Implant Materials

Timothy W. King

INTRODUCTION

While autogenous tissues are often the first choice, implant materials have wide application in plastic surgery including reconstruction or augmentation of soft-tissue defects, bony deformities, or the fixation of fractures. Selecting the implant material depends on the specific requirement for its application. For example, tissue ingrowth into a polypropylene mesh or the rigid incorporation of a bone substitute is often desirable, while the encapsulation (or lack of tissue ingrowth) of a silicone Hunter rod allows for free gliding of a subsequent tendon graft.

Autologous tissue may be more appropriate in many clinical scenarios including patients with a history of radiotherapy, marginal blood supply of the surrounding tissue, or tenuous soft-tissue coverage over the implant. In these cases, the risk of implant-related complications, including infection and implant extrusion, is significant, and the use of an alloplastic implant should be avoided if possible.

Implant materials, however, can be used as alternatives to autogenous tissue in selected cases and in specific situations are superior to autogenous tissue. Implant materials can be created to undergo no resorption and are preferable to autologous grafts that will resorb when used as onlay grafts. For example, implants have been successfully used as bone graft substitutes in orbital floor reconstruction, cranioplasty, and maxillofacial reconstruction. They have the advantage of avoiding operative time for graft harvesting and the absence of donor site morbidity.

HISTORY

The first recorded use of an artificial material can be traced to 30,000 B.C. where these materials were used as sutures. The first recorded implant was thought to date back to 3,000 B.C. where pre-Incan Peruvians used materials such as gold, silver, or nut shells to repair trephination defects. However, recent investigations have brought these reports into question. Regardless, over the next 5,000 years, implant use was sporadic and limited by infection and foreign body reaction. The modern era of medical implants is often attributed to British ophthalmologist Harold Ridley who noted that Spitfire canopy plastic unintentionally implanted in the eyes of pilots healed without adverse reaction. Based on this finding he developed and implanted the first artificial lens into a human in 1949. As surgeons, engineers, and scientists continued to create new implant materials, it became clear that there were certain properties that an ideal implant would impart. Cumberland1 and Scales2 described the properties of an ideal implant, which are shown in Table 7.1. Remarkably, although these criteria were published almost 60 years ago, they are still the fundamental properties that manufacturers of modern biomaterials attempt to achieve.

For the purpose of this chapter, implant materials will be divided into the following general categories: metals, polymers, ceramics, glues, skin substitutes, and bioprosthetic meshes (Table 7.2).

METALS

In order to achieve the mechanical and biophysical properties desired for applications in medicine, combinations of metals (alloys) have been developed. These alloys are designed to be inert and withstand the corrosive environment within the human body. Since metals cannot repair themselves after deformation or fatigue, they must have mechanical properties that exceed the properties of the natural tissue they are supporting or replacing (i.e., the metal must be both stronger and stiffer than the natural tissue).

Stainless Steel

Stainless steel has been used as a biological implant since the 1920s. Medical-grade stainless steel, alloys of iron-chromium-nickel, have a relatively high tensile strength but are easily deformed (bent). While this is useful in some applications, such as the application of arch bars for maxillomandibular fixation, overall these mechanical properties are less desirable than other currently available materials such as cobalt-chromium and titanium. In addition, stainless steel leaches metallic ions into the surrounding tissues, causing an inflammatory reaction and pain. Stainless steel is currently used in surgical wire and in arch bars. In the past, bone fixation systems utilized stainless steel, but other alloys have replaced stainless steel in this application.

Cobalt-Chromium

Historically, cobalt-chromium alloys have been one of the most significant biomaterials used in humans. Vitallium, a cobalt-chromium-molybdenum (Co-Cr-Mo) alloy, was first described in 1932 to address some of the problems experienced with stainless steel. Co-Cr-Mo alloy was used in early craniofacial miniplates and screws and revolutionized the field. The major disadvantage of Co-Cr-Mo alloys is the scatter artifact on computed tomography (CT) imaging. Because of this, and other benefits, titanium has essentially replaced Co-Cr-Mo alloys in most biomedical applications.

Titanium

Commercial-grade medical titanium implants were introduced in the early 1980s and have almost entirely replaced the other alloys in medical applications because they are stronger, are lighter, have higher resistance to corrosion, and cause less inflammation. Titanium also has less stiffness, which results in less stress shielding (localized osteopenia secondary to the implant protecting the bone from normal loading). More recently, some companies have also introduced titanium alloy implants. The alloys are stronger than the pure titanium, allowing for thinner plates without compromising their overall strength. Pure titanium or titanium alloys (which have less than 0.5% iron) have two additional beneficial properties: they do not set off metal detectors, and they do not create a significant artifact on CT or magnetic resonance imaging studies. Finally, titanium can form chemical bonds with the surrounding mineralized bone without fibrous tissue forming between the implant and the bone. This unique characteristic allows titanium to be used to create osseointegrated implants. Plastic surgery applications of these alloys include plates and screws for fixation of bone and titanium mesh for use in applications such as orbital wall reconstruction (see Figure 7.1).

TABLE 7.1 PROPERTIES OF AN IDEAL IMPLANT

Minimal foreign body reaction

Elastic or supple

Easily tailored

Good tissue incorporation

Allow collagen ingrowth

Promote permanent tissue repair

Good tensile strength

Tolerate infected environments

Minimal wound complications

Data from Cumberland VH. A preliminary report on the use of prefabricated nylon weave in the repair of ventral hernia. Med J Aust. 1952;1:143-144 and Scales JT. Materials for hernia repair. Proc R Soc Med. 1953;46:647-652.

TABLE 7.2 IMPLANT MATERIALS

Metals
	Stainless steel
	Cobalt-chromium
	Titanium
	Gold
	Platinum
Polymers
	Silicone
	Polytetrafluoroethylene
	Polyester
	Polypropylene (Prolene, Marlex)
	Polyethylene (Medpor)
	Polymethylmethacrylate
	Biodegradable polyesters
	Polyamides (Supramid, Nylamid)
Ceramics
	Hydroxyapatite
	Tricalcium phosphate
Adhesives and glues
	Fibrin tissue adhesives
	Cyanoacrylates
Biologic materials
	Skin substitutes
		Integra
		Epicel
		Dermagraft
		Apligraf
	Bioprosthetic mesh
		Small intestinal submucosa
		Human acellular dermal matrix
		Porcine acellular dermal matrix
		Bovine pericardium
		Bovine fetal dermis

FIGURE 7.1. Titanium plates for midface reconstruction. L-shaped and curvilinear 2.0 mm plates with a 7, 5, and 3 mm length 2.0 mm screw (left to right).

Gold

Although gold is chemically inert, it has poor mechanical properties in its pure form. When strength is required (for example, in dental fillings), a gold alloy is used. For applications such as eyelid weights in patients with lagophthalmos, where strength is not an issue, 24-carat gold alloy (99.9% w/w purity) is used to ensure chemical inertness.

Platinum

Platinum is an inert metal and is the material of choice for patients with gold sensitivity in need of eyelid implants for lagophthalmos. Platinum has a higher density than gold, thus the eyelid implants have a lower profile and are less noticeable than gold implants. Some formulations containing platinum, however, have been shown to be immunogenic and have raised concerns about long-term exposure. Platinum is also used as a catalyst in the formation of some polymers, including the production of medical-grade silicone used in gel breast implants.

POLYMERS

Polymers are molecules composed of repeating monomer subunits. The physical characteristics of a polymer are defined by the structure of the monomer, the number of monomer units in the polymer chain, and the degree of cross-linking. As polymer chains are cross-linked, the ability for them to move independently is decreased. Thus, a polymer with little cross-linking might exist as a liquid while the same polymer with abundant cross-linking becomes a “gel” or “solid.”

Silicone

Silicone is likely the most maligned and misunderstood implant material today secondary to its use in breast implants. Silicone gel-filled breast implants were first introduced in the United States in 1962. Multiple variations and modifications to the shell and gel were made over the years in an attempt to improve the outcomes of breast augmentation and reduce the associated complications. In 1992, the U.S. Food and Drug Administration (FDA) stated that there was “inadequate information to demonstrate that breast implants were safe and effective” and placed a moratorium on silicone gel breast implants for cosmetic purposes but allowed their continued use for reconstruction after mastectomy, correction of congenital
deformities, or replacement of ruptured silicone gel-filled implants due to medical or surgical reasons.3 The Department of Health and Human Services (HHS) subsequently appointed the Institute of Medicine (IOM) of the National Academy of Science to begin one of the most extensive research studies in medical history. In 1999, the IOM released a comprehensive report on both saline-filled and silicone gel-filled breast implants finding that “evidence suggests diseases or conditions such as connective tissue diseases, cancer, neurological diseases or other systemic complaints or conditions are no more common in women with breast implants than in women without implants.”3

Only gold members can continue reading. Log In or Register to continue