Bone Substitute Materials in Implant Dentistry

Abstract

Although bone autografts have been routinely used as “gold standard” for reconstruction/ replacement bone defects, because they have osteogenic, osteoinductive, osteoconductive properties, they have a high number of viable cells and are rich in growth factors. However, the use of autograft is limited by several factors, being one of them the insufficient amount of donor tissue. Therefore, bone substitute materials have been extensively studied in order to develop an ideal material for substitution of bone grafts, due to some disadvantages presented by autografts, allografts and xenografts, such as poor bone quality, an inadequate amount of bone and possible immunogenicity for allografts and xenografts, which limit the use of these grafts in specific surgical protocols. These disadvantages have led tissue engineering and biotechnology to develop new materials and promising methods for tissue repair, especially for bone tissue. Thus, bone substitutes, synthetic and/or biotechnologically processed have become potential materials for clinical applications in different areas of health. An ideal bone substitute (BS) material should provide a variety of shapes and sizes with suitable mechanical properties to be used in sites where there are impact loading; moreover, these materials should be biocompatible, osteoconductive, preferably being resorbable and replaced by new bone formation. In general, resorbable BS materials are preferred, since these materials are expected to preserve the increased bone volume during the reconstruction and simultaneously are gradually replaced by newly formed bone. Synthetic materials, denominated as alloplastics, may act as scaffolds for bone cells providing tissue growth inside the respective material. A scaffold must be highly porous with interconnected pores and have adequate mechanical properties. The surface of a scaffold should be similar to extracellular matrix (ECM). These properties enable the scaffold to act as a matrix for tissue regeneration to maintain and improve tissue/organs functions; therefore, it is considered the key element for the success in tissue engineering. Numerous physicochemical features of scaffolds, such as surface chemistry, surface roughness, topography, mechanical properties and interfacial free energy (hydropho‐ bic/hydrophilic balance) are important for cell attachment, proliferation and differentiation. These factors are also critically important to the overall biocompatibility and bioactivity of a particular material [1-3]. Resorption of a biomaterial is related to several factors, such as, particle size, porosity, chemical structure (composition and crystallinity), and pH of body fluids [4, 5]. Particles with nano‐ metric sizes are reabsorbed faster than micrometric particles, because osteoclasts or macro‐ phages act faster on a biomaterial surface. Biomaterial crystallinity also changes the resorption rate, since highly crystalline structures are more resistant to resorption than an amorphous or semi-crystalline structure. Moreover, the chemical composition is also important. Impurities such as calcium carbonate promote faster resorption [6]. The failure or the success of a material for bone fill or replacement may be related to the resorption rate of the material, as well as the regenerative capacity of bone tissue. This process can occur in three forms: 1. insufficient permanence of the material to promote bone apposition and to allow the osteoconductivity; 2. premature destabilization of newly formed bone due to the complete degradation of the material; 3. an exaggerated inflammatory response due to the degradation of the material [7]. Thus, bone substitute materials must have suitable resorption rate in accordance with the rate of tissue formation. Despite recent advances in the development of new BS for bone tissue engineering, there is still a search for a material or a composite with mechanical properties and physicochemical characteristics similar to autograft and a structure closer to the natural ECM.