What PFA is and why it exists as a separate procedure

For many patients with persistent front-of-knee pain, the prospect of a full knee replacement feels disproportionate — and in some cases, it is. Patellofemoral arthroplasty (PFA) exists precisely for that situation: it is a partial knee replacement that addresses only the patellofemoral compartment, the articulation between the undersurface of the kneecap and the trochlear groove at the lower end of the femur.

The knee has three distinct compartments. PFA resurfaces one of them, leaving the medial (inner) and lateral (outer) tibiofemoral surfaces completely undisturbed. Crucially, the cruciate and collateral ligaments are preserved, which means the joint continues to move in a more natural pattern than it would after a total knee replacement (TKR), where all three compartments are resurfaced regardless of whether disease is present in each one.

The principal advantage of this compartment-specific approach is bone and tissue conservation. When arthritis genuinely is confined to the patellofemoral compartment, replacing only that compartment avoids unnecessary disruption of healthy structures — and keeps the option of TKR open should disease progress elsewhere in the knee in later years.

Symptoms that point toward isolated patellofemoral arthritis

Pain at the front of the knee — behind or around the kneecap — is the hallmark of patellofemoral disease. It tends to flare on stairs (particularly going down), when squatting, kneeling, or rising from a chair, and after sitting with the knee bent for any length of time. This last pattern, sometimes called the 'theatre sign', reflects how compressive forces across the patellofemoral joint rise sharply with flexion.

This symptom picture helps distinguish patellofemoral arthritis from the more common medial or lateral tibiofemoral form, where discomfort tends to be felt at the inner or outer joint line rather than at the front. Patellofemoral arthritis affects an estimated 10–24% of adults over 55 with knee pain and is more common in women, though some patients present younger — particularly those with a history of kneecap tracking problems or a previous patellar injury.

Surgery only becomes relevant at the end-stage: when full-thickness cartilage loss is confirmed on imaging and when conservative management has been genuinely tried and found inadequate. PFA is not a treatment for early or moderate patellofemoral disease. Patients who reach surgical evaluation have typically worked through a sustained period of non-operative care; what that pathway involves, and what 'adequate trial' means in practice, is set out in the candidacy criteria that follow.

Who qualifies — and who does not

Imaging, not symptoms alone, determines whether PFA is the right operation — and strict candidacy criteria exist for good reason.

Inclusion criteria

The core requirement is end-stage osteoarthritis confined to the patellofemoral compartment, with the medial and lateral tibiofemoral surfaces showing well-preserved cartilage on MRI and weightbearing X-rays. Patients must also have completed an adequate trial of conservative management — physiotherapy, weight management, activity modification, bracing, and appropriate injection or analgesic therapy — before surgical referral is appropriate. Younger, active patients with isolated patellofemoral disease are often the profile best served by PFA; the bone and ligament conservation the procedure offers matters most to those who are likely to remain active for decades.

Exclusions that disqualify

Several conditions rule PFA out entirely:

• Any significant tibiofemoral osteoarthritis. PFA resurfaces only the patellofemoral compartment; it cannot address disease in the medial or lateral joint lines. A patient with tibiofemoral OA who undergoes PFA will continue to experience pain from the unresurfaced compartments — the procedure simply will not work for them.
• Inflammatory arthritis (rheumatoid arthritis and related conditions) affects the whole joint and requires a systemic treatment approach; compartmental surgery is not appropriate.
• Uncorrected patellofemoral malalignment or patellar instability. If the kneecap does not track centrally in the trochlear groove, the new implant is loaded asymmetrically and wears abnormally fast. Malalignment is therefore either a contraindication or must be surgically corrected during the same procedure.
• Significant patellar dysplasia — structural abnormality of the kneecap's shape — similarly compromises implant fit and function.

Because tibiofemoral involvement can be present without producing obvious symptoms at the time of assessment, imaging confirmation that both tibiofemoral compartments are genuinely healthy is an essential step before a patient proceeds to PFA.

Assessment and imaging before surgery

Before any decision about PFA is made, the surgeon needs a precise compartment-by-compartment picture of the knee — not just confirmation of patellofemoral damage, but clear evidence that the tibiofemoral cartilage is intact.

A plain X-ray is always part of the assessment. It establishes bony architecture, the degree of joint space narrowing, and any structural malalignment, but it cannot reliably show the condition of cartilage in the medial or lateral tibiofemoral compartments. That is the critical limitation: a patient can have a surprisingly normal-looking X-ray in those compartments while early cartilage loss is already present.

MRI fills that gap. It is the standard for direct cartilage visualisation and is used both to characterise the extent of patellofemoral damage and to confirm that the tibiofemoral surfaces are genuinely preserved. Where the clinic uses AI-assisted MRI analysis — the onMRI™ platform, which applies automated cartilage segmentation and T2 mapping to quantify cartilage health across compartments — this adds an objective, reproducible layer to that assessment beyond standard radiologist reporting alone.

CT scanning is added when bony detail matters more specifically: trochlear morphology, patellar height, and rotational alignment all show more clearly on CT, making it particularly relevant when malalignment is suspected.

All imaging data feeds into patient-specific templating — a pre-operative planning step in which the surgeon maps implant size, positioning, and alignment before entering the operating theatre. In diagnostically complex cases, dynamic needle arthroscopy can provide real-time direct visualisation of compartment status where imaging alone leaves uncertainty.

What the surgery involves

The procedure centres on two components working together. The femoral trochlear groove — the channel in which the kneecap runs — is reshaped and fitted with a cobalt-chrome metal implant that restores a smooth, correctly contoured gliding surface. The undersurface of the patella is simultaneously fitted with a polyethylene (plastic) button, creating a low-friction articulation between the two. The analogy of a dental crown is useful here: the worn surface is replaced, but the underlying structure is preserved and the surrounding anatomy is left undisturbed.

What is notably not touched is equally important. Both tibiofemoral compartments — the medial and lateral sides of the knee where the femur meets the tibia — remain completely intact. The anterior and posterior cruciate ligaments, and the collateral ligaments, are not cut or released. This is a fundamental difference from total knee replacement, where all three compartments are resurfaced and the cruciate ligaments may be sacrificed or substituted. Preserving this ligament architecture means the knee retains more of its natural load-distribution and proprioceptive feedback after surgery.

Because the scope of the operation is more limited, the incision and soft-tissue dissection are correspondingly smaller than for TKR. Patients generally receive either a general anaesthetic or a regional spinal block, and most are admitted as an inpatient for one to two nights.

For patients concerned about long-term durability, it is worth knowing that the bone conservation inherent in PFA has a practical consequence: if osteoarthritis eventually progresses into the tibiofemoral compartments in later years, conversion to a total knee replacement is typically straightforward using standard TKR implants — leaving all options open.

Recovery and long-term outlook

Recovery from PFA is generally faster than from total knee replacement — a direct consequence of the more limited surgical scope. Because neither tibiofemoral compartment is disturbed and the cruciate ligaments remain intact, the soft-tissue recovery period is shorter, and most patients begin mobilising with physiotherapy support within the first day or two after surgery. The principal functional gain, in appropriately selected patients, is meaningful relief of anterior knee pain during stair climbing, squatting, and prolonged sitting — the activities that patellofemoral disease makes most difficult. Return to high-impact activities is not guaranteed for everyone, and realistic goals should be agreed with the treating surgeon beforehand rather than assumed.

The most important long-term variable is whether osteoarthritis remains confined to the patellofemoral compartment. If disease progresses into the medial or lateral tibiofemoral compartments in subsequent years, conversion to total knee replacement is the established pathway — one that is typically straightforward given that the tibial surfaces and bone stock are preserved by PFA. Patient selection is therefore the single most influential factor in long-term prognosis: patients with genuinely isolated patellofemoral disease on pre-operative imaging carry a substantially better outlook than those in whom borderline tibiofemoral changes were not adequately excluded before surgery.

Published implant survival data for PFA vary considerably by design generation — first-generation implants performed less well than contemporary onlay and inlay designs — and outcomes also reflect surgeon case volume and technique. Because population-level registry figures do not translate simply to individual cases, a meaningful prognosis for any given patient is best discussed at surgical consultation with direct reference to the relevant data and that person's own imaging findings. What the evidence does support consistently is that the prerequisite for a durable result is the same at ten years as it is on the day of surgery: accurate patient selection.

1. [1] Unicompartmental knee arthroplasty. https://en.wikipedia.org/?curid=16991704 https://en.wikipedia.org/?curid=16991704
2. [2] Knee replacement. https://en.wikipedia.org/?curid=2830398 https://en.wikipedia.org/?curid=2830398
3. [3] Patellofemoral pain syndrome. https://en.wikipedia.org/?curid=12033023 https://en.wikipedia.org/?curid=12033023