It probably stands for polyethylene liner. This is a plastic insert that goes inside the cup or socket portion of the implant. It provides a cushion and surface
against which the ball of the joint can glide over and move around.
Studies show the PE liner is the weak link in hip joint replacement. The other parts of the implant hold up and wear well for five, 10, even 15 years and beyond. But thinning, cracking, and breaking of the PE requires implant revision.
Some companies are making a thicker liner for surgeons to try. The next step is to see how long the liners last in various hip implants. Do they last longer in a ceramic or metal prosthetic? Do they last longer in younger patients? These questions are under study today.