Patellofemoral Pain

For Medical Students

The Principle

A loaded joint can hurt without anything being broken, in the way a hand can ache from gripping too long.

Load that is too high, held too long or applied in the wrong place can all produce pain.

So the question is rarely whether the joint is damaged. It is usually what the joint is being asked to do.

At the Knee

Pain at the front of the knee is commonest in the activities that load this joint most: stairs, squatting, kneeling and sitting for long periods. 3.12.3 covers the pressures.

The cartilage on the back of the patella has no nerve supply, so the pain comes from elsewhere.

The bone under the cartilage, the synovium and the fat pad all have nerve endings and all respond to load.

That is why a scan can look normal in a painful knee, and worn in a comfortable one. 6.60 covers the condition.

What Changes It

The activities being done, and how much of them.

Body weight, which scales the force. 3.5.7 covers obesity.

Quadriceps strength and control, which change how the load is managed. Section 8 covers the training.

Hip and foot control, which change where the load goes. 3.12.7 covers dynamic valgus.

Clinical Relevance

The advice is about load rather than about damage, which is often reassuring to hear.

Most people improve with a change in loading and in strength, without any operation. Section 8 covers it.

A normal scan does not mean the pain is imagined. It means the pain is not coming from something a scan sees. 5.12 covers the imaging.

Section 4 covers how the joint is examined.

MRCS Level

The Principle

Mechanics predicts where load is highest and when. It does not predict who will feel pain at a given load.

That mismatch is the central difficulty of this condition and the reason purely mechanical models of it have disappointed.

The useful role of mechanics here is to explain the pattern of the symptoms, which it does well, rather than their presence.

At the Knee

The provocative activities map onto the arc of highest pressure, which is early to mid flexion under load. 3.12.3 covers the pressures.

Prolonged sitting loads the joint at a modest pressure for a long time, which is a different mechanical insult from a single squat.

Open chain extension near terminal extension produces peak pressure over a small contact area, which is why it is often provocative. 3.12.2 covers the areas.

Subchondral bone is the most plausible nociceptive source, and bone marrow signal change correlates with symptoms imperfectly. 5.12.7 covers imaging findings.

Chondromalacia is a cartilage finding rather than a diagnosis, and its relationship to pain is weak. 6.61 covers it.

What Changes It

Load magnitude, duration and rate, which are three separate variables often collapsed into one.

Contact area at the loaded angle, which decides the pressure. 3.12.2 covers it.

Quadriceps function, which changes both the force and its timing.

Proximal and distal control, which change the vector. 3.12.7 covers dynamic valgus.

Factors outside mechanics altogether, including sleep, mood and sensitisation, which change the threshold rather than the load.

How It Is Measured

Symptom provocation by activity, which is the clinical measurement and is subjective. 4.15.9 covers the assessment.

Estimated joint pressure from modelling, which gives a load and not a symptom. 3.16.4 covers modelling.

Magnetic resonance imaging of cartilage and bone marrow, which shows findings with a weak association to pain. 5.12 covers it.

Instrumented strength and control testing, which measures a contributor rather than the condition.

No measurement identifies the painful structure in an individual, which is why the diagnosis remains clinical.

Clinical Relevance

Use mechanics to explain the pattern of the symptoms and to guide loading, not to explain why the patient has them.

Separate load magnitude, duration and rate when taking the history, because the management differs.

Avoid attributing pain to a cartilage appearance, since the association is weak. 6.61 covers chondromalacia.

Explain a normal scan as evidence about structures rather than about the symptom. 5.12 covers the imaging.

FRCS Level

At the Knee

The defensible position is that patellofemoral pain has mechanical contributors and is not a mechanical diagnosis, and the literature has moved steadily in that direction.

Pressure based explanations predicted that correcting malalignment would relieve pain, and the results of realignment surgery for pain alone have been poor. 7.56 covers the operation.

Loading programmes outperform mechanical corrections for pain, which is itself a mechanical finding: it suggests tolerance rather than geometry is the modifiable variable. 8.29 covers them.

Tissue level mechanics has moved to the subchondral bone and to the rate and duration of loading rather than to peak pressure alone.

Conventional teaching has abandoned the single malalignment model, and the alternative is a multifactorial account rather than a competing single cause. 6.153 covers the controversies.

What Changes It

Cumulative load, which is magnitude multiplied by exposure and is the variable most often unmeasured.

Loading rate, which modelling suggests matters independently of peak pressure.

Tissue tolerance, which training changes and geometry does not.

Skeletal geometry, which sets the baseline load distribution. 3.12.11 covers it.

Central and psychosocial factors, which change the threshold and are not a diagnosis of exclusion.

How It Is Measured

Patient reported outcome measures, which are the primary endpoints in every trial and are not mechanical quantities.

Modelled joint stress from motion capture and inverse dynamics, which has the error chain described in 3.12.3.

Quantitative magnetic resonance imaging of cartilage composition, which is a research tool with no clinical threshold. 5.12 covers the imaging.

Load monitoring by wearables, which measures exposure rather than stress and is the most practical of these. 3.16.8 covers wearables.

The absence of a measurable link between a mechanical quantity and this symptom is the reason the condition resists mechanical solutions.

Clinical Relevance

Offer loading based management before considering any mechanical correction for pain alone. 8.29 covers the programmes.

Reserve realignment for instability rather than for pain, because that is where the evidence sits. 3.12.8 covers instability.

Quantify exposure as well as intensity when taking a history, since cumulative load is the likelier driver.

Explain the multifactorial account honestly rather than substituting a single mechanical cause the patient can act on.

Weigh the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations, because tolerance and adherence sit on that side of the balance rather than in the imaging.