What is a discoid meniscus?

What is a Discoid Meniscus?

A discoid meniscus is a congenital anatomical variant where the meniscus has an abnormally thick, disc-shaped morphology instead of the normal C-shaped crescent, most commonly affecting the lateral meniscus and occurring more frequently in Asian populations (10-13% incidence) compared to Western populations (3-5% incidence). 1, 2

Anatomical Characteristics

• The discoid meniscus covers more of the tibial plateau than a normal meniscus and has increased thickness, representing a spectrum of morphological variants rather than a single entity 3, 1
• The lateral meniscus is far more commonly affected than the medial meniscus, with discoid medial meniscus being extremely rare 4, 3
• Bilateral involvement occurs in more than 80% of cases, making examination of the contralateral knee essential 1, 2
• The ultrastructural features differ from normal menisci, with deranged collagen arrangement and altered vascularity, which has implications for healing after repair 2

Classification Systems

Traditional Watanabe Classification

• Complete discoid meniscus: covers the entire tibial plateau 1
• Incomplete discoid meniscus: covers more than normal but not the entire plateau 1
• Wrisberg variant (Type III): may have normal shape but lacks normal posterior tibial attachment, resulting in hypermobility 1, 5

Modern MRI-Based Classification

• This classification is based on peripheral detachment patterns and provides more useful information for surgical planning: no shift, anterocentral shift, posterocentral shift, and central shift 1

Clinical Presentation

• Symptomatic patients typically present with snapping, clicking, pain, swelling, and reduced range of knee movement 3, 1
• The Wrisberg variant specifically may cause the knee to snap or pop when flexed and lock in extension 5
• Symptoms usually arise from meniscal tears or peripheral detachment rather than the discoid morphology itself 2
• Many patients remain asymptomatic, with discoid meniscus discovered as an incidental finding 3

Diagnostic Approach

• Plain radiographs may show indirect signs but are frequently not definitive for diagnosis 1, 5
• MRI is essential for diagnosis and treatment planning, allowing identification of the discoid morphology, associated tears, and peripheral detachment patterns 3, 1
• The Wrisberg variant can be particularly challenging to diagnose on MRI when no tear exists, and may require imaging with the knee in the locked extended position for visualization 5

Pathophysiology and Risk

• The abnormal morphology and ultrastructure make discoid menisci prone to tearing due to unusual biomechanical stresses 1, 2, 5
• The Wrisberg variant lacks posterior attachment to the tibial plateau, resulting in hypermobility that increases tear risk 5
• The discoid meniscus has an impact on gait mechanics due to its abnormal shape and size 2

Management Principles

Asymptomatic Patients

• Asymptomatic patients require close follow-up without surgical treatment and preservation of the discoid meniscus is recommended 4, 1, 2

Symptomatic Patients Without Locking

• Conservative management is appropriate for symptomatic patients who do not have true mechanical locking 2

Surgical Indications and Techniques

• Partial meniscectomy (saucerization) is currently considered the treatment of choice, as total or subtotal meniscectomy leads to increased risk of degenerative arthritis 1, 2
• When peripheral detachment is present, it must be stabilized with meniscal repair while preserving the meniscus rim to allow transmission of hoop stresses 1, 2
• The preferred surgical principle is reduction, followed by reshaping with reference to the midbody of the medial meniscus, and repair as firm as possible 1
• Meniscectomy and reshaping for injured discoid medial meniscus produces promising short-term results 4

Outcomes and Prognosis

• Functional outcomes and onset of radiographic arthritis after saucerization and repair are better in the long term compared with subtotal meniscectomy 2
• However, there is no compelling evidence currently favoring repair as results deteriorate with increasing follow-up 2
• Poor prognosis is reported in patients undergoing total meniscectomy, those with higher age at presentation, and those with valgus malalignment 2
• Return to sports is possible after more than 4 months in patients undergoing repair 2

Common Pitfalls

• Failing to examine the contralateral knee, as bilateral involvement is common 1, 2
• Performing total or subtotal meniscectomy instead of partial meniscectomy with repair when indicated 1, 2
• Operating on asymptomatic discoid menisci, which should be preserved 4, 1
• Missing the diagnosis of Wrisberg variant on standard MRI when imaging is not performed with the knee in the locked position 5