There are two different meniscus replacement procedures -
Meniscal allograft (meniscus transplant)
In meniscus transplant a meniscus is taken from a cadaver (dead donor) and transplanted into the recipient patient's knee to replace their own meniscus ('meniscal allograft') when they have for some reason lost their own meniscus.
The surgeon dissects the meniscus (or menisci) away from the bone block, usually leaving a small amount of bone as pegs or a ridge to aid in fixation of the transplant.
The donor meniscus will have been tested for such conditions as HIV and Hepatitis, and will have been frozen and the cells will no longer be alive. Although this is a transplant, it is not necessary for the patient to take medications to prevent rejection - the transplant becomes replaced in time by the patient's own cells.
In the meniscus scaffold a synthetic meniscus (eg. collagen meniscus) is used as a scaffold for the body to replace the meniscus.
The scaffold, in the shape of a meniscus, is purchased commercially, and can be trimmed into shape to allow the scaffold to fill any size and shape of defect in the patient's meniscus. The scaffold is soaked in blood from the patient before it is inserted, and then during healing the patient's own cells migrate into the scaffold and new tissue is formed.
The original scaffold eventually disappears as it is biodegradable and resorbable.
These procedures are still only being done in a few centres around the world and not every orthopaedic surgeon is familiar with the finer details. Even in the best hands it is unreasonable to expect a perfect result in a knee which has already been subject to sufficient stress to have lost the meniscus (i.e. probably an injury, years of meniscal problems, maybe early arthritis of femur and tibia, and finally meniscectomy). But in some cases a perfect result is obtained - so one has to be guided by the surgeon involved.
Tissue engineering is a rapidly progressing science. With the progress being made in stem cell, progenitor cell, and matrix technologies it is only a matter of time until meniscal replaement is routine.