Evaluation of initial surface apposition in porous-coated acetabular components.

The initial surface apposition of the acetabular bone-implant interface was investigated for acetabular components of three different designs: hemispherical with spikes, hemispherical, and threaded hemispherical. Four acetabular components of each design were implanted into fresh anatomic specimen acetabula that were underreamed by 1 mm to the size of the component. Each acetabular specimen then was embedded in methylmethacrylate, sectioned, and examined under magnification. Although acetabular implantation was carefully performed by an experienced surgical team in an idealized laboratory environment, less than complete interface contact was achieved in most cases. Surface contact was limited by five factors: (1) bony anatomy, (2) asymmetric acetabular reaming, (3) retention of the subchondral plate, (4) acetabular component design, and (5) incorrect version of the acetabular component. The bony acetabulum is an incomplete hemisphere because of the acetabular notch and a deep acetabular fossa, and therefore cannot be machined to a perfect hemisphere. Asymmetric reaming occurs because of the anisotropic quality of bone. Retention of the subchondral plate causes incomplete seating of components with spikes or threads. The acetabular design determines the amount of porous coating available for bony apposition and varies depending on the surface area used for screw holes, apical holes, and thread segments. Increased, stable initial surface contact of acetabular components will increase the chances of stable biologic fixation. Improved surface contact will require changes in component design, instrumentation, and technique.