Patellofemoral Arthritis

For Medical Students

The Principle

Surfaces wear where they rub, and they rub hardest where the pressure is highest.

So the pattern of wear in a joint is a record of how that joint has been loaded.

Reading the pattern backwards tells you something about the mechanics that produced it.

At the Knee

Wear on the back of the patella usually starts on the outer facet. 2.4.1 covers the facets.

That is the facet that carries more load through most of the bending range. 3.12.2 covers contact areas.

The matching surface on the outer wall of the groove wears at the same time, because the two rub together.

The inner facet and the extreme inner edge are usually spared, because they are loaded only in deep bending.

What Changes It

How the joint has been loaded over years, rather than any single event.

Alignment, which changes which facet carries more. 3.12.11 covers it.

Previous dislocation, which damages cartilage directly. 6.53 covers it.

Body weight, which scales every load. 3.5.7 covers obesity.

Clinical Relevance

Pain on stairs and on rising from a chair fits wear in this joint rather than in the main part of the knee. 6.62 covers the condition.

The joint can be badly worn and still comfortable in full bend, because the quadriceps tendon shares the load there.

Wear here can occur with the rest of the knee intact, which changes what can be offered. 7.79 covers the arthroplasty.

5.12.7 covers how it looks on imaging.

MRCS Level

The Principle

Degeneration in a joint whose contact band moves is spatially patterned, and the pattern is informative in a way that a global grade is not.

The pattern reports which part of the arc the joint spent its load in, and therefore which mechanical factor dominated.

That makes the distribution of wear a better mechanical clue than its severity.

At the Knee

Lateral facet degeneration is the commonest pattern and matches the lateral load bias measured through the arc. 3.12.2 covers it.

Isolated patellofemoral degeneration occurs in a minority of arthritic knees and is commoner in women and in knees with a history of instability. 6.62 covers the condition.

Trochlear dysplasia and patellar instability are both associated with earlier and more lateral wear. 3.12.4 covers dysplasia.

Medial facet wear suggests overmedialisation, usually surgical, rather than a natural pattern. 7.53 covers tubercle transfer.

Central and panfacet wear is a later pattern and carries less mechanical information.

What Changes It

Alignment and rotation, which set which facet carries load. 3.12.11 covers them.

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

Previous dislocation or chondral injury, which starts wear at a site load alone would not have chosen. 6.59 covers the injury.

Previous realignment surgery, which can create a wear pattern of its own.

Cumulative load, which decides the rate rather than the site. 3.5.7 covers obesity.

How It Is Measured

Axial radiographs grade joint space narrowing and are insensitive early. 5.12.7 covers the appearances.

Magnetic resonance imaging grades cartilage lesions by facet and is the mechanically informative study.

Arthroscopic mapping is the reference standard for distribution and is invasive.

Grading systems record severity rather than distribution, which loses the mechanical information.

Correlation between any grade and symptoms is weak, as it is throughout this joint. 3.12.9 covers that.

Clinical Relevance

Record the facet as well as the grade, because the distribution carries the mechanical message.

Read medial facet wear as a sign of overcorrection rather than as natural progression. 7.53 covers the operation.

Expect symptoms in loaded flexion and relative comfort in full flexion, because of tendon load sharing. 3.12.2 covers it.

Avoid inferring severity of symptoms from severity of wear. 6.62 covers the condition.

FRCS Level

At the Knee

The mechanical account of patellofemoral degeneration is more secure than the mechanical account of patellofemoral pain, and the two should not be run together.

Site of wear follows contact pressure distribution reliably. Severity of symptoms follows neither site nor grade. 3.12.9 covers the pain.

That asymmetry is why offloading procedures have a defensible rationale for a mapped lesion and a poor record for undifferentiated pain. 7.53 covers them.

Isolated patellofemoral degeneration after instability is the clearest mechanical story in this joint: a known vector abnormality, a predicted site, and wear that appears there.

Conventional teaching now distinguishes the instability related pattern from primary degeneration, because they behave differently and respond differently.

What Changes It

The vector abnormality present, which predicts the site. 3.12.8 covers the factors.

Whether the joint has dislocated, which adds impact damage to load damage. 6.53 covers it.

Previous surgery, which may have moved the band deliberately or inadvertently.

Trochlear morphology, which concentrates contact where the groove is incongruent. 3.12.4 covers it.

Coronal and rotational alignment, which bias facet loading over decades. 3.13.8 covers alignment and osteoarthritis.

How It Is Measured

Facet specific cartilage grading on magnetic resonance imaging, which is the measurement that preserves the mechanical information. 5.12.7 covers it.

Radiographic grading, which is cheap, late and severity based.

Arthroscopic mapping, which is accurate and only available when operating.

Contact pressure modelling from patient specific geometry, which predicts a site and is a research tool. 3.16.4 covers modelling.

No prospective measurement establishes that a predicted high pressure site becomes a worn site in an individual knee, which is the claim the rationale rests on.

Clinical Relevance

Map the lesion by facet before offering an offloading procedure, because the direction of transfer depends on it. 7.53 covers the operation.

Separate instability related degeneration from primary degeneration in the assessment, since the history and the plan differ. 6.62 covers the condition.

Treat the link between predicted pressure and observed wear as a reasonable inference rather than a demonstrated sequence.

Expect offloading to help a mapped lesion more than an unmapped one, and counsel on that basis.

Weigh age, physiology, comorbidities, knee demand, psychological state and the patient's own ideas, concerns and expectations, because an isolated pattern on imaging does not settle what operation suits the person.