Abstract
The cemented acetabular component has been essentially abandoned, due to the reliable and durable fixation provided by bone ingrowth into cementless acetabular components of many different designs. A variety of porous surfaces including sintered beads, titanium fibermetal, plasma sprayed titanium, and ultraporous tantalum have been shown to result in significant osteointegration, and provide long term fixation of cementless acetabular components. New ultraporous metals will also likely prove to perform similarly, however, their advantages in the primary THA are unclear.
Most currently available cementless acetabular components rely on obtaining initial “interference” or “frictional” fit provided by relative underreaming. Many designs incorporate additional features such as screws, pegs, and fins to limit implant micromotion and augment initial fixation until early tissue ingrowth occurs. “Underreaming” by more than 1 mm has been associated with incomplete component seating and increased incidence of acetabular fracture. Knowledge of the geometry of the component by the surgeon is recommended, since some designs are elliptical and have a built-in degree of interference fit. Screws used to augment acetabular fixation in the primary THA can typically be restricted to the area of the acetabular dome (cluster configuration) and cups with multiple holes are usually unnecessary and may be undesirable as they allow access of wear debris to the acetabular implant-bone interface.
In order to minimize backside wear and dissociation of the acetabular liner, modular components need to have a well-designed locking mechanism. Retrieval studies have shown that the peripheral rim of the acetabular liner is most susceptible to oxidative degradation and the integrity of the locking mechanism in this area can be compromised with time. Non-modular, “one piece” components eliminate these concerns, but most of these designs rely on initial frictional fit alone for stability. In the event that the position of a nonmodular component needs to be changed intra-operatively, the quality of frictional fit after repositioning can be diminished and may not be sufficient for implant stability. Modular components that incorporate screws, allow for acetabular component repositioning and adjunctive fixation with screws. Many newer acetabular component designs can accommodate a modular liner for either a metal on polyethylene, ceramic on ceramic or metal on metal bearing.
