Abstract
The major failure mode of cemented or non-cemented acetabular fixation is osteolysis produced by polyethylene debris and biologic reaction to this material. A monoblock acetabular non-cemented component offers advantages in reducing the failure mechanism of acetabular cups. First, because the polyethylene is fixed to the metal shell there is no motion between the shell and the liner as is seen with modular components. Therefore extra-articular polyethylene wear debris is not generated. Secondly, there is no need for a locking mechanism which may fail and from which metallic debris may be produced. Thirdly, no screw holes are present on the back of the monoblock cup increasing the surface area for ingrowth and eliminating an entrance point for wear debris to access the floor of the acetabulum. Avoidance of the use of screws also prevents the possibility of neurovascular injury during screw insertion. Fourthly, by adding an elliptical configuration to a monoblock cup the dome of the shell is the same dimension as the reamed diameter allowing for improved coaptation of shell to acetabular floor. By increasing the diameter at the rim secure press fit is achieved without sacrificing contact at the dome.
There are disadvantages to a monoblock cup and these include the need for a revision liner mechanism should there be a need to replace the polyethylene liner. Additionally, if secure stabilisation is not achieved, the cup cannot be converted to screw fixation.
In a radiologic review of 661 acetabular components, 5.1% of cups were noted to have a polar dome acetabular gap of greater than 1.5 mm on the immediate postoperative radiograph. These patients were followed for a minimum of two years and there was noted shift in implant position in only one patient. Gaps tended to lessen in degree and fill in with bone in almost all cases. The clinical result was not compromised by the presence of a dome gap.
In a short-term follow-up of 6 years, 1843 elliptical monoblock acetabular cups have been inserted with greater than two-year follow-up in 972 hips. There have been no mechanical failures requiring revision. Two patients have been revised for instability and one for infection. The need to convert to a cup with screw fixation because of poor press fit is less than 2%.
The abstracts were prepared by Mrs Dorothy L. Granchi, Course Coordinator. Correspondence should be addressed to her at PMB 295, 8000 Plaza Boulevard, Mentor, Ohio 44060, USA.
