Contact Areas

For Medical Students

The Principle

Two curved surfaces pressed together touch over a patch rather than at a point, and the patch grows as the force rises.

A bigger patch spreads the same force over more tissue, which is the joint's own protection against the load it carries.

If the patch moves as the joint moves, different tissue takes the load at different angles, and wear follows the path rather than sitting in one place.

At the Knee

The back of the patella is divided into facets by a central ridge. 2.4.1 covers the anatomy.

Near extension the patella is barely touching the femur, because it has not entered the groove. 3.12.1 covers engagement.

As the knee bends, contact starts on the lower part of the patella and moves steadily upwards towards the top of it.

The area in contact grows with flexion, which is the joint's response to a force that is also growing. 3.12.3 covers the pressures.

In deep flexion the quadriceps tendon itself comes to lie against the femur, taking some of the load off the patella.

What Changes It

Flexion angle, which is the main thing that moves and grows the patch.

Quadriceps force, which presses the surfaces together harder.

Cartilage thickness and stiffness, because a softer surface spreads more. 2.4.1 covers the cartilage.

Trochlear shape, which decides how well the two surfaces match. 3.12.4 covers it.

Clinical Relevance

Pain felt on stairs and on rising from a chair is pain in mid flexion, which is where the force and the contact are both high.

Cartilage wear on the patella follows the contact path rather than covering the whole surface. 3.12.10 covers the wear pattern.

Chapter 3.5 covers the same question for the tibiofemoral joint, where the mechanics are different.

The pattern explains why a patient can be comfortable walking on the flat and sore on a staircase. 6.60 covers the condition.

MRCS Level

The Principle

Contact area is the denominator of joint pressure, so it decides how much a given force actually matters to the cartilage.

A joint that enlarges its contact area as the load rises is self protecting. A joint that does not is exposed exactly when it is working hardest.

The patellofemoral joint protects itself well in flexion and poorly near extension, which inverts the usual expectation.

At the Knee

At 20 degrees the contact band sits across the distal patella and is small. By 90 degrees it has moved proximally and is several times larger.

The medial and lateral facets load unequally through the arc, with the lateral facet generally carrying more. 2.4.1 covers the facets.

The odd facet, on the extreme medial edge, engages only in deep flexion and is unloaded for most of the arc.

Beyond about 90 degrees the quadriceps tendon articulates with the trochlea, and the load is shared between tendon and bone.

Near full extension the patella rests on soft tissue and the contact area is minimal, which is why the joint tolerates full extension well. 3.12.1 covers engagement.

What Changes It

Flexion angle, which is the dominant variable and moves the band proximally.

Quadriceps force, which increases area as well as pressure by deforming the cartilage.

Patellar height, because an alta patella engages late and loads a smaller area for longer. 3.12.12 covers it.

Trochlear dysplasia, which reduces congruence and concentrates contact. 3.12.4 covers it.

Cartilage loss, which changes both the geometry and the stiffness of the interface.

How It Is Measured

Dye staining and pressure sensitive film in cadaver knees give the classical maps, with the joint opened and the loading simulated. 3.16.1 covers cadaveric testing.

Thin film pressure sensors measure area and pressure together, and the sensor itself changes the interface it is measuring.

Magnetic resonance imaging at set flexion angles measures cartilage contact in the living knee, at low resolution and few angles. 5.12 covers the imaging.

Finite element models give continuous area through the arc from a geometry and a set of assumptions. 3.16.4 covers modelling.

No method measures contact area in a loaded, moving, living knee, which is the condition the clinical question is about.

Clinical Relevance

Expect symptoms in the arc where area is small and force is rising, rather than at the extreme of flexion.

Read a cadaveric contact map as a map of one loading condition, not of the joint.

Localise patellar cartilage lesions by the flexion angle that reproduces the pain, because the contact band is predictable. 4.15.9 covers the examination.

Avoid quoting a contact area figure without the flexion angle and the method attached.

FRCS Level

At the Knee

The clinically useful statement is that contact area and quadriceps force rise together through flexion, and the ratio between them is what the cartilage experiences.

That ratio is least favourable in early to mid flexion, where force has begun to rise and the band is still small.

The lateral facet bias through the arc is consistent across studies and matches where wear is found at arthroscopy. 3.12.10 covers the wear.

The quadriceps tendon contribution in deep flexion is a genuine load sharing mechanism and explains why full flexion is often comfortable in a worn joint.

Trochlear dysplasia and patellar alta both work by keeping the joint in its unfavourable ratio for longer. 3.12.4 and 3.12.12 cover them.

What Changes It

The flexion angle at which engagement completes, which sets the length of the unfavourable window. 3.12.1 covers it.

Patellar height and tubercle position, which move the window. 3.12.12 covers both.

Trochlear depth and facet asymmetry, which decide congruence. 3.12.4 covers them.

Cartilage status, which changes the interface stiffness and therefore the area at any load.

Realignment and offloading procedures, which work by moving the band rather than by reducing the force. 7.53 covers the operation.

How It Is Measured

The literature is largely cadaveric, and the figures depend on the loading rig, the quadriceps simulation and whether the retinaculum was left intact.

Open and closed chain simulations give different areas at the same angle, which is a common source of apparent disagreement. 3.4.3 covers joint reaction forces.

In vivo magnetic resonance measurement is the most clinically relevant and the least precise. 5.12 covers it.

Models calibrated against cadaveric data inherit that data's loading assumptions. 3.16.4 covers modelling.

Measurements of contact area are therefore comparable within a study and poorly comparable between studies.

Clinical Relevance

Reason about this joint in terms of pressure rather than force, because area changes faster than force does. 3.12.3 covers pressure.

Treat early to mid flexion as the vulnerable arc, and ask the history about it specifically.

Expect a realignment to change where load sits rather than how much there is. 3.12.12 covers tubercle position.

Compare contact area figures only within a single study, and say so when quoting them.

Read comfortable deep flexion in a worn joint as tendon load sharing rather than as preserved cartilage.