Biocompatibility of implants

Possible effects of biomaterials on the living environment due to a lack of blood compatibility are thrombogenicity, and the induction of hemolysis. In addition, the biomaterial must not be carcenogenic, immunogenic, antileukotactic or mutagenic. In turn, the environment should not cause degradation or corrosion of the biomaterial that would result in loss of physical and mechanical properties. No synthetic material will be completely harmonious or inert with the living environment, however, materials do have different levels of inertness.

There are many factors which influence implant biocompatibility such as implant size, shape, material composition, and surface wettability, roughness and charge. The following sections provide a brief overview of polymeric, ceramic, and metallic biomaterials along with a discussion of surface changes and drugs that promote biocompatibility.

For a material to be deemed biocompatible, any adverse reactions which may ensue at the blood/material or tissue/material interface must be minimal, while resistance to biodegeneration must be high. This requires a biomaterial to interact as a natural material would in the presence of blood and tissue. Implantable materials should not:

• Cause thrombus-formations
• Destroy or sensitize the cellular elements of blood
• Alter plasma proteins (including enzymes) so as to trigger undesirable reactions
• Cause adverse immune responses
• Cause cancer
• Cause teratological effects
• Produce toxic and allergic responses
• Deplete electrolytes
• Be affected by sterilization

To the present date, there are no known materials which totally satisfy these criteria so when a foreign material is placed into a biological environment, inevitable reactions occur which are detrimental to both host and material.

Implantable materials all possess inherent morphological, chemical, and electrical surface qualities which elicit reactionary responses from the surrounding biological environment. In fact, biocompatibility can be described as multifactorial in that simultaneous stimuli from any of these material properties can affect the host response.

Reactionary tissue response begins with inflammation at the site of implantation. Proximal capillaries grow in diameter (vasodilation) causing an increase in the permeability of their endothelial cell linings. This allows fluid to flow into surrounding tissues and cause swelling. It has been suggested that changes in blood flow at the site of implantation can be directly associated to the magnitude of inflammatory response. Shortly after inflammation begins, a variety of cells move into the area. Neutrophils (a type of leukocyte responsible for phagocytosis) are first to appear at the inflamed site. The accumulation of these and other leukocytes activate macrophages, which secrete mediators of inflammation and tissue destruction. This gives rise to foreign body giant cells and fibroblasts, both of which are key factors in the inflammatory process.