MODULAR ACETABULAR COMPONENTS: PROBLEM FREE AT LAST?

Abstract

Modular polyethylene acetabular bearings were initially introduced in conjunction with a metal-backed component designed for fixation with cement. It was anticipated that aseptic loosening would be eliminated by the new metal-backed component. A modular liner would allow a simple exchange of the bearing when it was worn. Although this technique has rarely been used with cemented sockets, the concept of modularity had merit for cementless sockets. Early studies of cementless sockets indicated that initial implant stability and increased bone ingrowth occurred with screw fixation. A modular liner allowed screws to be placed through the socket.

First-generation modular components were variably plagued by mechanically insufficient locking mechanisms, insufficient support of the poly by the metal shell, insufficient polyethylene thickness (variably oxidized) and excessive motion between the liner and the shell leading to an array of complications including frank dissociation of the liner, back-side wear, generally higher wear rates and more osteolysis, especially retro-acetabular.

There is nothing inherently good or bad about modularity; it wholly depends on how it is done. More recent offerings have, at least in theory, addressed the limitations of earlier designs. Current designs are characterised by stronger locking mechanisms and reduced liner-shell motion. Most systems offer choices for initial fixation (press-fit, screws, spikes, etc.), bearing diameters, and face geometries (neutral, face-changing, lipped or hooded). Some systems offer a choice of bearing material (polyethylene(s), metal, ceramic). It will simply take time to learn which material, design, and manufacturing advances translate into long-term clinical success.

In an era of hip arthroplasty featuring more wear resistant bearings, it is reasonable to question whether or not modularity is necessary. If satisfactory fixation can be obtained with a simple press-fit and the wear resistance of the bearing is high enough to last the patient’s lifetime, why not use a non-modular component? Modularity allows intraoperative flexibility at the primary surgery and in the event a revision is required. Further, it is not yet known if any or all of the new bearings will demonstrate the anticipated in vivo wear resistance.