SAN DIEGO - Materials used in orthopedics and maxillofacial surgery are finding ophthalmic applications in reconstructive surgery, offering implants with better biocompatibility and more natural looking movement.
The last major advance in ocular implants came in the 1980s when a doctor began using hydroxyapatite (HA) from sea coral. Arthur C. Perry, MD, used hydroxyapatite that had been made in a process that changes sea coral (calcium carbonate) into the mineral portion of human bone (calcium phosphate) through a hydrothermal reaction.
Not only was the new material more biocompatible than previously used glass or PMMA spheres, but the coral was porous, which allowed the body to vascularize through the implant. This offered less chance of implant migration and improved the range of motion of the artificial eye.
"With the muscles hooked onto it, the eye socket can have movement into it," said David R. Jordan, MD, a reconstructive surgeon in Ottawa. "Once you can translate eye movement into the socket, then you have an artificial eye that moves in conjunction with the real eye."
Dr. Perry noted that the coral-based material made by Integrated Orbital Implants (IOI), located here, has no blind channels in it. That is, he said, the natural coral has no dead-end pockets in which infections could accumulate, and which the body cannot vascularize.
Although HA is now considered the preferred material for implants, researchers are looking to reduce the cost of the material by synthesizing the calcium phosphate artificially, Dr. Jordan said.
In France, the company FCI has worked for the past 3 years to synthesize new hydroxyapatite materials that would cost half as much as the natural product. Synthetic HA is now distributed in Canada and Europe, although American patent laws prevent its domestic distribution.
Dr. Jordan noted that synthetic hydroxyapatite is "slightly less porous, but still vascularizes extremely well, allowing all the advantages of natural hydroxyapatite. Synthetic HA is not only cheaper but is easier to work with and environmentally conservative," he said.
FCI is also working on Alumina, a form of aluminum oxide that has been used in hip implantation for 25 years.
The material is stable in the body and biocompatible with the body's tissues. After the Alumina is implanted, the body coats it with proteins, Dr. Jordan said. The implant becomes immunologically camouflaged and is perceived by the body as its own, he said. He has completed preclinical work on the material and plans to start clinical trials soon in humans.
"Our feeling is that just being biocompatible is not good enough any more," Dr. Perry said. "We are looking for materials that are interactive with the body and become part of the patients' own tissue. In the future, medical implants are going to be more interactive with the body."
Researchers also are investigating materials that can be applied in and around implants in order to make them move and fit more naturally in the socket.
HA implants generally are wrapped before implantation because the porous surfaces are rough and act like Velcro when being inserted, Dr. Perry said.
Adding a layer around the implant ensures the spicules of HA do not abrade the overlying tissue. This may aid mobility as well.
"We've been recommending using eye bank sclera, but it's sometimes hard to get and sometimes the quality of that is irregular," Dr. Perry said. "We're looking at some other materials, such as bovine pericardium, to cover the implant."
Bovine pericardium is also being used because it is cheaper and easier to obtain. Manufacturers of bovine pericardium for other purposes are now making the material in ball form for ocular uses, Dr. Perry said.
The latest company to obtain marketing clearance is Bio-Vascular Inc. of St. Paul, Minn., which makes Ocu-Guard. Their brand of bovine pericardium is scheduled for release this year.
Dr. Jordan prefers Vicryl Mesh (Ethicon), a synthetic material made from the same material as sutures. It is an inexpensive and simple to use implant wrap, he said. "Not only does it help the implant slide into position, but it allows improved vascularization to the implant," he said. "In addition it is absorbable and disappears with time."
Other materials used with implants can also aid the natural movement of the eye. Titanium pegs have proved to bind to tissues better than other types of materials, Dr. Perry said. Porex Surgical, College Park, Ga., offers the motility couplin post (MCP) titanium screw kit for its Medpor implant. IOI also has a new titanium pegging system.
Like HA, Medpor has no blind channels. It is made from high density polyethylene, which is used by the National Institute of Standards to test the biocompatibility and foreign body reactions of new implant materials.
The MCP is placed in the Medpor implant and projects through the conjunctiva, allowing the placement of an artificial eye.
Titanium pegs are smaller and offer better adhesion, a result of the oxidation process the metal undergoes when exposed to air or water. A film forms on titanium that allows it to adhere to tissue.
Another material for orbital implantation that was designed to be more interactive with the body is demineralized bone, a bone graft that is prepared by a process patented by Osteotech Inc., of Eatontown, N.J. The material comes from human cadaver bones, usually rib segments, and is processed to remove the calcium and phosphorous and to leave behind the proteins.
The remaining material is flexible, can be sutured or cut easily with scissors, and interacts with the surrounding orbit to promote new growth, said Jeffrey Schiller, MD.
The material can be used to restore volume to sockets after enucleation or fractures, or repair damage in blowouts, for example.
The material interacts with the body by two methods: osteoinduction, where the body lays bone at the site of the proteins, and osteoconduction, when it is implanted next to bone and the bone migrates into it.
"When you look at the material 6 months later, the body has eliminated the foreign protein, while replacing it with fresh native bone," Dr. Schiller said. "There's nothing there but the patient's own bone."
Although the material has been used in orthopedics for almost a decade, it has only recently been implanted in the orbit. Dr. Schiller began to work with the material after assisting craniofacial surgeons who treated injuries close to the orbit.
Dr. Schiller said more than 15 patients have been treated to date with the material.
Demineralized bone is easier to obtain than the patient's own bone, which another surgeon would have to harvest, he added, with the pain and morbidity of a second surgical site. The material will not transmit viruses, such as AIDS, because of careful donor and material screening and extensive chemical processing.
However, Dr. Schiller said he uses the more traditional Medpor when he wants a thinner, more malleable sheet of material for periorbital fractures.
"Patients demand an eye that looks and acts like a normal eye," Dr. Perry said. "With integrated orbital implants, we're able to peg the implant and make it move similar to the other eye."
While price is a concern, many patients are willing and able to spend a few extra hundred dollars for a natural-looking eye. Also, the costs of the materials are only between 7% and 13% of the entire cost of replacing a lost eye.
"There's a big difference in the cost of a porous implant versus a silicone ball, but there's a big difference in the performance," he said. "With porous implants, there is a coupling mechanism, which aids in supporting the weight of the lower eyelid. Cost is going to be a concern, but at some point in time the patient ought to be aware that there are better implants out there rather than plastic and silicone balls."
For Your Information:
- Arthur C. Perry, MD, FACS, clinical professor at the University of California at San Diego and a consultant in ophthalmic plastic surgery for the U.S. Navy Regional Medical Center in San Diego, can be reached at Ste. 310, 9850 Genessee Ave., La Jolla, CA 92037; (619) 457-3010; fax: (619) 457-0028. Dr. Perry has a direct financial interest in products mentioned in this article. He is a paid consultant for Integrated Orbital Implants Inc.
- David R. Jordan, MD, practices at 340 McLeod St., Ste. 104, Ottawa, Ontario, K2P 1A4, Canada; (613) 563-3800; fax: (613) 563-1576. Dr. Jordan has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
- Jeffrey D. Schiller, MD, practices at 245 N. Gannon Ave., Staten Island, NY 10314; (718) 370-1001; fax: (718) 370-0945. Dr. Schiller has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.