Load Sharing

For Medical Students

The Principle

When two structures lie side by side in a load path, the load divides between them. The division depends on their shape and their stiffness.

In the knee the load divides between the menisci and the directly contacting cartilage. Both carry a share.

The share is not fixed. It changes with the position of the joint, because the geometry changes with it.

At the Knee

Conventional teaching gives the menisci roughly half of the tibiofemoral load in the extended knee, with the rest carried by direct cartilage contact.

In flexion the share rises. Conventional teaching puts it at around 85 to 90 per cent at 90 degrees, as contact moves onto the posterior horns.

The lateral meniscus carries a larger share of its compartment's load than the medial does of its, because the lateral plateau is convex. 2.6.1 covers the anatomy.

That is why losing a lateral meniscus is mechanically worse than losing a medial one, against the frequency with which each is injured. 3.5.5 covers meniscal loss.

What Changes It

Flexion angle, which is the largest factor.

Which compartment, with the lateral more meniscus dependent.

Meniscal integrity, since a torn or extruded meniscus carries less. 3.6.10 covers extrusion.

Cartilage state, since a thinned surface changes how the load divides.

Clinical Relevance

Deep flexion puts most of the compartment's load through the posterior horns, which is why posterior horn tears present in deep flexion. 3.5.4 covers flexion.

A patient with a meniscal problem feels it in positions where the meniscus carries most, which is a useful thing to ask about. Section 4 covers the tests.

Preserving a meniscus preserves a large share of the load path rather than a small one.

Any load sharing figure needs its flexion angle, since the number nearly doubles across the range.

MRCS Level

The Principle

The division follows from stiffness and geometry. The meniscus is interposed where the condyle is curved away from the plateau, so its share rises wherever that gap is larger.

In flexion the posterior condyle is smaller in radius and sits further back, so the gap it leaves is larger and the posterior horn fills more of the contact. 3.3.2 covers roll and glide.

Load share and contact area share are related and not identical. The meniscus can carry a given share of load over a different proportion of the area. 3.5.1 covers area.

At the Knee

Conventional teaching gives the meniscal share as roughly 50 per cent in extension rising towards 85 to 90 per cent at 90 degrees, from cadaveric contact studies.

The posterior horns are the loaded part in flexion, which is the mechanical reason posterior horn and root pathology dominates the clinical picture. 3.6.9 covers roots.

Meniscectomy transfers the lost share to the cartilage, which is the arithmetic behind the pressure rise. 3.5.9 covers it.

The meniscofemoral ligaments contribute to how the lateral meniscus is held in the contact during flexion. 2.6.3 covers them.

What Changes It

Flexion angle, as the dominant variable.

Root and rim integrity, which decide whether the meniscus stays in the load path.

Extrusion, which removes the meniscus from the contact without removing the tissue.

Alignment and load magnitude, which set the absolute load the share is taken from.

How It Is Measured

Cadaveric contact studies with sensors under and around the meniscus measure the division at chosen flexion angles.

Removing the meniscus in the same specimen and remeasuring is how the share is inferred, which is a subtraction rather than a direct reading.

Finite element modelling estimates the division continuously through the range, with its material assumptions.

No in vivo measurement of the meniscal share exists, so every figure is cadaveric or modelled.

Clinical Relevance

Quote the share with its flexion angle, since it nearly doubles across the range.

Expect posterior horn pathology to dominate, given where the load sits in flexion.

Read the share as a subtraction from a paired specimen rather than a direct measurement.

Treat extrusion as removing the meniscus from the share without removing tissue.

FRCS Level

At the Knee

Treat the flexion dependence as the reason posterior horn work matters out of proportion to its size. The tissue carrying 85 per cent of a compartment's load at 90 degrees is a small amount of tissue.

It is also why a posterior root repair is a load path operation rather than a tidying one, and why its indication rests on mechanics rather than on symptoms alone. 3.6.11 covers root tears.

The same dependence sets what a transplant has to achieve. Restoring area in extension is easier than restoring the flexion share, which depends on the horns and their fixation. 3.6.14 covers transplantation.

What Changes It

Posterior horn and root integrity, which carry the flexion share.

Fixation of a transplant's horns, which determines whether it participates in flexion.

Alignment, which sets the absolute load.

Activity, since deep flexion activities load the horns specifically.

How It Is Measured

Paired cadaveric studies at several flexion angles are the standard, and most report extension and 90 degrees rather than a continuous curve.

Deep flexion measurement is technically difficult, so the deepest part of the range is the least characterised.

Modelling fills the gap and depends on its assumptions.

Clinical outcome after posterior horn and root surgery is the available evidence that the mechanics matter.

Clinical Relevance

Repair posterior roots on load path grounds rather than on symptoms alone.

Fix a transplant's horns securely, since the flexion share depends on them.

Advise on deep flexion activity by what it loads, in a knee with posterior horn pathology.

Treat the deep flexion literature as thin and the mechanism as sound.