Shock Absorption

For Medical Students

The Principle

A material that deforms and springs straight back stores energy and gives it all back. A material that deforms and recovers slowly loses some of it as heat.

Losing energy on the way through is damping. It is what turns a sharp impact into a softer one.

Car tyres do this. They are springy enough to roll and lossy enough to take the edge off a bump.

At the Knee

Meniscal tissue is viscoelastic. Its stiffness depends on how fast it is loaded, and it takes time to recover its shape.

Under a fast load the meniscus deforms and the fluid in its matrix cannot move quickly, so it behaves stiffly and transmits more.

Conventional teaching gives the menisci a contribution of around a fifth of the knee's shock absorbing capacity, with muscle, cartilage and bone making up the rest. 3.4.10 covers shock absorption.

A meniscectomised knee is therefore a less well damped knee, which is a separate loss from the contact area loss.

What Changes It

The rate at which the load arrives, since the tissue's behaviour is rate dependent.

Tissue hydration and matrix integrity, which set how the fluid moves.

Degeneration, which changes both.

How much meniscus is present and whether it is in the load path.

Clinical Relevance

Losing a meniscus costs damping as well as area, and the two losses add.

Impact activities are the ones where damping matters, which bears on activity advice. Section 6 covers management.

Muscle is the largest shock absorber and the only trainable one, so the meniscal loss can be partly offset by strength. 3.4.8 covers energy absorption.

This is the second of the five meniscal functions, and it is genuinely secondary to load spreading. 3.6.1 covers the five.

MRCS Level

The Principle

The tissue is biphasic: a solid collagen and proteoglycan matrix with interstitial fluid that moves through it. Load is shared between the two phases, and the sharing depends on time.

Under instantaneous load the fluid carries most of it and the tissue is stiff. Under sustained load the fluid flows out, the solid matrix takes over, and the tissue creeps.

Energy is dissipated in that flow, which is where the damping comes from. It is hysteresis rather than plastic deformation.

At the Knee

Conventional teaching gives the meniscal contribution to shock absorption as roughly 20 per cent, from early mechanical work, and the figure is quoted more often than it is qualified.

Meniscectomised knees show increased impact transmission in cadaveric and in vivo work, which is the functional form of the same claim.

The contribution is rate dependent, so it is larger in impact activities than in walking. 3.4.10 covers loading rate.

Cartilage and subchondral bone damp too, and the menisci are one element of a system rather than its main component.

What Changes It

Loading rate, through the biphasic behaviour.

Matrix integrity and proteoglycan content, which set the fluid's resistance to flow.

Degeneration and age, which reduce hydration and change the properties.

Resection, in proportion to the tissue removed.

How It Is Measured

Confined and unconfined compression testing of meniscal plugs measures the biphasic properties, in excised tissue.

Impact transmission testing in cadaveric knees with and without the meniscus measures the functional contribution.

Tibial accelerometry in vivo estimates impact transmission in patients and measures acceleration rather than energy.

The commonly quoted 20 per cent figure comes from a small early literature and should be held loosely.

Clinical Relevance

Expect the damping loss after meniscectomy to show in impact activities rather than in walking.

Treat muscle as the modifiable shock absorber when the meniscal contribution has been lost.

Hold the 20 per cent figure loosely, since its evidence base is small.

Separate the damping loss from the contact area loss, since they have different consequences.

FRCS Level

At the Knee

Treat damping as the meniscal function with the weakest quantitative evidence and a sound mechanism. It is worth stating to a patient and not worth building an argument on.

The clinically useful consequence is activity advice: a meniscus deficient knee tolerates impact less well, which follows from the mechanism rather than from a trial.

A substitute's damping properties differ from native tissue, and conventional teaching holds that allograft and synthetic scaffolds do not reproduce them. 3.6.14 covers transplantation.

What Changes It

Amount and location of tissue remaining or restored.

Material properties of any substitute, which differ from native tissue.

Muscle capacity, which is the modifiable element of the damping system.

Activity and surface, which set the loading rate the knee meets.

How It Is Measured

Cadaveric impact testing compares intact, deficient and substituted states.

Material testing of scaffolds and allograft measures their properties against native tissue.

In vivo accelerometry estimates impact transmission and is not a measure of joint energy absorption.

No clinical measurement of a patient's meniscal damping exists.

Clinical Relevance

Advise on impact activity in a meniscus deficient knee on mechanistic grounds, and say so.

Expect a substitute not to reproduce native damping properties.

Build the argument for preservation on load distribution rather than on damping.

Target muscle capacity as the modifiable part of the damping system.