What are the menisci?

The menisci of the knee are two crescentic shaped cartilages found within the knee, situated between the ends of the femur and the tibia. There are two menisci in each knee; the medial meniscus and the lateral meniscus. They are wedge-shaped in cross-section and are made of elastic fibrocartilage, not too dissimilar to the outer part of the ear – almost like two potato wedges sandwiched between the bones.

Image showing a view of the top of the shin bone as seen from above (bottom of image = front of knee)

Same view of the top of the shin bone (the tibial plateau) with the menisci shown.

What is the role of the menisci?

In the past it was thought that the menisci were just the vestigial remnants of a muscle within the knee, and they were routinely excised completely in the presence of any kind of damage. There are still many patients around today with nice big scars on their knees, courtesy of previous open total meniscectomies. However, it is now recognised that the menisci have a number of very important functions within the knee:

• they act as shock absorbers
• they share load within the knee
• they are secondary stabilisers, of particular importance in the ACL-deficient knee
• they probably contribute to proprioception (sensory feedback and reflexes) within the knee
• they probably have a role lubrication of the joint, and they may play a part in nutrition of the articular cartilage

Image showing the knee with the menisci intact. As the curved end of the femur pushes down on the flat upper surface of the tibia with weight-bearing, the menisci act as shock absorbers and cushions between the bones, helping to share the weight and dissipate the forces.

…and with a meniscus removed: the curved femur sits on the flat tibia and there is point-loading, with a greatly reduced contact surface area and significantly increased contact pressures. These increased pressures cause damage to the articular cartilage covering the surfaces of the bones, and leads to wear and tear and arthritis.

How do meniscal tears occur?

Meniscal tears are the most common injury of the knee presenting to an Orthopaedic Surgeon. In younger patients, meniscal tears normally occur as a result of sports injuries, with the most common mechanism involving twisting on a loaded flexed knee with the knee then giving way. Typically, the giving way is then followed by pain and swelling in the knee. The most common sports associated with knee injuries of this kind are:

• football
• netball
• skiing

With age, the menisci degenerate and become less elastic, more friable and more liable to tearing. Degenerate meniscal tears may occur spontaneously, with no specific history of any one particular traumatic episode. Alternatively, degenerate meniscal tears may occur from simple movements such as squatting or getting up from kneeling.

What are the symptoms?

The typical symptoms of a meniscal tear are:

• knee pain – this may be felt either at the front or the back of the knee
• giving way – this is normally associated with episodes of sudden pain
• swelling – this may be permanent or may occur after episodes of giving way
• locking – this is where the knee gets stuck and cannot straighten fully.

A patient with a meniscal tear may not necessarily experience all of these symptoms, but can have any combination of the above.

How is the knee investigated?

If there are clinical signs of arthritis within the knee, then the appropriate investigation is to simply perform an X-ray.

In patients where the history and clinical examination are strongly suggestive of a meniscal tear, then the clinical diagnostic accuracy for determining meniscal tears without the need for any special investigations is approximately 90%, in the hands of an appropriately trained, qualified and experienced Orthopaedic Surgeon.

In patients where there may be doubt regarding the diagnosis, where there is a suspicion of potential alternative pathology or in cases where there has been major damage to the knee and where there may be multiple concomitant injuries such as ligament tears, then an MRI scan of the knee may be indicated.

MRI scans do give quite interesting and useful pictures of the knee. However, they are not 100% reliable, as the diagnostic accuracy of an MRI scan for a meniscal tear is only approximately 80 – 90%.

MRI scan showing a side-on view of the knee with a tear in the posterior horn of the lateral meniscus (shown by red arrow) – the meniscus should show up as a solid dark triangular wedge.

What is the treatment?

Some patients respond well to conservative treatment with rest, alteration of activities and physiotherapy. However, the menisci of the knee have very poor blood supplies and therefore they frequently fail to heal up. Despite this, if a patient’s symptoms are only relatively minor then potentially no further specific treatment may be required.

However, in those patients with significant ongoing symptoms, surgical treatment is indicated. Surgery is particularly indicated if a knee is repeatedly giving way, as each time the knee gives way there is the potential for further damage to be being caused. The surgical treatment of choice is to perform an arthroscopy of the knee (keyhole surgery). This is a relatively minor operation, performed under a general anaesthetic and normally lasting somewhere in the region of approximately half an hour.

Intra-operative arthroscopic view of the inside of a knee with a radial tear of a meniscus (shown by red arrow).

In approximately 25% of younger patients, where a meniscal tear is found to be relatively simple, fresh and in the region of the cartilage around its outer edge where the blood supply is reasonable, it may be possible to perform meniscal repair using tiny devices comprising of anchors and sutures.

However, in the remaining 75% of younger patients plus in all those patients with ragged degenerate tears, the only appropriate option is to excise the torn tissue and trim the meniscus, which is known as a partial meniscectomy.