Features May 2005 Issue

Surrogate Joints = Long-Term Relief

New materials and surgical techniques are producing artificial joints that last longer than ever before.

When joint pain continues to disrupt your life and doesn’t respond to medication, weight loss, exercise, or rest, then it’s time to consider joint replacement surgery.

“It’s the only real long-term cure,” says Brian Donley, M.D., a Cleveland Clinic orthopaedic surgeon. “It’s the only procedure in which everything that is, or could become, diseased—the cartilage, the linings, the ends of bones—is removed."

What you can expect from an artificial joint today continues to improve on what you could expect just a few years ago. Researchers are developing better implant designs that, eventually, will give you near-normal range of motion and function for the rest of your active life, whether it be for five or 50 years.

And that “someday” is almost here. The newest generation of implantable knees and hips, fingers and ankles lasts longer than ever before. Here are some reasons why.

Growing demand
The sheer number of Americans entering middle age is driving up the demand for artificial joints, as natural joints begin to give way to a lifetime of wear and tear. According to the American Academy of Orthopaedic Surgeons, from 1996 to 2001 the number of total knee replacements among 38- to 56-year-olds more than doubled.

Yet, this number could soon grow much larger. Only 10 percent of those who could benefit from total joint replacement seriously consider it as an immediate option, according to a report in the Journal of Bone and Joint Surgery. Many candidates repeatedly postpone the decision, wrongly assuming that current implants, like the previous generation of artificial knees and hips, need to be replaced every 10 to 12 years.

But this is no longer the case. A combination of better materials, designs and surgical techniques has resulted in artificial finger and ankle joints, as well as hips and knees, that last longer than ever before.

DePuy’s Agility Total Ankle System provides a 20-degree range of motion and is especially suited to older patients.

A material difference
Many of the newer implants used in weight-bearing joints are made from a combination of metal (titanium or cobalt-chrome alloy) and hardened plastic (high-density polyethylene), with the metal serving as main structure and anchor into the bone, and the plastic as the friction-bearing surface.

Since it is the plastic that wears out first, much work is currently being focused on developing tougher forms. The plastic now used in artificial hips, knees and ankles is 20 times more durable than the material used just a few years ago. This is made possible by creating more molecular crosslinks by exposing the plastic to radiation or special chemicals. And even tougher plastics are on the way. A joint venture between the U.S. and Israeli governments has produced a form of plastic (3DPE) that is reportedly 10 times stronger than steel and extremely resistant to wear.

The use of ceramics is also extending implant life. In some of the latest hip systems, the ball at the end of the femur stem, and the liner of the pelvic cup, are made of ceramic instead of plastic. This is not the ceramic of brittle pottery, but rather an extremely hard form of aluminum-alumina oxide. A French study that looked at ceramic implants of 116 patients found no detectable signs of wear on X-ray examination after 20 years of use. 

Metal-on-metal implant designs are also making a return. Advances in engineering now enable the adjoining metal surfaces to be ground to such precise smoothness that the components fit together with microscopic exactness, largely eliminating the friction that caused metal-on-metal designs of the 1960s to fail. 

Keeping it clean
Friction is the main enemy of implant life because particles produced by such friction can work their way between bone and implant, gradually eroding bone and loosening the artificial joint.

To help offset particle build-up, some experimental implant designs include a bleeder valve, a device to collect the particles as they come off the joint. Ideally, you would return to the hospital every so often to have the valve opened and debris removed. Another method being investigated is ferrography, a magnetic technique currently used to clean metal shavings out of aircraft engines. Through this device, doctors would be able to track the location of wear particles and know when to clean the implant before the joint loosens.

Better alignment
No matter how friction-free your implant or how durable the materials, a big part of how long it lasts depends upon the skill of the surgeon. Precise positioning within the bone is essential to get the angles right, allow optimal transfer of force, and reduce premature wear.

Though much of this precision comes from the eye of the surgeon, computers are also helping. Orthopaedic surgeons are increasingly using computer-assisted navigation systems to get exact measurements about joint and instrument positioning during the procedure. The technology, which uses global positioning systems, superimposes the position of the instruments as they are used in surgery onto images of the anatomy displayed on a monitor. 

Help for hands
Because hips and knees are the most commonly replaced joints in the body, they tend to be the testing ground for many of these advances. We’ve written about them in previous issues of Arthritis Advisor (Knees: Oct. 2004, Apr 2004, Dec 2003, June 2003 / Hips: Dec 2002). However, progress is also being made in hand and foot implants.

Artificial finger joints have been around since the 1970s, but they’ve largely been made of silicon, acting as spacers that kept bones from rubbing together but susceptible to breakage over time. Newer designs are pre-flexed, giving them a slightly bent position at rest and a more full grip.

More recent implants are similar to modern knee implants in that they are made of cobalt chrome alloy and hard plastic, allowing for greater motion and durability. The newest type of finger joint implant is made from hard, ceramic-coated graphite (PyroCarbon from Ascension Orthopedics) and appears to provide even greater durability. “These are best used in younger patients with post-traumatic arthritis or older patients with osteoarthritis who generate higher, more repetitive stress on their joints,” says Peter J. Evans, M.D., Ph.D., chief of the Section on Hand Surgery at The Cleveland Clinic.

Unfortunately, these newer carbon-based designs are only suitable for some patients because they require substantial remaining healthy bone and soft tissue. Subsequently, they are not recommended for those with advanced arthritis, especially rheumatoid arthritis.

Best foot forward
Advances in total ankle replacement have also come through several generations of improvement. “Creating an ankle implant that holds up is particularly challenging,” says Dr. Donley. “The ankle doesn’t just move in one plane. It has multiple axes of motion, creating a complex variety of stresses that can lead to premature failure.”

The newest designs overcome this challenge with a combination of technique and technology. “To reduce the stress on the implant, we first fuse the bones of the lower leg,” says Dr. Donley. The other part of the solution is the Agility Total Ankle System, the latest ankle implant design from DePuy Orthopaedics. This system, made of metal and polyethylene, provides a range of motion of 20 degrees and is best suited for less active, older patients with disabling arthritis.

Third-generation ankle systems, which are still going through clinical testing, promise further improvement because they leave intact more of the original bone. The more bone that remains, the more likely the surgeon can offer you a back-up plan (replace the implant or fuse the bones), should your new implant fail.