As we saw last week in Part I of Hearing International‘s review of Titanium, the list of titanium benefits is lengthy. This makes the metal incredibly useful for a number of different industries, such as the automotive, aerospace and architectural applications. Since titanium resists corrosion, is biocompatible and has an innate ability to join with human bone (osseointegration), it has become a staple in the medical field. From surgical titanium instruments to orthopedic titanium rods, pins and plates, and dental implants, titanium has truly become a fundamental material for medicine.

Osseointegration

At the heart of the medical application for titanium is its unmatched ability to bond with human tissue and bone. The term osseointegration is defined as the direct structural and functional connection between living bone and the surface of a load-bearing artificial implant. Most of these artificial implants are made of titanium. Osseointegration is virtually unique to titanium, and its use has enhanced the science of medical bone and joint replacement techniques. Retention of a craniofacial prosthesis such as an artificial ear (ear prosthesis), eye (orbital prosthesis), or nose (nose prosthesis) has been practiced for a number of years; however, osseointegration of a lower limb prosthesis is now on the horizon.

The path leading to osseointegration in humans started in 1952 when a Swedish professor, Per-Ingvar Brånemark, first began conducting experiments with titanium implant chambers to study blood flow in rabbit bone. He had attached a small titanium optic observation chamber to the leg bone of a rabbit so he could observe live blood cells. Everything went well until Brånemark discovered that the precious instrument could not be removed from the bone due to osseointegration, which was unknown at the time.

Brånemark was annoyed, but didn’t think much about it until some years later. By then, he held a professorship in anatomy at the University of Gothenburg and had progressed to studying blood flow in humans — in this case in 20 student volunteers who had titanium observation chambers inserted in their upper arms for several months. Astonishingly enough, no adverse reactions were noticed. This led Brånemark to decide that the biocompatible properties of titanium needed further study, but he also realized that he wouldn’t be able to do it alone. A good 40 years before multidisciplinary research became fashionable, the Brånemark constellation included engineers, instrument makers, metallurgists, surgeons, psychiatrists and dental experts, as well as medical researchers and biochemists. At one point, it even involved a professor in business studies.

The team’s first experiments with titanium screws as a basis for dental fixtures were conducted on beagles. It was discovered that the best results were achieved with very gentle surgical methods. If the screws were left unloaded for a couple of months, the bone would integrate with the titanium and create a strong anchorage. Once the bone had been allowed enough healing time, the fixture would only become stronger with use. Brånemark called the discovery “Osseointegration.”

In 1965, Gösta Larsson became the first human patient. Larsson was born with a deformed chin and jaw. The titanium fixtures that attached a set of false teeth to his jawbone meant that he could for the first time eat and talk normally. Larsson went on to live another 40 years during which the original implants remained in place and functioned well.

At first, few of his fellow scientists took Brånemark seriously and there was little acceptance of osseointegration as a viable treatment. In Sweden he was even openly ridiculed at scientific conferences. Over time, the wonderful results of the Brånemark team spread by word of mouth and when the method was deemed “lege artis” in 1975, osseointegration could finally be offered to the greater public — in Sweden and beyond. Eventually an emerging breed of young academics started to notice the work being performed in Sweden and at a Toronto Conference in 1983 the worldwide scientific community finally began accepting Brånemark’s work.

Today osseointegration is a highly reliable and commonplace treatment used in dental and orthopedic applications. The Brånemark method has been used for 40 years, and 95%-99% percent of the treatments have been successful. The Brånemark team developed a complete method, proven during many years and including not only an advanced product, but an educational program and a set protocol for handling the product and conducting the operation. (Click on the picture of the Dental Implant for a video on osseointegration).

Titanium and Hearing Devices

The first application of osseointegration in hearing devices occurred in 1977 when three patients were implanted with a bone conduction hearing solution by Dr. Anders Tjellström at the Ear –Nose- and Throat department at Sahlgrenska University Hospital in Gothenburg, Sweden. A 4-mm long titanium screw was inserted in the bone behind the ear and a specially designed bone conduction hearing aid was attached, based upon the osseointegration work of Dr. Brånemark. The Audiant Implant was developed by Xomed (now Medtronic Xomed Surgical, Inc)) and championed by the late Dr. Jack Hough and colleagues in the 1980s and early 90s. While it enjoyed minimal commercial success due to issues with benefit, the Audiant incorporated a magnet implanted with a titanium screw that osseointegrated over a few weeks and then the audio processor was fitted. Later, the rights to the Bone Anchored Hearing Aid (BAHA) were purchased and further developed by Cochlear Americas Corporation. The BAHA device and other similar instruments are a percutaneous osseointegrated bone-anchored hearing aid that involves the installation of a percutaneous titanium abutment. The initial results were very good and since then about 100,000 patients all over the world have been treated according to this bone-conduction principle.

The current version of these devices was cleared by the Food and Drug Administration (FDA) in 1996 to treat mixed and conductive hearing losses. FDA clearance is often used as a standard many other countries. In 1999, these instruments were cleared for use in children age five and older and in 2001, they were cleared for bilateral fittings. In 2002, clearance was obtained for use in patients with Single Sided Deafness (SSD).

Titanium encapsulates the magnets in another implantable bone-conduction device manufactured by Sophono, and titanium is also used to encapsulate middle ear implants such as the Floating Mass Transducer (FMT) of the Med-El Vibrant SoundBridge and the Middle Ear Transducer in Cochlear’s MET and Carina implants. The FMT is attached to the long process of the incus with a titanium clip and the transducer tips of the MET and Carina implants are composed of titanium. The Envoy Esteem fully implantable middle ear implant also encapsulates the device in titanium. All cochlear implants are secured in place with titanium screws that osseointegrate. Additionally, the Vistafix from Cochlear is designed to use titanium implants that bond with the bone using osseointegration, improving the attachment and cosmetic outcome of facial prostheses and allowing patients that require an auricular prostheses to quickly resume leading a normal life. The direct acoustic cochlear implant systems, such as the Otokinetics device (in development), or the device currently under development by Cochlear Corporation incorporate titanium in their actuators.