chemically bonded-ceramic cements, their physical strength properties are similar to other cement types such as zinc orthophosphate, calcium hydroxide, polycarboxylate, zinc oxide eugenol, and certain conventional glass ionomers and RMGIs. The place of the bioactive calcium silicate and calcium aluminate cements in the evolution of dental cements can be seen in Table 1.