Contact Pressures

For Medical Students

The Principle

Pressure is force divided by the area carrying it. The same force over a smaller area gives a higher pressure.

Cartilage responds to pressure rather than to force. A knee can carry more load safely if it spreads it.

That single relationship is why losing a meniscus matters, why malalignment matters, and why deep flexion is demanding. All three reduce area or concentrate load.

At the Knee

Conventional teaching gives peak tibiofemoral contact pressure in normal walking as roughly 1 to 3 megapascals, rising with more demanding activities.

Pressure is not uniform across the contact. There is a peak somewhere in it, and the peak is what damages cartilage rather than the average.

The menisci lower the peak by spreading load towards the periphery, which is their mechanical function stated in one sentence. 3.6 covers it.

Articular cartilage is well suited to this: it is smooth, it deforms, and it is lubricated, so it tolerates repeated high pressure for decades.

What Changes It

Contact area, which is the denominator. 3.5.1 covers it.

Joint reaction force, which is the numerator and is dominated by muscle. 3.4.3 covers it.

Flexion angle, which changes both at once.

Meniscal and cartilage integrity, which change how load is spread within the contact.

Clinical Relevance

Cartilage damage is a pressure story rather than a force story, which is why a knee can carry a heavy load safely and fail under a smaller one badly distributed.

Every mechanical treatment for cartilage is an attempt to lower peak pressure, by raising area or by moving load elsewhere.

Peak pressure, not average pressure, is the quantity to ask about in any study.

A patient's symptoms often follow activities that concentrate pressure rather than those that raise force.

MRCS Level

The Principle

Use the right word. Pressure is a fluid term and contact stress is the mechanical one, and the literature uses both for the same measurement. Peak contact stress is the quantity of interest.

Stress has components. Compressive stress is the one usually quoted, and shear stress at the cartilage to bone interface is a separate and clinically important quantity.

Cartilage failure in the laboratory is associated with a threshold of impact stress rather than with a load, which is the quantitative form of the force against stress distinction.

At the Knee

Conventional teaching puts normal peak contact stress in daily activity in the low megapascal range, and cadaveric work places the threshold for cartilage matrix damage from a single impact considerably higher, commonly quoted around 15 to 20 megapascals.

Between those figures lies the clinically important region: stresses too low to break cartilage at once and high enough to damage it over years.

Peak stress rises steeply when area falls, because area is a denominator. Halving the area doubles the stress for the same force.

Shear at the osteochondral junction is the proposed mechanism for delamination injuries, and it is not captured by a compressive pressure measurement. 6.48 covers them.

What Changes It

Area, as the denominator, and anything that reduces it.

Load rate, through cartilage's biphasic stiffness. A fast load gives a higher stress for the same force. 3.4.10 covers it.

Alignment, which moves load between compartments. 3.5.6 covers it.

Cartilage thickness and stiffness, which set how the load is distributed within the contact.

How It Is Measured

Pressure sensitive film gives peak and average stress in a cadaveric rig and is the source of most published figures.

Electronic sensor arrays give the same dynamically and alter the contact by their own thickness.

Instrumented implants give force rather than stress, so they bound the numerator and not the quantity itself.

Finite element modelling is the only route to in vivo stress estimates, and its figures inherit its material assumptions.

Clinical Relevance

Ask for peak rather than average contact stress when reading a comparative study.

Treat a halved contact area as a doubled stress, which is the arithmetic behind the meniscectomy literature.

Consider shear as well as compression when a delamination or osteochondral lesion is the problem.

Read an in vivo stress figure as a model output rather than a measurement.

FRCS Level

At the Knee

Treat peak contact stress as the common currency of every mechanical argument in the knee. Meniscectomy, malalignment, cartilage loss and deep flexion all raise it, and every mechanical treatment lowers it.

The honest limit is that no threshold is established in vivo. Cadaveric damage thresholds come from single impacts, and degeneration is a cumulative process, so the two literatures do not meet.

That is why a stress figure argues for a mechanism and a cohort argues for a treatment. Conflating them is the commonest overstatement in this field.

What Changes It

Area restoration, through meniscal repair, transplantation or cartilage procedures.

Alignment correction, which redistributes the force.

Body mass and activity, which set the numerator and the number of cycles.

Cartilage repair tissue stiffness, which determines whether the restored surface distributes load like the original.

How It Is Measured

Cadaveric contact studies compare a construct against the intact and the deficient state and are the standard for a mechanical claim.

Patient specific finite element modelling estimates the effect of a planned correction, as a research tool.

Cartilage imaging measures thickness and composition rather than stress. 5.11 covers it.

Registry and cohort outcomes measure the clinical consequence over the timescale the mechanics predict, which is a decade rather than a year.

Clinical Relevance

Argue mechanism from contact stress and treatment from outcome data, and keep the two apart.

Lower peak stress by restoring area and correcting alignment together where both are wrong.

Expect a cartilage repair of different stiffness to distribute load differently from the original.

State that no in vivo stress threshold is established when a patient asks how much load is safe.