Abstract
Periacetabular osteolysis is seen in response to particles (polyethylene, ceramic, metal or cement), at times in the presence of an unstable implant, and perhaps made worse by the unique host response to the particle burden. The impact of wear modes: due to either the primary bearing surface (MOP, MOM, COC) or unintended surfaces as seen in impingement, as well as the quality of the bearing counterface all influence the extent of the osteolytic response. The final common pathway appears to be via macrophage stimulation, an upregulation of cytokines leading to a resorption of bone.
The patterns of lysis range from linear resorption at the implant interface to more expansile patterns which can be more dramatic in size and may place the implant at jeopardy for loosening. Assessment of implant fixation as well as extent of the lytic process employs the use of plain radiographs (including oblique views), computerised tomography and magnetic resonance imaging. The utility of MRI for the quantification of bone loss as well as the newer phenomena of associated soft tissue lesions (pseudotumors, adverse tissue reactions) has turned out to be a valuable tool in helping determine timing and need for revision.
The basic principles in determining need for revision surgery revolve around: degree of lysis, integrity of the soft tissues, fixation of the implant, track record of the implant, as well as patient factors including symptoms, age and activity.
In cemented sockets, progressive bone loss, pain with or without overt loosening is indication for revision which is generally accomplished using an uncemented hemispherical acetabular component with bone graft and screw augmentation.
In the uncemented socket, the decision to revise is based upon a) implant stability, 2) the integrity of the locking mechanism, 3) degree of bone loss. With stable implants, polyethylene exchange and “lesional” treatment is appropriate. Well fixed implants with extensive lysis can be successfully managed with liner exchange and bone grafting as necessary. If the liner locking mechanism is compromised, cementing a liner into place is an excellent strategy. Removing a well fixed cup with extensive lysis runs the risk of encountering a large acetabular defect which may be difficult to reconstruct. Loose implants clearly require revision.
In the era of “hard bearings”, progressive soft tissue expansion leading to damage of the abductor and other soft tissue constraints about the hip is an indication for revision. Revision of MOM THR's may be performed by maintaining the femoral component and performing an isolated acetabular revision or in some instances of modular acetabular components, maintaining the shell and inserting a new liner. In all instances of implant retention, it is critical to confirm that the components are in optimised position: implants retained in suboptimal position are at risk for early failure.
