Discoid meniscus is a congenital anatomic variant where the lateral meniscus shows a broad disc or plate-shaped morphology instead of the normal crescent (C-shaped) configuration. It is a congenital anatomic variant with prevalence of 1-5% in Western populations and 10-15% in Asian populations. Bilateral cases occur in 10-20%. Three types are defined: complete (meniscus covers entire tibial plateau), incomplete (wider than normal but does not completely cover tibial plateau), and Wrisberg variant (posterior attachment only through Wrisberg ligament — most unstable type). Symptoms usually begin in childhood or adolescence: snapping knee syndrome (palpable and audible click during knee flexion/extension), pain, and locking. Asymptomatic discoid meniscus can be an incidental finding. MRI diagnosis is made by bow-tie sign on 3 or more consecutive sagittal slices — normal meniscus shows bow-tie in only 2 slices.
Age Range
5-30
Peak Age
15
Gender
Equal
Prevalence
Uncommon
Discoid meniscus results from failure of normal meniscal tissue resorption during embryological development. In normal development, the meniscus is initially disc-shaped and during the fetal period the central region resorbs to assume the crescent shape — in discoid meniscus this resorption is incomplete. However, current research questions this 'incomplete resorption' theory and suggests abnormal meniscofemoral attachment or vascular anomaly may be the primary cause. The pathophysiological significance of discoid meniscus is explained by several mechanisms: (1) Increased tissue thickness combined with decreased peripheral vascularity results in inadequate nutrition and mucoid degeneration in central regions. (2) Abnormal geometry causes irregular load distribution between femoral condyle and tibial plateau — especially the central thick region is subjected to excessive compression. (3) In Wrisberg variant, absence of stable capsular posterior horn attachment leads to hypermobility — the meniscus subluxates during flexion/extension creating the 'snapping' mechanism. The bow-tie sign on MRI reflects the width of meniscal tissue in the sagittal plane: the disc-shaped meniscus is 2-3 times wider than normal and gives body cross-section (bow-tie) in more sagittal slices.
On sagittal PD MRI, visualization of meniscal tissue in bow-tie shape (widest portion of C-shaped meniscus) in 3 or more consecutive slices through the lateral meniscus body region. Normal lateral meniscus shows bow-tie appearance in only 2 consecutive slices. This difference results from the discoid meniscus being 2-3 times wider than normal and is a pathognomonic diagnostic criterion. Threshold value may vary with slice thickness but at 4 mm slice thickness >=3 bow-tie slices establishes discoid meniscus diagnosis.
On sagittal PD images, bow-tie cross-section (thick portion where anterior and posterior horns converge at the body) is obtained in 3 or more consecutive slices through the lateral meniscus body. In normal lateral meniscus, bow-tie is seen in only 2 consecutive slices (8-10 mm width / ~4 mm slice thickness). In discoid meniscus, width can reach 15-25 mm with bow-tie seen in 4-6 consecutive slices. This finding is the most reliable diagnostic criterion for discoid meniscus.
Report Sentence
The lateral meniscus is seen in bow-tie configuration on ___ consecutive sagittal slices, consistent with discoid meniscus.
On coronal PD images, the lateral meniscus body appears notably thicker and wider than normal — meniscal tissue covers a large portion of the lateral tibial plateau and extends toward the tibial spines. In complete discoid type, meniscus completely covers the tibial plateau; incomplete type shows partial coverage. Meniscal thickness >5 mm is a typical finding. Irregular margins and heterogeneous internal signal of discoid meniscus reflect mucoid degeneration.
Report Sentence
The lateral meniscus body is notably thicker (approximately ___ mm) and wider than normal on coronal images, showing broad tibial plateau coverage consistent with discoid meniscus.
On PD fat-suppressed sequences, widespread Grade 1-2 intrameniscal signal increase is seen within the discoid meniscus — representing mucoid degeneration. Superimposed tear appears as Grade 3 signal (reaching articular surface) and is usually in horizontal or complex pattern. Degeneration is particularly concentrated in the central thick region — this area is in the avascular white zone with inadequate diffusion nutrition.
Report Sentence
Widespread intrameniscal degenerative signal increase is seen within the discoid meniscus, with Grade 3 signal reaching the articular surface in the ___ region consistent with tear.
In Wrisberg variant, the posterior horn of the meniscus lacks stable capsular attachment and is held only by the meniscofemoral ligament (Wrisberg). On T2 sagittal and coronal images, the posterior horn may appear displaced laterally or anteriorly (meniscal shift). During knee flexion, the meniscus subluxates between lateral femoral condyle and tibial plateau — this mechanical instability creates the 'snapping' symptom. Absence of posterior capsular attachment on MRI is diagnostic.
Report Sentence
The lateral meniscus posterior horn appears displaced laterally with non-visualization of posterior capsular attachment, consistent with Wrisberg variant discoid meniscus.
As associated findings with discoid meniscus, chondral changes (cartilage thinning, irregularity) and joint effusion may be seen in the lateral compartment. Irregular load distribution of discoid meniscus leads to early degeneration of lateral tibial plateau and lateral femoral condyle cartilage. Focal bone marrow edema in the lateral compartment may also be seen. These associated findings are important in surgical decision-making for symptomatic discoid meniscus.
Report Sentence
Chondral changes (Grade ___ cartilage loss) and minimal joint effusion are seen in the lateral compartment associated with discoid meniscus.
Criteria
Meniscal tissue completely covers tibial plateau — extends to tibial eminences (spines) on coronal images
Distinct Features
Widest type, 5-6+ bow-tie slices on sagittal, high symptom probability, highest risk of degeneration and tear
Criteria
Meniscal tissue wider than normal but does not completely cover tibial plateau — 3-4 bow-tie slices on sagittal
Distinct Features
Most common type (70-80%), usually asymptomatic and incidental finding, lower degeneration risk than complete type
Criteria
Posterior horn lacks capsular attachment — held only by Wrisberg (posterior meniscofemoral) ligament
Distinct Features
Most unstable type, most prominent snapping knee syndrome, posterior horn subluxation, most frequent surgical indication, childhood presentation
Distinguishing Feature
Lateral meniscus tear in normal morphology shows 2 bow-tie slices with normal meniscal width; discoid meniscus shows >=3 bow-tie slices and increased thickness
Distinguishing Feature
Chondral lesions are articular cartilage surface defects; discoid meniscus is meniscal morphology anomaly — separate pathologies but discoid meniscus predisposes to lateral compartment chondral degeneration
Distinguishing Feature
Baker cyst is synovial fluid accumulation in popliteal fossa completely different from meniscal pathology; however may develop secondary to lateral compartment pathology associated with discoid meniscus
Urgency
routineManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralAsymptomatic discoid meniscus requires no treatment and is followed with observation. Symptomatic discoid meniscus (pain, locking, snapping) requires surgical treatment. Standard surgical approach is arthroscopic saucerization — discoid meniscal tissue is partially resected to approximate normal crescent geometry. Any existing tear is repaired or partial meniscectomy is performed along with saucerization. Wrisberg variant requires posterior stabilization (peripheral repair/capsular suture). Total meniscectomy should be avoided as lateral compartment osteoarthritis risk increases dramatically. Meniscal tissue preservation is especially important in pediatric patients. Postoperative rehabilitation begins with 4-6 weeks partial weight bearing and full return to sports is targeted at 3-4 months.
When discoid meniscus is symptomatic (pain, locking, clicking) or has an associated tear, surgical treatment (arthroscopic partial meniscectomy + saucerization) may be needed. Asymptomatic discoid meniscus can be monitored. Wrisberg type more frequently requires surgery due to instability.