Discoid Lateral Meniscus

Patient information for parents and carers — Dr Christopher Spelman, Orthopaedic Surgeon

What is a discoid meniscus

The menisci are two crescent (C-shaped) wedges of cartilage that sit between the thigh bone and shin bone in the knee, acting as shock absorbers and helping to spread load and stabilise the joint. A discoid meniscus is a meniscus that has an abnormal shape from birth – thicker and more disc-like than the normal crescent, covering more of the joint surface than it should. It nearly always affects the outer (lateral) side of the knee, and is one of the more common structural variations of the growing knee. Both knees are affected in a proportion of children.

As well as being the wrong shape, a discoid meniscus often has abnormal internal structure and a less reliable blood supply, particularly in its central portion. This makes it both more likely to cause mechanical symptoms and more prone to tearing than a normal meniscus.

Types (Watanabe classification)

Discoid menisci are grouped into three types, which guide treatment:

●      Complete – the disc of cartilage covers the entire lateral surface of the shin bone. This is the most common type.

●      Incomplete – the meniscus is broader and thicker than normal but does not cover the whole surface.

●      Wrisberg (hypermobile) type – the least common but most important to recognise. The normal firm attachment at the back of the meniscus is missing, so the meniscus is unstable and can flip in and out of position, held only by a single ligament (the ligament of Wrisberg). This type typically causes a snapping or clunking knee in a young child.

Symptoms

Many children with a discoid meniscus have no symptoms at all, and it is found by chance. When it does cause problems, typical features include:

●      Clicking, snapping or a visible or audible “clunk” on the outer side of the knee, sometimes with movement – classically the “snapping knee” of a young child (often the unstable Wrisberg type)

●      Pain on the outer side of the knee, especially with activity

●      Catching, locking, or the knee not fully straightening – more likely once the meniscus has torn

●      Swelling after activity

Symptoms may appear in early childhood (particularly the unstable type) or later, when a discoid meniscus tears during sport in an older child or adolescent.

Diagnosis

Assessment begins with the history and examination, looking for the characteristic clunk and outer-knee tenderness. X-rays are usually normal but may show subtle secondary changes. MRI is the key investigation – it shows the abnormal disc shape (for example, the meniscus appearing as a continuous band, the “bow-tie sign”, across more slices than a normal meniscus), and detects any tear and, in some cases, instability of the meniscus.

Treatment

Treatment depends on whether the meniscus is causing symptoms and on its type:

An incidental, asymptomatic discoid meniscus

A discoid meniscus found by chance, causing no symptoms, is left alone and simply observed. Surgery is not performed on a meniscus that is not causing problems.

A symptomatic or torn discoid meniscus

Where a discoid meniscus causes mechanical symptoms or has torn, keyhole (arthroscopic) surgery is used. The modern principle is to preserve as much healthy meniscus as possible – historically the whole meniscus was removed, but this is now avoided because it markedly increases the risk of arthritis in later life. The operation involves:

●      Saucerisation – reshaping the abnormal disc back towards a normal C-shape by trimming the excess central portion, while leaving a healthy peripheral rim (usually around 6–8 mm) intact.

●      Repair – where the meniscus is torn or unstable (particularly the Wrisberg type, or where the peripheral attachment is deficient), the rim is repaired and stabilised to the capsule with sutures rather than removed.

Recovery

After a simple saucerisation, recovery is relatively quick, with a graduated return to activity over several weeks. Where the meniscus has been repaired, a more protected rehabilitation is needed – often a period of limited weight-bearing and restricted deep bending for around six weeks – because the repaired tissue needs time to heal. A structured physiotherapy programme guides the return to sport.

Outcomes and what the evidence shows

Arthroscopic saucerisation, with repair of the rim where indicated, gives excellent or good results in the large majority of children. In one series of children under 12, all patients achieved excellent or good outcomes at a minimum of two years, and studies with longer (five-year) follow-up confirm durable results. Preserving a stable peripheral rim is central to a good long-term outcome, because a meniscus that is completely removed or left unstable is associated with poorer results and a higher long-term risk of arthritis.

Complications

Discoid meniscus surgery is keyhole surgery and is generally safe, but considerations include:

●      Re-tear of the meniscus, or persistence/recurrence of symptoms, sometimes requiring further surgery.

●      Residual instability of the rim, particularly in the Wrisberg type, if not adequately stabilised.

●      Long-term risk of arthritis in the outer compartment – lower than in the past now that the meniscus is preserved rather than removed, but still increased compared with a normal knee, particularly if a large amount of meniscus has to be trimmed.

●      The usual low risks of knee arthroscopy – infection, blood clots, stiffness and small areas of numbness around the incisions.

Summary

A discoid lateral meniscus is a common, usually harmless variation in the shape of the outer meniscus of the knee. When it causes symptoms or tears, the modern treatment is arthroscopic saucerisation to reshape it while preserving a healthy rim, with repair where the meniscus is torn or unstable – and total removal is avoided. Outcomes in children are very good. Dr Spelman will assess the type of discoid meniscus and tailor treatment, preserving as much meniscus as possible.