Shock Absorption

For Medical Students

The Principle

Shock is a load that arrives quickly. The same peak force spread over a longer time is far less damaging.

Absorbing shock means slowing the arrival. Anything that deforms while the load builds does that.

A dropped egg on concrete and on a cushion meets the same force in total. Only the arrival time differs, and that decides the outcome.

At the Knee

Four things absorb shock at the knee. Muscle does it actively by lengthening under control. Cartilage, the menisci and subchondral bone do it passively by deforming.

Muscle is the largest and the only adjustable contributor, which is why technique and strength change impact more than any tissue property. 3.4.8 covers energy absorption.

Cartilage deforms and recovers slowly, so it is better at sustained load than at a fast one. The menisci deform more readily and contribute at speed.

Subchondral bone is stiffer than cartilage and takes what reaches it. A stiffer plate passes more back to the cartilage above.

What Changes It

Muscle capacity and timing, which set the active contribution.

Surface and footwear, which change the arrival time before the limb is involved.

Meniscal and cartilage integrity, which set the passive contribution.

Fatigue, which reduces the active share late in an activity.

Clinical Relevance

Impact symptoms respond to changing how a load arrives rather than only how much it is, which is why technique, footwear and surface all matter.

A knee that has lost its menisci has lost part of its passive absorption, and the remaining structures take it. 3.6 covers meniscal biomechanics.

Strengthening improves the largest contributor, which is the mechanical case for rehabilitation in impact related pain. Section 6 covers management.

Advising a patient to change surface or footwear is a mechanical intervention with a stated target.

MRCS Level

The Principle

Loading rate is the quantity, not peak force. It is the slope of the force curve and it is modifiable separately from the peak. 3.4.2 covers the ground reaction force.

The tissues are viscoelastic, so their stiffness depends on the rate at which they are loaded. Cartilage is stiffer when loaded fast, which means it deforms less and transmits more.

That rate dependence is why a fast load is a different mechanical event from a slow one of the same size, rather than a more severe version of it.

At the Knee

Cartilage absorbs by fluid flow through its matrix, which takes time. Under a fast load the fluid cannot move and the tissue behaves stiffly.

The menisci are more compliant and deform faster, so they contribute proportionally more at speed, which adds to the argument for preserving them.

Subchondral bone absorbs through trabecular deformation and remodels in response to the load it sees, so sclerosis is both a consequence and a cause of altered absorption.

Eccentric muscle work is the dominant contributor in any activity with a flight phase. 3.3.10 covers running.

What Changes It

Loading rate itself, through technique, surface and footwear.

Cartilage hydration and matrix integrity, which set its rate dependent stiffness.

Meniscal status, which changes the compliant element.

Subchondral sclerosis, which stiffens the deeper path and reduces its share.

How It Is Measured

Force plate loading rate is the practical in vivo measure and is activity specific.

Accelerometry at the tibia measures impact related acceleration, which correlates with loading rate and is not the same quantity.

Cadaveric and material testing measures tissue rate dependence directly, in excised tissue.

No clinical measurement of a patient's shock absorbing capacity exists, so it is inferred from strength, tissue state and symptoms.

Clinical Relevance

Target loading rate rather than peak force when managing impact related symptoms.

Expect a meniscectomised knee to absorb fast loads less well, and advise on activity accordingly.

Read subchondral sclerosis as evidence of concentrated load and look for its cause.

Treat accelerometry as a proxy for loading rate rather than a measurement of it.

FRCS Level

At the Knee

Treat shock absorption as the mechanical account of activity advice. A patient with a meniscectomised, malaligned or cartilage deficient compartment has lost absorbing capacity, and the advice follows from which part.

Rate dependence is why impact sports and occupational kneeling differ in their consequences despite both loading the knee heavily. One is a rate problem and the other a cumulative one.

Subchondral bone has moved from background to target, with marrow lesions treated as markers of where absorption has failed. 6.47 covers them.

What Changes It

Which absorbing structure has been lost, and whether it can be substituted.

Alignment, which concentrates the rate dependent load in one compartment.

Activity selection, which sets both the rate and the number of cycles.

Muscle capacity, which is the modifiable contributor and the one rehabilitation reaches.

How It Is Measured

Loading rate from gait and landing analysis quantifies the exposure, in a laboratory.

Weight bearing imaging and marrow lesion assessment show where load has concentrated, after the fact.

Wearable monitoring estimates cumulative cycles and impact magnitude outside the laboratory, through a model.

No measurement predicts which patient will tolerate a given activity, so advice is based on mechanism and on symptoms.

Clinical Relevance

Base activity advice on which absorbing structure has been lost rather than on a diagnosis alone.

Separate rate dependent exposure from cumulative exposure when advising, since they respond differently.

Investigate a marrow lesion as a marker of concentrated load rather than as an incidental finding.

Target muscle capacity, since it is the absorbing contributor that treatment can change.