What is a discoid meniscus of the knee?
The menisci are crescent-shaped cartilage structures located between the thigh bone (femur) and shin bone (tibia) in the knee. There are two menisci in the knee, an inner and an outer one. Their shape compensates for the mismatch between the joint surfaces of the femur and tibia. In doing so, they increase the contact area between the joint surfaces and help transfer load, reduce pressure on the joint cartilage, and contribute to the stability and distribution of joint fluid within the knee. A discoid meniscus is a congenital anatomical variant of the knee meniscus. Instead of the normal crescent shape, the meniscus has an altered shape, resembling a ring or disc. A discoid meniscus is thicker and made of lower-quality tissue. A meniscus altered in this way is usually unstable and responds less appropriately to mechanical loads in the knee, making it more prone to tearing. It was once believed that all meniscal injuries in children were related to the discoid shape. However, an increasing number of young athletes are presenting to paediatric orthopaedic clinics with tears of a normally shaped meniscus.
How is a discoid meniscus diagnosed in children?
In children, the clinical presentation of a discoid meniscus ranges from completely asymptomatic to a markedly painful knee. Clicking, pain at joint level, giving way and restricted knee movement are common symptoms. Children under 10 years of age are considered to have an almost characteristic sign of a discoid meniscus – clicking together with episodes of locked movement, known as snapping knee syndrome. Sometimes children have a contracture – a locked knee – and limp, which can indicate that the discoid meniscus has torn and shifted from its normal position. Such knees often have an effusion, and the characteristic tests for meniscal damage are positive. Since menisci are not visible on X-ray, an MRI scan is the imaging test used to confirm a discoid meniscus.
How is a discoid meniscus treated in children?
A discoid meniscus that does not cause problems in the knee is not operated on. These children are advised to avoid activities involving sudden changes of direction and rotation around the knee. However, if the discoid meniscus causes mechanical problems in the knee, or causes pain and swelling after a tear, it needs to be treated surgically. The current method of choice is knee arthroscopy with reshaping (saucerisation) of the discoid meniscus so that its final shape is as close as possible to normal. Instability of the meniscus is often found during reshaping, in which case the meniscus is sutured to the joint capsule. Surgery is followed by physical therapy to regain full range of motion and restore muscle strength according to prescribed rehabilitation protocols. Children are usually advised to wait six months before returning freely to sports activities.
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