Implant materials in surgery

Although prosthetic materials in surgery are constantly being improved, none are ideal in regard to tissue compatibility, permanent fixation, and resistance to infection.

Two principles are paramount: biocompatibility and a fabric that is incorporated into tissue.

Biocompatibility is the foremost consideration. Both specific and nonspecific immune mechanisms are involved in the inflammatory reaction to foreign materials. Highly incompatible materials, such as wood splinters, are rejected immediately with an acute inflammatory process that includes massive local release of proteolytic enzymes. Consequently, the foreign body is never incorporated and lies loosely in a fibrous pocket. Mechanical irritation may induce a similar reaction, which accounts for the occasional spontaneous rejection of large (usually larger than 00), stiff suture materials from soft tissue sites. In less severe incompatibility, rejection is not so vigorous and proteolysis not so prominent. Mononuclear cells and lymphocytes — the major components of wound inflammatory tissue — direct a response that creates a fibrous capsule which may be acceptable in a joint replacement but it may distort a breast reconstruction severely.

Most implant material in surgery must become anchored to adjacent normal tissues by allowing ingrowth of fibrous tissue or bone. This requires biocompatibility and interstices large enough to incite ingrowth just as in a wound and to allow pedicles of vascularized tissue to enter and join similar units. In bone this imparts stability. In vascular grafts, the invading tissue supports neointima formation, which retards mural thrombosis and distal embolization. Soft tissue will grow into pores larger than about 50 m in diameter and even faster into larger ones. Of the vascular prostheses, woven Dacron is best for tissue incorporation. In bone, sintered, porous metallic surfaces are best. Large-screen polypropylene mesh can be used to support the abdominal wall or chest even in the presence of infection and is usually well incorporated into the granulation tissue that penetrates the mesh. Microporous polytetrafluoroethylene (Teflon) sheets are often not well incorporated and are not suitable for use in infected tissues.

Unfortunately, when the mechanical properties of the implant do not match those of the host tissue, shear forces may overcome delicate biologic unions, with loosening of anchoring sites, especially with orthopedic prostheses.

The implantation space remains vulnerable to infection for years and is a particular problem in implants that cross the body surface. Mesh cuffs around vascular access devices that incite incorporation have successfully forestalled infection for months, but infections that arise from bacteria entering the body along “permanently” implanted foreign bodies, which traverse the skin surface, remain an unsolved problem.

Plastic implants are often chosen for texture and flexibility. Silastic materials are highly compatible, but fixation and material fatigue are problems. Cosmetic implants—particularly silicone breast implants—have a low but troublesome incidence of deforming fibrotic capsule formation. In addition to idiosyncratic healing responses, there is also the problem of toxic responses to trace components such as plasticizers and hardeners. Potential complications include cancers if materials such as asbestos are present even in small amounts. The case for systemic complications such as connective tissue diseases from Silastic implants has not been proved.

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