Meniscus Structure and Function

The menisci are two crescent-shaped fibrocartilaginous structures interposed between the femoral condyles and the tibial plateaus, one medial and one lateral. They differ in shape, attachment and mobility.

The medial meniscus is C-shaped and covers approximately 60% of the medial tibial plateau. Its anterior horn attaches to the plateau anterior to the ACL, and its posterior horn posterior to the ACL and anterior to the PCL. Peripherally it is attached to the joint capsule and the deep MCL, which makes it the less mobile of the two: it translates approximately 5mm during knee flexion.

The lateral meniscus is more circular and covers 80% of the lateral plateau. Its anterior horn attaches anterior to the tibial eminence, near the ACL, and its posterior horn posterior to the eminence. It has no attachment to the LCL, from which the popliteus tendon separates it, and it translates approximately 10-12mm during flexion.

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The two meniscofemoral ligaments run from the posterior horn of the lateral meniscus to the lateral aspect of the medial femoral condyle, straddling the posterior cruciate ligament. They arise only from the lateral meniscus, a key asymmetry between the two menisci, and are present in most knees: at least one in the large majority, both in a minority.

What they do. They tether the posterior horn of the lateral meniscus and restrain its radial extrusion under load. That is why a lateral posterior root tear is not biomechanically equivalent to lateral meniscectomy (Bao): the ligaments keep the torn meniscus partly load-bearing. The medial side has no such backup, hence Allaire's finding that a medial root tear behaves like total meniscectomy (see Meniscal Root Tears below).

On imaging. On MRI or at arthroscopy a meniscofemoral ligament can be mistaken for a tear fragment of the posterior horn, or for the PCL.

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Composition. The meniscus is made up of:

• 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. The key structural feature is the circumferential arrangement of type I collagen, whose bundles resist hoop stress. Radial tie fibres interconnect the circumferential bundles and prevent longitudinal splitting. At the articular surface the fibres form a randomly oriented mesh, and the organisation runs from structured in the outer meniscus to random in the inner.

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Hoop stress. When axial load is applied across the knee, the wedge-shaped meniscus is squeezed radially outward. The peripheral attachments prevent extrusion, so the circumferential fibres convert the compressive force into circumferential (hoop) tension, distributing load over a larger contact area and reducing peak stresses on the articular cartilage. The mechanism is lost if the meniscus is excised or extruded.

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Vascular zones. Vascularity is zonal, and it determines healing potential. The periphery is supplied by the perimeniscal capillary plexus, from the superior and inferior geniculate arteries:

• Red-red (peripheral 0-3mm): vascularised from the perimeniscal plexus
• Red-white (middle 3-5mm): transitional, with a variable blood supply
• White-white (central, over 5mm from the periphery): avascular

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

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Load transmission. This is the primary function. The menisci transmit 50-70% of the compressive load across the knee in extension and 85% in flexion. After total meniscectomy contact stress rises sharply: the conventionally quoted figure is 235%, credited to Ahmed and Burke, though their abstract gives no percentage. The directly verifiable comparator is Allaire's root-tear result, set out under Meniscal Root Tears below.

Partial loss. After partial meniscectomy, contact stress rises broadly in proportion to the amount of tissue removed.

Shock absorption. The meniscus deforms under cyclic loading and dissipates energy, and intact menisci reduce peak impact forces during gait by approximately 20%. How it does so is its viscoelastic, biphasic behaviour, set out in the next section.

Joint stability. The menisci deepen the tibial plateaus, increasing their concavity, and their wedge resists anterior-posterior translation. In the ACL-deficient knee they are a secondary restraint to anterior tibial translation, the posterior horn of the medial meniscus especially.

After ACL rupture. The medial meniscus becomes a primary stabiliser, which is why combined ACL and medial meniscus injury has a particularly poor prognosis.

Lubrication and proprioception. The meniscus spreads synovial fluid across the articular cartilage surface. Mechanoreceptors in the meniscal tissue, especially in the anterior and posterior horns, contribute to joint position sense.

Two phases. The meniscus is a biphasic (poroelastic) material. Its solid phase is the collagen-proteoglycan matrix; its fluid phase is the interstitial water, held by the negative charge of the proteoglycans. The general biphasic theory is developed for cartilage in Articular Cartilage Structure; here it is applied to the meniscus.

Fluid load support. 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 and only flows out slowly through the matrix, and that fluid flow is the source of the time-dependent (viscoelastic) behaviour.

How it absorbs shock. 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.

Why speed matters. Rapid (impulsive) loading is more dangerous than slow loading: there is no time for the fluid to redistribute, so peak stresses are higher.

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Vascular Zone and Healing

Zone decides repair. Only the red-red zone has consistent healing potential. Peripheral vertical tears can therefore be repaired, whereas central horizontal cleavage tears heal poorly and were traditionally resected.

Loss of Meniscal Function

Extrusion. A meniscus displaced over 3mm beyond the edge of the tibial plateau is extruded, through loss of the radial tie fibres or a root tear. It cannot generate hoop stress, so load transmission is lost and cartilage degeneration accelerates. The meniscus is physically present but biomechanically non-functional, functionally equivalent to meniscectomy, and meniscal root repair or transplantation should be considered.

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After meniscectomy. The degenerative sequence after meniscectomy runs:

• 6 months: cartilage fibrillation visible on MRI
• 5-10 years: radiographic osteoarthritis
• 10-20 years: symptomatic osteoarthritis requiring arthroplasty

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Meniscal Root Tears

Why the roots matter. The root attachments of the 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.

The medial root. The posterior medial root tear is the most common, often degenerative in middle-aged knees. In cadaver knees it raised peak medial contact pressure by about 25%, not significantly different from total medial meniscectomy, and anatomic repair restored contact pressure and kinematics to normal (Allaire et al.).

The lateral root. 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. The "root tear = total meniscectomy" rule is therefore robust for the medial posterior root but less absolute laterally, and side matters when counselling on the urgency of repair.

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MRI signs. Look for three:

• Ghost meniscus: the root is not visualised on sagittal images
• Radial linear defect at the root
• Meniscal extrusion over 3mm on coronal images

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Repair. Transtibial pull-out or suture-anchor root repair aims to restore hoop stress.

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From Structure to Management

The principle. Preserve the meniscus whenever possible, because meniscus loss raises contact stress and osteoarthritis risk. The algorithm below applies the structure-function principles to a symptomatic knee.

Algorithm

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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

Surgery for degenerative tears. Multiple RCTs, including the ESCAPE trial, show no clinically meaningful benefit of arthroscopic partial meniscectomy over structured exercise for degenerative tears without locking. Debate persists over the minority with true mechanical symptoms, who may still benefit.

How much resection is safe. No precise threshold defines a "safe" resection volume. The pragmatic principle is to preserve every fibre possible and protect the peripheral rim that carries hoop stress.

Biologic augmentation. Fibrin clot, PRP, marrow venting and scaffolds aim to extend repair into the red-white zone, but high-level evidence remains limited and outcomes are heterogeneous.

Meniscal allograft transplantation has a clear but narrow indication: a symptomatic meniscus-deficient compartment in an aligned, stable, non-arthritic knee.

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.