|Researchers expect the low-wear characteristic of ceramic-on-ceramic implants to reduce osteolysis, a condition that develops when the debris given off from implant wear causes the bone surrounding the implant to break down.|
The clinical success of biocompatible ceramic materials has led to remarkable advances in the quality of life for millions of people. In hip replacements, low-wear ceramic-on-ceramic bearings extend life. As coatings on metal replacements, ceramic materials stimulate bone growth, promote tissue formation, and provide protection from the immune system. And on the cancer-fighting front, tiny glass microspheres deliver large, localized doses of radiation to diseased organs in the body with minimal damage to healthy tissue.
Increasingly, surgeons use ceramics to repair and replace human knees, shoulders, wrists, fingers, teeth, heart valves, and hips. After all, what is the hip? Just a body bearing that survives a lifetime of variable, multiaxial, cyclical, mechanical loads while immersed in a corrosive saline solution at 37C. Fact is, the U.S.-standard metal and polyethylene hip bearings wear with every step, and eventually lead to osteolysis, a condition that develops when the debris given off from implant wear causes the bone surrounding the implant to break down.
Ceramic is smoother and harder than materials currently used in hip implants. Researchers expect the low-wear characteristic of ceramic-on-ceramic implant bearings to reduce osteolysis. The recently-publicized Jack Nicklaus hip-replacement surgery has really put ceramic hip bearings in the news. Nicklaus agreed to participate in an Investigation Device Exemption (IDE) study of the use of ceramics for hip implants that will provide data to the U.S. Food and Drug Administration.
FDA expects to issue PMA (Pre-market approval) for ceramic-on-ceramic bearings for hip replacements this year, even though the technology has been used abroad for decades. According to Ed Levadnuk, product manager at ceramic material supplier Norton Demarquest (Vincennes, France), 40% of all hip replacements in Europe use ceramics, while only 5% use ceramics in the U.S.. A typical hip joint replacement consists of four major components:
A stem, made of titanium or cobalt-chrome molybdenum, that is inserted into the thigh bone (femur)
A ball-shaped head or cap, constructed of metal or ceramic, mounted on top of the stem
A hemispherical shell or cup, usually made of titanium, that mounts in the hip socket
And an insert or glider that is made of ultra high molecular weight polyethylene or ceramic.
"The problem with the polyethylene glider is that it wears with every step, releasing particles that build up in the joint over time," explains Joe Contiliano, member of the acetabular team at Allendale, NJ-based Howmedica Osteonics Corp. Osteonics manufactures Nicklaus' hip implant, and develops, manufactures, and markets other orthopedic reconstructive products, such as knee, upper extremity, trauma and spinal implants for the U.S. and international markets. "Eventually the polymer glider must be replaced. But patients may only get one shot at that because after enough particles accumulate in the joint, the bones break down and the implants loosen," says Contiliano.
To address this problem, Nicklaus' ceramic-on-ceramic hip implant uses a very low wear, alumina-on-alumina bearing. "It's designed to extend implant life beyond 20 years, which is a major benefit for younger or more active patients," says Contiliano.
From coral to "bone bondo." Interpore Cross International Co. (Irvine, CA) processes harvested marine coral into the natural mineral content of human bone. Called hydroxyapatite (HA), the biocompatible human bone graft and tissue substitute is not only used as a coating to stimulate bone growth on metal implants, but is also used in thousands of spinal fusion, bone graft, and other procedures each year.
|In the case of liver cancer, localizing the dosage of radiation with microscopic glass-ceramic beads prevents the damage of healthy tissues.|
Interpore's HA, called Pro Osteon 500 Porous Hydroxyapatite Bone Graft Substitute, offers a safe and viable alternative to autograft (bone directly from the patient) or cadaver bone graft procedures. According to Interpore's VP of Research and New Technology Edwin Shors, autograft procedures experience a 10-20% complication rate at the bone harvest site (often the hip or leg area). As for cadaver bone grafts, Shors asserts that the risk of disease transmission exists, and bone materials often become less than ideal during sterilization in such procedures.
Conversely, because Pro Osteon possesses the same mineral content in human bones and is 100% biocompatible, a rejection in the human body has yet to be reported, says Shors. The material's complete biocompatibility makes it attractive for use in orthopedic, oral/maxillafacial, and ophthalmic surgical procedures and in spinal implants to treat degenerative conditions and deformities in the spine.
Shors reports that Pro Osteon is also used in trauma situations such as badly broken or fractured leg bones. Used in 20,000 of the 500,000 bone grafts performed in the U.S. last year, Pro Osteon patients experience equivalent healing rates to autogenous bone grafts. "Upon healing," says Shors, "the material is as strong as bone."
Tiny cancer killers. Clinical studies recently began on a promising cancer treatment that uses radioactive glass microspheres, injected directly into diseased areas, to deliver large, localized doses of radiation to diseased organs in the body. University of Missouri-Rolla Professor of Ceramic Engineering at the Materials Research Center Delbert Day pioneered and co-developed the promising new treatment for deadly liver cancer. Sent directly into the liver to attack cancerous tumors, the beads deliver a potent treatment while leaving the surrounding healthy tissue undamaged.
According to Day, the key was developing a material that could contain the radiation. Early designs used a radioactive coating. The problem with this method was that once in the body fluids dissolve the coating and allow blood to transport radiation throughout the body. "Now we chemically dissolve the radioactive element on an atomic scale, within the glass bead," explains Day, "minimizing the amount of isotope exposed on the bead's surface. Using a glass microsphere that's insoluble in blood or body fluids, we can keep the radiation where we want it."
External beam radiation, says Day, requires about ten treatments over a 30-day period to deliver a total dose of 2,000 to 5,000 rads. Unfortunately, this dose is too small to be completely effective, but any larger amount would cause too much damage to healthy tissue. By contrast, radioactive microspheres injected into the liver safely deliver an average dose of 15,000 rads in one treatment with minimal collateral damage.
A single injection of microspheres, through a catheter inserted into a major artery, takes about one minute. After a few hours of observation, and if no complications develop, the patient can go home the same day and resume normal activity. Most patients rarely suffer side effects, though a few may have a low-grade fever that lasts for 24 hours, he adds. The liver, which continues to function normally, remains radioactive for about four weeks, but the microspheres contain the radiation, says Day.
The microspheres are incredibly small, about one-third the diameter of a strand of hair. Yet, the five-to-ten million used in each injection are still too large to pass through the liver, which acts as a filter to prevent them from traveling into other parts of the body.
The microsphere treatment is now in the experimental stage in the U.S., says Stephen Freiman president of the Westerville, OH-based American Ceramics Society (ACerS), but a decision to allow broadened use is expected from the FDA later this year. Day notes that glass microspheres show strong potential for other successful medical applications, such as injecting radioactive microspheres directly into the joints of patients suffering from rheumatoid arthritis to reduce pain and inflammation.
Freiman says the therapy is already used overseas and in Canada, and that ACerS is naming it as one of the greatest medical advances in its 100-year history. Currently, the treatment is finding success in fighting liver cancer, but may also be used for other cancers, such as brain tumors, where surgery or traditional radiation treatments present extraordinary risks.