Contact Pressures

For Medical Students

The Principle

Pressure is force divided by the area carrying it. The same force through a smaller patch does more damage.

That is why a drawing pin pierces skin and a thumb does not. The force is similar and the area is not.

To know whether a joint is in trouble you need both numbers, and the force alone tells you very little.

At the Knee

The patella is pressed against the femur by the quadriceps pulling above it and the patellar tendon pulling below.

As the knee bends, the angle between those two pulls closes, and the force squeezing the patella into the groove rises steeply.

Walking on the flat loads this joint lightly. Stairs, squatting and rising from a low chair load it heavily. 3.3.12 covers stair kinematics.

Because the contact area grows at the same time, the pressure rises less steeply than the force does. 3.12.2 covers the areas.

What Changes It

Flexion angle, which changes both the force and the area.

Body weight, which scales the force in every activity.

The activity itself, because a deep squat and a flat walk are different problems.

Quadriceps strength and how the muscle is used, which changes the pull. 8.9.4 covers quadriceps activation.

Clinical Relevance

Advice to avoid deep squatting and stairs is advice about pressure, not about the cartilage being fragile.

Losing weight reduces the force in proportion, which is why it helps this joint noticeably. 3.5.7 covers obesity.

Pain that comes on sitting for a long time with the knee bent is a pressure problem held for a long time. 6.60 covers the condition.

Section 8 covers the rehabilitation that works around the loaded arc.

MRCS Level

The Principle

Patellofemoral joint reaction force is a resultant. It comes from the quadriceps tension and the patellar tendon tension acting at an angle to each other.

As flexion increases that angle closes, so the resultant grows even if the muscle tension does not. 3.4.3 covers joint reaction forces.

Dividing the resultant by the contact area gives the quantity that matters to cartilage, and both terms are moving at once.

At the Knee

Quoted joint reaction forces are roughly half body weight in level walking, around three times body weight on stairs and higher still in a deep squat.

Peak contact pressures in cadaver studies reach several megapascals, with the exact figure depending heavily on the rig.

Pressure is not uniform across the contact band. The lateral facet carries more through most of the arc. 2.4.1 covers the facets.

Open chain extension against resistance loads the joint most near full extension, where the area is smallest. Closed chain loading does the opposite.

That inversion is the mechanical basis for prescribing one kind of exercise over another. 8.29 covers the rehabilitation.

What Changes It

Flexion angle, through both the resultant and the area.

Open against closed chain loading, which moves the peak to a different part of the arc.

Body mass, which scales the resultant directly. 3.5.7 covers obesity.

Patellar height, which keeps a small area under a rising force. 3.12.12 covers it.

Cartilage and subchondral stiffness, which change how the pressure is distributed within the band.

How It Is Measured

Pressure sensitive film in cadaver knees gives peak and mean pressures for one loading condition. 3.16.1 covers cadaveric testing.

Instrumented implants measure force in vivo in replaced knees, which is a different joint from the one in question.

Inverse dynamics with a musculoskeletal model estimates the resultant from motion capture and ground reaction force. 3.16.7 covers motion capture.

Finite element models convert an estimated force into a pressure distribution, inheriting the model's assumptions. 3.16.4 covers modelling.

The figures from these methods differ by more than the differences between activities, which is a caution rather than a reason to ignore them.

Clinical Relevance

Prescribe exercise by where the peak pressure sits in the arc rather than by muscle group. Section 8 covers the programmes.

Expect open chain extension near full extension to be the provocative manoeuvre in an irritable joint.

Quote a pressure figure with its activity, its flexion angle and its method, or not at all.

Counsel weight loss as a proportional reduction in force, which is a mechanical argument the patient can follow.

FRCS Level

At the Knee

The defensible statement is directional rather than numerical. Pressure rises with flexion, rises faster in open chain extension near terminal extension, and is highest where area is smallest relative to force.

Absolute peak pressure figures are rig dependent and should not be carried between studies. The direction of change between conditions is what survives.

The lateral facet bias is consistent, which is why lateral facet wear is the common finding and why lateral release was once thought to address it. 3.12.10 covers the wear.

Reducing pressure surgically is achieved by enlarging or moving the contact area, not by reducing extensor force, which cannot be reduced without losing function.

That is the mechanical rationale behind tubercle transfer, and the reason its effect is modest and direction dependent. 7.53 covers the operation.

What Changes It

Chain and arc of loading, which together decide where the peak sits.

Congruence, which decides how much of the nominal area is actually carrying load. 3.12.4 covers trochlear shape.

Patellar height and tubercle position, which move the band and change the lever arm. 3.12.12 covers them.

Alignment in the coronal and transverse planes, which alters the lateral vector. 3.12.11 covers it.

Cartilage loss, which raises local pressure by removing the compliant layer that spreads it.

How It Is Measured

Cadaveric film measurement remains the reference for distribution and is unphysiological in its loading.

In vivo estimation combines motion capture, inverse dynamics and a model, so the error accumulates across three steps. 3.4.3 covers the forces.

Instrumented implant data is the only direct in vivo force measurement and comes from replaced joints.

Cartilage contact area measured on magnetic resonance imaging at discrete angles gives the denominator but not the numerator. 5.12 covers the imaging.

There is no method that gives pressure in a loaded, moving, native, living patellofemoral joint, and the figures in common use all substitute for that.

Clinical Relevance

Reason in directions and ratios rather than in absolute pressures.

Expect a procedure that moves or enlarges contact to help more than one aimed at the force. 3.12.12 covers tubercle position.

Stage rehabilitation by the arc and the chain rather than by a resistance figure. 8.29 covers the programmes.

Avoid citing a peak pressure from one cadaveric rig as a property of the joint.

State plainly that native in vivo pressure is not measurable, when the question is asked.