This article covers key concerns about the torn knee meniscus.
What is a meniscus tear?
A meniscus tear is a disruption of the structure of the meniscus affecting its integrity and stability, and thus its ability to absorb shock.
The knee meniscus is the wedge-shaped shock absorber of the knee between the flat upper tibia and the rounded lower ends of the femur bone. Each meniscus is composed of a cartilagenous matrix with embedded fibres. The intact meniscus allows vertical forces from the body's weight to be partially displaced to the outer rim - what is called hoop stress - so that there is less pressure on the vital articular cartilage of the joint.
How do you tear a knee meniscus?
Classically meniscus tears occur when the upper body is forcibly twisted while the weight is being taken by the one leg with the foot planted on the ground. Tears may also occur from deep squats or lifting heavy weights. In older people, degenerative tears may simply be the result of wear-and-tear.
Are there different kinds of meniscus tear?
The meniscal damage may simply be a frayed edge - seen as ragged fronds on the sharp edge of the meniscus.
A frayed inner meniscal rim, seen frequently, is usually of little consequence. Surgeons usually trim away the area to tidy it up and prevent enzyme release from the area. All the surgeon can do here is to trim away the edge and hope that the fraying is contained.
More problematic is the degenerate meniscus, which refers to when the whole meniscus undergoes internal change, and simply collapses in a ragged fashion. The shock-absorber function is grossly impaired and the cartilage at the ends of the long bones of femur and tibia become stressed. Arthritis can ensue.
A sharp split from the medial rim towards the lateral rim (across the radius) is known as a radial tear. This is amenable to suture (stitching), but the inner part may not heal because of the poor blood supply in that area, so some trimming may be done by the surgeon. Still it is worth trying to get the outer part to heal, as this part is most important in the meniscus.
A longitudinal or circumferential tear extends along the length of the meniscus (along the circumference). These tears run in the same direction as the main fibre bundles, so the impact may be less than a radial tear. The longitudinal tears are amenable to suture, particularly if they are towards the outer rim, where the blood supply is good.
A bucket-handle tear renders the meniscus fairly incompetent, and the changed bio-mechanics of the knee may propagate the tear and prevent healing. These types of tear may not go through the full depth of the meniscus, but if they do, they are called 'bucket-handle- tears'.
The importance of the bucket-handle tear is that the 'handle' may flip over and be caught on the other side of the rounded condyle of the femur (rounded end of the thigh bone) and lock the joint, preventing full extension (straightening) and causing pain.
Each locking episode will stress the tear and cause it to get worse.
Again bucket handles in the white-on-white area (the inner area where blood supply is poor) are usually removed surgically as they do not heal, while surgeons may suture (sew) white-on-red and almost certainly will try to preserve red-on-red, which may even heal by themselves. The red-on-red area is the outer rim of the meniscus where blood supply is good.
horizontal cleavage tear
A horizontal split in the body of the meniscus is usually called a horizontal cleavage tear. These tears are a bit unusual. The tear usually begins quite hidden from view in the inner aspect of the body of the meniscus, although it may be evident on an MRI scan.
It most likely starts after a minor injury, followed by a degenerative process which starts in the damaged area. Later the meniscus into top and bottom sections, so eventually the tear is exposed at the inner aspect and reveals itself. Here it can act like the valve of a clam, forcing fluid in and then closing and holding the fluid under pressure, where eventually it can disrupt most of the meniscus and force its way to the periphery where it may form a meniscal cyst.
It may be a shock to an inexperienced knee surgeon, who opens the knee, to see what looks like a minor tear on the inner aspect. He/she trims this away, only to find the deeper and more drastic damage which just seems to go on and on.
A neglected radial tear, usually an oblique one, may try to heal itself and round off into a rounded beak like a parrot's beak - the parrot-beak tear, and this can catch in the joint.
The flap tear is also horizontal but at the surface of the meniscus rather than in the middle. The flap tends to flick over from time to time, causing symptoms.
The tear is fairly easily dealt with. The flap section is simply trimmed away - there is sufficient body of the meniscus to heal the defect and provide shock absorption to the knee.
meniscal root tear
A root tear cuts right across the anterior or posterior horn of the mensicus, separating the body of the meniscus from the root, so that the meniscus becomes markedly unstable. It is similar to a meniscal root avulsion. These tears are difficult to manage.
Meniscus tears and acute knee locking
Knee locking is a sudden inability to fully straighten the knee after some incident, which may at the time be trivial although there is often a story of a previous meniscus tear.
A classical cause of acute knee locking is a displaced bucket-handle tear, where the inner part of a longitudinal tear flips under the rounded condyle of the femur. In the case of the bucket-handle tear, reduction of the displaced part should immediately allow full extension.
The diagnostic value of the stump impingement reflex sign for determining anterior cruciate ligament stump impingement as a cause of knee locking. Carmont MR, Gilbert RE,Marquis C, Mei-Dan O and Rees D. Sports Med Arthrosc Rehabil Ther Technol. 2012; 4: 29.
Medial versus lateral meniscus tears
There is a significant anatomical and functional difference between the meniscus on the inner aspect of the knee (the medial meniscus), and the one on the outer aspect (the lateral meniscus) -
- The medial meniscus is tightly bound to the capsular wall of the joint around its outer edge - it does not allow much sliding movement forwards, backwards, clockwise or anti-clockwise.
- The lateral meniscus is quite different - it is more tightly rounded, and there is a section where the meniscus is not attached to the capsular wall of the joint. It can slip around on the top of the shinbone and is likely to move rather than tear if abnormal forces are applied to it.
- The medial, therefore, takes the brunt of all forces. It is usually the medial which tears rather than the lateral meniscus.
How is a torn meniscus diagnosed?
Tears are sometimes identified on MRI scans as an incidental finding, but usually there is a story of an injury. Classically the doctor goes through the steps of taking a 'history', doing an examination, and then requesting (if necessary) special investigations:
Although every meniscal injury is different, there is often a story of a twisting injury, followed by swelling of the knee and joint line pain. Commonly the injury occurs in the 'planted' knee - imagine a football player landing on the knee ('planted') while the other knee follows through with the kick, with the full body continuing the momentum and continuing to rotate.
There may be immediate sharp pain, and perhaps a 'pop!' and swelling. Later the patient may complain of persistent pain in the joint line, and episodes of catching or locking. The knee may be locked in flexion (bent) at the first injury and may need to be reduced in the operating theatre (OR).
During examination of the knee, the doctor will be alert for signs suggesting a meniscal tear. There may be swelling of the knee and focal tenderness along the joint line.
The doctor may also perform some specific tests, which may offer further weight to the suspicion of a meniscal tear -
MRI is one of the most frequent investigations these days for meniscal tears, helping the radiologist identify the type and extent of the tear. MRI scans may be difficult to interpret, but a good radiologist should be able to describe the presence and extent of a tear of the meniscus.
X-rays are generally unhelpful for recent tears, although a standing weight-bearing X-ray may show a diminished gap between the long bones a meniscus is missing or totally incompetent.
McMurray's Test and Joint Line Tenderness for Medial Meniscus Tear: Are They Accurate? Gupta Y, Mahara D and Lamichhane A. Ethiop J Health Sci. 2016 Nov; 26(6): 567–572.
Avulsions causing meniscus instability
Meniscus instability may also be present if the mooring of the meniscus lifts away, even though the meniscus itself is not torn. This is called an avulsion, for example:
meniscal root avulsion
It is possible for the meniscus to be undamaged after and injury, but for the root of the meniscus to be pulled right out of its bony moorings in the tibia. This may affect the posterior or the anterior root. The sudden loss of stability and shock absorption may cause the meniscus to extrude over the edge of the tibia, making it completely incompetent - but the observant radiologist should be able to pick up the finding on an MRI or CT scan, offering the surgeon a change to repair the root before irreparable damage is done to the joint surfaces.
meniscotibial and meniscofemoral ligament avulsion
The outer edges of each menisci have attachments to the femur bone above and the tibial bone below by meniscofemoral and meniscotibial ligaments. These are more complete on the medial meniscus. A violent injury may tear the ligament from the bone - that is, avulse it. The meniscus itself is undamaged, but becomes incompetent. The radiologist should be able to identify the meniscotibial avulsion as it gives the sign of a floating meniscus.
A ramp lesion is another form of avulsion, except that this time the outer rim of the meniscus tears away from the capsule, rather than from bone. The meniscus itself is undamaged and it may be difficult to make the diagnosis. Only fairly recently have knee surgeons become very aware of ramp lesions and should probe for them during routine arthroscopy. They can be repaired and this should prevent them from propogating.
When the meniscus becomes suddenly incompetent - such as with a root tear or a root avulsion - the meniscus can extrude over the edge of the top of the tibia, and forms a bulge in the joint line which is evident on MRI scan. This then renders the meniscus totally incompetent as a shock absorber and, if not repaired urgently, can lead to rapid destruction of the joint cartilage of both femur and tibia.
Factors Associated with Meniscal Extrusion in Knees with or at Risk for Osteoarthritis: The Multicenter Osteoarthritis Study. Crema MD et al. Radiology. 2012 Aug; 264(2): 494–503.