Implantable polymeric prostheses

The application of polymeric materials in medicine is a fairly specialized area with a wide range of specific applications and requirements. Although the total volume of polymers used in this application may be small compared to the annual production of polyethylene, for example, the total amount of money spent annually on prosthetic and biomedical devices exceeds 16 billion in the United States alone. These applications include over a million dentures, nearly a half billion dental fillings, about six million contact lenses, over a million replacement joints (hip, knee, finger, etc.), about a half million plastic surgery operations (breast prosthesis, facial reconstruction, etc.), over 25,000 heart valves, and 60,000 pacemaker implantations. In addition, over AO,000 patients are on hemodialysis units (artificial kidney) on a regular basis, and over 90,000 coronary bypass operations (often using synthetic polymers) are performed each year (]J. 

Dental Polymers. Every year nearly a half billion dental fillings are done, and over a million dentures are constructed. Most of the materials used in each of these cases are polymeric. In addition, over 300,000 dental implants are made each year with either ceramics or polymers (1). The majority of the dental fillings and dentures are made from various copolymers of methyl methacrylate with other acrylics, although some other polymers, such as polyurethanes, vinyl chloride-vinyl acetate-methacrylate copolymers, vulcanized rubber, and epoxies, have been used to some extent. One major problem is aesthetics—the prosthesis must look natural and not discolor (by photoinduction or staining) to any great extent. 

The first implanted synthetic polymeric biomaterial appears to be PMMA, which was used as a hip prosthesis in 1947 (see USP XVIII, The Pharmacopia of the USA, (18th Revision), US Pharmacopoeial Convention, Inc., Rockville, MD, 1 September 1980). Polyethylene, and then other polymers, were used as implants in the middle ear in the early 1950s, yielding good initial results, but local inflammation limited the use of these materials. 

This principle can be applied to bone cements. After polymerization of the (hydrophilic) monomer inside the femoral cavity and insertion of the prosthesis, the hydrogel would start absorbing water, due to the same constraints referred to above, fixing the implant in place. Since the bone cement must fulfil some minimum requirements in terms of mechanical properties, the combination of hydrophilic and hydrophobic (MMA) monomers will allow for the tailoring of water uptake and, consequently, the mechanical properties of the system. 

Bone reaction in the implant bed is characterized by the formation of a layer of necrotic tissue some 0.5—3.0 mm deep. After the initial necrotic phase of 2—3 weeks, the repair phase follows, which may take up to one year, and stabilization will only be complete after a maximum of 2 years. As necrotic changes may lead to the loosening of the prosthesis, their possible causes must be understood interference with bone vascularization at operation is one of them, alongside polymerization temperature and cytotoxicity of the monomer. 

Two examples of this phenomenon, a vascular graft and a total hip replacement, show how the fibrous capsule which develops around the implant may involve and extrinsically compress other organs. In these two cases, the fibrous encapsulation of an aorto-femoral vascular graft and polymethylmethacrylate bone cement from the acetabular component of the total hip prosthesis, respectively, have also encapsulated a ureter leading to obstruction of the urine flow from the kidney to the bladder. In each of these cases, the fibrous capsule formation is normal, that is, it is no different histologically from fibrous capsules which form around biocompatible polymeric prostheses. 

In describing the components of total joint prostheses and their interaction with adjacent tissues, many authors identify the articulating interface (metal/polymer), the metallic prosthesis - polymethylmethacrylate bone cement interface and the bone cement-bone interface. Unfortunately, the bone cement-bone interface may be transient with a fibrous capsule eventually interposing itself between the bone cement and the bone to create the bone cement-fibrous capsule-bone interface (19,20). As described earlier, the formation of a fibrous capsule around implants is a common occurrence. In the case of total joint prostheses where situ polymerizing methyl methacrylate is used as bone cement, this fibrous capsule formation may be accelerated by the heat of polymerization or the toxicity of the monomer, both of which may lead to localized tissue destruction and cell death (21,22). 

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