What Are The Components Made Of?

Most hip replacements are made up of a combination of metal and plastic. The femoral head and femoral stem are metal. The acetabular component is a durable plastic. Together, these materials work together to create a strong joint that moves smoothly as you sit, stand and walk. There is very little friction as the ball moves inside the socket.

The two surfaces that rub against each other inside the joint are called bearing surfaces. The most common bearing surfaces are metal on polyethylene.

The metals used for both the femoral head bearing surface and the femoral stem are alloys such as stainless steel, cobalt-chromium, and titanium. Alloys are a combination of metals that are put together for maximum strength. Ideally, metal that is used in a hip replacement should be inert, corrosion resistant, nonmagnetic and biocompatible. It should have good longevity and be resistant to bending or breakage.

The first femoral components designed by Charnley were stainless steel. Now most components are made of cobalt-chromium and titanium. Cobalt-chromium and titanium have a longer fatigue life than stainless steel.

They are also more adaptable to having porous surfaces, which are now used with bone ingrowth components. Cobalt-chromium is the best metal for the bearing surface of the femoral head. It is relatively resistant to wear and to corrosion. It is less likely to breakdown at the surface. Titanium is a better alloy for a femoral stem because its elastic properties are closer to those of bone. Most modular components now have a cobalt-chromium head and a titanium stem.

The bearing surface for the socket or acetabulum is ultra high molecular weight polyethylene (UHMWPE), a strong plastic that is chemically inert. Polyethylene is a widely used plastic. It is a polymer made up of repeating units of ethylene chemically bonded together. UHMWPE is an especially durable polyethylene made up of large molecules. It has good resistance to deformity or creep but does give off some wear particles into the hip joint.

The first acetabular components were completely polyethylene and made to be implanted with cement. Most acetabular components are now a combination of an inner UHMWPE liner and an outer metal shell.

The shell is typically made of titanium. The metal shell reinforces the polyethylene and prevents any deformity. It can also have a porous surface for bone ingrowth so that cement is not needed to hold it in place. A newer type of cross-linked polyethylene may be more resist-ant to wear and thus permit the use of larger femoral head sizes.

Research continues into the development of more durable materials and better bearing surfaces for joint replacement components.