What makes PFA different from a full knee replacement

Not everyone with knee pain needs their entire knee replaced. For patients whose arthritis is confined strictly to the kneecap and its groove on the front of the thigh bone, a patellofemoral arthroplasty (PFA) resurfaces only those two worn surfaces — the underside of the patella and the femoral trochlear groove — leaving the inner (medial) and outer (lateral) compartments of the knee completely untouched.

The implant itself uses metal and polyethylene components to replace the damaged cartilage, but because the surgical footprint is far smaller than a total knee replacement (TKR), there is less disruption to the surrounding soft tissue and bone stock. That preserved tissue is what underpins the faster recovery and earlier return to daily activity that published series consistently report.

PFA is not a scaled-down or inferior version of TKR — it carries a different clinical indication, not a broader one. A TKR is the right operation when arthritis has spread across multiple compartments; PFA is the right operation when disease is genuinely isolated to the patellofemoral joint. That distinction makes patient selection the single most critical determinant of whether the procedure succeeds.

Who qualifies — and who does not

Qualifying for PFA begins with a single non-negotiable finding: arthritis confined entirely to the patellofemoral joint, confirmed on imaging. Clinically, that means bone-on-bone change at Iwano stage 3 or 4, or trochlear dysplasia causing symptomatic joint damage. The cartilage in the inner and outer sections of the knee — the medial and lateral compartments — must be healthy. Even early degenerative change in those areas rules PFA out.

Beyond the imaging findings, surgeons typically look for all of the following before proceeding:

• Symptoms that have not responded adequately to at least three to six months of conservative care — physiotherapy, activity modification, anti-inflammatory medication, and injections
• Intact anterior and posterior cruciate ligaments, and stable collateral ligaments
• Broadly neutral limb alignment without significant deformity
• A BMI broadly under 30 kg/m²
• No inflammatory joint disease such as rheumatoid arthritis or gout

The main contraindications are multi-compartment osteoarthritis, major ligament instability, fixed stiffness, and significant coronal deformity. These are not arbitrary thresholds — they reflect the principal reason PFA revisions occur: progression of arthritis into compartments that were already compromised at the time of surgery.

Not meeting these criteria is not a dead end. Patients with more widespread arthritis, significant deformity, or inflammatory disease are candidates for other procedures at a different point on the treatment pathway, including unicompartmental or total knee replacement. A consultant assessment establishes which option fits the individual's joint, alignment, and activity profile.

Age, activity level and how they shape candidacy

Published survivorship data from a 2025 study of 120 procedures (mean follow-up 8.6 years) contains a finding that runs counter to the conventional wisdom: patients aged over 76 achieved 100% survivorship — the same as those under 55 — while the 56–65 group fared worst, with 90% survivorship driven by osteoarthritis spreading into the tibiofemoral compartments. The implication is that age itself is a proxy variable. What the numbers are actually measuring is compartment health.

Typically, PFA patients are younger than those undergoing total knee replacement — pooled data report a mean surgical age of approximately 49–55 years, with women making up around 65% of cohorts. Trochlear dysplasia, post-traumatic change, and idiopathic degeneration are the three commonest underlying causes. A broad working range of 40–65 years is sometimes cited, but the 2025 evidence suggests this is an oversimplification: younger, active patients who have preserved medial and lateral cartilage benefit most, and appropriately selected older patients can do equally well.

Activity level is relevant — a more active patient places greater demand on the implant — but it does not override the structural criteria. A consultant review of imaging and alignment, rather than age alone, determines whether the patellofemoral compartment is truly isolated and whether the remaining knee is robust enough to sustain the procedure long term.

What recovery looks like week by week

The first hours after PFA set a different pace from total knee replacement. Most patients are standing and moving with crutches by the end of the operating day — early mobilisation is a deliberate part of the recovery, made possible by the procedure's smaller surgical footprint and the preservation of surrounding soft tissue.

Weeks one to six

Walking aids are typically no longer needed by around two to six weeks. Physiotherapy begins promptly: early sessions concentrate on activating the quadriceps, controlling swelling, and restoring range of movement. Over the following weeks the programme progresses to strengthening and gait work, with the pace guided by individual response rather than a fixed timetable.

Weeks six to twelve

Desk-based work and routine daily activities are generally manageable by this stage. Driving usually resumes at around six to eight weeks for right-leg procedures — once the leg responds quickly enough for reliable braking — and often earlier for left-leg cases in an automatic vehicle.

Three to six months

Low-impact activities — cycling, swimming, and brisk walking — are the most commonly resumed sports and tend to dominate the activity profile after PFA. A systematic review of 265 patients across seven studies reports return-to-sport rates of 65–91%, with approximately 59% returning within six months and around 75% reaching or exceeding their pre-operative activity level; because definitions of 'return to sport' varied across those studies, the figures serve as a realistic guide rather than a predictive guarantee for any individual. Data on high-impact sport after PFA remain limited.

How long PFA lasts and what influences survivorship

Survivorship figures from pooled registry data give a realistic picture: approximately 90% of PFA implants remain in place at five years, 82% at ten years, and around 73–74% at fifteen to twenty years — at which point long-term evidence becomes sparse.

Conversion to total knee arthroplasty occurs in roughly 6–13% of cases at mid-term follow-up. This conversion almost invariably reflects osteoarthritis spreading beyond the patellofemoral compartment rather than a mechanical problem with the implant itself — a distinction that shifts the longevity question away from the hardware and towards two factors that can be influenced: who performs the procedure and which implant design is used.

Surgeon training has a measurable effect. Registry data from 482 procedures (JAMA Network Open, 2025) show a six-year cumulative revision rate of 8% for surgeons with focused PFA training, compared with 26% for non-specialist surgeons — a three-fold difference that underlines why referral to an experienced patellofemoral surgeon matters.

Implant design also plays a role. Onlay PFA implants are associated with lower rates of instability, malposition, stiffness, and conversion to TKA than inlay designs; inlay designs show marginally better WOMAC pain scores, though at the cost of higher overall complication rates. Where robotic assistance is used, evidence from comparative series suggests superior patellar tilt correction compared with conventional technique, with functional outcomes and revision rates remaining broadly comparable across approaches.

Getting a precise assessment before deciding

Confirming that arthritis is strictly confined to the patellofemoral compartment requires more than a standing X-ray. MRI capable of characterising cartilage across all three knee compartments is the current standard for candidacy assessment; AI-assisted analysis such as onMRI™ can add granular detail on cartilage depth and meniscal integrity that plain radiographs routinely miss. Biomechanical and gait assessment adds a further layer — identifying loading and alignment patterns that inform the surgical plan and may surface correction needs, such as a malalignment tendency, that should be addressed alongside the arthroplasty itself.

When PFA proves unsuitable, the same diagnostic pathway should indicate clearly what is appropriate — whether that is a joint-preservation procedure such as osteotomy, an alternative arthroplasty, or continuation of conservative management.

Lincolnshire Knee, part of the MSK Doctors group, accepts patients without GP referral, with assessment and imaging available at Sleaford (NG34) and Grantham (NG31). Book at lincolnshireknee.co.uk.

1. [1] Impact of Age on Patellofemoral Arthroplasty Outcomes, Osteoarthritis Progression, and Survivorship (2025). (2025). https://doi.org/10.1016/j.arth.2025.06.064 https://doi.org/10.1016/j.arth.2025.06.064
2. [2] Outcomes of patellofemoral joint arthroplasty: a systematic review of revision timelines and complication rates (2025). (2025). https://doi.org/10.1186/s13018-025-05592-8 https://doi.org/10.1186/s13018-025-05592-8
3. [3] Return to sports and recreational activities after patellofemoral arthroplasty: A systematic review. (2025). https://doi.org/10.1016/j.jisako.2025.100925 https://doi.org/10.1016/j.jisako.2025.100925
4. [4] Patellofemoral Arthroplasty Is an Efficient Strategy for Isolated Patellofemoral Osteoarthritis with or without Robotic-Assisted System (2023). (2023). https://doi.org/10.3390/jpm13040625 https://doi.org/10.3390/jpm13040625
5. [5] Surgeon Training and Revision Rates After Patellofemoral Arthroplasty (JAMA Network Open, 2025). (2025). https://doi.org/10.1001/jamanetworkopen.2025.17825 https://doi.org/10.1001/jamanetworkopen.2025.17825