Histology

For Medical Students

Structure and Location

The meniscus is fibrocartilage: a dense collagen framework with cells scattered in it and a great deal of water held by the matrix between.

Conventional teaching puts the water content at about 70 per cent of the wet weight, with collagen making up most of the remaining dry weight and type I collagen predominating.

The collagen is not arranged randomly. The great majority of the fibres run circumferentially, following the curve of the C from one horn to the other. A smaller number run radially, tying the circumferential bundles together.

Relations

At the surfaces, a thin layer of fibres runs in a mesh rather than in bundles, which gives a smooth articulating surface.

At the peripheral border the collagen continues into the capsule and into the coronary ligament, so there is no sharp boundary between meniscus and capsule.

At the horns the circumferential bundles converge and insert into bone through a transitional zone of fibrocartilage and mineralised fibrocartilage, in the same way a tendon inserts.

Function

The circumferential fibres carry the hoop tension that resists the meniscus being extruded when the wedge is compressed. They are the load bearing element.

The radial fibres stop the circumferential bundles splitting apart from one another. They are the reason the meniscus does not simply unravel.

The water held in the matrix makes the tissue stiff under rapid load and allows it to deform slowly under sustained load, which is why it behaves differently in a jump and in a long squat.

Clinical Relevance

The fibre direction predicts the tear pattern. A longitudinal tear runs between circumferential bundles and leaves the hoop partly intact. A radial tear cuts across them and breaks the hoop.

That is why a small radial tear can matter more than a large longitudinal one.

Degenerate tissue loses water and the fibre organisation becomes less regular, which is why an older meniscus tears under loads a young one tolerates.

MRCS Level

Structure and Location

Three collagen layers are conventionally described. A superficial mesh at both surfaces. A lamellar layer beneath it with fibres crossing obliquely. And the deep central layer, which is the bulk of the tissue and is almost entirely circumferential.

Radial, or tie, fibres run from the periphery towards the free edge through the central layer and are more numerous in the posterior horn than elsewhere.

Cells differ by depth. The surface cells are flatter and more fibroblast like. The deeper cells are rounder, sit in lacunae and resemble chondrocytes, and are often called fibrochondrocytes.

The matrix between contains proteoglycan, which is present in much smaller quantity than in articular cartilage but concentrated in the inner part, where the tissue is loaded in compression rather than tension.

Relations

The enthesis at each horn has the four zone structure of a direct tendon insertion: ligament, uncalcified fibrocartilage, calcified fibrocartilage, bone. That is why a root avulsion is an enthesis failure rather than a tissue tear.

The peripheral border blends into the capsule, and the perimeniscal plexus lies in that transitional tissue rather than in the meniscus itself.

The free edge has no vascular or neural supply and no perichondrium, so it has no mechanism for repair.

Blood Supply and Innervation

Vessels enter from the perimeniscal plexus and penetrate radially a limited distance into the peripheral tissue, with a corresponding gradient of cellularity: the peripheral tissue is more cellular and more vascular, the inner tissue less of both.

Nerve fibres accompany the vessels. Mechanoreceptors, including Ruffini endings and Golgi type structures, are concentrated at the horns and in the peripheral third, which is why the horns contribute to proprioception.

The avascular inner tissue depends entirely on diffusion. That limits its thickness as much as its healing: nutrition by diffusion only works over a short distance, which is part of why the free edge is thin.

Function

The tissue is anisotropic: far stiffer along the circumferential fibres than across them. Load applied along the fibres is carried; load applied across them is resisted much less well.

Proteoglycan concentrated inwards means the inner part behaves more like cartilage and the outer part more like tendon, which matches what each part does.

Viscoelasticity comes from fluid flow through the matrix. Rapid load gives a stiff response because the fluid cannot move; sustained load allows creep.

3.6 owns the material properties as numbers.

Clinical Relevance

Repair relies on the vascular peripheral tissue and on the cellularity that goes with it. Both fall off towards the free edge, and they fall off together.

Suture holds in circumferentially organised tissue and pulls through tissue that is degenerate or predominantly radial, which is part of why posterior horn repairs behave differently from body repairs.

Because the horn insertion is an enthesis, root repair is fixing tendon to bone and behaves like it: the construct has to survive while the enthesis reforms.

Allograft and scaffold replacement have to reproduce a circumferentially organised tissue to transmit load, which is the main reason they are difficult.

FRCS Level

Relations

The structural relation that governs reconstruction is the continuity between the meniscus, the capsule and the horn entheses. A meniscus is not a free body; it is one end of a continuous fibrous system.

A replacement that is not anchored at both horns and not attached peripherally is not a meniscus, whatever it is made of.

Function

Because the tissue is anisotropic and the hoop is the mechanism, any repair or replacement has to restore fibre continuity in the circumferential direction. Restoring bulk does not restore function.

That is the anatomical argument behind the difficulty of meniscal scaffolds: growing tissue into a defect is achievable, and growing correctly oriented load bearing collagen into it is a different problem.

Clinical Relevance

Tear pattern, location and tissue quality decide repairability, and all three are histological properties. The decision is made at arthroscopy on what the tissue looks and feels like.

In allograft transplantation, sizing and both horn fixations matter because the hoop has to be reconstituted. Section 7 owns the procedure.

Biological augmentation aims at the cellularity gradient on this page rather than at the mechanics, which is why its effect is on healing rate rather than on load transmission.

The composition figures on this page are standard teaching values, registered and unchecked, and they vary with age and with the part of the meniscus sampled.