Building New Body Parts

     Producing a material that can function intimately with living tissue, with minimal adverse reaction, is quite a challenge for engineers and scientists.  Biomaterials are designed to perform specific functions in the body and, at times, are used to replace parts of living systems.  Some common implants include knee and hip joint replacements, spinal implants, and bone reinforcement devices.   Also popular are artificial heart valves, soft tissue replacements, and a variety of dental implants.  Each of these devices must be constructed of special materials that are uniquely suited for their respective tasks.  Properties such as mechanical integrity, corrosion resistance, and biocompatibility must be evaluated for any biomaterials.

     In recent years, the study of biomaterials has become increasingly important with the increase in the number of implant operations being performed.  According to the Journal of Long-Term Effects of Medical Implants, in 1998 there were 674,000 adults in the U.S. using 811,000 artificial hips.  The same journal also reported that 170,000 people worldwide received artificial heart valves in 1994.

Orthopedic Biomaterials

     As an alternative course of treatment, after unsuccessful attempts at physical therapy and non-surgical treatments, surgeons often turn to artificial devices to replace deteriorated joints.  

Bone plates, introduced in the early 1900's, were among the earliest successful biomedical implants.

     Artificial joints consists of a plastic cup made of ultrahigh molecular weight polyethylene and a metal or ceramic ball attached to a metal stem.  The metals of choice are titanium or a cobalt-chromium alloy while ceramic materials are typically made up of aluminum or zirconium oxides.

     Artificial joints can replace hips, knees, shoulders, wrists, fingers, or toe joints.  Replacement can help restore function to a joint that has been impaired by a degenerative disease or trauma from sports injuries or other accidents.  In the U.S. alone, an estimated 300,000 patients receive joint implants every year. 

     Material researchers are trying to find other materials to replace the ultrahigh molecular weight polyethylene in joint replacements.   On average, the joint cups lasts about 10 years and researchers believe the joints can be made to last even longer.  In fact, nearly everyone involved in orthopedic replacements agrees that the polymer cup is the final hurdle on the way to perfecting artificial joints.  As the polymer wears due to friction, small particles (called "wear" particles) can break off and become embedded in surrounding tissue.   The body's immune system counters by releasing enzymes in a vain attempt to digest the wear particles.  Instead, the result is the eventual death of adjacent bone cells.  Over time, sufficient bone is resorbed around the implant causing loosening which requires implant replacement or adjustments.
     To illustrate the importance of material choice for joint implants, consider the use of polytetraflouroethylene (PTFE or Teflon™) to construct temporomandibular joints (TMJ) in the 1980's.  The choice seemed to be a good one as PTFE exhibits a low coefficient of friction and had been used extensively as a bearing surface in other engineering applications.  In 1983, the Federal Drug Administration (FDA) approved the use of PTFE as a viable TMJ replacement joint material.  However, of the more than 25,000 PTFE TMJ implants received by patients, most failed.  The problem was in the manner that PTFE is able to maintain its low coefficient of friction.  A thin film of PTFE is continuously transferred to the opposing material.  In the TMJ   implants, surrounding tissue quickly became overwhelmed by wear particles.

Heart Valve Biomaterials

     Heart valves are typically made of one of two types of material:  "soft" bioprosthetic materials such as denatured porcine or "hard" synthetic materials such as pyrolytic carbon.  Both types of material exhibit problems when implanted.

     Bioprosthetic valves often fail due to calcification which can result in mechanical malfunctions and vascular obstructions.   Bioprosthetic valves are also prone to mechanical fatigue.  Cyclic loading can result in cracking followed by catastrophic failure.  The most common problem with mechanical heart valves is thrombosis (clotting) which can cause serious health complications and even death.  Coating mechanical valves with pyrolytic carbon has become very popular due to its excellent resistance to clotting.  Pyrolytic carbon is currently being studied to assess its resistance to fatigue.

Parting Word

     It's easy to see that just because a material performs a function well in the world of engineering outside the body, this is not easily translated to success inside the body.  Our bodies are the most complex engineering "system" in the world and it takes specialist to develop the materials that will help better our way of life in the future.  Scientist, working together with engineers, continue to develop biomaterials that will enhance life for everyone and help us enjoy longer, more prosperous days.

References:

Blanchard, Cheryl R., "Biomaterials: Body Parts of the Future," Technology Today, Fall 1995, Southwest Research Institute.

Sharkness, C.M. et al., "Prevalence of Artificial Hips in the United States," Journal of Long-Term Effects of Medical Implants, 2 (1), 1-8, 1992.

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