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What is a knee meniscus?

Menisci of the knee
Menisci of the knee
© KNEEguru

The concepts surrounding the knee meniscus are not difficult to understand, but they are not always clearly explained. This video should help to put a lot of issues into a clearer perspective. Below the video you will find the narrative that accompanies the video.

Narrative

This video is about a structure in the knee called the knee meniscus and I am going to do my best to explain to you what the meniscus is, what it does, how it gets injured and what can be done about that.

A meniscus is a wedge-shaped semi-circular structure that sits on the flattened area on the top of the shinbone or tibia, fitting snugly between it and the thighbone or femur. The plural of meniscus is menisci and each knee has a pair of menisci, somewhat different in shape and in function, but both act as shock absorbers, accommodating the rounded femur to the flattened tibia.

By virtue of their physical and chemical structure and their shape, the menisci spread the load in this region, so that the important joint cartilage surfaces at the ends of these bones are not stressed during normal activities such as walking, running or jumping.

Shock absorption relies both on the shape and also the particular structure of the meniscus, which is composed of a cartilagenous matrix containing relatively few cells, but packed with an arrangement of very fine collagen fibres in several planes. One set of fibres is arranged circumferentially and packed more tightly around the outer edge, a second set are radial in arrangement and packed more tightly towards the upper surface, while a third set is more of a weave and ties the other fibres together.

When the body bears weight, the matrix and the fibres spread the load - so that what would otherwise be a vertical compressive force becomes instead distributed into a circumferential one as you might imagine from a car tyre or the hoops of a barrel. If the meniscus and its fibres are disrupted, then this shock-absorbing function is greatly reduced and the joint cartilage may become damaged resulting in arthritic changes there.

So we’ve had a look at shock absorption. Let’s progress now to the second key function of the menisci, which is that of stabilisation.

Stability of a joint is primarily achieved via ligaments, which are strong fibrous stays holding the bones together. Ligaments may also act as stays or anchors for other structures in the body. The menisci have a number of ligaments associated with them, but in discussing them we need to also go into some of the differences between the two menisci.

Let’s take a closer look. Note that we are looking down at the two menisci from above.

The two ends of each meniscus are anchored to the tibia centrally via ligamentous ‘roots’. Between the two menisci at the front of the knee is an intermeniscal ligament.

The two menisci are different shapes, and are anchored differently around their outer edge. The medial meniscus is larger, and more C-shaped than the lateral meniscus. Around its outer edge, ligaments attach it to the tibia bone and there are also fibrous connections in this region to the joint capsule. These outer tethers make the medial meniscus relatively immobile compared to the other side, and that renders it particularly vulnerable to injury because it has trouble absorbing any shearing stress.

The lateral meniscus is smaller, and more O-shaped. It is also more mobile and less vulnerable to injury than the medial meniscus. It is not fixed with outer ligaments in the same way as the medial meniscus. It has instead a partnership with a rather strange muscle and tendon complex called the popliteus, which allows it considerable movement but in a controlled way.

The popliteus is a short muscle whose main function seems to be to release the knee from the straight position, and allow it to bend. Its other key function is as a tensioner of the lateral meniscus, pulling it out of danger from crushing during a deep knee bend or squat. The best way to appreciate it is to look at a picture, but let’s turn our image around because we are now looking at the back of the right knee rather than the front.

You can see that the structure is complex, but all you need to appreciate right now is that there is a fibrous attachment from the popliteus to the upper rim of the lateral meniscus, and what happens to the popliteus affects this lateral meniscus also. Regarding the complicated anatomy, the important issue for this discussion is simply that injuries here may go unrecognised, or the surgeon may be uncertain how to repair them, and the consequence for the patient may be loss of stability and shock absorption and premature arthritic changes in the joint.

Now let’s spin our view around and return to the previous image to introduce a few new words so that we can build up vocabulary in order to discuss meniscus tears. The main part of the meniscus is called the body of the meniscus, while its outer edge is referred to as the rim. The two ends of each meniscus are called the horns. The one towards the front of the knee is called the anterior horn and one towards the back is called the posterior horn.

The area already referred to where the horns are tethered centrally to the underlying tibial bone is called the root, and where the root splays out could be called the footprint. Sometimes there are tears of the root or the outer tether can actually pull away or avulse the meniscus rim from the bone, creating instability and diminished shock absorption without the actual substance of the meniscus being injured. In a similar way the outer edge of the medial meniscus can become avulsed from the tibia without any damage to the meniscus itself. Likewise the lateral meniscus can become pulled away from its relationship with the popliteus, but not itself suffer any actual tear.

Avulsions are relatively uncommon. Usually it is the substance of the meniscus that is injured in the form of a tear, which disrupts the fibres and also leads to instability and diminished shock absorption. The commonest area for tears to occur are the front part of the body and anterior horn of the medial meniscus, although tears may occur anywhere and in either meniscus. Let’s take a look now at the nature of common tears.

Tears may go along the length of the meniscus - these are called circumferential tears - or they may go across the wedge of the meniscus - these are called radial tears. A third kind of tear may occur horizontally - these are called horizontal cleavage tears. The first of these tear types may be relatively easy to repair, while the latter two are problematic, but before we discuss this it might be useful to look at the factors that are important in meniscus healing.

You see, there are very few cells in a meniscus, as most of the structure consists of a fibre-filled matrix in which these few cells are suspended. So there is not the usual cellular response to injury that one sees in other damaged tissues such as the skin, which heals readily.

One of the most important factors in the healing of meniscus tears is the blood supply of the meniscus. The blood vessels enter to supply the meniscus from its wider outer rim and they rapidly break up into a fine capillary network. This network, however, does not extend to the inner narrow edge, which is completely without a blood supply! It is not difficult to understand, then, why tears affecting the outer rim heal more readily than tears of the inner edge.

Surgeons refer to tears in the vascular part as being in the ‘red’ zone, and those of the inner edge as being in the ‘white’ zone. So a tear which is ‘red-on-red’ could be expected to have a good blood supply and to heal well, while a tear which is ‘white-on-white’ could be expected to have no blood supply and to heal poorly, if at all. In between them, red-on-white tears may or may not have sufficient blood supply to heal.

The next important factor in meniscal healing is the direction of the tears. Circumferential tears may heal on their own, and are also more amenable to surgical repair with stitches or devices that pull the edges together. Radial tears, although not difficult to repair, need protecting for some weeks by using crutches and not bearing full weight on the joint, or else the torn edges may splay apart and break the repair. Horizontal tears are problematic, because of a valve-like effect that tends to force the joint fluid into the tear with each step. Indeed, this may be so extreme, that a fluid-filled chamber extrudes from the outer edge to form a meniscal cyst.

So, to summarise all that, I could say that meniscal tears in general heal more poorly than many other tissues, but the best opportunity for healing are circumferential tears towards the outer rim where the blood supply is good.

Of the circumferential tears, there is a special circumstance that I need to draw to your attention. If the tear is long enough, an awkward movement might allow the looser end to flip over on itself, and become trapped on the other side of the rounded end or condyle of the femur bone. This so-called bucket-handle prevents the person from straightening the leg - we call this a locked knee.

Unskilled efforts to unlock the knee can cause the tear to extend, and eventually the flipped-over bit may break on one side and move around in the fluid of the joint space, with intermittent episodes of joint catching or locking. Should the other end break also, then the fragment will float free in the knee cavity as a loose body which often becomes sequestered in a fold of the capsule at the back of the knee, popping back out from time to time in a distressing episode of catching or locking.

Of the radial tears, one has also earned itself a special name - the parrot-beak tear. This is when the tear is a bit oblique, rather than truly radial, so that one half is pointed and facing the middle. It sometimes happens that this part attempts to heal, but instead it just becomes a bit larger and rounded, coming to resemble the larger upper bill of a parrot’s beak. Like all radial tears healing can be problematic and it will need surgical intervention to remove the beak and restore the anatomy as best as possible.

You may be wondering by this time what causes a tear of the meniscus? Almost invariably it happens when there is a compressive force associated with a twisting motion or torque.So the person is upright, with the foot planted on the ground, while the body twists above the knee. It may not necessarily involve a great deal of force if the internal fibrous structure is already damaged from previous smaller incidents. Sometimes a direct blow to the side of the knee may bend the knee sideways, tearing the supportive ligaments and crushing the meniscus, but this is less common than the twisting form of injury.

Either way the symptoms tend to be the same. The joint line on the affected side becomes painful and tender. The knee may fill up with fluid and swell. The situation may settle, but the patient may consider that the knee is no longer trustworthy because of a feeling of instability and perhaps intermittent episodes of catching or giving way. With a horizontal tear a meniscal cyst may appear as a tender bump on the side of the joint, which sometimes seems to disappear and then reappear at a later date depending on whether it is filled with fluid or whether it has emptied.

So we now have a good idea of the anatomy of the meniscus, and how injuries can disrupt the structure and function, causing symptoms and an untrustworthy knee. How does the doctor make a definitive diagnosis?

The story of an injury episode, usually involving a twisting action and followed by pain - particularly in the joint line - and swelling, are highly suggestive of a tear. A history of episodes of giving way, catching or locking add further weight to the diagnosis. Examination tends to focus on identifying focal joint tenderness and eliciting pain there by various manoeuvres which generally involve holding the femur still while applying twisting stresses to the tibia.

Ordinary routine X-rays are not likely to be terribly helpful because it is not easy to see any detail of the non-bony structures. MRI scans are much more useful, although they need interpretation by someone skilled in orthopaedic radiology, otherwise tears may be missed or other problems may be mis-interpreted as tears. It should always be possible to identify tears during arthroscopy, but it is a sad fact that inexperienced surgeons may miss some types of tear or avulsion, because it takes skill to examine and interpret the posterior horns, the undersurface, the ligament stays and the popliteus region. That all sounds gloomy, and my intention is to alert you to the need for finding an experienced meniscus surgeon if you feel from your symptoms that it is likely that you have damage in this area. There is another reason for seeking out an experienced surgeon, and that will become clear as we move to the topic of management of meniscal injuries.

If you understand the consequences of untreated meniscus damage, then you will understand the management issues. We have talked about shock absorption and stability, and reduction in these two functions may lead to stress on the shiny white articular cartilage on the rounded ends of the femur and on the flattened top of the tibia, with consequent arthritis.

Joint cartilage, like meniscus cartilage, has a matrix and fibres, although the nature of the fibres is different. It also has only a few cells, which again have a blood supply on the deeper aspect but none on the exposed surface. Like the meniscus the layers remote from the blood supply will be the poorest with respect to healing.

Stresses to the joint cartilage will affect the composition of the matrix, and it may become softened, a condition called chondromalacia. The person may become aware of a crackly noise in the joint when bending or straightening the knee - this is called crepitus. With further stress, the fibrillar matrix may start to disintegrate, and the shiny white surface may fissure and break down to expose areas of wavy fronds. These may later break off and float into the joint cavity, which may by now contain more fluid than normal and the knee may feel swollen. As more and more cartilage disintegrates in this way, patches of the underlying bone may become exposed. The junction between bone and cartilage may become undermined so that bigger chunks of cartilage loosen away from the underlying bone, and eventually may break free and float in the joint as a loose body, with episodes of catching and giving way.

As the bone becomes affected, a different kind of pain in the knee may make itself felt and this is characteristically worse at night, on negotiating stairs and when the atmospheric pressure goes down. The pain may be nagging or pulsing in nature, and experienced vaguely over the front of the knee - quite unlike the sharp initial meniscus pain which is usually focal and in the joint line on the affected side.

So when I talk about managing the meniscus - yes the surgeon is managing the meniscus, but what he is really trying to do is to prevent this terrible progression to arthritis. So the principles of management of meniscus problems focus on restoring and maintaining shock absorption and stability, but also include monitoring the knee over time for any signs of early arthritis so that that, too, can be proactively managed.

Now, let’s move on to considering the management of tears and avulsions.

Tears that have a better chance of healing, that is circumferential tears in the red zone, should be repaired. It’s a bit much to go into the various methods here, but basically this is done via stitching or by using special harpooning devices, and the surgeon is likely to have his favourite technique. These days, the stitches or devices are likely to be bio-absorbable, so they just get absorbed over time and leave no trace. The surgeon may feel the need to trim off bits that are unlikely to heal, may roughen the contact edges or poke small holes in the meniscus to improve bleeding, and may use biologic glues to make the torn edges sticky and help to hold them together. Stem cell preparations may also be used to encourage cellular healing.

Clearly, this is more of a challenge than removing the torn part, but tell me, who wants to choose an option that is more likely to progress to painful arthritis? There is a critical difference in the length and complexity of rehabilitation of a repair compared to a partial meniscectomy and this is a very important issue. Sports players, in particular, are keen to get back to their sport, and may dismiss the option of a repair, because the rehabilitation may take several months longer than a partial meniscectomy. In my opinion this is a tragedy because the long term outcome of a meniscectomy is likely to be considerably worse than after a successful meniscus repair.

So what happens to those tears that have a poor chance of healing? Here, the surgeon has a dilemma. Most surgeons would probably trim away the loose bits, using their judgement as to how much to trim and how much to retain. It would be termed a trimming of the meniscus if it was a small trim, and a partial meniscectomy if it was a larger trim.

Now another big problem is that of complex tears, where the meniscus is torn in more than one direction and area, and full reduction and repair is impossible. The principle here would be to focus on retaining at least the outer rim, which is more critical than the inner part of the meniscus when it comes to protecting the joint from arthritis. This might be called a sub-total meniscectomy. If even this is impossible and the whole meniscus had to be removed the procedure would be called a total meniscectomy, which is something of a disaster for the patient from the point of view of eventual arthritis.

Before we move on, let’s look briefly at avulsions. A major issue is that the avulsion may be missed during arthroscopy, and the meniscus declared to be normal. An experienced surgeon will know to carefully probe the rims and tug at the roots. The avulsion of the medial meniscus rim can be relatively easily repaired with stitches, but root and popliteal avulsions may be a surgical challenge even to an expert surgeon.

So these are the other reasons why I caution people to be sure that their surgeon is experienced. The surgeon must feel confident to perform a repair on both tears and avulsions, and to be confident about what must be trimmed and what can be retained, and what risk the patient carries after surgery for later complications.

Before we close, there are four further management options that we must discuss, so that you have a complete overview. The first of the surgical options is a meniscus transplantation, the second is a meniscus scaffold and the third is a realignment osteotomy. A relevant non-surgical option related to an osteotomy is an unloader brace.

Meniscus transplant may be recommended in the younger patient who has had a total meniscectomy and in whom the joint surfaces are still in good shape. The whole meniscus and a bit of the tibia bone is transplanted from a donor, and the meniscus stitched into place while the bone is seated into a prepared hole in the recipient bone. It is not a procedure to be lightly undertaken because of the usual issues associated with transplants, such as rejection and the transmission of infective agents such as HIV, but in experienced units the patient may have a good outcome.

A meniscus scaffold is a bio-engineered material, in a wedge shape, that is trimmed and sewn into place where there is a large defect in the meniscus. The idea is that the cells can grow into the scaffold and replicate, and produce matrix and fibres with sufficient resilience to offer some protection to the joint cartilage.

A realignment osteotomy is a procedure where one or other or both of the long bones are cut and fixed at a new angulation, so that the load is shifted over to the good side and the affected meniscus area is relieved of the stresses that would otherwise damage the joint surface.

Angulation may also be obtained by the wearing of an unloader brace, which again relieves the stresses on the joint cartilage and offers protection to the affected area while decisions are being made about future management.

So that is the topic of the meniscus in a nutshell. I do hope that this presentation has helped to put all the many issues into some kind of perspective for you. Thank you for listening.

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Further reading:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC155528/

Updated: 27 Dec, 2014
ABOUT THE AUTHOR

Dr Sheila Strover

Clinical Editor
Degrees: 
BSc (Hons)
MB BCh
MBA

Dr Sheila Strover is the founder of the KNEEguru website. Although not a knee surgeon, she has a sound understanding of knee surgery and...

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