Biologic Classification of Biomaterials

Biomaterial is defined, in the broader sense, as any pharmacologically inert material that is capable of interacting with a living organism without causing adverse reactions either at the site of the implant or across the whole organism.When an artificial material is placed in the human body, tissue reacts in a variety of ways depending on the material type thereby, affecting the mechanism of tissue attachment to that surface. Biologic classification is based on tissue response and systemic toxicity effects of the implant and is divided into three classes of biomaterials:

•  Biotolerant
•  Bioinert
•  Bioactive

Biologic Classification of Biomaterials

Biotolerant materials, are characterized by a thin fibrous tissue interface. The fibrous tissue layer develops as a result of the chemical products from leaching processes, leading to irritation of the surrounding tissues such as Poly(Methyl Methacrylate) acrylic resin. (PMMA).

Bioinert materials are chemically inert in the body and exhibit minimal chemical interaction with adjacent tissue such as titanium and alumina (Al 2 O 3 ). Tissue attachment with inert materials is through tissue growth into surface irregularities, by bone cement, or by press fitting into a defect.

Bioactive materials: Certain glasses, ceramics, and glass-ceramics that contain oxides of silicon, sodium, calcium and phosphorus (SiO 2 , Na 2 O, CaO, and P 2 O 5 ) form a chemical bond with bone, resulting in a strong mechanical implant bone bond.These materials do bond to bone through a time-dependent, kinetic modification of the surface triggered by their implantation within living bone. An ion-exchange reaction between the bioactive implant and surrounding body fluids results in the formation of a biologically active calcium phosphate layer on the implant. This layer is chemically and crystallographically equivalent to the mineral phase in bone which is responsible for the strong interfacial bonding.It is necessary to understand the material science of biomaterials and their special requirements as dental implants. Let us discuss the general implant materials in aspect of their physical, chemical and mechanical properties.

General requirements of implant materials
1) Biologically compatibility: an ideal implant material will elicit mainly physiological reactions within the surrounding tissues (bone, connective tissue and epithelium). The interactions between the implant materials and tissues must not lead to secondary alterations in the organism or to biologic instability of the implant due to corrosion or dissolution and resorption of the implant surface.

2) Mechanical requirements

• Dental implants must be able to sustain and transfer loads –It should have adequate mechanical strength in order to distribute the forces uniformly.
• The modulus of elasticity of the implant material should be as close as that of bone in order to prevent peri-implant bone loss which occurs due to a phenomenon called stress-shielding. It is a phenomenon of loss of bone occurring as the result of removal of normal stress from the bone by an implant

3) Chemical requirements

• It must withstand corrosion by body fluids
• It should not have any direct or indirect negative effects on the surrounding tissue and organism

4) Functionality: It should take maximum advantage of available bone and permit the maximum amount of forces to be transmitted through the implant within physiologic limits.

5) It should provide desired esthetics and be conducive to oral hygiene maintenance.