Abstract
When a constrained liner is used in a non-cemented cup it is advisable to add screw fixation to the cup even if the cup has an excellent press-fit because there is more pull-out pressure on the liner with a constrained cup. It also is necessary for the cup to be in the correct anteversion/inclination. It is not advisable to use a constrained liner to “make up” for poor cup position. The patient can still dislocate and then that will require an open reduction.
Our most common use with constrained sockets is to cement a liner into a well fixed cemented cup. We also will cement the liner into two-stage infections to keep it stable between those operations. Failure with the cemented liner into a non-cemented cup only occurs with poor surgical technique. There is only one correct surgical technique and violation of this can cause disassociation of the liner from the cup or dislocation of the head from the liner. The correct technique is: 1.) Preferably there is no hood on the liner because that can increase impingement. 2.) The liner size must have a press-fit of the liner edge to the edge of the metal shell. This is absolutely critical. The liner size cannot sink into the shell or be proud of the shell. 3.) The liner cannot be tilted in the shell to change anteversion or inclination. 4.) The backside of the polyethylene liner must be roughened with a high speed bur preferably in a spider web design. 5.) The inside of the cup should be roughened with a carbide bit of a high speed drill. The screw holes should be cleared of fibrous tissue. 6.) The cement thickness is not a critical factor and 1–2mm is always sufficient. 7.) Maintain pressure on the liner with one size smaller pusher (28mm for 32 inner diameter liner) until the cement is hard.
