New technology in knee replacements

by Nathan Wei, MD, FACP, FACR

Nathan Wei is a nationally known board-certified rheumatologist and author of the Second Opinion Arthritis Treatment Kit. It's available exclusively at this website... not available in stores.

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Total knee replacement is often prescribed for patients with end stage arthritis.

Implants only last between 10 and 15 years, and then they must be replaced. This procedure is called a revision arthroplasty.

However, Smith and Nephew Orthopedics has designed an implant that lasts 85 percent longer in tests that simulate the stresses inside the knee. The prosthetic components are made of zirconium that are heated and infused with oxygen to create a ceramic surface that is more durable than the conventional cobalt chrome alloy prosthetics. The material, which was approved by the Food and Drug Administration in 1996, has gone through two years of testing at New York's Hospital for Special Surgery (HSS) and recently received the hospital's approval for use in knees.

More than 600,000 knee replacements are performed annually. Knee replacement can often reduce or eliminate the pain caused by degenerative joint conditions such as rheumatoid or osteoarthritis.

"Unfortunately, we are seeing more and more patients with severe arthritis earlier in life," Dr. Richard Laskin, chief of the Arthroplasty Council at the Hospital for Special Surgery in New York, said in a statement. "Until now, these patients were advised to postpone the replacement surgery and deal with the pain." Now, however, surgeons can implant ceramic knees in 40- or 50-year-old patients, drastically reducing their pain earlier in life.

In addition, the implants offer added benefits. The metal alloy used in current implants can scratch or pit, causing them to break down faster. According to the HSS, the ceramic implant is scratch-resistant. When it is wet, ceramic can glide smoothly along plastic, making for smoother motion than with metal-on-plastic, the statement said. Lastly, patients who are allergic to nickel, the metal most often used in the current implants, cannot currently receive total knee replacements. The ceramic implant, however, can be implanted safely in patients with nickel allergies, since it uses a zirconium alloy with no nickel. "We think it will last significantly longer than cobalt chrome implants," said Angie Craig, spokeswoman for Smith and Nephew.

Three- dimensional imaging

VectorVision, a “global-positioning system for the body,” allows surgeons to use preoperative diagnostic images to visualize and track the position of their choice of surgical instruments intraoperatively through a touch-screen interface.

The VectorVision system allows surgeons to select the optimal implant during total joint replacement surgery by using a patient’s 3-D image data. The system ensures that the ligaments have the appropriate tension after the operation, allowing the patient’s leg to move smoothly after knee replacement. “This technology is the biggest advancement in total knee replacement in several years,” said James R. Larson, MD.

For the increasing number of people needing joint replacement surgery, the device offers patients hope of less pain and a shortened recovery time after surgery. “Image-guided knee replacement is a natural transition from minimally-invasive procedures,” said Tilok Ghose, MD. “The system provides improved accuracy, which is especially important since the average age of patients requiring knee replacements is getting younger.”

Uni-compartment knee replacement

Uni-knee compartimental knee implant has been shown to markedly decrease pain and to restore knee function, alignment and stability. This is achieved by replacing missing cartilage with a single metallic implant. The patient’s own knee is preserved and a patient’s body weight does not appear to be an issue. There is no dramatic bone resection or any metallic/cement fixation required, as a patient’s own ligaments allow for implant and knee stability. The knee is not compromised with regard to possible future complete (or even partial) knee replacement operation(s).

Combining the UniSpacer with arthroscopic removal of the chronically inflamed deep synovial tissue of the knee further ensures dramatic relief from arthritic pain/swelling. Optimized surgical technique, including post-operative protocols, tends towards correspondingly improved patient results.

Post-operative recovery appears to be relatively more rapid than with knee replacement, as the minimally invasive Uni-knee/UniSpacer incision is usually 2-3 inches in length and over the inner aspect of the knee. Non-braced weight bearing as tolerated is typically allowed immediately. A continuous passive motion (CPM) machine is utilized short-term to assist in rapid return of knee motion, as is specifically designed physiotherapy utilized to promote strength and functional motion.

A team of 12 Australian engineers, surgeons and material scientists has designed the first largely ceramic prosthetic knee that, by reducing the effects of friction and wear, is expected to have a much longer lifetime than existing technology.

Project leader Dr Besim Ben-Nissan and PhD student Warwick Payten from the UTS Faculty of Science are working with three major universities and the CSIRO to develop the new technology. The research team has already secured an international patent and is now seeking corporate support to renew the patent.

"The Australian design is the first of its kind because it is primarily ceramic with a stiff titanium alloy base plate attached to the ceramic using clips," Dr Ben-Nissan said.

"The titanium alloy reinforces the prosthesis so that it can take all the stresses that a knee would take. This design is intended to last much longer than existing metallic knee prostheses which have a life-span of 10 to 15 years."

Dr Ben-Nissan said the ceramic knee was specifically developed to withstand transient loading should the position of the knee joint temporarily become misaligned.

"Any knee replacement design must be able to withstand repeated loads of up to seven times body weight for normal activities like walking, in addition to transient loads that can be much higher.

"Before our design there had only been one purely ceramic knee prosthesis developed by Japanese researchers, but it had not taken into account transient loading and hence had failed miserably."

Dr Ben-Nissan said the new ceramic design had passed strict mechanical testing and would undergo further testing in the lead-up to clinical trials. He said increased confidence in the use of knee prostheses had meant the demand for knee replacements was rising at about 17 per cent each year, with around 300,000 knee joints replaced in the United States alone.

"Unfortunately, however, long term clinical results are scattered and do not support this confidence. Therefore while the overall rate of failure has been reasonably low, it still remains unacceptable," he said.

"In addition, there are growing numbers of younger and more active patients who require total knee replacement. Their increased activity plus longer usage is expected to result in a higher incidence of eventual failure of conventional knee replacements."

Dr Ben-Nissan said the main aims of knee replacement surgery was for pain relief, the correction of joint deformity, the provision of a good range of movement, and the improvement of joint stability.

He said investigations had revealed that the majority of the failures of total or revision knee replacements were caused by loosening of the replacement, or friction as a result of high stress on the knee joint.

"One of the major factors affecting previous designs was the inability of material scientists and engineers to overcome some of the shortcomings of ceramics arising from their inherent brittleness and the scatter of strength data.

"To use ceramics in biomedical applications, stress and strains must be much better understood than for similar applications of metals," he said.

"Our design incorporates mathematical modelling using the 'finite element analysis'. This can be used to accurately determine the stresses that often occur on knee replacement components and determine the suitability of ceramic knee prosthesis prior to its production."

Dr Ben-Nissan said the Australian design involved a number of prosthesis sizes and components available to cover primary knee replacements, previously failed replacements, severe femoral and tibial fractures, and partial destruction of the knee.

Minimally invasive knee replacement

Minimally-invasive quadriceps-sparing total knee replacement starts with a well-positioned skin incision – typically about 4” in length. This varies with the patient’s size and is made down the front of the knee adjacent to the kneecap, and the knee joint is inspected and preliminary ligament balancing is performed.

Next, specially-designed alignment rods and cutting jigs – which are smaller and easier to pass through the smaller incision than those used for traditional total knee replacement – are used to remove enough bone from the end of the femur (thigh bone), the top of the tibia (shin bone), and the underside of the patella (kneecap) to allow placement of the joint replacement implants. Proper sizing and alignment of the implants, as well as final balancing of the knee ligaments, all are critical for normal post-operative function and good pain relief. Provisional (trial) implant components are placed, without bone cement, to make sure they fit well against the bones and are well aligned; at this time, good function--including full flexion (bend), extension (straightening), and ligament balance--is verified.

Finally, the bone is cleaned using saline solution and the joint replacement components are cemented into place using polymethylmethacrylate bone cement. The surgical incision is closed using stitches and staples.

Total knee replacement may be performed under epidural, spinal, or general anesthesia.

No two knee replacements are alike, and there is some variability in operative times, but a typical total knee replacement takes about 60-80 minutes to perform when traditional techniques are used, because the wider exposure permits more rapid progress through the technical steps of the procedure.

Dr. Richard Berger, an assistant professor of orthopedic surgery at Rush Presbyterian Medical Center in Chicago, is a pioneer of the new technique. He predicts that five years from now, almost all surgeons will be performing the minimally invasive procedure. "There's really no downside, only an upside," he said.

In traditional knee-replacement surgery, Berger explained, the length of the incision varies, typically between 8 inches to 10 inches. "The quadriceps (front of the thigh) muscle is cut or split, the kneecap is flipped over and that exposes the knee. The bone is cut and sized, and the [artificial] knee replacement is put in." With the new technique, "we make an incision that is 3.5 inches or so," Berger said. "We don't cut muscle. We take out the bad bone and cartilage but without cutting any of the muscle or tendon." Then the prosthetic replacement is put in place.

People who need knee replacement typically experience significant pain that can worsen as the cartilage that cushions the joint deteriorates. While pain medications and walking aids, such as a cane, may help temporarily, the only long-term solution is usually replacement.

The traditional approach requires 12 to 14 weeks of physical therapy, Berger said. With the new approach, patients are often walking with a cane and even driving within about 12 days, says another pioneer, Dr. Peter Bonutti, a fellow of the American Academy of Orthopaedic Surgeons.

Another big plus with the minimally invasive technique is the reduction in post-operative pain, according to proponents.

The key to the operation -- besides the doctor's skill -- is the development of new surgical instruments, said Bonutti. "What we developed are new, smaller instruments, to allow us to accurately position the joints without having to dislocate the knee cap to perform the procedure."

Added Berger, "We get the knee moving more rapidly because it doesn't hurt as much."

The hospitalization time with traditional surgery is about four days, Berger said. He said he does the new surgery on an outpatient basis, but other doctors admit their patients to a hospital for the operation.

Even advocates such as Bonutti and Berger acknowledge that not everyone is a candidate for the surgery. People who have already had extensive surgery on their knee may not be good choices, said Berger. "And those who are morbidly obese may not be," he said.

But for most patients, the new approach represents a breakthough, Berger said. Both he and Bonutti agreed, however, that it's crucial to find a surgeon who has performed the new technique enough to be proficient at it.

Several companies are now making the instruments for the new surgery, and more doctors are training in the technique. A surgeon who has done 30 to 50 such operations would probably have a sufficient level of experience, Bonutti said.

Newer approaches to hip replacement using a smaller incision have also been developed.

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