How Are The Components Implanted?

The components of a total hip replacement may be either press fit or cemented into bone. Press fit means that the specific component is impacted tightly into a space of the same size as the component. At first, all total hip components were cemented in place. The cement acts as a filler or grout, not as a glue.

It fills the space between the component and bone.

In early total hip systems, there were only two or three cup sizes and three or four femoral stem sizes available for use. A tight fit was not possible and cement was required to fill the space between component and bone.

It was noted that over time the interface between cement and bone could break down and the components could become loose. This would cause the need for a revision procedure. Attempts were therefore made to find better fixation for components.

The concept of press fit had been around for a long period of time. Most hemi-arthroplasties done for fractures such as the Austin Moore prosthesis were simply impacted or press fit into bone. It was therefore felt that this might provide better long-term fixation in healthy bone. Noncemented fixation became more popular as implant manufacturers created better instruments and a wider range of component sizes that made a tight press fit possible. Along with this came the development of porous rather than smooth surfaces on the implants.

Porous coating allows for ingrowth of bone directly into the implant surface. The optimal pore size for bone ingrowth was determined by laboratory studies.

At present, most acetabular components have porous surfaces along the entire circumference of their interface with bone. Femoral stems may be partially porous coated in some areas or fully coated for the length of the stem.

To insert the acetabular component, the socket is reamed to appropriate size. The metal shell of the acetabular component is then impacted into place. If necessary, an additional one or two screws are placed through holes in the metal shell into the iliac bone of the pelvis to secure fixation. The polyethylene liner of the acetabular component is then placed inside the metal shell and held by a clip or locking mechanism.

In similar fashion, the femoral canal is reamed size for size to accept the stem of the femoral component. The femoral head component is then impacted on to the upper metal portion of the stem called a Morse taper. The femoral head forms a metal to metal bond with the Morse taper called a cold weld.

While most components at this point in time are press fit, cement remains a useful option in some situations such as soft osteoporotic bone. A total hip that has a noncemented acetabular component and a cemented femoral component is called a hybrid replacement.