While the cancer slowly, steadily consumes the liver, it too falls under attack. Millions of glass microspheres bombard the malignant tumor with high-dose beta radiation. Injected directly into the blood that flows directly into the organ, each one of the tiny glass spheres irradiates surrounding tissue, killing cancer cells and reducing tumor size. One month later, their captive radioisotope decays and the microspheres remain harmlessly in the body.
Epilepsy-induced seizures stifle movement, convulse the body, or block consciousness. Intermittent electric pulses, transmitted to the brain through the vagus nerve, have been shown to decrease the frequency of seizures or abolish them in some patients by conditioning the brain to better react to the interruptions in brain function common in epilepsy. How is this accomplished? With an externally programmed pulse generator system implanted in the chest. Delivering intermittent stimulation on a 24-hour basis, its leads extend under the skin to the lower neck where it wraps around the vagus nerve.
What used to seem extraordinary is now becoming ordinary thanks to medical implant technology. Heart pacemakers are, perhaps, the best known example. A bold experiment 30 years ago, pacemakers today better the lives of two million users worldwide.
The idea and implementation of medical implants, however, is hardly new. Consider teeth. Around the time Louis XVI roamed the halls of Versailles and King George III ruled England, dentures were commonly constructed from the teeth of sheep and other animals. The cream of society sipped champagne through more exotic materials--teeth carved from elephant or hippopotamus tusks.
Titanium and ceramic are today's materials of choice; computer aided design and other tools perfect their use. As an example, Sweden's Nobel Biocare, known for its Branemark dental implant system, points to the company's Procera Division as its fastest growing business unit. Laboratory technicians using a personal computer and Procera scanner can precisely model a titanium/ceramic anchor and crown, and send measurements to Nobel Biocare for manufacture. Turnaround time is 36 hours. Already available in Europe, the procedure was introduced in the U.S. and Canada in late 1997.
In addition, Nobel Biocare has recently turned its expertise in titanium implant technology to facial reconstruction and hearing aids. Titanium facial components, the company reports, can form a bridge between the patient's bone and the prosthetic device--usually silicon--for better mobility and less discomfort.
In hearing applications, titanium implants fuse with the skull and ear bones; vibrations from sound waves are then transported to the inner ear via the skull bone. The technique is for people whose type of hearing loss can't be corrected with ordinary hearing aids.
Reconstructing the nervous system may prove to be the ultimate application for medical implant technology, integrating materials, electronics, and software. At the Fraunhofer-Institute for Biomedicine Technology (St. Ingbert, Germany), researchers are investigating flexible nerve plates for integration into multi-contacting nerves. Preliminary configurations involve a thin polyimide flexible plate with electrodes and connecting leads of platinum and iridium. Fitted to one or both sides of the plate, the electrodes facilitate charge transfer during nerve stimulation.
A similar application is the Intelligent Neural Interface project, which is the subject of cooperative research under the European ESPRIT program. Here, the electrode microsystem consists of a grid structure through which the axons or nerve fibers can grow, an integrated signal conditioning system, and guide channels for the nerve. Currently in the early stages of development, this system is being tested on cell structures. The ultimate objective: enabling patients to control prosthetic devices using nerve currents.
Commercially available devices may not be so sweeping in function, but still do amazing things. The following products exemplify the range and scope of medical implant technology:
Radiation delivery vehicles
Microspheres of rare earth aluminosilicate (REAS) glass, when used together as a radiation delivery vehicle inside the body, initiate a chain of positive events.
1) Since the tumors are irradiated in situ, weaker and more localized beta radiation replaces the more penetrating gamma radiation of external beam treatment systems.
2) Because shorter-range beta radiation is less likely to damage nearby healthy tissue, higher radiation doses can be tolerated.
3) Higher, localized doses increase the probability of destroying malignant cells.
Typically, REAS glass contains only four elements: oxygen, aluminum, silicon, and the desired rare earth. Neutron bombardment turns the rare earth element into a radioactive isotope such as Y-90 or Sm-153.
Alkali-free and high in alumina/silica content, REAS glass microspheres are basically insoluble in physiological liquids. This, says Delbert Day, co-developer of the REAS microsphere technology and Curator's professor of ceramic engineering at the University of Missouri-Rolla, prevents escape of the rare earth radioisotope from the target site in the body.
Day adds that because the rare earth element can account for as much as 60% of the glass microsphere by weight, the maximum radiation dose delivered to a target organ can be customized. Microsphere content can also be tailored to match the size of a specific organ, since tissue penetration varies with different rare earth elements.
Glass microsphere technology is the basis of a commercial product called TheraSphere(TM). Made by Nordion International (Ontario, Canada), TheraSphere is approved in Canada for treating patients with either primary or metastatic liver cancer. Treatment time takes two to four hours, and patients experience little discomfort--usually just a slight fever.
In the U.S., the procedure awaits FDA approval and remains experimental. REAS research, however, is ongoing. Other goals: combating cancer in other organs such as the kidney and brain, and treatments for rheumatoid arthritis.
Pacemaker for the brain
After brain injury, epilepsy is the world's most prevalent neurological disorder, affecting an estimated 50 million people. An alternative treatment promises hope to those who have not found relief from seizures using commonly prescribed antiepileptic drug therapies.
Developed by Cyberonics Inc. (Houston, TX), the NeuroCybernetic Prosthesis (NCP) System encompasses an implantable pulse generator and lead, and an external programming system used to change stimulation settings. The latter includes a programming wand, programming software, and a PC.
A hermetically sealed titanium case, with specially designed feedthroughs, houses the pulse generator's CMOS integrated circuits and microprocessor. Six components define the circuitry's major functions:
- A voltage regulator controls system power supply.
- A crystal oscillator provides the timing reference.
- Logic and control oversees overall function, receives and implements programming commands, and collects/stores telemetry information.
- Output develops and modulates the signal delivered to the lead.
- The antenna receives programming signals and transmits telemetry information to the programming wand.
- A reed switch places the pulse generator in "magnet mode."
Power comes from either a Wilson Greatbatch lithium thionyl chloride battery (3.7V, 2.8 amp-hr) or a Wilson Greatbatch lithium carbon monofluoride battery (3.3V, 2.3 amp-hr). Recently approved by the U.S. Food & Drug Administration, the NCP System is available in all member countries of the European Union, Canada, and other markets.
Implantable heart monitor
While many people experience syncope, or fainting, as an isolated event, some are subject to recurrent episodes. Repeat occurrences may reflect underlying cardiac conditions, but because syncope can occur without warning, such causes often remain undetermined and untreated. These syncope patients live in fear of passing out while driving, working, or simply walking in the park.
The Reveal(reg) Insertable Loop Recorder (ILR), designed by Medtronic Inc. (Minneapolis, MN), aids in syncope diagnosis. Comprising an implanted recorder, hand-held activator, and programmer, the ILR system continually monitors the heart's electrical activity and records it in the form of an electrocardiogram (ECG). This information, in turn, can be used by physicians to identify or rule out a heart rhythm problem.
The implanted recorder is a programmable device containing two surface electrodes spaced approximately 3.5 cm apart. It continuously records subcutaneous ECG into its looping memory, replacing old ECG information with new data. Constructed of the same tissue-contacting materials as used in Medtronic pacemakers, the implanted recorder also contains a lithium power source, custom IC, and static RAM mounted on a multilayer pc board.
The activator, a battery-operated telemetry device, communicates with the implanted recorder through the skin. When signaled by the activator, the ILR stores up to 42 minutes of ECG data. Stored data can reflect a period of time before and after activation, ensuring capture of symptoms leading up to the event. The programmer retrieves stored data via RF telemetry.
Bone and cartilage repair
In the 1990s, synthetic bone graft materials have offered advantages over traditional methods of bone repair and replacement. Bone grafts taken from elsewhere in the patient's body, for example, require two operations, while bones from cadavers can prove incompatible with the host bone.
Today, the trend is towards next-generation materials for bone and cartilage repair. One example: Pro Osteon(reg), the first synthetic bone graft material to be cleared for use by orthopedic surgeons by the U.S. Food & Drug Administration. Interpore International (Irvine, CA), maker of Pro Osteon, is testing three new products:
1) Clinical trials are underway to evaluate the use of Pro Osteon as a graft material for weight-bearing application of cervical spine fusion. The company reports testing a new titanium cage design, filled with Pro Osteon, for spinal interbody fusion procedures.
2) Growth factors are naturally occurring proteins that recruit and convert certain cells in the body to stimulate bone formation. Combining these with Pro Osteon not only promises the necessary scaffolding for bone ingrowth, but a means of accelerating or stimulating such growth. Consequently, Interpore has signed a license and development agreement with Quantic Biomedical Inc. of San Rafael, CA, for rights to technology which produces a material with concentrated tissue growth factors derived from the patient's own blood.
3) Many surgeons would like a bone graft substitute that entirely resorbs after bone ingrowth is complete. Having met the requirements of its European Notified Body, Interpore International has recently announced the launch of Pro Osteon Resorbable Bone Graft Substitute in the European Community. The implant is primarily composed of calcium carbonate, with the exposed surfaces throughout the interconnected porosity covered by a very thin outer layer of calcium phosphate. Once implanted, the calcium phosphate outer layer will slowly resorb, delaying exposure of the underlying and faster resorbing calcium carbonate.
ENABLING TECHNOLOGIES
- Therasphere cancer treatment: Alkali-free, rare earth aluminosilicate glass can be made radioactive and won't dissolve in the body.
- NeuroCybernetic Prosthesis System: Custom integrated circuits and programming software stimulate the vagus nerve.
- Pro Osteon bone graft material: Thermochemical process converts calcium carbonite of coral into the mineral composition of bone.
- Reveal Insertable Loop Recorder: Custom IC, RAM, RF telemetry, and application software monitor the heart's electrical activity.
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