Features June 2006 Issue

New Ways To New Joints

New techniques, more durable materials, improvements in implant alignment all herald advances in joint replacemen

Though replacing worn joints is nothing new, the art and science of doing it less invasively, with longer-lasting results, continues to evolve. Some of the prosthetic systems designed to replace the knee and hip, the most commonly replaced joints, have now reached their third generation, while a few of those designed to replace the ankle, often considered the most challenging joint to replace, have only just been introduced.

The improvements in these systems over their predecessors is usually not due to one element, but to a combination of factors, such as better design, improved materials, more advanced surgical techniques, and new tools to help assure optimum alignment between bone and implant.

Here’s a look at some of these advances in joint replacement.

Shoulders: More options
Though shoulder replacement may get less media attention than hip arthroplasty, it also involves repair of a ball-and-socket joint, and progress in this area has been equally impressive.

Delta CTA Reverse Shoulder System transposes shoulder components, allowing existing muscles to hold implant together more tightly.

Approximately 20,000 to 25,000 shoulder arthroplasties are performed each year in the U.S., and that number is growing 12-15 percent a year due to an aging    population.

“Historically, shoulder replacement was mostly only an option for older, inactive patients with advanced osteoarthritis and an intact rotator cuff,” says Joseph Iannotti, M.D., Ph.D., chairman of the department of orthopaedic surgery at The Cleveland Clinic. “But in the last few years, we’ve been able to offer a wider range of shoulder options matched to the age, disease state, and activity level of each patient.”

One of those new options is joint resurfacing. In shoulder resurfacing, instead of the need to remove the entire end of the upper arm bone (humerus) and replace it with a metallic ball and stem, the natural bone stem is left in place and the ball is reshaped and fitted with a metallic cap. In addition, the natural indentation in the glenoid cavity, in which the humerus sits to create the shoulder joint, is not replaced with a hard plastic polyethylene cup. Rather, the surgeon creates a cup made from material taken from elsewhere in your body (such as a piece of fibrous tissue from your outer thigh).

The use of such a natural-tissue cup reduces the risk of implant failure due to friction particles. Friction is the main enemy of implant life because particles produced by friction can work their way between bone and implant, gradually eroding bone and loosening the artificial joint.

Such extended longevity of a shoulder implant is particularly important in the resurfacing approach since the technique is largely reserved for younger, more active patients.

But other advances, such as the reverse shoulder system, are helping older patients retain greater shoulder mobility. In this system (the Delta CTA Reverse Shoulder System from DePuy), the normal positioning of the shoulder components are transposed, with the metal ball (glenosphere) and stem fitted into the glenoid cavity and the hard plastic cup fitted into the top of the humerus.

This reverse system is designed for older, less active patients whose rotator cuff muscle is so weak or torn that it can no longer hold the shoulder together. This novel design allows the intact and still strong muscles in other parts of the shoulder, such as the deltoid muscles, to take over the work of the damaged rotator cuff and helps hold the implant together more tightly than the injured shoulder can. The result is a more stable shoulder that is less likely to dislocate.

Hips: Smaller incision, better materials
Though joint resurfacing also helps some hip-replacement patients keep more of their natural bone, what has changed the way patients and surgeons think about hip surgery is the mini-hip.

In traditional total hip replacement, a 10- to 12-inch incision is needed, but with the mini-hip, the incision is often only three to four inches long. Yet, a smaller incision is only part of the story. To be truly minimally invasive, the surgeon must also reduce the number of underlying muscles and tendons that are cut. Rather than sever these tissues, the surgeon cuts away only a few of them, and works around the remaining muscle bundles, to position the implant.

Such navigation requires an experienced surgeon, one who can confidently navigate within a restricted field of view, yet still precisely position the implant’s parts.

Another major advance in hip implants is in the use of more durable weight-bearing surfaces. The latest hip implant systems have surfaces made of ceramic instead of plastic, which reduces wear and increases longevity. The ceramic is an extremely hard form of aluminum-alumina oxide that, in various clinical studies, has shown no detectable wear even after 20 years of use.These newer ceramic designs resemble current implants in that a metal stem is still inserted into the top of the diseased femur, and a metal cup is still placed within the worn-out pelvic socket. But the difference is in the articulating surfaces. In the new designs, the ball at the end of the femur stem, and the liner of the pelvic cup, are made of ceramic.

Knees: Improved geometry
Longer-lasting materials and less invasive surgical approaches are also transforming the art of total knee replacement.

Rather than waiting until their mid-60s before getting new knees, people with active lifestyles are opting for knee replacement while in their 50s.

These new designs offer more than just added durability. The geometry of knee implants keeps improving, with shapes that more closely resemble a biologic knee, providing better stability and range of motion, as well as a feeling that is closer to that of the original.

As with the mini-hip, mini-incision total knee replacement is a less invasive technique that requires less cutting of muscles and tendons, helping to reduce pain and shorten recovery. Instead of cutting through the quadriceps to expose the knee, the surgeon spreads the muscle fibers apart and uses smaller instruments to make cuts to the bones. The initial incision is three to five inches long, instead of eight-to-12.

Agility Total Ankle system, designed for advanced stages of arthritis, provides wide base of support, allows 20 degrees of rotation.

To ensure that cuts are made with precision, some surgeons are beginning to use computer-assisted navigational tools. Electronic markers—placed on the tibia, femur, and cutting blocks used to reshape the ends of your bones—send out infrared signals to nearby computers, which help determine and display the optimal cutting angles.

Ankles: Greater durability
The last major weight-bearing joint to resist a successful, long-term replacement has been the ankle. Though a variety of systems have been tried over the years, creating an ankle implant that can last is challenging because your ankle doesn’t move in only one plane. It has multiple axes of motion, creating a complex variety of stresses that can lead to premature failure.

However, several new designs have emerged in the last few years to brighten the picture. The Agility Total Ankle (from DePuy) was the first system to be approved by the U.S. Federal Drug Administration (FDA). Now in its second generation, this system allows 20-degree range of motion, and has shown to be 89 percent successful after nine years of use. Still, it is intended as an option only if you have advanced or end-stage ankle arthritis, meaning you have difficulty standing or walking even with the help of a cane.

Another new ankle system awaiting approval by the FDA is the Salto system (from Tornier). Like the Agility Total Ankle, it is intended for final-stage arthritis and has shown excellent results in patients with up to eight-year follow up. Its difference lies in its more anatomic design. A third system, the STAR (Scandinavian Total Ankle Replacement), is a three-part ankle system that is being used in Europe but is awaiting FDA approval for use in the U.S. Preliminary studies have shown little or no wear after seven years of use.

Whichever weight-bearing joint may be the source of your pain, there are more new options than ever—due to a combination of new surgical approaches, better materials, and improved alignment—for relieving that pain and providing you with a replacement that can help restore your mobility.