Abstract
There are many types and articulating surfaces in acetabular cups. Most of the designs currently available are modular, the liner snapping into a locking mechanism of some type. These modular inserts may be polyethylene, usually highly crosslinked polyethylene, or ceramic. Metal shells used in metal on metal devices are usually of a monoblock design.
The elliptical monoblock design has been available for 20 years and was originally made of Titanium with a compression molded polyethylene liner. Tantalum (trabecular metal) was used as the shell material in the more recent designs and the polyethylene is actually molded directly into the tantalum framework. Monoblock acetabular components have a number of advantages. They do not allow access to the ilium because there are no holes in the socket shell with the monoblock construct. They require no locking mechanism which may increase metallic debris. No back surface liner wear can occur because all motion is eliminated at the liner/shell interface. However, because of this absence of screw holes there is an inability to visualise the floor of the acetabulum and perfect coaptation between the shell and the acetabular floor may not occur. The presence of dome gaps of greater than 1.5mm have been noted in 5% of these components but these have not compromised implant stability and in a review of over 600 cups there has been no change in implant position. The elliptical shape of the cup makes the mouth of the acetabular component 2mm greater than the dome so that an exceptionally strong acetabular rim fit results.
Results with over 258 monoblock cups with a minimum of 10-year follow up (10–15 years) have been excellent (Poultsides, et al.). The incidence of pelvic osteolysis was not seen in any patient in this series. There were 3 revisions for instability but none for mechanical failure. There were three femoral revisions for loosening but the cup was intact and not revised in these patients. Utilising the Livermore measurement method polyethylene wear averages 0.08mm per year (0.06mm-0.13mm) and there have been no revisions for wear. Radiographic evaluation demonstrates stable bony interface in all patients. At minimum 10-year follow up the monoblock acetabular component with compression molded polyethylene confirms the theoretical advantages of this design and results have been excellent. Moen et al. have demonstrated no osteolysis in CT scans in tantalum monoblock cups at a follow up of 10.3 years.
