Each knee has an inside (medial) and an outside (lateral) meniscus. The menisci play several key roles that are vital in maintaining the health of the knee. Specifically, they act as shock absorbers and load sharers, increase the stability of the knee, and provide lubrication and nutrition to the bearing surface (articular cartilage) of the knee.
They were once thought of as vestigial structures that served no real purpose. If injured and problematic, they were routinely excised through a procedure called meniscectomy. It is now known that a knee joint without healthy menisci is at significantly increased risk of developing wear and tear arthritis (post-traumatic orosteoarthritis). The arthritis is a result of the increased contact forces and shear that results from loss of shock absorption and stability after meniscectomy. For this reason, current surgical strategies are focused on preserving as much of the meniscus as possible or replacing it if necessary.
Certain meniscal tears are repairable with sutures, predominantly those that are freshly torn and involve healthy tissue. The closer tear is to the peripheral blood supply the higher the likelihood of successful repair. Patients with unrepairable meniscal injuries usually have symptoms of pain, catching, swelling or locking in the knee. The surgeon may perform a partial or complete meniscectomy to alleviate the symptoms in the short-term. The more meniscal tissue removed, the higher the likelihood of subsequently developing arthritis.
Most people who are meniscus deficient already have some arthritic changes in their knee. Early reports of meniscus transplantation done in arthritic knees suggested a higher incidence of transplantation failure if the irregular cartilage surfaces were not simultaneously addressed with cartilage grafting techniques. Therefore, the standard orthopaedic literature recommended that meniscus transplantation be performed in meniscus deficient patients only if they are young and free from arthritis of the knee.
Indications for meniscal transplantation include:
Contraindications include:
Meniscal allograft processing, sterilization and storage procedures vary from center to center. Some surgeons, particularly in Europe, prefer to harvest the meniscal graft themselves in a sterile fashion and use them when they are fresh, usually within two weeks of procurement. On the other hand, some American centers harvest the graft outside of a sterile operating room environment and then perform a sterilization wash. These grafts are then packaged and frozen at -80 °C, until they are to be transplanted. To decrease the risk of disease transmission, irradiating the graft has been used in the past to enhance sterilization. However, it has been shown to degrade most collagen-based tissues and the meniscus is particularly susceptible. Tissue preservation techniques such as cryo-preservation and freeze-drying have shown little benefit and have generally been abandoned except by a few tissue banks.
Matching the size of the donor knee to the size of the recipient knee is crucial for successful meniscus transplantation. Studies by Pollard et al. noted the radiographic (x-ray) measurements provided an indication of meniscus size based on the width of the tibial plateau. However, inherent variability in both the positioning of the knee and the direction of the x-ray beam, as well as human error contribute to some inaccuracy of these measurements. Sizing has been made more accurate with the use of MRI. Matching by sex, height, and weight has been shown to be nearly as accurate as radiographic techniques (0.72 correlation of height to tibial width) and has been adopted by a number of centers.
Various meniscal transplant techniques have been described, all of which are considered surgically demanding. All involve arthroscopy and some require open surgery, as well. Some surgeons leave the allograft anchored to its bony attachments and fix these bone bridges or plugs into size matched slots, troughs or holes. Other surgeons use tunnels through which they pass sutures that hold the allograft in place. Additional sutures are also used to attach the allograft to the remnant of native meniscus. Important points include obtaining stable and anatomic fixation of the horns of the meniscus and securing the meniscus rim to the tibia. Securing the graft in this way preserves the hoop (concentric) stresses of the meniscus. Meniscus extrusion or shrinking has been noted and may be in-part a function of sewing the meniscus too tightly to the synovium rather than restoring the meniscus tibial ligamentous attachments.
Post-operatively, meniscus transplant patients enter specific rehabilitation programs, aimed at decreasing pain and swelling, optimizing range of motion and strength, while avoiding injury to the healing meniscal allograft. Most patients are allowed partial weight bearing in extension immediately as it stabilizes the meniscus on the surface of the tibia.
