Chondromalacia and PFOA — two stages of the same problem

Kneecap cartilage problems and knee arthritis sit on the same spectrum — they are not two separate diseases, but two stages of the same process. Understanding where on that spectrum a patient sits makes a real difference to what treatment can achieve.

The earlier stage is chondromalacia patellae: the cartilage on the underside of the kneecap softens and, over time, develops surface cracks and deeper fissures. At the mild end this process is often reversible with the right management. Left unaddressed, progressive fragmentation and eventual full-thickness cartilage loss can occur — at which point the condition is better described as patellofemoral osteoarthritis (PFOA), the structural, degenerative endpoint where bone begins to contact bone beneath the kneecap.

Critically, the patellofemoral joint — the articulation between the kneecap and the groove at the front of the femur — is anatomically separate from the medial and lateral compartments of the knee where tibiofemoral arthritis develops. Significant retropatellar wear can exist alongside entirely healthy inner- and outer-compartment cartilage. That distinction matters because it shapes both the diagnosis and, importantly, the surgical options available if conservative measures do not provide sufficient relief.

How kneecap pain feels different from other knee OA

Stairs tend to be the give-away. Most people with retropatellar cartilage wear find descent harder than ascent — the kneecap is pressed more forcefully against the femoral groove as the knee bends under load. Squatting and kneeling produce the same anterior ache, and prolonged sitting with the knee bent — at the cinema, on a long drive, in a meeting — can bring on a deep, nagging discomfort that eases within a few minutes of standing up. This is sometimes called the 'theatre sign' or 'movie sign', and it is one of the more recognisable features of patellofemoral disease.

Tibiofemoral OA — wear in the inner or outer compartment of the knee — tends to feel quite different. The pain sits on the medial or lateral side of the joint line, and it worsens with sustained walking or prolonged standing rather than with sitting. If level walking is the main aggravator and stairs feel only marginally worse, the picture more often points to the tibiofemoral compartment.

A grinding or clicking sensation on bending the knee (crepitus) is a common clinical feature of retropatellar cartilage change. In early chondromalacia this, and the associated pain, may come and go with activity levels; in established PFOA the discomfort tends to be more persistent, including at rest.

It is worth noting, however, that anterior knee pain can arise from several sources — patellofemoral pain syndrome, patellar tendinopathy, and bursitis among them. Symptom pattern alone cannot confirm cartilage wear, and formal assessment including examination and targeted imaging is needed to identify the cause reliably.

Why patellofemoral cartilage wear happens

The underlying cause is almost always mechanical — the kneecap running off-centre in its groove rather than cartilage wearing out with age and body weight, which is the more familiar pattern in tibiofemoral arthritis.

The femoral trochlea guides the patella through its full range of movement. When tracking is central, load spreads evenly across the retropatellar surface. When the kneecap drifts laterally, pressure concentrates on one edge and focal cartilage stress builds with repetition. Over months or years, that concentrated load is what drives softening and then fissuring.

Several factors cause or worsen the lateral drift. The VMO — the teardrop-shaped muscle on the inner thigh — acts as the main soft-tissue check against the quadriceps' outward pull; when it is weak relative to the outer thigh muscles, the kneecap shifts off-centre under load. Hip abductor weakness compounds this by allowing the femur to rotate inward, amplifying lateral displacement at the patellofemoral joint.

Women are more susceptible to patellofemoral wear partly because of anatomy: a wider pelvis increases the Q-angle — the angle between the line of quadriceps pull and the patellar tendon axis. A larger Q-angle increases the lateral force on the kneecap with every step, which goes some way towards explaining the higher female prevalence.

Foot alignment and tibial rotation alter loading through the lower limb and can elevate patellofemoral joint reaction forces even when the knee itself looks structurally normal on standard imaging. Direct trauma — a fall onto the kneecap or a dashboard-type impact — can initiate cartilage softening in younger patients who have no pre-existing maltracking at all.

Getting the diagnosis right

A routine knee X-ray — taken standing, looking at the joint from the front — gives almost no information about the patellofemoral compartment. The kneecap overlies the trochlea in that view and simply cannot be assessed there. Confirming retropatellar cartilage wear requires dedicated skyline (Merchant or sunrise) radiographs, which angle the beam along the joint surface and reveal patellar tilt, lateral displacement, and any visible joint-space narrowing.

MRI goes further. T2-weighted cartilage mapping sequences can measure retropatellar cartilage thickness, detect early signal change (softening before structural loss), and identify subchondral bone marrow lesions beneath damaged areas — information that is invisible on plain X-ray at any angle. Some specialist services now use AI-assisted MRI analysis to standardise cartilage segmentation and reduce reader variability; onMRI™, available through the MSK Doctors assessment pathway, applies this approach to patellofemoral cartilage evaluation.

Clinical examination runs alongside imaging. A specialist will assess patellar tracking through range of movement, measure or estimate the Q-angle, test VMO activation under load, and apply provocation tests — including Clarke's sign and patellar grind — to reproduce and localise the patient's symptoms. Where loading pattern and gait asymmetry need to be quantified objectively, MAI Motion® gait analysis can capture how the kneecap is actually loaded during movement, adding a dynamic layer that static imaging cannot provide.

Oncefound, cartilage damage is described using the Outerbridge scale (Grades I–IV, from softening to full-thickness bone exposure). This grading is not merely descriptive — lesion depth and location directly influence whether treatment stays conservative, moves to a cartilage repair procedure, or requires a bony realignment operation such as tibial tubercle osteotomy.

Conservative care and injection options

Physiotherapy comes first, regardless of how far cartilage damage has progressed. The core targets are the VMO and hip abductors — the muscles whose weakness drives the maltracking pattern described in the previous section. VMO-biased quadriceps exercises (terminal knee extensions, shallow single-leg squats) combined with hip abductor strengthening progressively reduce the lateral pull on the kneecap and lower retropatellar contact pressure over time.

Patellar taping using the McConnell technique, and where relevant Q-angle-correcting foot orthotics, can reduce pain enough during the early rehabilitation phase to allow effective exercise. Both are adjuncts that support the mechanical correction — they do not replace the strengthening work.

When symptoms remain disruptive despite a structured programme, intra-articular injections are a reasonable next step. Corticosteroid provides short-term relief and is appropriate for acute flares; it does not alter cartilage and is not a long-term strategy. Hyaluronic acid (viscosupplementation) has a longer clinical record in knee OA, though the evidence base for its use in the patellofemoral compartment specifically is thinner than for tibiofemoral disease. For patients with non-regenerative PFOA — where cartilage can no longer repair itself — polyacrylamide hydrogel (Arthrosamid/iPAAG) is an emerging option: a published case series by Maulana, Cole & Lee (2022) reported reduction in patellofemoral bone marrow lesions following a single injection. Larger controlled studies are still needed to define its place in the pathway. Where some regenerative potential remains, ChondroFiller — a biodegradable collagen scaffold that recruits repair cells — may be more appropriate than a permanent cushioning agent.

If six to twelve weeks of consistent physiotherapy and load management fail to produce meaningful improvement in pain or function, or if symptoms are severe enough to restrict daily activity from the outset, the next step is specialist assessment to evaluate whether structural intervention is warranted.

When surgery becomes the right option

Surgery for patellofemoral cartilage wear falls into two distinct categories, and which applies depends on how much damage has accumulated and where.

Tibial tubercle osteotomy

For patients with focal damage — Outerbridge Grade III or below — concentrated at the distal pole of the patella, a tibial tubercle osteotomy (the Fulkerson procedure) offers a joint-preserving option. The tibial tubercle, where the patellar tendon inserts, is detached and repositioned medially and anteriorly, shifting load away from the damaged cartilage zone. Work by Pidoriano and colleagues established that lesion location on the patella correlates directly with surgical outcomes, which is why grading and mapping the damage guides the decision rather than symptom severity alone.

Patellofemoral arthroplasty

Once cartilage loss has reached Grade IV — full-thickness erosion to subchondral bone — and extends beyond a focal area, patellofemoral arthroplasty (PFA) becomes the more appropriate surgical route. PFA resurfaces the kneecap and femoral trochlea only, leaving both tibiofemoral compartments and the native ligaments untouched. This makes it meaningfully different from total knee replacement, which many patients assume is the only surgical option for severe kneecap arthritis.

PFA is appropriate only when PFOA is genuinely isolated. The presence of significant medial or lateral tibiofemoral degeneration on the same side shifts the decision toward total knee replacement; resurfacing one compartment in a more broadly arthritic knee tends not to produce adequate relief. A 2019 systematic review in Knee Surgery, Sports Traumatology, Arthroscopy noted ongoing evolution in criteria for these procedures, and where the threshold between PFA and TKR lies in any individual case requires whole-joint assessment rather than evaluation of the patellofemoral compartment alone.

1. [1] Chondromalacia patellae. https://en.wikipedia.org/?curid=1944613 https://en.wikipedia.org/?curid=1944613
2. [2] Patellofemoral pain syndrome. https://en.wikipedia.org/?curid=12033023 https://en.wikipedia.org/?curid=12033023
3. [3] Knee replacement. https://en.wikipedia.org/?curid=2830398 https://en.wikipedia.org/?curid=2830398