Metal is a tried and true material used in a variety of applications in the human body because it offers strength, durability, and reliability. Unfortunately, like most materials, it also has its downside and new materials being introduced may become better alternatives. This month, MDT asked several industry experts for their thoughts on the use of metal as medicine.

Q: Will metal ever be replaced completely as a material used in healthcare within the human body, what benefits might be realized, and what material(s) might take its place for various applications?

David Z. Pokvitis
We have served the medical industry for 54 years, and have been involved in some of the most profound breakthroughs in technology over that span of time. From some of the first heart pacemakers to cutting/biopsy instruments to implants, our many customers have long pitted metals against other materials to find the safest, most reliable, most cost effective solutions.

David Z. Pokvitis Co-Owner, Able Electropolishing Co.

In the last several years, I have personally seen some advanced implantable products that are MEMS-based, offering incredible miniaturization for precise drug delivery or organ assistance, including brain/eyesight/neural systems. While not privy to all aspects of those devices, metals are all part of those devices. Certainly advanced plastics and ceramics will share "space" and electronics will provide much of the "smarts," there is almost always a large metal component.

When properly fabricated, decontaminated, deburred, or smoothed, it is hard to replace literally thousands of years of experience in metal alloying. There is far too much known about metals, their stability, strength/weight, conductivity, and other attributes to ignore now or into the near future. In the end, companies faced with safety and health of their customers will lean toward the tried and true, and proceed with great caution on newer materials.

Nobody can say with certainty where the industry will be in 20 years, but from the devices I have seen that are due on the market in the next three to ten years, metals will retain an important role in some of the most significant healthcare breakthroughs. We are very excited and challenged to be involved in those breakthroughs.

Ashok Khandkar, Ph.D. CEO, Amedica Corp

Ashok Khandkar, Ph.D.

CEO, Amedica Corp.

Orthopedic implant technology will continue to progress towards materials that improve durability, longevity, biocompatibility, and patient fit. While metals have and will continue to play an important role in the human body, the future points to advanced materials with the promise of addressing the many limitations of metals as discussed in the context of hip and spine implants.Modern hip implants have evolved towards alternate bearings with newer materials for improved implant durability and function. Such materials include highly cross-linked polyethylene and ceramic bearings. These materials are able to reduce risks associated with conventional bearings such as bone loss and allergic response to metal wear particles.

In the spinal fusion arena, inter-vertebral spacers, which are used to stabilize the spine and relieve pain, have evolved from allograft bone grafts to titanium and polyetheretherketone cages to porous tantalum metal spacers. Each successive generation of the spacers is designed to offer an improvement over the previous version. However, the current breed of metal spacers lack the imaging clarity desired by surgeons, limiting their ability to radiographically evaluate bone fusion.

The next generation in orthopedic implant materials may lie in novel silicon nitride-based ceramic technologya ceramic first developed for use in demanding, mission-critical aerospace and defense applications. This ceramic is advantageous because of its proven ability to be substantially stronger and more reliable than previous generations of ceramics. Silicon nitride also offers the biocompatibility required for in-vivo use, and when made in a porous form that mimics trabecular bone, offers the ability to promote bone attachment. Interestingly, inter-vertebral spacers made from silicon nitride possess bone-like imaging characteristics, enabling surgeons to have accurate and distortion-free imaging capability.

The superior wear resistance, strength, and durability of silicon nitride can enable a wider range of implant design and sizing options, thereby providing surgeons the ability to give the patient better anatomic fit and improved clinical outcomes. Thus with this new ceramic, for the first time, surgeons have the option of spinal devices that meet all structural, biologic, bio-mechanical, and imaging requirements.

Deborah Schenberger, Ph.D. Analyst, Nerac Inc.

Deborah Schenberger, Ph.D.

Analyst, Nerac Inc.

Metal will remain the material of choice for fracture repair and total joint replacement for many years, even as other materials are gaining acceptance. In fact, new alloys being developed ensure metal's continued use in the human body.Bone is a living material that adapts to the loads it experiences. When metal is placed within bone, such as during a hip implant, the metal's stiffness is significantly greater than that of bone. As a result, the metal carries most of the load, creating a condition called stress shielding. Bone reacts to this change in loading by resorbing, which eventually loosens the hip implant and leads to revision surgery with a longer implant.

One way to reduce stress shielding, and thus bone resorption, is to decrease the stiffness of the metal in the implant or plate. Metallurgists accomplish this by changing alloy composition, inducing porosity in the metal, or creating a composite of metal with glass, ceramic, or a biomaterial. This is an area of continual and active research, and orthopedic device manufacturers have already developed numerous new orthopedic implants and fixation devices with reduced stiffness materials. Therefore, it is unlikely metal will ever be replaced completely within the field of orthopedics.

Nonmetal materials, such as polymers or ceramics, show promise, but they still do not measure up to metal. One bioabsorbable polymer material is being developed for temporary fixation, for instance in plates and anchor screws. Scaffolding, often a highly porous ceramic material, shows great promise and allows bony in-growth. Eventually, bone is completely incorporated into the scaffold, mimicking the patient's natural bone. As exciting as these new approaches may be, none will be commercially available for years.

Jerry Burke Analyst, Nerac Inc.

Jerry Burke

Analyst, Nerac Inc.

Metal in the human body is normally discussed in the context of implants or appliances inserted somewhere to replace or repair bone tissue and the biological implications of using these metals. At its most basic, however, the body requires certain levels of metalscopper, zinc, iron, manganese, calciumto function properly.As far as whether metal will ever be replaced as an implant or appliance material, the answer is simple: not in the foreseeable future. In fact, current developments in this area are geared more toward making metals more easily accepted in the body by employing coatings for lubricity, biocompatibility, or adhesion. The downside of metal implants and appliances is that the patient outlives the device and revision surgery is required. Replacing metal materials may not eliminate this issue, and for now the harmful effects are minimal when compared to the benefits.

Economically, metal is the most effective material to use. It is a known material, easily obtained, and is easily machined and finished. It has a long history of being used in the body. That is not to say that other materials should not be developed or experimented with. But until the replacement material has demonstrated a cost benefit, it will never be accepted by the medical community, much less insurance companies.

Still, there are exciting materials replacing metals in some applications. For example, bone matrix proteins to regenerate bone growth have been demonstrated in the treatment of osteoporosis and spine degeneration. Even so, it is better to treat the disease than to excise it and replace the bone with metal or some other material. There are also agonists to reverse bone loss, but the question is whether such materials can be extended to allow the body to grow replacement bone. Probably, but that is still generations away.

David Z. PokvitisCo-Owner, Able Electropolishing Co.