Discoid meniscus: current concepts

Key points

• Discoid meniscus is the most frequent congenital malformation of the menisci; it primarily affects the lateral meniscus and is highly prevalent in Asian populations.
• Its anatomic, vascular and ultrastructural features make it susceptible to complex tears.
• Current recommendations grade discoid menisci by morphology (complete vs incomplete), peripheral stability (stable vs unstable) and the presence or absence of tears.
• Initial workup includes plain X-rays and MRI, while arthroscopy confirms the shape and stability of the meniscus.
• Asymptomatic patients are observed; symptomatic ones are treated with saucerization, tear repair and stable fixation, preserving the meniscus whenever possible.

Overview

This EFORT Open Reviews article summarises current concepts on discoid meniscus, a congenital variant with central hypertrophy, a larger than normal diameter and loss of the typical C shape. Lateral discoid meniscus has an incidence of 3% to 5% in the United States and is present in up to 15% of Asian populations, with many asymptomatic cases; medial discoid meniscus has an estimated incidence of 0.06%, and both knees are affected in 15% to 25% of cases.

Structure and classification

The discoid meniscus has lower collagen density with a disorganised network, mucinous changes and less peripheral vascularisation, which predispose it to tears. The Watanabe classification describes type I (complete), type II (incomplete, covering no more than 80% of the tibial surface) and type III, the unstable Wrisberg variant. Because it does not capture shape, tears and stability together, and because instability can affect the anterior and middle thirds, the authors favour grading by morphology, peripheral stability and tears; an MRI classification by Ahn et al based on meniscal shift is also described.

Presentation and imaging

Discoid menisci are usually asymptomatic unless unstable or torn; in children there is often no previous trauma and symptoms progress insidiously. Complex degenerative and bucket-handle tears are the most common patterns, and pain from a tear lasting more than six months doubles the risk of associated cartilage lesions. The McMurray manoeuvre is 98% specific for meniscal injury and the Thessaly test has a precision of 96% for lateral tears. X-ray signs include squaring of the femoral condyle, a cupped tibial plateau, joint space above 11 mm and hypoplasia of the lateral tibial spine, with a positive predictive value of 76.2% in children aged 10-16 years. MRI confirms the diagnosis (Silverman criteria) but had lower sensitivity (61.7% vs 78.2%) and specificity (90.2% vs 95.5%) in children under 12 than in those aged 12-16 years.

Treatment

Surgery is recommended for persistent pain, blocking, oedema or limitation of sports attributable to the discoid meniscus. Total meniscectomy is now avoided in children because of its association with osteoarthritis; arthroscopy assesses shape, stability and tears, followed by saucerization leaving an intact peripheral rim of at least 6-8 mm, suture repair of amenable tears and fixation of an unstable periphery. No suture technique has proven superior. Meniscal allograft transplantation is an option for young patients with symptoms after total or subtotal meniscectomy, with reported functional improvement of 70% at 7-14 years and failure close to 10%.

Frequently asked questions

Does an asymptomatic discoid meniscus need surgery?

No. Observation is the only current recommendation, and no benefit has been shown from operating on an asymptomatic contralateral knee; prompt evaluation is advised if knee symptoms develop.

Why is the meniscus preserved whenever possible?

In the lateral compartment 70% of the load is transmitted through the meniscus, and in its absence the forces on the joint surface increase by up to 200%.

What affects long-term results?

Probably the most important factor is the patient's age at surgery, with significantly lower expectations over 30 years of age.

Source

Discoid meniscus: current concepts. EFORT Open Rev 2020;5:371-379. DOI: 10.1302/2058-5241.5.190023. Open access under the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) licence. Summary prepared by Medpresso from the original publication; it is not a substitute for the full text or for medical advice.