What is the Meniscus?
This is the biological shock absorber that sits between the end of the thigh bone and top of the shin bone. It is commonly referred to as “the cartilage”. There are two menisci in each knee, one on the inside (medial meniscus) and one on the outside (lateral meniscus).
What is the structure of the menisci?
The menisci are horse shoe shaped. They feel like soft rubber.
The collagen bundles are arranged in a circumferential pattern that allows them to absorb compressive loads, like the hoops on a barrel. Radial fibres are woven between the circumferential fibres, which help to provide structural integrity.
The menisci are triangular in cross section, the outer border of each is thick and attached to the inner lining of the knee. From where it obtains a blood supply (allowing healing). The inner border tapers to a thin free edge. This has no blood supply and so cannot heal if injured.
What is the function of the menisci?
The menisci have a number of vital functions
- Shock absorption – They are viscoelastic (like rubber) and absorb compressive forces between the thigh and shin bone. They decrease the force between the surface of the bones as the menisci cover most of the end of the bones, like a washer.
- Joint stability – The menisci ( along with other structures) stabilise the knee joint.
- Lubrication of the joint.
- Aids proprioception (orientation of the limb).
How is the meniscus injured?
Classically the meniscus is injured by twisting and turning movements (torsional forces) when there is weight going through the knee. However patients often cannot remember any specific incident.
The knee is the most frequently injured joint during sporting activity. Of all the possible injuries, meniscal injuries are the most common.
The menisci are like pieces of rubber. When we are young they are incredibly strong. As we get older they become weaker and are prone to splitting – a degenerative tear.
When twisting and turning the end of the thigh bone and top of the shin bone can act like a nut cracker, trapping the meniscus and splitting/tearing it.
What problems / symptoms may a tear cause?
Symptoms are varied but may include pain, swelling, locking and giving way. These may come and go. In some cases there are no symptoms.
In the athlete, injury usually results from twisting forces applied to a loaded knee although in older individuals the insult may be trivial. If the tear is in the vascular periphery of the meniscus the knee will rapidly swell as it fills with blood. Usually the swelling comes on over a several hours or days.
The knee may be painful, stiff and difficult to move. The pain is usually sharp in nature, aggravated by twisting and turning. If some of the damaged cartilage gets caught between the bone ends ( a bit like a door stop) the knee may lock (jam) or give way. It may be impossible to straighten the knee.
How is the diagnosis of a meniscal tear made?
All diagnosis are made by taking a history of the problem, examining the knee and then undertaking tests.
- History – The history given is usually of twisting or turning the knee (playing football/getting out of a car), sometimes a pop /snap/tearing sensation. The knee is painful and swells straight away or a little time after. The pain may come and go but is usually made worse by twisting/ turning or catching your foot.
- Examination – The knee is usually swollen (an effusion) and this may restrict how far it can bend. Sometimes it is locked (cannot be fully straighten). It is usually painful when the gap between the ends of the bone are examined with a finger. There is increased pain when the knee is twisted (McMurray’s manoeuvre).
- Investigation – A meniscal tear cannot be seen on an xray, but it may show some background arthritic changes. It is usual to undertake and MRI scan which is very good at showing tears (72-92% accurate).
How are meniscal tears treated?
There are a number of ways of treating torn menisci depending on the age of the patient, symptoms, type of tear, type of patient, sport played and to what level and an coexisting knee problems.
Observation (conservative management) – Some tears may become asymptomatic or heal spontaneously. Sometimes the symptoms only occur infrequently and a patient chooses not to do anything.
Arthroscopy and examination of the knee with:
- Removal of part of the meniscus (partial menisectomy).
- Removal of all of the meniscus (total menisectomy).
- Repair of the meniscus.
- Meniscal replacement.
What is a partial menisectomy?
This is when only some of the meniscus is taken away.
The meniscus has a very important function in shock absorption, transmitting force and stabilizing the knee. Removing any of the cartilage may affect these functions, therefore as small a piece of cartilage as possible is removed.
Most tears occur at the inner edge of the meniscus. This area does not have a blood supply and cannot heal and so the damaged piece is remove.
Removal of the damaged piece does not in itself go on to cause arthritis. The piece has no function due to the damage and causes symptoms so it is worse than useless.
How can a meniscus be repaired?
Only the thick outer edge of the meniscus has a blood supply and this is needed for healing to occur. All the repair techniques involve holding the damaged area back in place and keeping it there while it hopefully heals.
The first meniscal repair was described in 1885 with the first arthroscopic repair performed in Tokyo in 1969 .
There are a number of different surgical techniques used.
I currently use an “all inside technique” using a special suture (stitch) device.
Reports of the success of meniscal repair vary but healing rates of over 90% have been reported. It is more successful if it is undertaken at the same time as an ACL reconstruction.
What is the rehabilitation following a meniscal repair?
There is no agreed rehabilitation regime following a meniscal repair.
The repair is usually at the back (posterior horn) of the meniscus and needs protecting while it heals. I ask patients to wear a brace with a range of movement from 0-60º.
During this time crutches should be used, partial weight bearing (PWB) for 2 weeks, full weight bearing (FWB) for the remaining 4 weeks. The brace should be worn night and day, taking it off regularly to make sure you can fully straighten the leg.
During this time you will not be able to drive unless it is your left leg and you drive an automatic car.
Following this 6 weeks the brace is removed but you should not squat under load for a further 6 weeks.
What is the rehabilitation following a meniscal repair?
Part or of the meniscus can be replaced, but these techniques are still experimental.
Replacement of part of the meniscus involves placing an artificial polyurethane material in the area missing. This creates a scaffold for blood vessels and new tissue to grow into. The new tissue is not like the original and does not have its specialised function, but it may be “better than nothing”. There have been no long term results of how successful this is.
It is possible to replace the whole of the meniscus. The meniscus is surgically placed in the knee and help in places with lots of stitches. This is still experimental and is used in young patients who have had their meniscus removed and are having pain or reduced function. It is not used for patients who already have arthritis changes, have unrealistic expectations or will not comply with the post operative rehabilitation.
There are a number of specific problems. It is expensive, technically demanding and has a 30% reoperation rate. The meniscus is obtained from a dead donor and there are risks of disease transmission.
There is no long term information on how successful it is.