As life happens, so do injuries, especially those that occur when we least expect them. And sometimes these injuries need special care and attention. 

A torn meniscus in your knee, for example, whether the result of a quick pivot during a basketball game, an unexpected fall, or just wear and tear over time, can have long-lasting effects. If the injury was so severe that surgery was required to remove a large portion of the meniscus—a shock absorber critical to healthy knee function—your knee joint may be prone to a chronic, painful condition called osteoarthritis.

In this case, your orthopaedist might suggest that you undergo a treatment known as meniscal allograft transplantation. In this procedure, a donor meniscus is inserted into the knee joint, which can allow young, active people to return to daily activities and can even help injured athletes return to their sport.

“In the case of extensive meniscus tissue loss in the knee, there are treatment options such as meniscus allograft transplantation or osteotomy, or even nonsurgical treatments such as the use of unloader braces,” says Christina Allen, MD, chief of Yale Medicine Orthopaedics & Rehabilitation Sports Medicine and head team physician for Yale Athletics. “The treatment options really depend on the patient’s age, activity level, and the condition of the articular cartilage above and below the meniscus.”

A meniscus is a “c”-shaped wedge of rubbery cartilage that sits in the knee joint between the ends of the tibia (shinbone) and femur (thighbone). Each knee contains a medial (inner) and lateral (outer) meniscus, which together are called menisci (the plural of meniscus).

The menisci are critical to proper functioning of the knee. The knee joint is where the ends of the tibia and femur come together; during physical activities like walking or running, forces are generated between these bones. The menisci act as shock absorbers that distribute forces across the joint and prevent the bones (and the layer of articular cartilage that covers the ends of these bones) from rubbing against one another. The menisci also help keep the knee joint stable.

But sometimes, the menisci can get injured or damaged. These injuries—usually referred to as meniscus tears—are often the result of an acute injury that might occur while participating in sports like soccer or football, but they can also be caused by several decades of wear and tear. Meniscus tears typically cause a range of symptoms that may include pain, swelling, and stiffness.

In a meniscus transplantation, a surgeon implants a new meniscus (taken from a cadaver) into a patient’s knee joint. This procedure is technically known as a meniscal allograft transplantation. Allograft is a term that simply means “the transplantation of tissue from one person (a donor) to another.”

Sometimes, a torn meniscus heals on its own, but often surgery is required. In many cases, meniscus injuries can be successfully treated with a partial meniscectomy, in which the torn meniscus tissue is trimmed away. In other cases, especially in younger patients, the surgeon might perform a meniscal repair, a surgery in which the torn meniscus is stitched together. These treatments keep as much healthy meniscus tissue (and its shock-absorbing capacity) intact as possible.

In some cases, however, the entire (or almost all of the) meniscus must be removed because of the severe degree of tearing of meniscus cartilage; this is done during a surgical procedure known as a total meniscectomy. But without this critical shock-absorbing component, the articular cartilage that allows bones in the knee joint to glide smoothly against one another wears thin. 

This increases the chances of developing osteoarthritis, a painful form of arthritis that may require knee replacement surgery. A meniscal allograft transplantation can preserve proper knee function and reduce the odds that osteoarthritis will occur.

It’s important to note that transplant rejection—when the patient’s immune system rejects transplanted tissue—does not occur with meniscus transplants. This means that patients who undergo meniscus transplantation surgery do not need to take anti-rejection medications that suppress the immune system.

In general, the best candidates for meniscus transplantation meet the following criteria:

• 40 years of age or younger
• Fit and motivated to undergo a rigorous 8-10 month rehabilitation program
• Previous partial (subtotal) or total meniscectomy in which part or all of meniscus was removed
• Ongoing knee pain that limits physical activity
• Articular cartilage loss in the knee joint, if present, is isolated to one small area (the articular cartilage may be repaired during the meniscal transplantation procedure)
• Normal or correctable (by osteotomy, a surgical procedure that straightens the leg) limb alignment
• Body Mass Index (BMI) under 30

Meniscal allograft transplantation is not recommended for patients who meet the following criteria:

• Over 40 years of age
• BMI of 30 or higher
• Osteoarthritis or inflammatory arthritis in affected knee joint
• Poor limb alignment that cannot be corrected
• Uncorrected ligament injury that causes instability of the knee

Once a patient is deemed a good candidate for meniscal allograft transplantation, the doctor will order tests, such as X-rays and magnetic resonance imaging (MRI) studies, to determine the size of the patient’s meniscus. It is crucial that the size of the donor meniscus match the patient’s meniscus.

Meniscal allograft transplantation surgery is usually done via knee arthroscopy, a minimally invasive surgical technique that minimizes scarring and usually allows for an earlier start to physical therapy. 

During an arthroscopy, a surgeon makes 2 to 3 small incisions on the patient’s knee. He or she then inserts a thin tube equipped with a camera and surgical tools into these incisions. The camera is connected to a monitor in the operating room, which allows the surgeon to see inside the knee. The procedure also requires one larger incision that the surgeon will use to tie the sutures that are passed through the donor meniscus and joint capsule.  

Typically, the first step of the procedure involves clearing debris from the joint and removing almost all of the remaining meniscal tissue, leaving a 2 to 3 mm rim of meniscus tissue. Next, the surgeon will implant the donor meniscus.

To attach it, the surgeon may need to drill tiny holes into the end of the patient’s shinbone. In some cases, the surgeon may remove a small channel from the top end of the shinbone. The front and back tips of the “c”-shaped donor meniscus may be attached to small plugs of bone or to a tiny bridge of bone. 

The surgeon will implant these bony structures into the holes or channel in the patient’s shinbone, then sew the meniscus into place using stitches. Sometimes the meniscus can be sewn into place without the use of the bone plugs or bridge.

Other parts of the patient’s knee may also require treatment. In some cases, the surgeon will perform an ACL reconstruction or revision (re-do), correct limb alignment, or repair articular cartilage damage in combination with the meniscal transplantation.  

In general, meniscus transplants last longer and are more successful when the patient’s knee joint is stable and properly aligned. That’s why it is important that the surgeon restore knee ligament stability and alignment as part of the meniscus transplant surgery. This minimizes wear on articular cartilage and keeps the transplanted meniscus from being exposed to excessive stress and abuse. 

Certain complications have been associated with meniscal allograft transplantation including:

• Tear of the transplanted meniscus
• Infection (both bacterial and viral). The risk of disease transmission due to meniscus allograft transplantation is extremely low; donors are carefully screened and the tissue is carefully tested.
• Nerve damage
• Knee effusion (swelling and stiffness due to buildup of excess fluid in the knee joint)
• Arthrofibrosis (formation of excess scar tissue in knee joint following surgery)
• Synovitis (inflammation of the membrane that lines the knee joint)
• Decreased range of motion of the knee joint

Around a third of people who undergo a meniscal allograft transplantation will eventually require additional surgery to repair tears or other failures of the transplanted meniscus.

Rehabilitation from meniscal allograft transplantation typically requires 8-10 or more months of physical therapy. Typically this will be divided into four or five phases with the goal of a gradual and safe return to normal life, including—for athletes—getting back to their sport. Though they may vary from one patient to the next, here’s a summary of what those phases may look like:

• Phase 1. A brace and crutches are required after surgery. Patients should avoid placing any body weight on the affected leg for 6 weeks. Under the guidance of a physical therapist, they may undertake light non-weight-bearing exercises to strengthen the quadriceps and to begin to move the knee through a limited range of motion. This phase usually takes 6 weeks to complete.
• Phase 2. Starting around 7 weeks after surgery, patients can begin to support their full body weight and wean off of crutches. Exercises and stretches aim to build strength and flexibility, and improve range of motion. A stationary bike may be used during therapy sessions, and exercises may involve reduced-weight leg presses and leg extensions. Patients can begin to wean off of their postoperative knee brace, and may begin using an unloader brace that reduces stress on the transplanted meniscus.
• Phase 3. Around 3 months after surgery, patients continue strengthening the muscles around the knee joint and in the hip, and using the stationary bike. Patients may now walk forwards and backwards on a treadmill. The focus at this stage is on developing balance and proprioception (awareness of the position and movement of the body).
• Phase 4. Between 4 and 6 months following surgery, patients may perform squats and progressively more difficult strength-training exercises. Toward the end of this phase, jogging may be added. At 6 months, patients who meet functional testing requirements may begin to perform exercises that involve jumping and pivoting.
• Phase 5. After around 7 months of rehabilitation, patients who wish to return to a sport or other physically demanding activity will continue to strengthen the muscles in their knee, leg, and hip. Exercises in this phase will likely entail running, jumping, and pivoting, as well as exercises tailored to the demands of the patient’s sport or activity of choice. With active participation in physical therapy, patients may be tested for a return to their sport at 8-10 months, often with the use of the unloader brace to protect the meniscus transplant.

For those who meet the criteria for meniscus transplantation and are able to fully engage with the rehabilitation program, the outlook is good. But it’s important to realize that rehabilitation requires 8 months or more of active participation—and, for competitive athletes, perhaps longer.

In the vast majority of cases, patients are able to return to daily activities with less pain and fewer symptoms. Studies suggest that up to 80 or 90% of athletes are able to return to pre-injury sporting activities following meniscal transplantation.

The transplant is not as durable as the patient’s normal meniscus and over time, the transplanted meniscus is subject to wear and tear or injury. If that happens, another surgical procedure may be required. This is an important consideration in counseling patients on their long-term sports and activity goals.