What patellofemoral arthroplasty actually does

Patellofemoral arthroplasty replaces just two surfaces: the trochlear groove — the channel on the front of the thigh bone along which the kneecap slides — and the underside of the kneecap itself. The rest of the joint is left entirely alone. The cartilage, ligaments, and bone of the inner and outer compartments between the femur and tibia are untouched throughout.

That distinction is the whole point of the procedure. A total knee replacement removes and replaces all three compartments of the knee, regardless of how much cartilage each contains. Patellofemoral arthroplasty works only where the problem is confined: when the kneecap joint has broken down but the rest of the knee remains healthy. Preserving intact tissue is why the operation is classed as a partial, or bone-conserving, replacement — and why indication, recovery, and longevity all differ meaningfully from total knee replacement.

Two implant families are used — inlay and onlay designs — and the choice between them bears directly on complication and revision rates. That distinction is explored in detail later; for now, it is worth knowing that onlay implants account for the large majority of procedures performed today.

Which patients are the right candidates

The central question a surgeon needs to answer before recommending patellofemoral arthroplasty is precise: has the arthritis remained confined to the kneecap joint, leaving the inner and outer parts of the knee — where the thigh bone meets the shin bone — genuinely intact?

When imaging confirms this, three patterns of kneecap OA are commonly found in patients considered for PFA: post-traumatic damage (accounting for roughly a third of cases), trochlear dysplasia in which abnormal groove shape accelerates wear (around 39%), and idiopathic degeneration with no identifiable cause (approximately 28%). All three can be appropriate indications, provided the inner and outer knee compartments show no significant cartilage loss.

Age and activity level also factor in. PFA tends to suit patients under 65 who remain physically active — people for whom a full total knee replacement would needlessly sacrifice healthy joint tissue. The procedure is by design a joint-preserving option rather than a universal lifetime solution.

When PFA is not appropriate

Three situations rule the procedure out entirely: arthritis affecting the inner or outer knee compartments, inflammatory joint disease such as rheumatoid arthritis, and severe malalignment that cannot be corrected. Where patellar tracking problems or milder malalignment are present, these may be addressed at the same time — sometimes through a tibial tubercle osteotomy — without excluding a patient from PFA outright.

Obesity represents a meaningful, though not absolute, relative barrier. Research by Marullo and colleagues found that patients with a BMI above 30 face significantly higher revision rates, even when early functional improvements are comparable to those of non-obese patients. Weight optimisation before surgery is therefore a sensible discussion at the assessment stage.

What mid-term survival data actually show

Survival figures from the current evidence base give a reasonably clear picture at each time horizon. A 2025 systematic review pooling data from 21 international registry studies placed five-year implant survival at 90.30% (95% CI 88.32–92.27) and ten-year survival at 82.23% (95% CI 78.90–85.56). By 15 to 20 years, pooled survival had declined to approximately 73%. An independent centre series published in 2022 reported comparable results — 91.1% survival at mid-term follow-up, with more than 80% of patients achieving good or excellent functional scores.

These numbers are honest rather than alarming when read in context. PFA is not designed as a permanent lifetime solution for every patient who receives it; it is a joint-preserving procedure calibrated to delay — or, in some cases, avoid — total knee replacement in people with intact tibiofemoral compartments.

Early and late failure: different causes

Failure mode depends heavily on timing. Within the first two years, persistent anterior knee pain is the dominant reason for revision — a pattern that remains incompletely understood and is not reliably resolved even by conversion to total knee replacement. One five-year series of 36 knees found that all six revisions occurred in patients who had the patella resurfaced as part of the original procedure, and that pain frequently continued afterwards. This observation has prompted ongoing clinical debate about whether routine patellar resurfacing is advisable within the index operation; it is not a settled question, and the finding comes from a single series.

Beyond the two-year mark, the predominant failure mode shifts. Progression of osteoarthritis into the tibiofemoral compartments — the inner or outer knee — is the most common reason patients eventually require conversion to total knee replacement. This reflects the natural course of the underlying joint disease rather than implant failure, and reinforces why demonstrably intact tibiofemoral cartilage at the time of surgery is an absolute prerequisite for the procedure.

How PFA compares with total knee replacement

Against total knee replacement — the standard comparator for the same indication — PFA presents a consistent pattern: meaningfully lower perioperative risk, broadly comparable function, and a durability trade-off that belongs in any honest pre-operative conversation.

Perioperative safety and range of motion

A UK National Joint Registry analysis covering 7,819 PFAs and 662,141 TKAs performed between 2003 and 2021 found that PFA carried markedly lower 30-day complication risk across every category examined: DVT or pulmonary embolism (risk ratio 0.35), respiratory infection (0.65), urinary tract infection (0.59), wound infection (0.73), and death (0.36). Ligament and compartment preservation also translates into consistently better postoperative range of motion — a finding replicated across multiple randomised trials and cohort studies — because neither the cruciate ligaments nor the healthy tibiofemoral cartilage are disturbed.

Function over time

For patient-reported outcomes at 6 to 18 months, NJR data show median Oxford Knee Scores of 35 for PFA and 38 for TKA. The Oxford Knee Score runs from 0 to 48, where higher values indicate better function; the three-point gap is small and unlikely to represent a meaningful difference in day-to-day experience. A 10-year randomised trial follow-up found no statistically significant difference in physical activity levels, though PFA patients averaged an 8.6% higher step count.

The durability trade-off

The honest counterweight is longevity. Ten-year implant survival is approximately 85% for PFA against around 95% for TKA, and the revision hazard ratio for PFA is approximately 3.4 compared with TKA — a number worth stating plainly rather than softening. That said, an 85% ten-year survival rate is not a forecast of failure; for many patients it represents a decade of preserved joint tissue and lower surgical risk. When revision does occur, it most commonly means conversion to TKA rather than a more complex re-do procedure. The practical question, then, is whether the perioperative safety advantage and joint-preservation benefit are worthwhile given a patient's age, activity level, and likelihood of tibiofemoral progression — a calculation that requires individual clinical assessment rather than a blanket recommendation.

Implant design and the role of surgeon experience

Two technical variables shape PFA outcomes in ways that bare survivorship statistics can obscure: the choice of implant design, and the experience of the surgeon performing the procedure.

Onlay versus inlay implants

Contemporary PFA implants fall into two families. Inlay designs sit flush within a milled recess in the trochlear groove; onlay designs cap the anterior femur in a manner analogous to the femoral component in total knee replacement. A systematic review comparing the two found that onlay designs were associated with lower rates of instability, stiffness, deep infection, patellar wear, and conversion to TKA. Inlay designs showed modestly better WOMAC scores in some analyses, but their overall complication and revision profile was significantly worse. The weight of current evidence has led onlay designs to dominate practice for this reason, though neither design has been evaluated in a head-to-head randomised trial.

The surgeon-training effect

Of arguably greater practical consequence is the gap between trained and untrained operators. A registry-based cohort study published in JAMA Network Open in 2025 found a six-year cumulative revision rate of 8% for surgeons with focused PFA training, compared with 26% for those without — a threefold difference. This finding matters when interpreting registry averages: a pooled revision rate blends high-volume specialists with occasional operators, and the resulting figure may understate what a trained surgeon can achieve and overstate what an experienced one risks. For patients, it underlines the value of seeking surgeons with documented, high-volume PFA practice rather than treating the procedure as generically equivalent across operators.

Robotic assistance

Robotic-assisted PFA is an emerging refinement, showing superior patellar tilt correction compared with conventional technique in comparative series. Functional outcomes at three to six years are encouraging — one 18-knee series recorded Oxford Knee Score improvement from 17.3 to 46.3 — though the evidence comparing robotic and conventional approaches is still maturing, and robotic guidance is best understood as a precision aid to implant positioning rather than a guarantee of superior results.

What remains uncertain and how to approach next steps

Several questions in this evidence base remain genuinely open — and a clear account of them is part of honest patient information.

The optimal patient age for PFA over total knee replacement has not been established. Registry data suggest older recipients tend to achieve better outcomes, yet the procedure's bone-preserving rationale makes it conceptually suited to younger, active patients. This tension sits with clinical judgement and shared decision-making rather than a fixed threshold.

Whether patellar resurfacing should form a routine part of PFA or be used selectively is similarly unsettled. The available data come from single-centre series with limited follow-up, and no consensus recommendation currently exists.

Long-term survival figures beyond ten years derive largely from older implant generations; whether second-generation onlay designs will perform meaningfully better at 15 to 20 years is unknown. The same applies to robotic assistance: mid-term functional outcomes are encouraging, but whether the technique produces a durable advantage over conventional PFA at longer follow-up has not yet been demonstrated.

Despite these open questions, the diagnostic pathway for a patient considering PFA is well defined. Confirming isolated patellofemoral involvement requires weight-bearing knee X-rays — including a skyline patellar view — MRI to assess compartment integrity and cartilage quality, and, where patellar tracking or alignment is in question, objective biomechanical assessment. These investigations determine whether PFA is genuinely the right procedure or whether a different approach would serve better; no surgical recommendation can be made without them.

Seeking an assessment

Lincolnshire Knee is part of the MSK Doctors group and accepts patients without referral. An initial assessment can be arranged at lincolnshireknee.co.uk.

1. [1] Anterior Knee Pain After Patellofemoral Joint Replacement: Revision Is Not the Answer — Five-Year Survivorship and Functional Outcomes. (2025). https://doi.org/10.1302/1358-992x.2025.13.060 https://doi.org/10.1302/1358-992x.2025.13.060
2. [2] Outcomes of patellofemoral joint arthroplasty: a systematic review of revision timelines and complication rates. (2025). https://doi.org/10.1186/s13018-025-05592-8 https://doi.org/10.1186/s13018-025-05592-8
3. [3] Patellofemoral Arthroplasty Is an Efficient Strategy for Isolated Patellofemoral Osteoarthritis with or without Robotic-Assisted System. (2023). https://doi.org/10.3390/jpm13040625 https://doi.org/10.3390/jpm13040625
4. [4] Surgeon Training and Revision Rates After Patellofemoral Arthroplasty. (2025). https://doi.org/10.1001/jamanetworkopen.2025.17825 https://doi.org/10.1001/jamanetworkopen.2025.17825
5. [5] Robotic-assisted patellofemoral arthroplasty provides excellent implant survivorship and high patient satisfaction at mid-term follow-up. (2024). https://doi.org/10.1007/s00264-024-06224-2 https://doi.org/10.1007/s00264-024-06224-2
6. [6] Outcomes and complications of inlay versus onlay patellofemoral arthroplasty: A systematic review. (2023). https://doi.org/10.1016/j.knee.2023.01.001 https://doi.org/10.1016/j.knee.2023.01.001