High Yield Overview

MENISCUS STRUCTURE AND FUNCTION

Fibrocartilage | Load Distribution | Shock Absorption | Knee Stability

50-70%load transmission in extension

85%load transmission in flexion

Type Icollagen predominant type

0.3%vascularity (central zone)

MENISCAL ZONES

Red-Red

PatternPeripheral vascular zone (0-3mm)

TreatmentExcellent healing potential

Red-White

PatternMiddle transitional zone (3-5mm)

TreatmentVariable healing potential

White-White

PatternCentral avascular zone (over 5mm)

TreatmentPoor healing potential

Critical Must-Knows

Menisci transmit 50-70% of compressive load in extension, 85% in flexion
Type I collagen arranged circumferentially provides hoop stress resistance
Red-red zone (peripheral 3mm) has blood supply from perimeniscal capillary plexus
Meniscectomy increases contact stress by 235%, accelerating osteoarthritis
Lateral meniscus more mobile (10-12mm) than medial (5mm) during knee flexion

Examiner's Pearls

"
Viva starter: 'Draw the meniscus showing fiber orientation and vascular zones'
"
Key biomechanical function is load transmission and shock absorption
"
Complete meniscectomy → 235% increase in contact stress → OA within 10-20 years
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Meniscal extrusion (over 3mm) → loss of hoop stress function → degenerative changes

Clinical Imaging

Imaging Gallery

The anatomy of the articular surface of kangaroo tibial plateau, and the creep behaviour of fibrous cartilage and hyaline cartilage.Menisci cover approximately 70% of the medial tibial plateau and 80% — The anatomy of the articular surface of kangaroo tibial plateau, and the creep behaviour of fibrous cartilage and hyaline cartilage.Menisci cover apprCredit: He B et al. via PLoS ONE via Open-i (NIH) (Open Access (CC BY))

The pathologic features were observed using optical microscope. Attachment between the undersurface of medial meniscus and tibial plateau was not identified in all cases. There is some gap (black arro — The pathologic features were observed using optical microscope. Attachment between the undersurface of medial meniscus and tibial plateau was not idenCredit: Hong JH et al. via Knee Surg Relat Res via Open-i (NIH) (Open Access (CC BY))

Representative case of a patient with a large defect filled with a corticocancellous graft. This is a 26-year-old male professional baseball player with OCD without a fragment due to previous surgical — Representative case of a patient with a large defect filled with a corticocancellous graft. This is a 26-year-old male professional baseball player wiCredit: Johnson LL et al. via Cartilage via Open-i (NIH) (Open Access (CC BY))

Critical Meniscus Exam Points

Microstructure

Type I collagen arranged circumferentially. Radial tie fibers prevent longitudinal splitting. This architecture resists hoop stresses generated during load transmission.

Vascular Zones

Peripheral 20-30% is vascular (red-red zone). Middle 10-25% is transitional (red-white). Central 60-70% is avascular (white-white). Determines healing potential.

Biomechanical Function

Load transmission, shock absorption, joint stability. Transmits 50-70% load in extension, 85% in flexion. Meniscectomy increases contact stress by 235%.

Clinical Consequence

Meniscus extrusion over 3mm → loss of hoop stress function. Total meniscectomy → OA develops in 10-20 years. Preserve tissue whenever possible.

Mnemonic

LASSMeniscal Functions

Load transmission

50-70% in extension, 85% in flexion

Absorb shock

Deformation distributes impact forces

Stability

Deepens tibial plateau, resists translation

Secondary stabilizer

Especially in ACL-deficient knee

Memory Hook:Think of meniscus as the knee's shock-absorbing LASS-o that distributes LOAD!

Mnemonic

RRWMeniscal Zones and Healing

Red-Red zone (0-3mm)

Peripheral vascular - heals well

Red-White zone (3-5mm)

Transitional - variable healing

White-White zone (over 5mm)

Central avascular - poor healing

Memory Hook:Red blood means healing! RRW zones from peripheral (vascular) to central (avascular).

Overview and Introduction

Meniscus structure and function — Meniscus anatomy: Circumferential collagen fibers, zones, and load distribution.Credit: OrthoVellum

Gross Anatomy

The menisci are crescent-shaped fibrocartilaginous structures interposed between the femoral condyles and tibial plateaus. Each knee contains two menisci:

Medial Meniscus:

C-shaped, covering approximately 60% of medial tibial plateau
Anterior horn attaches to tibial plateau anterior to ACL
Posterior horn attaches posterior to ACL, anterior to PCL
Peripheral attachment to joint capsule and deep MCL (less mobile)
Translates approximately 5mm during knee flexion

Lateral Meniscus:

More circular (covers 80% of lateral plateau)
Anterior horn attaches anterior to tibial eminence (near ACL)
Posterior horn attaches posterior to tibial eminence
No attachment to LCL (popliteus tendon separates them)
Translates approximately 10-12mm during knee flexion (more mobile)

Microstructure

Composition

Water: 70-75% of wet weight
Collagen: 15-25% (predominantly Type I)
Proteoglycans: 1-2% (aggrecan, decorin)
Cells: Fibrochondrocytes (outer) and chondrocytes (inner)

Collagen Architecture

Circumferential fibers (Type I): Resist hoop stress
Radial tie fibers: Prevent longitudinal splitting
Surface mesh: Random orientation at articular surface
Organized outer to inner: structured to random

The circumferential arrangement of Type I collagen is the key structural feature. During weight-bearing, axial loads convert to radial displacement of the meniscus. The peripheral attachments prevent extrusion, generating hoop stresses (tensile forces) in the circumferential fibers. This mechanism is lost if the meniscus is excised or extruded.

Vascular Supply

Vascular Zones Determine Healing Potential

The meniscus has zonal vascularity that profoundly affects tear management:

Red-Red Zone (peripheral 0-3mm): Vascularized from perimeniscal capillary plexus (superior and inferior geniculate arteries)
Red-White Zone (middle 3-5mm): Transitional zone with variable blood supply
White-White Zone (central, over 5mm from periphery): Avascular, relies on synovial fluid diffusion

Only the red-red zone has consistent healing potential. This is why peripheral vertical tears can be repaired, while central horizontal cleavage tears cannot heal and require partial meniscectomy.

In children, the entire meniscus is vascular. By age 10, only the peripheral 20-30% retains vascularity. By adulthood, the central two-thirds is completely avascular.

Concepts and Biomechanical Principles

Core Functional Concepts

Circumferential Fiber Architecture:

The meniscus is predominantly composed of Type I collagen arranged in circumferential bundles. This orientation is critical for resisting hoop stresses generated during axial loading. Radial tie fibers interconnect the circumferential bundles, preventing longitudinal splitting.

Hoop Stress Mechanism:

When axial load is applied across the knee, the wedge-shaped meniscus is squeezed radially outward. The circumferential collagen fibers convert this compressive force into circumferential (hoop) tension, distributing load over a larger contact area and reducing peak stresses on articular cartilage.

Clinical Implication: Meniscal extrusion (over 3mm) disrupts hoop stress function → loss of load distribution → accelerated cartilage degeneration

Biomechanical Functions

Load Transmission

The primary function of the meniscus is load transmission across the knee joint:

In extension: Menisci transmit 50-70% of compressive load
In flexion: Menisci transmit 85% of compressive load
After total meniscectomy: Contact stress increases by 235%
After partial meniscectomy: Contact stress increases proportional to amount removed

Effect of Meniscectomy on Joint Mechanics

Condition	Contact Area	Contact Stress	Clinical Consequence
Intact meniscus	100% (baseline)	100% (baseline)	Normal joint mechanics
Partial meniscectomy	Reduced 20-50%	Increased 100-200%	Accelerated cartilage wear
Total meniscectomy	Reduced 50-70%	Increased 235%	OA in 10-20 years

Shock Absorption

The meniscus deforms under load, absorbing and dissipating energy. The viscoelastic properties of the fibrocartilage allow it to:

Deform under cyclic loading (time-dependent behavior)
Recover shape after load removal
Dissipate energy through internal friction

Peak impact forces during gait are reduced by approximately 20% by intact menisci.

Joint Stability

The menisci contribute to knee stability by:

Deepening the tibial plateaus (increases concavity)
Resisting anterior-posterior translation (wedge effect)
Secondary restraint to anterior tibial translation in ACL-deficient knee (especially posterior horn of medial meniscus)

The medial meniscus becomes a primary stabilizer after ACL rupture. This is why combined ACL and medial meniscus injury has particularly poor prognosis.

Lubrication and Nutrition

Articular cartilage lubrication: Meniscus spreads synovial fluid across cartilage surface
Proprioception: Mechanoreceptors in meniscal tissue (especially anterior and posterior horns) contribute to joint position sense

Clinical Relevance and Applications

Meniscal Tears and Treatment Implications

Vascular Zone Determines Treatment

Location	Blood Supply	Healing Potential	Treatment Options
Red-Red (0-3mm)	Excellent	90% healing rate	Repair preferred
Red-White (3-5mm)	Moderate	50-70% healing rate	Repair with augmentation
White-White (over 5mm)	None	Under 10% healing rate	Resection/meniscectomy

Exam Point: Repair vs Resection

Q: What determines whether a meniscal tear should be repaired or resected?

A: Location relative to vascular zones is the primary determinant. Red-red zone tears (peripheral 3mm) should be repaired due to excellent healing potential (90%). White-white zone tears (central avascular) should be resected as they cannot heal (under 10%). Red-white zone (transitional) may be repaired with augmentation techniques (fibrin clot, PRP) with 50-70% success.

Meniscal Root Tears

The meniscal root attachments (anterior and posterior horns) are critical for hoop stress generation. A root tear allows the meniscus to extrude under load, losing its biomechanical function:

Posterior medial root tear: Most common, often degenerative
Biomechanical consequence: Equivalent to total meniscectomy
MRI sign: "Ghost meniscus" - meniscus appears normal on sagittal but absent on coronal
Treatment: Root repair restores hoop stress function

Evidence Base

Meniscal Load Transmission: In Vitro Biomechanical Study

Ahmed AM, Burke DL • J Biomech (1983)

Key Findings:

Classic study measuring contact pressures with and without menisci
Intact menisci transmit 50-70% of load in extension, 85% in flexion
Total meniscectomy increased peak contact stress by 235%
Partial meniscectomy increased stress proportional to amount removed

Clinical Implication: Preserve meniscal tissue whenever possible - even partial preservation reduces contact stress.

Limitation: In vitro cadaveric study - may not reflect in vivo loading conditions.

Meniscal Vascularity and Healing Potential

Arnoczky SP, Warren RF • Am J Sports Med (1982)

Key Findings:

Defined vascular zones: peripheral 20-30% vascularized (red zone)
Middle 10-25% has variable vascularity (red-white zone)
Central 60-70% is avascular (white zone)
Healing potential correlates directly with vascularity

Clinical Implication: Tear location relative to vascular zones determines repairability - red-red zone tears should be repaired.

Limitation: Descriptive anatomical study - does not provide clinical repair outcomes.

Long-term Outcomes After Meniscectomy

Fairbank TJ • J Bone Joint Surg Br (1948)

Key Findings:

Described classic radiographic changes after meniscectomy
Flattening of femoral condyle, ridge formation, joint space narrowing
Degenerative changes developed within 10-20 years
Changes now known as 'Fairbank changes'

Clinical Implication: Meniscectomy leads to predictable osteoarthritic changes - paradigm shifted toward meniscal preservation.

Limitation: Observational case series without controls - predates modern imaging and repair techniques.

Exam Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

Scenario 1: Basic Structure and Function

EXAMINER

"You are shown a diagram of the knee in cross-section. The examiner asks: 'Please describe the structure and function of the meniscus.'"

EXCEPTIONAL ANSWER

The meniscus is a crescent-shaped fibrocartilaginous structure consisting predominantly of Type I collagen arranged circumferentially with radial tie fibers. The medial meniscus is C-shaped covering 60% of the plateau, while the lateral is more circular covering 80%. The key biomechanical functions are load transmission (50-70% in extension, 85% in flexion), shock absorption through viscoelastic deformation, and joint stability by deepening the tibial plateau. The meniscus has zonal vascularity - the peripheral 20-30% (red zone) is vascular from the perimeniscal plexus, the middle 10-25% (red-white zone) is transitional, and the central 60-70% (white zone) is avascular, relying on synovial diffusion. This vascular pattern determines healing potential and treatment strategies.

KEY POINTS TO SCORE

Microstructure: Type I collagen arranged circumferentially (resists hoop stress)

Functions: Load transmission (50-70% extension, 85% flexion), shock absorption, stability

Vascular zones: Red (0-3mm), red-white (3-5mm), white (over 5mm from periphery)

Clinical correlation: Vascularity determines repairability

COMMON TRAPS

✗Forgetting to mention circumferential fiber orientation (key to hoop stress mechanism)

✗Not quantifying load transmission percentages

✗Confusing medial and lateral meniscus shapes and mobility

LIKELY FOLLOW-UPS

"Why is the circumferential fiber arrangement important?"

"What happens to contact stress after total meniscectomy?"

"Which meniscus is more mobile and why does this matter clinically?"

VIVA SCENARIOChallenging

Scenario 2: Biomechanical Consequences of Meniscectomy

EXAMINER

"A patient has undergone total medial meniscectomy 15 years ago and now presents with medial knee pain and early osteoarthritis. The examiner asks: 'Explain the biomechanical basis for this patient's degenerative changes.'"

EXCEPTIONAL ANSWER

This patient demonstrates the predictable consequence of meniscectomy, described historically as Fairbank changes. The meniscus normally transmits 50-70% of load in extension and 85% in flexion through its circumferentially arranged Type I collagen fibers generating hoop stress. When the meniscus is removed, this load distribution mechanism is lost. Biomechanical studies by Ahmed and Burke showed that total meniscectomy increases peak contact stress by 235% and reduces contact area by 50-70%. This concentrated stress leads to cartilage breakdown. Initially, there is cartilage fibrillation visible on MRI within 6 months. Radiographic changes develop over 5-10 years including joint space narrowing, femoral condyle flattening, and ridge formation at the condyle edge. Symptomatic osteoarthritis typically develops within 10-20 years. This is why current practice emphasizes meniscal preservation - even partial preservation reduces contact stress compared to total meniscectomy.

KEY POINTS TO SCORE

Normal meniscus transmits 50-70% load (extension) and 85% (flexion) via hoop stress

Total meniscectomy → 235% increase in peak contact stress (Ahmed and Burke study)

Timeline: Cartilage damage 6 months → Radiographic OA 5-10 years → Symptomatic OA 10-20 years

Fairbank changes: Joint space narrowing, condyle flattening, ridge formation

COMMON TRAPS

✗Not citing specific biomechanical data (235% increase in stress)

✗Forgetting to mention the hoop stress mechanism

✗Not explaining the timeline of degenerative progression

LIKELY FOLLOW-UPS

"What are Fairbank changes?"

"How much meniscus needs to be preserved to maintain function?"

"Would you consider meniscal transplantation for this patient?"

MCQ Practice Points

Collagen Type Question

Q: What is the predominant collagen type in the meniscus and how is it arranged?

A: Type I collagen arranged circumferentially with radial tie fibers. This architecture allows the meniscus to resist hoop stresses generated during weight-bearing, which is the mechanical basis for load transmission.

Load Transmission Question

Q: What percentage of load is transmitted through the menisci in knee extension versus flexion?

A: 50-70% in extension, 85% in flexion. This increased load transmission in flexion explains why meniscal tears are more symptomatic with activities involving knee flexion under load (squatting, pivoting).

Vascular Zone Question

Q: A vertical longitudinal tear is identified 2mm from the peripheral edge. What is the expected healing potential with repair?

A: Excellent (approximately 90%). This tear is in the red-red zone (0-3mm from periphery) which has vascular supply from the perimeniscal capillary plexus and demonstrates excellent healing after repair.

Biomechanical Consequence Question

Q: By how much does total meniscectomy increase peak contact stress in the knee?

A: 235% (Ahmed and Burke, 1983). This dramatic increase in contact stress explains the predictable development of osteoarthritis within 10-20 years after total meniscectomy.

Meniscal Extrusion Question

Q: What is meniscal extrusion and why is it biomechanically significant?

A: Displacement of the meniscus over 3mm beyond the tibial plateau edge. This prevents the meniscus from generating hoop stress, making it biomechanically equivalent to total meniscectomy despite the tissue being physically present.