The meniscus is a crescent-shaped piece of fibrocartilage inside the knee joint, acting as a natural shock absorber and load distributor between the thigh bone and the shin bone. When this tissue is severely damaged or lost, the joint’s natural cushioning fails, often leading to chronic pain and degeneration. A meniscus can be replaced through a specialized surgical procedure known as Meniscus Allograft Transplantation (MAT). This technique involves implanting donor tissue to restore joint function and protect the remaining cartilage.
When Is Replacement Necessary?
Meniscus replacement is a highly specific procedure reserved for patients who have lost a significant portion of their own meniscal tissue. This condition, known as meniscal deficiency, usually follows a prior total or near-total meniscectomy (surgical removal of the damaged meniscus section). Replacement is necessary when there is not enough tissue remaining to protect the joint surfaces, unlike a simple meniscal repair where the torn cartilage is stitched back together.
The primary goal of the replacement procedure is to address persistent, localized pain and prevent the rapid onset of early osteoarthritis. When the meniscus is absent, the load-bearing pressure on the knee’s articular cartilage increases, accelerating wear and tear. Patients typically experience pain, swelling, and instability, especially during activity. This intervention focuses on younger, active individuals who have many years of expected use remaining for their knee joint.
Meniscus Allograft Transplantation
The replacement tissue used is an allograft, meaning it is a human meniscus harvested from a deceased donor (cadaver). The donor tissue undergoes rigorous screening and sterilization to eliminate the risk of disease transmission before cryopreservation. Because the allograft consists mainly of a collagen matrix with no living cells, the recipient does not require immunosuppressive drugs to prevent rejection, unlike organ transplants.
A precise step in the procedure is matching the size of the donor meniscus to the recipient’s knee, determined using pre-operative X-rays or Magnetic Resonance Imaging (MRI). The surgery is performed using minimally invasive arthroscopic techniques, involving small incisions around the knee joint. During the operation, the surgeon prepares the area by removing any remaining meniscal remnants and secures the donor tissue into the joint.
The allograft is anchored to the shin bone (tibia) using specialized techniques that secure its “roots,” the points where the native meniscus attaches to the bone. Common fixation methods involve using bone plugs or a bridge-in-slot technique, where a portion of the donor bone is inserted into a prepared channel in the tibia. The outer edge of the C-shaped meniscus is sewn into the joint capsule with strong sutures to ensure a secure fit. This restores its hoop-stress function, allowing it to act as a shock absorber.
Patient Candidacy and Success Factors
Meniscus Allograft Transplantation requires meeting strict criteria to maximize the likelihood of a successful outcome. The procedure is primarily indicated for patients under the age of 50 to 55 who have high functional demands and minimal existing joint damage. A prerequisite is having minimal or no progression of arthritis in the knee compartment receiving the allograft. The procedure is intended to prevent or delay arthritis, not to treat advanced joint destruction.
Proper leg alignment is a non-negotiable factor for candidacy, as abnormal alignment (such as severe bow-leggedness or knock-knees) places uneven stress on the transplanted tissue. If a patient has significant malalignment, an osteotomy—a procedure to realign the leg—must often be performed before or at the same time as the MAT. The knee must also be stable, often requiring an intact or successfully reconstructed anterior cruciate ligament (ACL) to prevent excessive motion that could damage the graft.
When patient selection is optimal, the long-term prognosis is favorable, offering substantial pain relief and improved function. Studies show graft survival rates often exceed 70% at five years and range from 45% to 70% at the ten-year mark. The procedure improves symptoms in a high percentage of patients, aiming to delay or avoid the need for a total knee replacement, sometimes for more than a decade.
The Recovery Process
Recovery from Meniscus Allograft Transplantation is a lengthy, structured process designed to protect the healing graft from undue stress. Immediately following surgery, the knee is placed in a hinged brace, and the patient is restricted to non-weight bearing for four to six weeks. This initial period of rest allows the meniscal root attachments and peripheral sutures to heal securely to the bone and soft tissues.
Structured physical therapy (PT) begins early, focusing on restoring the knee’s range of motion, though flexion is limited to 90 degrees or less initially to protect the repair. Weight bearing is gradually introduced after the initial healing phase, often transitioning from crutches to full weight bearing over several weeks. Rehabilitation then shifts focus to restoring muscle strength, particularly in the quadriceps and hamstrings, and improving proprioception.
Low-impact activities, such as cycling or using an elliptical machine, may be introduced around the three-month mark, but the return to high-impact sports is delayed. The entire rehabilitation process, from surgery to a full return to unrestricted activity, can take nine to twelve months or longer. The slow progression is necessary because the transplanted meniscus requires many months to fully integrate and adapt to the mechanical loads of the knee joint.