Surgical Protocol And Form Of The Implant

The surgical protocol implemented to assess compatibility of biomaterials and devices in tissues depends on the size and shape of the implant and certain characteristics of the tissue. The surgical trauma associated with implantation will initiate a wound healing response that has a profound effect on the subsequent assessment of the compatibility of the implant in the tissue. Therefore, surgical techniques that minimize trauma to the tissue should be employed. The greater the amount of maceration of the tissue, the more extensive the formation of scar tissue. Power tools with high-speed burrs, drills, and blades that are used to produce implant sites in bone can generate excessive heat that devitalizes adjacent tissue; show-speed instruments with irrigation should be employed to minimize this effect.

When implementing surgical protocols to assess the tissue compatibility of devices, there is a need for adherence to strict sterile technique. Stringent control of sterility is required for implant surgery because of the propensity of bacteria to colonize the implant surface. Care needs be exercised to prevent the introduction of extraneous material, such as talc from surgical gloves, into the implant site.

Studies have indicated that conditioning of an implant in aqueous solution prior to its introduction into the body could be important. Air nuclei on the surface of implants have been found to increase the activation of complement molecules and therefore could influence the tissue response.

The size of the implant is particularly important. Substances in particulate form, of a size that can be phagocytosed, are likely to elicit an inflammatory response because of the release of proinflammatory cytokines and eicosanoids by phagocytes (namely, macrophages) during phagocytosis. Implants (and particulate debris) of a size larger than the mononuclear phagocyte might be covered by the same cell type but do not provoke as much of an inflammatory response because the cells are not capable of phagocytosing the implant. Macrophage activity on the surface of these larger implants generally leads to fusion of the cells to form multinucleated foreign body giant cells (Behling and Spector, 1986).

There can be cases where the form of the implant in a particular animal model elicits an unfavorable response that is not found in implants of the same type in human subjects. Films of a wide range of materials implanted subcutaneously in rats were found to cause malignant tumors at the site of implantation (Oppenheimer et al., 1958). This process has been related to the uninterrupted plane surface area of the implant in this particular in vivo model (Brand et al., 1975), and has not been observed in human subjects.

Several methods have been employed for exposing biomaterials to tissues in vivo. Generally the biomaterial is placed in direct contact with host tissue as it is implanted into the surgical site produced by an open procedure. In some cases the material has been contained within a stainless steel cage (Marchant et al., 1983) or gelatin capsule in order to investigate the biological response to biomaterials in vivo without allowing direct contact of tissue with the implant, which could affect the response. Because these methods introduce a second foreign body into the implant site, controls comprising the carrier implant alone are critical. Materials in particle form or in the form of small-diameter rods, able to fit through the needle of a syringe, have been injected percutaneously into tissue sites. The advantage of the percutaneous route is the lesser degree of surgical trauma which can confound interpretation of the response to the material.

One of the most important variables of a surgical protocol is the "dose" of the biomaterial. Studies of the pharmacologic response to drugs and biologies are not considered complete without an assessment of dose response. In the case of soluble substances (namely, drugs), dose is generally determined by weight (and in some cases associated "activity units"). However, dose-response analyses of biomaterials are rarely performed because generally the dose of a biomaterial is dependent on several variables that could affect the response: weight, surface area, bulk size, number of implants (particles), topography. These parameters are interrelated, confounding experimental designs to determine the dose response; changing the dose according to one of these parameters also changes the values of the other parameters in a way that could alter the tissue reaction. In investigations of the biological response to agents that might be released from an implant, surface area might be considered the controlling variable. Surface area is also an important variable in studies assaying the interactions of biological molecules and/or cells with the biomaterial surface. However, because surface area is strongly influenced by rugosity (at the nano- and microstructural levels as well as macroscopically), methods should be implemented to quantify surface area.

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