Medically Reviewed by Eric Heimberger, MD

Medically reviewed by Eric Heimberger, MD McLeod Orthopaedics Seacoast

The fact that our hip or knee joints wear out shouldn’t surprise us. Consider this:  forces 4 to 8 times our body weight are exerted millions of times each year on our hip and knee joints. If you are considering hip or knee joint surgery, you may also be interested in learning more about the materials used in hip and knee joint implants, along with some advantages and disadvantages of each.

KNEE REPLACEMENT MATERIALS

The knee is a more complicated joint than the hip. There are more than 150 different knee implant designs available. Most use polyethylene that has been treated to interlace and tightly bind the molecules for the platform on the top of the lower leg (tibia) and the bottom of the upper leg (patella).

This Ultra-High Molecular-Weight Polyethylene (UHMWPE) has more than 40 years of clinical history.

The metal parts, onto which the plastic is bonded, usually are comprised of titanium or cobalt-chromium-based alloys.

Most knee joint replacements can last 10 to 15 years or even longer, depending on your activity level.

BASIC HIP JOINT MATERIALS

Developments continue to emerge that improve the life of a hip replacement by reducing wear and improving strength. Today’s hip joint implant has 4 parts:

• The stem fits into the upper leg (thigh) bone and is usually metal.
• The head or ball sits on top of the stem and can be metal or ceramic.
• A shell or cup is placed into the hip. Bone grows into the shell’s metal foundation.
• The liner fits into this cup and locks on the ball. It is made metal, ceramic or plastic (UHMWPE).

The stem and shell pieces are either cemented into the bone or inserted in a way that the bone will grow into and hold the metal pieces into place.

There are 4 basic combinations that are used in making hip implants:

• Metal-on-Metal. The first metal-on-metal hip replacements were used in 1955, but not approved for use in the U.S. until 1999.
  • Advantages: Potentially greatly reduces wear.
  • Disadvantages: In 2011, The US Food & Drug Administration issued a communication regarding concern about metal fragments in the body. Some joints were recalled.
• Metal-on-Plastic is the longest used combination with a metal ball and polyethylene socket. It was used in hip replacements as early as 1960 and is probably the most commonly used today.
  • Advantages: Durability, performance and the UHMWPE plastic wears less than traditional polyethylene.
  • Disadvantages: Particles of the plastic may interact with the body, leading to inflammation and potential changes in the bone, which could loosen the implant.
• Ceramic-on-Ceramic is called by some the “21st century answer” to hip replacement material, especially for younger patients. The metal ball and polyethylene liner are replaced with a ceramic bearing.
  • Advantages:  Clinical studies show excellent performance over time with extremely little wear.
  • Disadvantages:  In the 1980s, there was a problem with the ceramic shattering. However, changes since then have largely eliminated the problem. A common complaint of ceramic-on-ceramic joints is a “squeaking,” although occasionally it lessens over time.
• Ceramic-on-Plastic is often the choice for younger patients, because of its durability. In this implant, the ball is made of ceramic material and the liner socket is polyethylene.
  • Advantages:  Durability. Ceramic remains better lubricated than other materials.
  • Disadvantages:  It is more costly that other implant materials.

FINAL THOUGHT

Be an informed patient. Ask your Orthopedic Specialist what he or she plans to use in your joint replacement and why. Your overall health, fitness, weight and age will play a role in the ultimate decision, as will your surgeon’s experience with various manufacturers, designs and implant materials.

Find an Orthopedic Specialist near you.

Sources include:  McLeod Health, National Institutes of Health, American Academy of Orthopaedic Surgeons, Center for Orthopaedics, Bonesmart.org