• Mobile-bearing vs. fixed-bearing. A fixed-bearing prosthesis has the poly-
ethylene insert of the tibial component attached firmly in the metal
tray. In contrast, a mobile-bearing implant's cushion can move more
freely in the tray. This means that mobile-bearing implants allow for a
few more degrees of rotation compared to fixed-bearing options. Most
patients get fixed-bearing implants, but surgeons will often use
mobile-bearing options in younger, more active patients since the
devices require more support from surrounding soft tissue.
In the hip
The hip is a "ball-and-socket" joint made up of the acetabulum (the
socket) and the femoral head (the ball). Hip implants replicate this
anatomy with a metal or ceramic ball and a cup made of polyethylene,
ceramic or metal.
The key difference in these implants is whether metal, ceramic or
plastic is used and in what combination. While most hip prostheses
tend to perform the same in patients, the bearing surface — or the
area where the 2 components meet — can impact a surgeon's choice.
There are 3 major types of hip implants:
• Metal on metal. While popular several years ago, metal-on-metal
implants have fallen out of favor as studies have shown that they can
break down over time and send metal particles into the surrounding
soft tissue to cause adverse reactions.
• Ceramic on ceramic. Ceramic-on-ceramic implants are also less com-
monly used, since they can be harder to place, can become brittle
over time and are very expensive.
• Ceramic on polyethylene or metal on polyethylene. Most surgeons prefer poly-
ethylene implants. Though original designs had wear-and-tear issues
of their own, most modern implants use a cross-linked polyethylene
that's much more durable. This is good news for cost-conscious facili-
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