Three terms, one area of the knee — what they each mean

If you have been told you have 'runner's knee' by one clinician, 'patellofemoral pain syndrome' by another, and 'early arthritis behind the kneecap' by a third, you are not alone — and none of them were necessarily wrong. These three labels describe different points along the same spectrum of patellofemoral joint trouble.

Patellofemoral pain syndrome (PFPS) is the broadest of the three. It describes anterior knee pain arising from the joint between the kneecap (patella) and the thigh bone (femur), when no definitive structural cartilage damage has yet been confirmed. It is a functional diagnosis — a description of the pain pattern rather than a structural verdict.

Chondromalacia patellae takes things a step further: it means the articular cartilage on the underside of the kneecap has begun to soften and break down. Think of it as surface wear — the cartilage loses its smooth, firm quality and starts to fray. It is most common in younger, active people, which is why 'runner's knee' has stuck as a shorthand.

Patellofemoral osteoarthritis (PFOA) represents established degenerative disease across the whole patellofemoral compartment. The cartilage loss is more advanced, the underlying bone develops changes (including what imaging identifies as bone marrow lesions), and the damage is progressive rather than early or potentially reversible.

These three conditions form a spectrum: PFPS can exist with minimal structural change, chondromalacia reflects early structural change, and PFOA reflects advanced structural change. One can lead to another over years if the underlying mechanical cause — such as poor patellar tracking or muscle imbalance — is not addressed.

One important caveat: a structural finding on MRI does not automatically explain a patient's pain. Clinical correlation — matching imaging to symptoms and examination — is what guides treatment. The distinction between these labels matters because it shapes the pathway forward, not simply the name on a letter.

Symptoms: what patellofemoral pain actually feels like

The defining feature of patellofemoral pain is discomfort felt at the front of the knee — behind or around the kneecap rather than deep inside the joint. It rarely announces itself all at once; more often, you notice it building during activities that load the patellofemoral joint.

Stairs are the classic trigger, particularly descending, when the kneecap bears the greatest compressive force. Squatting and kneeling produce a similar effect. Sitting for an extended period — in a cinema, a car, or at a desk — can leave the knee stiff and achy when you finally stand; clinicians call this the 'theatre sign', and most patients recognise it immediately once it is named.

A grinding or clicking sensation under the kneecap (crepitus) is common and can be unsettling, but on its own it does not indicate how advanced the damage is. Swelling, when present, tends to be mild — noticeably less prominent than in arthritis affecting the knee's inner or outer compartments — and is more often a feature of established disease than early-stage changes.

Pain intensity does not map reliably onto cartilage grade. A patient with Grade 1 ICRS changes — surface softening only — may experience more daily discomfort than one with Grade 3 partial-thickness loss. Pain is a signal worth taking seriously regardless of whether imaging has yet confirmed structural change.

Symptoms that warrant earlier specialist assessment include pain that persists at rest, episodes of locking or giving way, or discomfort that seems disproportionate to your activity level. These patterns may indicate pathology beyond the patellofemoral compartment that needs ruling out.

Why it develops: risk factors and how the kneecap goes wrong

During stair descent, forces several times bodyweight pass through the patellofemoral joint — concentrated through a relatively small area of articular cartilage on the underside of the kneecap. How evenly that load is distributed depends largely on whether the kneecap tracks centrally within the femoral groove (the trochlea).

When the kneecap drifts laterally instead of gliding centrally — a pattern called patellar maltracking — compressive force concentrates on one edge of the cartilage surface rather than spreading across it. Over time, this uneven loading softens and erodes cartilage in ways that correctly distributed load would not. Maltracking can arise from structural factors: a high Q-angle (the wider-than-usual alignment angle between hip and knee), flat feet that alter how load travels up the leg, or the lasting mechanical effects of a previous knee injury or surgery.

Muscle balance is the other major driver — and a modifiable one. Weak quadriceps, particularly the inner portion (vastus medialis oblique), and poor hip stabiliser strength both reduce dynamic control of the kneecap's path. Without that muscular support, each step places more stress on the joint and the patella is left to drift rather than glide.

Cartilage compounds the problem by being poorly equipped to recover. It is avascular — it has no direct blood supply — so once softening or surface erosion begins, natural healing is severely limited. Clinicians use the ICRS grading system on MRI (Grades 1 to 4, from surface softening at one end to full-thickness loss reaching the bone beneath at the other) to describe how far this process has advanced. The practical implication is that correcting the mechanical drivers — maltracking and muscle weakness — early removes the conditions under which further damage tends to occur.

How the diagnosis is made

Assessment begins not with a scan but with a conversation. A specialist will ask about the pattern of your symptoms — when pain starts, what relieves it, which activities trigger it — and combine that with a physical examination: watching how your kneecap tracks as you bend and straighten the knee, measuring your Q-angle, and assessing the strength of the muscles that govern patellar movement. This clinical picture often points firmly towards a diagnosis before any imaging is ordered.

Standard X-ray is of limited value for patellofemoral cartilage problems — cartilage does not show up on plain film, so early or moderate changes are invisible. MRI is the preferred investigation because it depicts cartilage thickness, surface integrity, and the condition of the subchondral bone beneath. Where early change is the clinical question, advanced sequences — T2 mapping, dGEMRIC, and T1ρ imaging — assess the biochemical composition of cartilage rather than its shape alone, detecting deterioration before it becomes morphologically obvious. AI-assisted cartilage segmentation software (onMRI™, a tool used within MSK Doctors clinics) can support interpretation of these sequences, helping to quantify T2-map data in a reproducible and consistent way.

In cases where MRI findings and symptoms do not align — a not-uncommon situation — dynamic needle arthroscopy provides real-time, minimally invasive visualisation of the joint during a clinic appointment, clarifying the picture where imaging alone leaves uncertainty.

The consultant's role is to weigh all three inputs together: history, examination, and imaging. No single test resolves the diagnosis on its own.

Treatment options from first steps to surgery

For most people with chondromalacia or mild-to-moderate patellofemoral OA, the pathway starts with physiotherapy — and for a substantial proportion it goes no further. Targeted strengthening of the quadriceps (particularly vastus medialis oblique) and the hip stabilisers corrects the muscle imbalance that allows the kneecap to drift and concentrate load unevenly, while activity modification reduces joint irritation during recovery. Programmes that address foot alignment and hip mechanics alongside the knee tend to produce more durable results than those focused on the joint alone.

Injection therapies

Where physiotherapy has not achieved adequate improvement, intra-articular injections can reduce pain and support the joint environment. Three types are used clinically:

• Viscosupplementation (hyaluronic acid): supplements and restores joint fluid; meta-analyses support its use in early-to-moderate OA to manage symptoms.
• Platelet-rich plasma (PRP): concentrates growth factors from the patient's own blood to reduce inflammation and support cartilage biology.
• Polyacrylamide hydrogel (e.g. Arthrosamid®): a longer-acting cushioning agent suited to mild-to-moderate cartilage wear rather than bone-on-bone disease. A case series published in the Journal of Arthritis (Maulana, Cole, and Lee, 2022) reported reduction of patellofemoral bone marrow lesions following a single injection in advanced patellofemoral OA — a preliminary finding, though a case series rather than a controlled trial.

No randomised trial data currently compare these injection types directly within the patellofemoral compartment. Which approach is appropriate depends on disease stage and individual patient factors — a specialist clinical judgement, not a formula. For focal cartilage defects, injectable cartilage-regeneration gels (e.g. ChondroFiller®) offer an outpatient option with low reported complication rates, though the supporting evidence is at an earlier stage than for viscosupplementation.

Surgical escalation

Surgery is considered when conservative and injection-based care has not achieved adequate relief, or when structural maltracking requires direct mechanical correction. Tibial tubercle transfer — repositioning the kneecap's attachment point on the tibia — is appropriate where documented malalignment underlies a specific cartilage lesion.

At the end-stage, patellofemoral arthroplasty resurfaces the kneecap and the femoral groove it glides within, leaving the rest of the knee untouched. This is not the same as a total knee replacement — a distinction that matters to many patients weighing the decision.

Across every tier, correcting the mechanical cause of the problem is not optional: symptom relief without it leaves the joint vulnerable to recurrence or further deterioration.

When to see a specialist and what to expect

Most patellofemoral pain settles with activity modification and a structured exercise programme over several weeks. When it does not — or when the pattern changes in a concerning way — a specialist assessment is more productive than extended self-management alone.

Consider seeking specialist advice if:

• Symptoms have persisted beyond six to eight weeks despite rest and basic physiotherapy
• Swelling develops or recurs, particularly after activity
• The knee locks, catches, or gives way unexpectedly
• Pain is present at rest or disturbs sleep
• Symptoms are worsening rather than gradually settling

A first appointment is not automatically about ordering a scan. A consultant will take a detailed history of the symptom pattern, carry out a physical examination — including observing how the kneecap tracks during movement and assessing the surrounding muscle balance — and use those findings to decide whether, and what type of, imaging would add anything meaningful. That conversation alone often narrows the diagnosis more usefully than an early MRI requested without clinical context. The goal is a clear shared picture and a plan, not a test result in isolation. Patients who do not wish to wait for a GP referral can access specialist assessment directly: Lincolnshire Knee is part of the MSK Doctors group and accepts patients without referral. Book an assessment at lincolnshireknee.co.uk.

1. [1] Patellofemoral pain syndrome. https://en.wikipedia.org/?curid=12033023 https://en.wikipedia.org/?curid=12033023
2. [2] Chondromalacia patellae. https://en.wikipedia.org/?curid=1944613 https://en.wikipedia.org/?curid=1944613