Meniscus Structure and Function

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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

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

In children, the entire meniscus is vascular. By skeletal maturity only the peripheral 10-25% retains vascularity (Arnoczky & Warren), and the central two-thirds is avascular, relying on synovial-fluid diffusion.

The root-tear section and the Bao finite-element card both invoke the "meniscofemoral ligaments" as the reason a lateral root tear is not equivalent to meniscectomy - but the ligaments themselves are never described. They are a discrete, high-yield piece of meniscal anatomy.

The two ligaments run from the posterior horn of the LATERAL meniscus to the lateral aspect of the medial femoral condyle, straddling the posterior cruciate ligament:

Why they matter (structure → function):

• They are present in most knees (at least one in the large majority; both in a minority) and arise only from the lateral meniscus - a key asymmetry between the two menisci.
• Functionally they tether the posterior horn of the lateral meniscus and restrain its radial extrusion under load. This is precisely why a lateral posterior root tear is not biomechanically equivalent to lateral meniscectomy (Bao): the meniscofemoral ligaments keep the torn meniscus partly load-bearing, whereas the medial side has no such backup (hence the Allaire "medial root tear = meniscectomy" finding).
• On MRI or arthroscopy a meniscofemoral ligament can be mistaken for a tear fragment of the posterior horn or for the PCL.

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

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

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

The topic repeatedly calls the meniscus "viscoelastic" and "time-dependent" and the figure above shows its creep curve, but the underlying material science is never explained. It is the deep "function" answer an examiner is fishing for when they ask how a meniscus absorbs shock.

The meniscus is a biphasic (poroelastic) material. It has two phases:

• a solid phase - the collagen-proteoglycan matrix, and
• a fluid phase - interstitial water (~70-75% of wet weight), held by the negative charge of the proteoglycans.

Under load, the matrix has low permeability, so the water cannot escape instantly. The pressurised interstitial fluid carries most of the load in the first moments (fluid load support), and only flows out slowly through the matrix. This fluid flow is the source of the time-dependent (viscoelastic) behaviour:

How this produces shock absorption: energy is dissipated through fluid flow and internal friction (and through the viscoelastic matrix) rather than being transmitted intact to the cartilage. On unloading, the proteoglycans osmotically re-imbibe water and the meniscus recovers its shape, ready for the next cycle. This is why the meniscus damps impact loads and why rapid (impulsive) loading is more dangerous than slow loading - there is no time for fluid to redistribute, so peak stresses are higher. (The general biphasic theory of articular cartilage is developed in articular-cartilage-structure; here it is applied to the meniscus.)

Meniscal Tears and Treatment Implications

Meniscal Root Tears

The meniscal root attachments (anterior and posterior horns) anchor the circumferential fibres and are essential for hoop-stress generation. A root avulsion lets the meniscus extrude under load, abolishing its biomechanical function:

• Posterior medial root tear: Most common; often degenerative in middle-aged knees
• Biomechanical consequence (medial): A posterior medial root tear raises peak medial contact pressure by ~25% and is not significantly different from total medial meniscectomy (Allaire et al.). Anatomic repair restores contact pressure and kinematics to normal.
• Lateral side is different: Finite-element work (Bao et al.) suggests a lateral root tear is not fully equivalent to lateral meniscectomy because the meniscofemoral ligaments restrain radial extrusion - one reason lateral root tears are repaired aggressively, especially alongside ACL reconstruction.
• MRI signs: "Ghost meniscus" (meniscus seen on sagittal but absent on coronal), radial linear defect at the root, and meniscal extrusion over 3mm on coronal images
• Treatment: Transtibial pull-out or suture-anchor root repair to restore hoop stress

Differential Diagnosis of the Painful, "Meniscus-Like" Knee

Several entities mimic a symptomatic meniscal tear or extrusion and are common viva discriminators.

Controversies and Areas of Uncertainty

Global Epidemiology

• Meniscal injury is among the most common knee pathologies, with surgical incidence reported around 60-70 per 100,000 person-years in high-income populations; arthroscopic meniscus procedures remain among the most frequently performed orthopaedic operations worldwide.
• Bimodal pattern: traumatic tears in young, active people (often sport- and pivot-related, frequently with ACL injury) versus degenerative tears in those over 45, where prevalence rises with age and obesity and tears are commonly incidental on MRI.
• Medial more than lateral in degenerative disease; lateral and root/discoid pathology feature more in younger and paediatric cohorts.

Side-by-Side Guideline Positions

Registry and Practice Variation

• Large national datasets (e.g. UK and Scandinavian arthroscopy/knee registries) have tracked a decline in arthroscopic meniscectomy for degenerative tears following the landmark RCTs, alongside a rising share of meniscal repair - a measurable shift toward preservation.
• Resource-rich settings: ready access to MRI, arthroscopic repair implants, and allograft transplantation; greater use of root repair and biologic augmentation.
• Resource-limited settings: MRI and arthroscopy access may be constrained, so diagnosis leans on clinical tests (joint-line tenderness, McMurray, Thessaly) and treatment emphasises rehabilitation; when surgery is needed, repair capability and allograft availability are often limited, favouring tissue-sparing techniques and conservative care.