Cement Fixation

In 1962, Sir John Charnley used a small (22mm) stainless steel ball on a long stem which was inserted into the bone to replace the femoral (ball) side of the joint and a high density plastic socket to replace the acetabular (socket) side. Both of these components were secured to bone with a self-curing acrylic polymer commonly referred to as bone cement (methyl methacrylate).

Several generations of designs have evolved from this original Charnley prosthesis. The ball is now modular thereby allowing balls of different sizes, materials, and neck lengths to be placed onto the stem. Most balls are now made of either a cobalt chrome metal alloy or a ceramic material. Stems are now made of either cobalt chrome or titanium. The socket component is usually titanium with a bone ingrowth porous surface and an exchangeable bearing liner. The liner can be made of polyethelene (plastic), cobalt-chrome (metal), or ceramic. Sockets fixed with cement have largely been abandoned in the US. Cement fixation of the stem is now used in less than 10% of hip replacements, usually in older weaker bone.

Bone Ingrowth Fixation

We are now in an era with widespread use of devices that are designed to attach to bone without the use of cement. Bone will attach to a metal implant if the surface of the metal has a rough or porous surface. This process is called bone ingrowth or osseointegration. The bone must be prepared precisely for these devices. For successful bone ingrowth to occur the implant must achieve an initial stable press-fit when implanted and the porous coating must sit right up against live bone. In general, these devices are larger, longer, and stiffer than those used with cement but are proportional to the size of the individual bone. Surface coatings, such as hydroxyapatite, are also being utilized in an effort to hasten and/or enhance bone fixation. Many different devices using cementless fixation have been utilized since their introduction in the U.S. in 1977. It is now generally accepted that these implants remain fixed to bone longer than cemented devices. The theoretical downside is occasional failure of bone ingrowth, but this is a very rare problem, except possibly for very old weak bone. There is one other problem with these implants. In 2-5% of patients activity related thigh pain may develop. Even with well-fixed implants, there can be pain due to the stiffer modulus of these implants. There is no solution to this problem.

Hybrid Fixation

Hybrid fixation is when one component is inserted without cement, usually the socket, and one component is inserted with cement, usually the stem.