Ceramics replace body parts

When it comes to innovations in the medical world, check out ceramics. Surgeons are using bioceramic materials for repair and replacement of human hips, knees, shoulders, elbows, fingers, and wrists. The materials are also helping to restore diseased heart valves. Dentists are using ceramics for tooth-replacement implants and braces. Research is even being done on a ceramic-based gum thought to retard the growth of plaque.

When introduced in the human body as implants, or even as coatings to metal replacements, ceramic material can stimulate bone growth, generate tissue formation, and provide protection for the immune system. Glass microspheres smaller than a human hair are being used to deliver large, localized amounts of radiation to diseased organs in the body. In fact, ceramics are one of the few materials durable and stable enough to withstand the corrosive effect of bodily fluids. Here's a look at some of the latest medical applications for these versatile materials.

Heart-rendering ceramics. Compared to healthy natural heart valves, contemporary mechanical heart-valve replacements are deficient in three important respects. First, the small aortic sizes of the valves do not match those of the natural valves they replace. Second, all current valves, no matter what the size, must be closed by reverse flow. The latter deficiency is especially pronounced for larger mitral prostheses. Third, mechanical prosthetic valves require anticoagulation therapy.

With these deficiencies in mind, the Medical Carbon Research Institute (MCRI), Austin, TX, called on current advances in material and design technologies to create an enhanced heart valve. MCRI's objective: to optimize valve design to deliver maximum net forward flow with small pressure losses over the entire size range, while maintaining the highest structural integrity and surface quality. The result: the On-X^TM series of prosthetic heart valves.

The valves rely on MCRI's On-X pyrolytic carbon, which has unique mechanical properties that broadened the valve-design possibilities. Until now, producing pyrolytic carbon orifice shapes that optimize nozzle efficiency had not been possible, says Axel D. Haubold, MCRI vice president, research and development.

Specifically, shapes other than straight cylinders could not be stretched sufficiently without fracture to allow leaflet insertion. On-X carbon has a fracture strain 25% higher than other pyrolytic carbons. That allows orifices with flared inlets, preventing deformation of flow separation elastically during assembly. Further, the carbon orifices with flared inlets can be made thin, conserving orifice area. And, by virtue of their shape, flared orifices are inherently stiff, eliminating the need for bulky metallic stiffener rings.

Based on this material's capabilities, MCRI designed its valve replacements for the small aortic root with improved hemodynamics. The key: increased orifice internal diameter. Knowing that the flow through an orifice depends on the fourth power of its internal diameter, consider this example. If an orifice internal diameter is increased by 10%, flow increases by 50%.

For larger valve sizes, limiting the internal diameter to the optimum geometric orifice area (OA) achieves pressure losses that are clinically insignificant, says Haubold. Increasing the internal diameter beyond that of the OA provides a small gain in pressure loss, which is nullified by a disproportionate increase in negative factors.

Key features of the "new generation" of the X-On heart valves include:

A patented elongated oriface that organizes flow and reduces turbulence. It also allows for a small leaflet excursion angle, reducing regurgitant flow, cavitation potential, and noise.

With these design features working for it, Haubold feels that MCRI has achieved a notable advance in heart valves. "Through innovation in materials, science, and design, On-X valves offer an optimized hemodynamic benefit to the patient by providing maximum net forward flow with minimal pressure losses and low turbulent shear stress," he enthuses.

Liver treatments. In the case of liver cancer, localizing the dosage of radiation prevents the damage of healthy tissues. This is where microscopic glass-ceramic beads (microspheres) play a major role. The beads are radiated and then injected through a catheter into the body to treat the cancer. This new treatment, currently in use in Canada, is being evaluated for use in the U.S.

With this type of treatment, the patient can be given higher radiation dosages, which shortens the treatment periods, according to Professor Delbert Day of the Ceramic Engineering Department and Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO. In addition, localizing the dosage minimizes the side effects associated with other forms of radiation treatments.

Day is a co-developer of the procedure. It is licensed by the university under the tradename of the glass microspheres, TheraSphere.

The safe delivery of {beta} and {gamma} radiation to diseased internal organs has been a major concern for medical researchers over the years. The in situ irradiation of these organs has several advantages over external radiation sources: more localized radiation, less damage to healthy tissue, higher radiation doses, shorter treatment periods, and less patient discomfort.

Dissolving a {beta}-emitting radioisotope in chemical insoluble glass microspheres holds much promise for such in situ irradiation treatments. The glass microspheres are biocompatible and nontoxic to the body, insoluble (no release of the radioactive material to the body), free of unwanted elements that would become radioactive after neutron bombardment, and sized to lodge in the capillary bed of the organ to be treated.

At the University of Michigan, 24 liver-cancer patients were treated in research designed to determine the toxicity of the Yttria Aluminosilicate (YAS) glass microspheres--not their effectiveness--for in situ radiation. Only six of the patients received a dose exceeding 10,000 rads, considered the minimum therapeutic dose.

Four months after treatment with the microspheres, 16 of the 24 patients showed positive response; i.e., tumor growth either stopped or decreased. Follow-ups consisted of physical examinations and laboratory tests of liver function every week for the first eight weeks, and then at eight-week intervals until disease progression was observed. Chest radiographs and abdominal computed tomography examinations were conducted every eight weeks, again until disease progression was observed.

Tumor growth continued in four patients immediately after injection; it did not occur for six months in about one-half of the patients. Ten patients were alive 19 months after treatment. The mean survival time for those patients with colorectal cancer treated with the radioactive YAS glass microspheres was 60 weeks. The median survival time for persons with untreated metastases from colorectal cancer is 4 to 40 weeks.

From these tests, Day concludes, these results can be noted:

Doses of up to 15,000 rads can be safely delivered by {beta}-emitting radioactive glass microspheres.

Improving impaired hearing. In yet another application, glass ceramics are used to replace the small bones (ossicles) in the middle ear, which help conduct sound waves to the inner ear. Most important, these special materials bond to both hard and soft tissue. And they are biocompatible, strong, durable, and nontoxic.

The benefits derived from this application include enabling the patient to hear more clearly; strong bonding to bones/tissues prevents the implant from moving, falling out, or cutting the ear drum, which has presented a problem with plastic implants.

U.S. Biomaterials Corp., Baltimore, MD, makes such a product based on Bioglass, developed by Professor Larry Hench at the University of Florida. The first clinical application of Bioglass was as a middle ear prosthesis for the treatment of conductive hearing loss. Such hearing loss is commonly due to chronic otomastoiditis, but may also be due to trauma and to congenital abnormality.

Many materials previously used for treatment were unsuccessful in long-term use because of encapsulation by scar tissue, which dampens the sound before it reaches the oval window, but mainly because the materials did not bond to the ear drum.

Any implant damaged during implantation must be rejected, since soft tissue will grow into the cracks and, in time, widen them sufficiently to fail. Even so, in eight or more years of successful clinical performance the Bioglass ceramic device proved superior to that of any other available device. But that was not good enough.

The latest design of this device is U.S. Biomaterials' truncated cone, the Douek-MED(R), which can be shaped at the time of surgery to accommodate any anatomical variation found in the middle ear. The new implant can be contoured to ensure that it does not come in contact with tissues, such as the facial nerve, where bonding must be avoided. And it fits well over the tapes or around the malleus, where those ossicles remain, eliminating the vulnerable junction, a key improvement to the conical implant design.

Even more recently, U.S. Biomaterials announced it has developed another promising ceramic material that could help millions of people who suffer from hypersensitive teeth. It hopes to launch human clinical trails this year, and to present that data to the FDA by early next year.

Orthopedic substitutes. The area where ceramics have won the widest acceptance, however, is their use in orthopedic prostheses for the replacement of joints in hips, knees, and fingers. Ceramics' bonding capabilities keep the implant from dislocating. Also,ceramic coatings allow the implants to wear longer with fewer future complications due to their ability to lubricate the joint.

Ceramic ball joints can better withstand complex stress states and constant movement put upon them. Their strength, wear, and corrosion resistance allow for longer use, more comfort, and less breakage than metal, where problems have arisen because the metal particles migrate into surrounding tissues.

A recent improvement to this type of treatment was introduced by XYLON Ceramic Materials, Alfred, NY. The company recently scored a milestone when it become the first U.S. producer to gain federal Food and Drug Administration registration for a ceramic hip ball-joint replacement that can go directly into the recipient's hip socket.

Previous hip ball-joint replacement systems, including XYLON's, which in 1991 become the first American-made ceramic femoral head to gain FDA registration, required the ball to be inserted into a polyethylene cup. XYLON's ceramic ball caused less wear of the polyethylene cup than metal replacement joint, making it better for use in people who lead active lifestyles.

But the new, larger ceramic ball, says XYLON's Paul Johnson, can be used against the natural tissue of the patient's own hip socket, eliminating the need for a polyurethane cap. Since it has received FDA approval, surgeons can use the ball in patients who do not require a total hip replacement.

XYLON will soon submit for FDA registration a shoulder implant, and is working on developing replacement joints for the thumb, knee, and big toe, as well as expanding into other wear- and corrosion-resistant applications.

How glass is used in the body

For the most part, glass can be used in the body in three ways. So reports Professor Delbert Day of the Ceramic Engineering Department and Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO.

The first is that of biologically inert alkali-- alkaline-earth- aluminosilicate glasses used traditionally in dental restoration--caps, crowns, bridges, and veneers. Newer glass-ceramics and glass/polymer composites are replacing metal amalgams used for fillings.

The second body use is that of modified soda-lime-silica glasses that are bioactive and have the capacity to bond to living soft and hard tissues. These glasses, called BioGlass^TM, were discovered by Larry Hench at the University of Florida. Soda-lime-silica glasses modified with phosphorus oxide are used as dental implants, for the treatment of periodontal disease, and to restore hearing.

The third use of glass in the body is that of rare-earth alumininosilicate glasses to deliver large doses of localized radiation to diseased organs. They are used to treat patients with liver cancer, which is almost always fatal. These glasses differ from other glasses because they contain no alkali or alkaline-earth oxides--and are radioactive when placed in the body.

Eye-opening implants

Ceramics are also used to make artificial eyes (optical implants). The ceramics bond with existing muscles, tissue, and blood vessels, and are nontoxic, biocompatible, and permit the artificial eye to move more naturally.

Interpore International, Irvine, CA, produces such a material from a coralline hydroxyapatitie it calls Pro Osteon(R) Interpore 200(R). The porous ceramic is derived from coral using a proprietary Replamineform^TM process. The exoskeleton of marine biocompatible material (tiny sea animals called polyps) has interconnected porosity, architecture, and chemical composition similar to that of human bone, facilitating bone and tissue growth.

The orbital implants replace damaged or diseased eyes, artificial eyes that have extruded or migrated, or existing artificial eyes to improve tracking and movement. Interpore claims that these orbital implants provide a superior alternative to other artificial eyes because the implant is physically attached to the patient's soft tissue. As a result, the patient typically experiences integration of the soft tissue into the orbital implant. This, in turn, improves the tracking of the artificial eye with the patient's companion eye.

Pro Osteon was the first commercially available synthetic bone void filler for orthopedic applications in the U.S., according to David Mercer, Interpore's president and CEO. In clinical studies, the material was demonstrated radiographically and clinically to have healing time and complication rates comparable to autograft procedures.

In an autograft, bone material is harvested from another part of the patient's skeleton and grafted to the site of the bone deficit. This procedure increases total operating time and expense, and can lead to complications, such as infection, chronic pain, deformity, and excess blood loss.

Pro Osteon, says Mercer, provides an attractive alternative to autograft and allograft (cadaver bones) materials. It is approved for the repair of acute metaphyseal (where the bones meet)defects in conjunction with rigid internal fixation (plates and screws) when there is no bone-donor site available. He does not believe that any symptoms or conditions will materially limit the use of Pro Osteon.

Other applications