The meniscus of the knee has undergone a great change in fortune in the last 20 years. Before its critical importance in contributing towards shock absorption and knee stability was fully understood, surgeons were pretty 'gung-ho' about removing a torn meniscus. It resolved the patient's immediate problem of pain and maybe locking, but what the surgeon failed initially to realise was that many of these patients were being condemned to later joint cartilage destruction, arthritis, joint deformity, knee instability and probable total knee replacement.
This procedure of removing a meniscus is called 'meniscectomy'. The surgery may involve trimming a bit of torn meniscus, or may involve a wider removal of tissue - 'partial meniscectomy' - which leaves intact the important outer rim of the meniscus, which plays the greatest role in shock absorption and knee stability.
When the whole of the meniscus is removed, the procedure is called 'total meniscectomy' - and modern knee surgeons know that this should be avoided at all cost.
The lateral meniscus in O-shaped. Its outer rim is only loosely attached to the joint capsule, and the meniscus moves freely, its main tethering being to the flattened top of the tibia bone at the anterior and posterior horns. At the back of the lateral meniscus the capsular attachment is completely interrupted by a tendon, called the popliteus tendon. This tendon tensions the lateral meniscus when the knee is bent, and prevents it being crushed by the femur.
Any tear or surgical resection into the rim of the lateral meniscus affects its 'hoop integrity' and there will be a significant disruption of its function.
The medial meniscus is different from the lateral meniscus in several respects. It is larger, and it is C-shaped rather than O-shaped. There is no popliteus tendon interrupting its outer capsular attachment, which is much more evident and stronger than than on the lateral side. All this tethering means that the medical meniscus is less able to move, and is more frequently injured.
Modern knee surgeons should be competent in the procedures of meniscal repair, where sutures or fixation devices are used to bring together the torn ends of a meniscus, possibly after some struggle to reduce a displaced part of the tear. Today's knee surgeon should also have the knowledge to understand which type and location of tear is likely to heal on its own, and which type and location of tear will prove amenable to surgical repair.
There are some tears, however, that defy repair whatever the skill and understanding of the surgeon. In these patients, meniscal tissue removal will be necessary to avoid leaving a loose flap that can extend the tear and aggravate the symptoms. Scaffolds are really the subject for another paper, but suffice it to say that artificial constructs of materials such as collagen are available to be used to replace excised meniscus tissue. These scaffolds are 'seeded' with cells grown in a laboratory setting before being sewn into the defect.
A meniscus transplant is a surgical procedure to replace the entire meniscus and its rim with a donor meniscus purchased from a tissue bank. Such a procedure is still in the domain of the super-specialist and will not be offered at most orthopaedic units. The procedure is becoming more mainstream but meniscal transplant is still young, and the jury is still out regarding many of the key issues.
At the moment the realistic objectives are to relieve pain, improve function and thereby protect the joint cartilage of the femur and tibia bone from further degradation. It is not realistic that patients are told that they will get back to full competitive sporting activities. Patients need to understand they are protecting the joint until a later date in the future when they may be able to benefit from improved techniques. Any patient who does indeed find that they are non-symptomatic and are able to return to full function are the lucky ones.
In deciding to undertake this procedure, the knee surgeon will screen the patient to see that things are ideal -
The surgeon is likely to refuse to perform a meniscal transplant if -
Donor menisci are supplied by tissue banks who are reponsible to the processing and sterilisation. The procedures of tissue processing and secondary sterilisation are still evolving, and 'products' include fresh frozen, irradiated, or cryopreserved grafts, as well as those prepared via proprietary chemical techniques. The so-called sterilisation processes may not prevent contamination and do not guarantee a sterile graft. The patient is likely to be given prophylactic antibiotics intravenously during the surgery, and the patient will also be carefully monitored after surgery for any signs of infection.
As yet there is also no standard protocol to determine the size of the donor meniscius The size is generally estimated from X-rays or MRI scans sent to the tissue bank, who also receive details of the recipient's height, weight and gender. Of course it is important to make sure that the donor graft is correctly medial or lateral - the lateral donor meniscus should preferably be more than 10% larger than the native one Photographs can also be sent from the tissue bank to the surgeon with the potential graft placed against a metric ruler so that the surgeon makes final choice. Some surgeons will show the graft to the patient before putting them to sleep for the operation.
Surgical technique is another area where there is no standardisation. Some surgeons transplant the meniscus without any bone attached to it, while others advocate that a bony bridge or bone plugs on the meniscal horns are used to ensure a good strong fixation of the graft.
If bony fixation is used, there is usually a difference between the medial and the lateral meniscus. In the medial meniscus the surgeon will prepare a central bone bridge or two bone plugs matched to two tunnels to into which the bone plugs will be pulled and fixed. In the case of the lateral meniscus a central bone bridge will be prepared with a corresponding slot in the tibial plateau, and the two will be dovetailed together.
In the short term, the majority of meniscal transplant patients can expect decreased pain and a certain amount of protection of the joint cartilage. Unfortunately in the long term quite a high percentage of transplants deteriorate, tear or shrink, leading to decrease in function, and very few really completely restore the functionality of the original meniscus.
As already mentioned, correction of leg alignment and ligament instability are critical to the long term functioning of a meniscal transplant.
It is not easy to assess the progress of the graft, but MRI scans are more commonly used as a relatively reliable and noninvasive evaluation method.
Orthopedic surgeons should keep in mind that transplanted menisci cannot restore perfectly the normal meniscal function but just improve function with a possible cartilage protection effect in the meniscectomised knee. The patient and the family should be educated beforehand about modifying expectations and changing their sporting activities.
Also surgeons must be absolutely sure that they cannot save a meniscus before subjecting a young patient to a meniscectomy. It is imperative that any surgeon dealing with the knee be skilled in procedures of meniscal repair.
The young patient who has had a meniscectomy presents a special problem. Surgeons are likely to be reluctant to perform a meniscal graft when the young patient has high expectations but there are so many known issues, especially if the patient has no symptoms yet. Young patients will probably be advised to cut back on their expectations and activities, and understand that they need to buy time until better procedures are available to them.
1. Lee BS, Kim JM, Sohn DW, Bin SI. Knee Surg Relat Res. 2013 Mar;25(1):1-6. Review of Meniscal Allograft Transplantation Focusing on Long-term Results and Evaluation Methods.
3. Noyes FR, Barber-Westin SD. Instr Course Lect. 2005;54:341-53. Meniscus transplantation: indications, techniques, clinical outcomes.