Increasing Prosthetic Durability
New Technology in Total Joint Arthroplasty
By Michael Mycoskie, M.D.
Since the inception of total joint arthroplasty 9replacement) more than 30 years ago, continuous improvements in technology and technique have appeared. Today, there is little change to the basic design. Research and development is now mostly aimed at increasing the durability of the materials used.
Original prosthetic devises were made from metal that articulate with a plastic insert, thus replacing the joint cushion. The expected life of this original design averaged approximately15 years. For elderly patients, the durability is fine. But, in fact, arthroplasty is now offered to patients as young as 40 years of age, sometimes younger.
Research is now aimed at making this procedure more attractive to younger patients as well as to the surgeon caring for them. Fifteen years is not an acceptable expectation for my younger, active patients who need joint replacements. Advances in materials used, the availability of prostheses with improved mobility, and the use of computer navigation systems for prosthesis insertion are all improvements changing our prosthesis-duration expectations.
Knee Innovation
Intriguing developments in knees include a rotating platform total knee arthroplasty that closely mimics the human knee. The rotating platform wears extremely well and is currently used by me and other physicians at Arlington Orthopedic Associates (AOA). However, depending on the patient’s age and the knee’s deformity, this may not prove a suitable alternative. In these cases, a fixed-bearing prosthesis is often chosen. Certainly, the prosthesis chosen is determined by the individual factors of each patient.
Perhaps the most exciting advancements in total knee arthroplasty is computer assisted surgery. Teamed with Arlington Memorial Hospital, AOA is the first in Tarrant County and one of the few in the country to offer this new technology. While the surgeon still performs the surgery, a computer navigation system is used to map the patient’s knee and guide the surgeon as he or she makes the cuts in the bone.
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It is anticipated that this will allow the surgeon to more accurately fit the replacement parts to the patient’s bones and provide overall alignment and range of motion in a much more predictable way. This assures the longest prosthesis duration possible. I was skeptical at first, but now that I have used it, I am sold on the advantages the computer brings to the operating table.
Hip Innovations
Three new bearing surfaces for hips illustrate the trade-offs inherent in fresh approaches. New plastic, or high-density polyethylene, shows an 80-90 percent improvement in wear compared with traditional metal on plastic constructs. Most manufacturers offer them.
Some researchers consider plastic an inferior surface and they have developed a metal-on-metal articulation. Interestingly, this method was actually tried in the early days of total joint arthroplasty and abandoned because the materials could not be produced with precision.
The advent of computer-generated machining tools in the last decade enables manufacturers to make perfectly compatible metal-on-metal couples, so such technology may circumvent osteolysis, a side effect of plastic inserts. I short, a metal-on-metal replacement is now considered an excellent alternative.
Despite the promise of metal-on-metal articulation, unanswered questions about the long-term systemic effects of metal ions temper enthusiasm. So the latest articulation, which was released by the Food and Drug Administration (FDA) in spring 2005, has generated excitement. It is a ceramic-on-ceramic coupling. The wear debris from the ceramic surfaces is inert and has no potential for harming the patient through systemic exposure. It also resists wear much better than the plastic-on-metal or metal-on-metal couplings.
I am quick to point out that all these materials have proven successful. Long-term studies are not yet available because the procedures are simply too new. One widely used advancement in hip replacement I am excited about is the use of a larger ball portion of the prosthesis. The larger femoral head designs are working great. Advantages include less wear, which results in longer prosthesis duration and a much smaller chance of postoperative dislocation.