Biomechanics

For Medical Students

Structure and Location

The mechanical argument rests on three features of shape. The meniscus is a wedge, thick peripherally and thin centrally. It is a ring, anchored at both ends. And it sits between a convex femoral condyle and a nearly flat tibial surface.

Those three together make it a load spreading device that cannot escape sideways.

Relations

Above, the femoral condyle presses into the concave upper surface. Below, the flat lower surface sits on the plateau. Peripherally, the coronary ligament and the deep collateral hold it to bone.

At the horns, the circumferential collagen inserts into the intercondylar area, and those insertions take the tension the ring generates.

Function

When the knee is loaded, the femoral condyle pushes down into the wedge. The wedge tries to squirt outwards. The circumferential fibres stop it, and in stopping it they are put into tension. That tension is hoop stress, and it is how the meniscus converts a vertical load into a circumferential one.

The practical result is a larger contact area and a lower peak pressure on the cartilage. Conventional teaching has the menisci carrying about half the load across the joint in extension and a greater share in flexion, as the contact point moves back onto the posterior horns.

The medial meniscus also contributes to stability. Its posterior horn is a wedge behind the condyle and resists the tibia sliding forwards, making it a secondary restraint to anterior translation.

It contributes to joint lubrication and to proprioception through the receptors in its horns and periphery.

Clinical Relevance

Removing meniscal tissue reduces contact area and raises peak pressure, and the relationship is steep: conventional teaching has total meniscectomy roughly doubling or more the peak contact stress.

That is the mechanical account of why meniscectomy is associated with later cartilage loss, and why preservation is preferred wherever the tissue can be saved.

Breaking the hoop has the same mechanical effect as removing the meniscus, which is why a radial or root tear is a mechanical emergency in a way a longitudinal tear is not.

MRCS Level

Structure and Location

Three geometric variables govern the mechanics: wedge height, which sets how much congruence the meniscus supplies; radial width, which sets contact area; and horn attachment position, which sets the direction the hoop tension acts in.

The posterior horn is the widest and deepest part, and the contact point moves onto it with flexion. Load and tissue are therefore matched.

Relations

The coronary ligament is short medially, so the meniscus moves little under load. Movement under load is how the lateral meniscus accommodates translation; the medial meniscus cannot do that and is loaded instead.

The deep collateral attachment means the medial meniscus moves with the capsule when the knee is stressed in valgus, so a valgus injury loads the meniscal attachment as well as the ligament.

The posterior horn lies directly behind the femoral condyle in flexion, which is the position in which it acts as a restraint.

Blood Supply and Innervation

Mechanical loading and perfusion interact. The peripheral tissue that carries hoop tension is also the vascular tissue, so the load bearing element and the healing element are the same tissue.

Proprioceptive receptors in the horns are loaded when the hoop is tensioned, which is the proposed mechanism by which the meniscus contributes to joint position sense.

Loss of that input after meniscectomy is part of the explanation offered for altered neuromuscular control afterwards. 3.6 and Section 11 own the evidence.

Function

Contact mechanics. The meniscus increases contact area and so reduces pressure for a given load. Partial meniscectomy reduces the area roughly in proportion to the tissue removed, and pressure rises faster than area falls because the remaining contact is on a smaller and more curved region.

Stability. The medial posterior horn provides a bony buttress effect. In an intact knee its contribution is secondary; in a cruciate deficient knee it becomes primary and is loaded accordingly.

Shock absorption. The viscoelastic matrix dissipates energy under rapid load, and a knee without menisci transmits more of an impact to bone.

3.6 owns the quantified versions of all three.

Clinical Relevance

Partial meniscectomy is not mechanically neutral, and the amount removed matters. That is the mechanical case for the smallest resection that leaves a stable rim.

A stable peripheral rim preserves the hoop and therefore most of the function, which is why rim integrity is described rather than the quantity resected.

In cruciate reconstruction, a torn medial posterior horn should be addressed because it is a restraint the graft will otherwise share load with.

The load share and contact pressure figures on this page are standard teaching, registered and unchecked. They come from cadaveric work and the values differ between studies.

FRCS Level

Relations

The mechanically relevant relation in a degenerate knee is between meniscal position and the tibial margin. An extruded meniscus is not transmitting load wherever it sits, and extrusion implies either root failure or circumferential disruption.

In a varus knee the medial meniscus is loaded more and extrudes more, so alignment and meniscal function are not independent problems.

Function

The meniscus is a load sharing structure whose function depends on a continuous tensile ring. Treating it as a cushion predicts the wrong things: a cushion can be trimmed, a ring cannot be cut.

Secondary stability means the medial posterior horn is loaded in proportion to how much the primary restraint has failed. In a cruciate deficient knee it is doing the job badly and wearing out.

In a malaligned knee the meniscus is loaded asymmetrically, and no meniscal procedure corrects that.

Clinical Relevance

In the varus knee with medial meniscal pathology, meniscal surgery alone addresses the symptom and not the loading. Osteotomy and meniscal procedures are considered together for that anatomical reason, and Section 7 owns the combination.

In meniscal allograft transplantation the mechanical aim is to restore the hoop, which is why both horn fixations are the critical part of the procedure rather than the graft itself.

Extrusion is a functional finding. A meniscus that is present and extruded is mechanically closer to absent than to intact.

Section 3.6 carries the quantitative mechanics. Nothing on this page should be used as a number for calculation; the figures here are orientation values and are unchecked.