Contact Areas

For Medical Students

The Principle

Two curved surfaces pressed together touch over an area, not at a point. How large that area is depends on the load, on the shape of the surfaces and on how soft they are.

Contact area matters because it is a denominator. The same force spread over twice the area gives half the pressure.

Standing on a drawing pin and standing on a floorboard involve the same body weight. Only the area differs, and that is the whole difference.

At the Knee

The femoral condyles are convex and the tibial surfaces are shallow, so the bone on bone contact area would be small. The menisci sit between them and enlarge it.

Conventional teaching gives total tibiofemoral contact area in the loaded, extended knee as roughly 1000 to 1200 square millimetres, with the menisci carrying a substantial share of that.

The medial contact area is larger than the lateral, which fits the medial compartment's larger share of the load. 3.5.3 covers the distribution.

Cartilage is soft enough to deform under load, which enlarges the area further. A stiff joint surface would have a smaller contact area and a higher pressure.

What Changes It

Load. A more heavily loaded joint deforms more and touches over a larger area, which partly offsets the rise in force.

Flexion angle, which moves the contact onto differently shaped parts of the condyles. 3.5.4 covers it.

The menisci. Losing one reduces the area in that compartment substantially. 3.5.5 covers it.

Cartilage health, since degenerate cartilage is both thinner and mechanically different.

Clinical Relevance

Contact area is the quantity most knee operations are really trying to protect, even when they are described in terms of tissue.

Preserving a meniscus preserves area, which is why the mechanical argument for repair is stronger than the amount of tissue involved suggests. 3.6 covers meniscal biomechanics.

A smaller contact area means a higher pressure for the same activity, which is the route from a mechanical change to cartilage damage. 3.5.2 covers pressure.

Any pressure figure quoted without its area is half a measurement.

MRCS Level

The Principle

Separate the geometric contact area from the load bearing area. Two surfaces can touch over a region while the load is concentrated in part of it, and the second is what matters.

Contact mechanics in a joint is a soft contact problem. The surfaces are compliant and fluid filled, so the area grows with load in a non linear way rather than staying fixed.

Cartilage is biphasic. Under a short load the fluid carries much of it and the contact behaves stiffly; under a sustained load the fluid flows out and the area enlarges. 3.4.10 covers rate dependence.

At the Knee

Conventional teaching gives the menisci roughly half to two thirds of the tibiofemoral contact area in the extended knee, with the remainder as direct cartilage on cartilage contact.

The lateral compartment depends on its meniscus more, because the lateral tibial plateau is convex and offers no concavity of its own. 2.6.1 covers the anatomy.

Contact area falls with flexion as the smaller radius posterior condyles come into contact, which is why deep flexion is a high stress position. 3.5.4 covers it.

Reported areas vary widely by method, because the measurement depends on the load applied, the flexion angle, and whether the specimen was fresh.

What Changes It

Applied load and its duration, through the biphasic behaviour.

Flexion angle, through condylar radius.

Meniscal integrity, including a root tear or extrusion that takes the meniscus out of the contact without removing it. 6.36 covers extrusion.

Cartilage thickness and stiffness, which set how much the surfaces deform.

How It Is Measured

Pressure sensitive film between the surfaces in a cadaveric rig is the classical method and gives area and pressure together, under a load the investigator chose.

Thin film electronic sensors measure the same quantities dynamically, and the sensor's own thickness alters the contact it is measuring.

Magnetic resonance imaging under load estimates area in vivo without a sensor in the joint, at a resolution that limits precision.

Finite element modelling computes area from geometry and material properties and depends on both assumptions.

Clinical Relevance

Read a contact area figure with its load, flexion angle and method attached, since all three change it.

Treat the lateral compartment as the more meniscus dependent one, on the ground that it has no bony concavity.

Expect extrusion or a root tear to reduce contact area without removing tissue.

Expect deep flexion to be a small area position, and therefore a high stress one.

FRCS Level

At the Knee

Treat contact area as the quantity a reconstruction restores and a measurement cannot confirm. Meniscal repair, transplantation and cartilage procedures each aim at area, and nothing in clinical practice measures it.

That gap is why the evidence for these procedures rests on cadaveric contact studies for mechanism and on clinical cohorts for outcome, with nothing in between.

Conventional teaching holds that restoring the meniscal ring restores much of the lost area, and that a transplant restores less than a native meniscus. The difference is mechanical rather than biological.

What Changes It

Whether the meniscal ring is intact and anchored, which decides whether tissue present is tissue working.

Alignment, which concentrates the available area in one compartment. 3.13 covers it.

Cartilage procedure type, since a repair tissue of different stiffness gives a different contact area under the same load.

Graft sizing in transplantation, which sets how much area is restored and how it is distributed.

How It Is Measured

Cadaveric contact studies with pressure film or sensors are where every area figure comes from, and the sensor alters the contact it measures.

Loaded magnetic resonance imaging estimates area in vivo and is a research technique.

Finite element models scaled to an individual estimate the effect of a planned procedure, with material assumptions as the main limitation.

No clinical measurement of contact area exists, so it is inferred from structure on imaging. 5.11 covers cartilage imaging.

Clinical Relevance

Size a meniscal transplant carefully, since area restoration depends on it.

Repair rather than resect where the ring can be restored, on area grounds.

Correct alignment alongside any area restoring procedure, since malalignment concentrates load regardless.

State that contact area cannot be measured clinically when explaining why a procedure's mechanical case rests on cadaveric work.