When a person has a total knee replacement, the tibia and femur are cut as shown in the illustration below. The tibia is cut in such a way that the entire tibial plate including the meniscus is removed. If a patellar button is to be placed (when there is arthritis in the patella area), it is shaved to accept the plastic button.
The entire meniscus is removed during surgery. Depending on the condition of the anterior and posterior cruciate ligaments, they may also be removed. If this is the case, a special knee prosthesis will be used to compensate for their loss. The other ligaments in the knee joint are retained and the surgeon must ensure a careful balancing of those ligaments to ensure a well-functioning knee. This is where the surgeon's expertise comes in and it's the reason we advise our BoneSmarties to find a surgeon who does at least 150 knee replacements (not just "knee surgeries") in a year.
A new tibial plate and a plastic spacer are inserted as part of the knee replacement that completely replaces the function of the meniscus to cushion the joint.
I hope this explains the knee replacement process for you in a way that helps. If you have any specific questions, please don't hesitate to ask. You may also want to discuss this with your surgeon to see what his typical surgical procedures are.