Hard tissue response

Pastes of calcium hydroxide with water have been used as pulp-capping materials for many years and it is the material of choice for this application (Granath, 1982). Its favourable tissue responses have been known for many years (Zander, 1939). It has a healing effect, for it induces the formation of hard tissues of reparative dentine when pulp has been exposed (Eidelman, Finn Koulourides, 1965). This action seems to be associated with its high alkalinity (pH 12-5) and consequent bactericidal and proteinlysing effect (Fisher, 1977). 

The capacity of hard tissue such as bone to generate potentials in response to mechanical stress has been known from the beginning of the nineteenth century. A piezoelectric theory to account for the electric potential observed in dry bone on deformation was proposed by Fukada and Yasuda in 1957 and subsequently explored by many others in the 1950s and 1960s as well as by Friedenberg et al. in 1971. 

Bioadhesion, i.e. biofihn formation resulting in a fouling surface, is required for biomaterial to be considered as a part of the body (e.g., orthopedic prosthesis, hard tissue) to enhance its incorporation and its biomechanical response. Examples are in the rebuilding of bones, recolonization, and hybrid implants composed of two parts, a synthetic one (with polymers as the mechanical sub-... 

When hardened. Biodentine shows good biocompatibihty with the oral tissues and, in particular, exerts minimal influence on cells of the adjacent pulp. Some loss of cell viability has been reported, but this has been attributed to apoptosis and necrosis, rather than the toxicity of the material [71]. In one study of the performance of Biodentine, it was found that the material caused complete dentinal bridge formation in molars while creating no inflammatory response on the cells of the pulp [72]. Both of these features suggest that this material has promise for application in pulp capping. Its ability to promote regeneration of the hard tissue is particularly noteworthy, and will be considered in more detail in Chapter 9. 

Zinc oxide-eugenol is a somewhat old-fashioned material, but it is widely used as an endodontic sealer [18]. It has relatively poor mechanical properties, but is easy to use in the dental clinic [19] and outcomes are good, which explains its continuing popularity. When set, it is biocompatible towards dental hard tissues, though it is cytotoxic towards soft tissues [20]. Zinc oxide-eugenol is susceptible to hydrolysis, which causes the material to decompose and release eugenol. It is this latter substance which is responsible for the cement s adverse effects on soft tissues, but which also makes the material bactericidal. 

MTA has been found to give good biological results when used for root-end filling in vivo. It promotes only low peri-radicular inflammation [114] and frequently causes cementum to form on its surface [115], It has also been found to induce the formation of apical hard tissue [116] and, in non-infected teeth, to support almost complete regeneration of the peri-radicular periodontum [117]. Thus, MTA is not only biocompatible, it is also bioactive and generally promotes a positive response in the tissues with which it comes into contact [106,118]. 

Stress Shielding. Beyond the traditional biocompatibility issues, hard tissue biomaterials must also be designed to minimize a phenomenon known as stress shielding. Due to the response of bone remodeling to the loading environment, as described by Wolffs law, it is important to maintain the stress levels in bone as close to the preimplant state as possible. When an implant is in parallel with bone, such as in a bone plate or a hip stem, the engineered material takes a portion of the load— which then reduces the load, and as a result, the stress, in the remaining bone. When the implant and bone are sufficiently well bonded, it can be assumed that the materials deform to the same extent and therefore experience the same strain. In this isostrain condition, the stress in one of the components of a two-phase composite can be calculated from the equation ... 

Furthermore, these scaffolds have been shown to be both biodegradable and biocompatible in vivo. Results indicated that PPF is biocompatible within both soft and hard tissues, minimal fibrous encapsulation of the scaffolds occurred, and tissue response appeared to improve with implantation time. A progressive reduction in inflammatory cell density and a continued organization of connective tissue with the interstitial space was observed, even if scaffold microstmcture did not seem to play a key role. ... 

Fisher, l.P. et al.. Soft and hard tissue response to photocrosslinked poly(propylene fumarate) scaffolds in a rabbit model, /. Biomed. Mater. Res. 59,547,2002. 

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