The medial and lateral meniscii sit on the upper surface of the tibia to accommodate some rotation between the femur and the tibia. They are crescent shaped, and taper down from the outer edge. The lateral meniscus has a tighter radius than the medial, and has an area posteriorly that is not attached to the capsule of the knee. They are made of tough fibrocartilage, predominantly arranged in rings around the edge. They are very flexible when young,, but become more brittle with age, meaning degenerate attrition tears are common in the over 40s, compared to more traumatic tears in the young.
Injuries to the meniscii commonly occur with rotation on the weight-bearing knee. Degenerate tears tend to be more horizontal (laminar, or 'fish-mouth' tears) as opposed to traumatic tears such as 'parrot beak' tears. The final type of tear is one where the tear extends around the curve of the meniscus, such that the separated edge flips across the femoral condyle and thus blocks knee extension (true locking)
The overwhelming majority of meniscal tears happen in the inner 75% which does not have a blood supply, and as such cannot heal. These are best treated by excision of the torn fragment and smoothing of the edge (partial meniscectomy). Since the edge tapers, excision of moderate areas of the inner edge does not cause a problem, but nevertheless as little of the unstable meniscus as possible should be resected, even if this means leaving a laminar split int he remnant.
Parrot beak tear (left) and after trimming (right)
Fish-mouth tear (laminar) tear.
Laminar splits such as these may extend to the edge of the meniscus, where they can act like a valve allowing joint fluid to pass through to the edge of the joint, but not to return. This leads to a small cystic fluid collection, a parameniscal cyst. These may need resection of either the upper or lower leaf of the tear.
A bucket handle tear runs across the edge of the meniscus, and the fragment can displace into the knee. Smaller tears such as this can be resected leaving a stable rim.
A large bucket handle tear may displace across the front of the knee to block knee extension. If these are peripheral enough, and in the area with a blood supply, they can be repaired. Unfortunately this is only successful in about 80% if cases, and only in the under 40s since the area of the meniscus with a blood supply decreases with age.
Meniscal repair can be performed using sutures attached to small plastic clips that are passed through the meniscal tear and through the capsule of the knee.
Knee arthroscopy is a day case procedure, usually under general anaesthetic. A tourniquet is used to keep blood out of the joint; two small 5mm incisions allow the introduction of a small rigid telescope into the joint, and instruments through the other. A meniscectomy takes about 20 to 30 minutes, and the wounds are closed with sutures.
Post operatively crutches are not usually necessary; a bulky bandage to keep swelling down is applied for about 48 hours, agfter which the stitches are covered until their removal at 10 days post op.
Exercises start immediately to work on range of movement, with icing if the joint swells. Working out in the gym or exercise bike can start at 2 weeks, but impact activity such as running may cause knee swelling and inflammation so typically takes 6 weeks.