[Impacted acetabular cups]

The failure modes of impacted acetabular cups are: deterioration of the polyethylene insert, disassembly of the modular components, excessive polyethylene wear, severe peri-prosthetic osteolysis. These failures can be explained; solutions are suggested.Primary stability: Screws are pathways for polyethylene debris; impacting elastic cups increases the press-fit effect in order to provide stability without screw fixation.Polyethylene thickness: by increasing polyethylene thickness from a minimum 8 to 10 mm, the risk of insert deterioration is decreased and so too is creep of the core which can induce micromotion between the insert and its metal backing.Liner stability: Tilting of the liner is the most common mecanism of disassembly in two-piece acetabular cups. The solution we suggest is to use a liner with a cylindrical periphery.Polyethylene wear: Excess polyethylene wear in metal backed acetabular cups was reported. Conversly, other authors concluded to no excess wear in impacted cups. This may be due to differences in prosthetic components, as well as patient selection. We noticed no significant wear in our series (0.10 mm per year). Studies are contradictory and it is not obvious that excessive wear in impacted cups has to be feared.Osteointegration is mandatory for obtaining long term stability and is provided by porous coating. The osteoconductive properties of hydroxyapatite have been demonstrated by a large number of studies, including fundamental research, animal experiments and clinical applications. Within a few weeks, hydroxyapatite brings about bonding with the host bone without any intervening fibrous tissue layer. We expect hydroxyapatite-induced osteointegration to prevent particulate migration. Biodegradation of the hydroxyapatite coating occurs after 5 to 10 years. Bone ingrowth later relies on the surface roughness of the substrate.In conclusion, we suggest that the combination of impacted elastic acetabular cups with a thick polyethylene liner and bioactive coating can provide long term durability.