The kneecap can be replaced, and it is done routinely in orthopedic surgery, though not quite the way most people imagine. Rather than swapping the entire bone for a prosthetic one, surgeons resurface the underside of the patella with a smooth plastic button, sometimes alongside a metal trochlear component that lines the groove the kneecap rides in on the thighbone. This happens in two broad scenarios: as part of a total knee replacement, where the kneecap surface is resurfaced along with the rest of the joint, or as a standalone procedure called patellofemoral arthroplasty, which targets only the kneecap-and-groove compartment while leaving the rest of the knee untouched. The choice between these options, and whether to resurface the patella at all, turns out to be one of the more contested questions in knee surgery.
Why the Kneecap Matters So Much
The patella is not just a protective shield over the front of the knee. It acts as a dynamic fulcrum for the quadriceps muscle, increasing the leverage your thigh can exert when you straighten your leg. Without it, the quadriceps would have to work considerably harder to produce the same force. The kneecap increases the extensor lever arm in the critical range of about 30 to 70 degrees of bending, where activities like climbing stairs, rising from a chair, and walking on slopes demand the most from the joint.1Clinical Orthopaedics and Related Research. Biomechanics of the patellofemoral joint At low bending angles, the quadriceps actually has a mechanical advantage transmitted through the patella, with the force ratio exceeding 1.0 at knee angles of about 20 degrees or less.2PubMed Central. The effective quadriceps and patellar tendon moment arms relative to the tibiofemoral finite helical axis
Historically, surgeons sometimes removed the entire patella (a total patellectomy) to treat severe fractures or inflammatory arthritis. Those patients ended up with significant extensor weakness and poor long-term function, which is a big part of why modern surgery focuses on preserving the bone and resurfacing its joint surface instead of taking it out altogether.
When the Kneecap Needs Attention
The patellofemoral compartment, the groove where the kneecap slides against the femur, can develop its own arthritis independently of the rest of the knee. The three main causes of isolated patellofemoral osteoarthritis are primary age-related wear, a shallow or misshapen groove (trochlear dysplasia), and post-traumatic damage following a patellar fracture.3PubMed. Patello-femoral arthroplasty- indications and contraindications Moderate to severe cases tend to produce distinctive symptoms: dramatic swelling, a knock-kneed (valgus) alignment, significantly reduced quadriceps strength, and pain when the kneecap is compressed against the femur.4PubMed Central. Clinical features of symptomatic patellofemoral joint osteoarthritis Difficulty going downstairs and a grating sensation (coarse crepitus) are often early complaints, though in mild cases these barely distinguish the condition from a normal aging knee.
Most people with kneecap arthritis first notice trouble with stairs, squatting, and sitting for long periods. The pain tends to be at the front of the knee and worsens with activities that load the patellofemoral joint. When the rest of the knee is relatively healthy but the kneecap compartment is bone-on-bone, that is where the question of isolated replacement comes into play.
Two Paths to Replacing the Kneecap Surface
There are two fundamentally different clinical situations in which the patellar surface gets replaced. Understanding the distinction matters because the decision-making is quite different for each.
Patellar Resurfacing During Total Knee Replacement
When you get a total knee replacement, the damaged surfaces of the femur and tibia are replaced with metal and plastic components. The surgeon then faces a choice: resurface the underside of the kneecap with a polyethylene (plastic) button, or leave it alone. This decision is surprisingly controversial, and practice varies widely by country and surgeon preference. In the United States, most surgeons resurface the patella routinely. In parts of Europe and Australia, many prefer to leave it unresurfaced.
The evidence, when pooled across many studies, is somewhat anticlimactic. A review comparing the two approaches found no significant difference in patient satisfaction or functional knee scores between resurfaced and non-resurfaced groups. However, complications specific to resurfacing, including infection, anterior knee pain, stiffness, and loosening of the patellar component, were observed. The majority of meta-analyses showed no clear clinical advantage to resurfacing.5PubMed Central. Resurfacing Versus Non-resurfacing Patella in Total Knee Replacement: When and What to Choose One earlier meta-analysis found that reoperation for patellofemoral problems was more likely when the patella was left unresurfaced, but when only the highest-quality trials were analyzed separately, even that advantage disappeared.6The Knee. Is patellar resurfacing superior than nonresurfacing in total knee arthroplasty? A meta-analysis of randomized trials
So the short version: resurfacing the patella during a total knee replacement might slightly reduce the chance of needing a second surgery for kneecap pain, but it does not appear to make a meaningful difference to how the knee feels or functions day to day. The tradeoff is a slightly longer operation, additional cost, and the introduction of a component that can itself fail.
Isolated Patellofemoral Arthroplasty
This is the true “kneecap replacement” in the sense that it targets only the patellofemoral compartment. The surgeon replaces the underside of the patella with a polyethylene component and lines the femoral groove with a metal trochlear implant, creating a smooth new surface for the kneecap to glide against. The rest of the knee, the surfaces where the femur meets the tibia, is left completely intact.7PubMed Central. Patellofemoral Arthroplasty
The ideal candidate is typically 60 or older, with severe disability from patellofemoral arthritis (especially with trochlear dysplasia), a body mass index in the normal range, and a well-preserved tibiofemoral joint.8Journal of ISAKOS. Isolated patellofemoral arthroplasty-surgical technique and tips: current concepts Among the three main causes of isolated patellofemoral arthritis, trochlear dysplasia is considered the best indication for this procedure.3PubMed. Patello-femoral arthroplasty- indications and contraindications The appeal is that it preserves the healthy parts of the knee, which generally means a more natural feel and faster recovery than a total replacement. The downside is that arthritis can later spread to the rest of the knee, which is in fact the leading reason these implants eventually need revision.
What the Surgery Involves
In a patellofemoral arthroplasty, the general principles are consistent across different implant brands. The surgeon opens the front of the knee, prepares the trochlear groove on the femur, and fits a metal component that recreates the groove’s shape. The undersurface of the patella is then prepared and capped with a polyethylene component, metal on the femoral side and plastic on the patellar side. Careful attention goes toward creating a smooth transition from normal cartilage to the new trochlear component and ensuring the kneecap tracks properly through its range of motion.7PubMed Central. Patellofemoral Arthroplasty
Tracking, meaning how the patella glides centrally in the groove rather than tilting or pulling to one side, is critical to a good outcome. Surgeons assess this intraoperatively using what is called the “no-thumb technique”: the knee is taken through its range of motion without the surgeon’s thumb holding the kneecap in place, to see if it tracks naturally. If the kneecap tilts or shifts laterally, a lateral retinacular release may be performed in progressive steps, carefully cutting tight tissue on the outer side of the kneecap to allow it to center itself.9PubMed Central. Surgical Technique: Lateral Retinaculum Release in Knee Arthroplasty Using a Stepwise, Outside-in Technique
Implant Design Choices
When it comes to the patellar component itself, there are different design philosophies. The two main categories are inset (also called inlay) designs, which sit within a prepared recess in the bone, and onset (onlay) designs, which sit on top of the cut surface. In biomechanical testing, inset patellae showed about 25% greater shear strength than onlay versions, meaning they resisted displacement forces more effectively.10The Knee. Fixation strength comparison of onlay and inset patellar implants However, inset designs tended to shift and tilt laterally slightly more during motion in cadaver studies.11PubMed. Patellar tracking in total knee arthroplasty: inset versus onset design
A randomized trial comparing three commonly used designs, an inlay, a round onlay, and an oval onlay, found no significant difference in the primary patient-reported outcome at two years. The oval onlay design did achieve better bone coverage and required less trimming of the lateral facet. Patellar component design did not influence overall patellofemoral outcomes or implant survivorship over that time frame.12Journal of Bone and Joint Surgery. Does Choice of Patellar Implant in Total Knee Arthroplasty Matter? A Randomized Comparative Trial of 3 Commonly Used Designs In practical terms, all modern designs work reasonably well, and surgeon familiarity with a given system tends to matter more than which specific shape is chosen.
How Long Does a Kneecap Replacement Last?
For isolated patellofemoral arthroplasty, data from the Norwegian Arthroplasty Register showed 10-year survival of about 85%, meaning roughly 15 out of 100 implants needed revision within a decade. For comparison, total knee replacement in the same registry survived at about 94% at 10 years.13PubMed Central. Patellofemoral arthroplasty-patient demographics and revision causes compared with total and medial unicompartmental knee arthroplasty In younger patients (under 60), the 10-year survival of patellofemoral arthroplasty was about 84%, which actually compared favorably to unicompartmental (partial) knee replacement in the same age group, which survived at about 79%.
The leading reason for revision of patellofemoral arthroplasty is progression of arthritis into the tibiofemoral compartment, accounting for roughly 42 to 49% of all failures depending on the data source.14PubMed Central. Failure modes of patellofemoral arthroplasty—registries vs. clinical studies: a systematic review 13PubMed Central. Patellofemoral arthroplasty-patient demographics and revision causes compared with total and medial unicompartmental knee arthroplasty Other failure modes include persistent pain (about 16%), loosening of the implant from bone (about 13%), and surgical error (about 12%).14PubMed Central. Failure modes of patellofemoral arthroplasty—registries vs. clinical studies: a systematic review When a patellofemoral arthroplasty does fail, conversion to a total knee replacement is straightforward in most cases, which is one reason surgeons consider it a reasonable first step for younger patients with isolated kneecap arthritis.
Risks and Complications Specific to the Patella
Beyond the general surgical risks of any knee replacement (infection, blood clots, stiffness), there are complications unique to the patellar component.
Patellar clunk syndrome is a distinctive problem that can develop after posterior-stabilized total knee replacements. It involves a painful, audible clunking sensation as the knee moves from a bent to a straight position. The cause is a fibrous nodule that forms at the top of the patella where it meets the quadriceps tendon.15PubMed Central. Patellar Clunk Syndrome Following Posterior Stabilized Total Knee Replacement: Report of Two Cases This nodule catches on the intercondylar box of the femoral component during motion, creating symptoms that range from mild crepitation to a frank clunk. Prosthesis design plays a significant role in whether this occurs; newer implant designs have reduced the size of the intercondylar box to minimize the contact that triggers nodule formation.16PubMed Central. Systematic review of the etiology behind patellar clunk syndrome Treatment typically involves arthroscopic removal of the fibrous tissue.
Periprosthetic patellar fracture, a break in the kneecap bone around the implant, is another recognized complication. Risk factors include lateral release during surgery, excessive bone removal during preparation, and improper tracking of the kneecap. Over half of these fractures are associated with a loose implant, which complicates management considerably.17Injury. Management of periprosthetic patellar fractures: A systematic review of literature Treatment depends on whether the implant is still well-fixed and whether the extensor mechanism (the quadriceps-patella-patellar tendon chain) is still functional. Some fractures can be treated without surgery; others require revision of the patellar component or even removal.
Kneeling After Kneecap Replacement
One of the most common concerns patients raise is whether they will be able to kneel after surgery. This matters for gardening, prayer, playing with children, and various occupations. The evidence is genuinely mixed. A systematic review found that two studies reached opposite conclusions: one showed improved kneeling with patellar resurfacing, and the other reported the opposite.18PubMed Central. Patellar resurfacing and kneeling ability after total knee arthroplasty: a systematic review
A study that looked specifically at kneeling ability at mid-term follow-up found that all patients reported some worsening of their kneeling ability after total knee replacement, regardless of whether their patella had been resurfaced. Patients who did not have kneecap arthritis before surgery and then received patellar resurfacing were actually more likely to be dissatisfied with their kneeling ability afterward.19Journal of Orthopaedics. Higher rate of kneeling after primary knee arthroplasty without patellar resurfacing at midterm review Despite the worsening ability, overall satisfaction with kneeling remained high across all groups. The takeaway for patients: do not expect kneeling to improve after knee replacement, but most people adjust and remain satisfied with the result.
Non-Surgical Options Before Replacement
Surgery is not the first step for patellofemoral arthritis. A range of non-operative treatments can provide relief, particularly in mild to moderate cases. Physical therapy has the strongest evidence base among non-drug approaches, with high-quality evidence supporting its use, though the recommendation is described as “weak” because the benefits tend to be modest and short-lived. Taping the kneecap to improve its position in the groove has moderate-quality evidence behind it. Injection therapies, including corticosteroids, hyaluronic acid, and platelet-rich plasma, have less robust evidence but are widely used.20PubMed Central. Isolated patellofemoral osteoarthritis
Weight loss deserves special mention. The patellofemoral joint bears forces several times body weight during activities like stair climbing and squatting, so even modest weight reduction can substantially decrease the load on an arthritic kneecap. Anti-inflammatory medications, bracing, and shoe orthotics are other options that can be combined with exercise programs. Newer approaches like extracorporeal shockwave therapy and regenerative injections using bone marrow concentrate or mesenchymal stem cells are under investigation but do not yet have strong evidence supporting their routine use.21PubMed Central. Treatment Options for Patellofemoral Arthritis
Why Anatomy Varies and Why It Matters for Implants
One reason patellar replacement is not as straightforward as, say, a hip replacement is that kneecaps vary enormously in size and shape between individuals. Sex-based differences are particularly pronounced. Male patellae are significantly thicker, wider, and taller than female patellae, with average native patellar thickness of about 25 mm in men compared to about 22 mm in women.22The Knee. Gender optimized patellar component designs are needed to better match female patellar anatomy The largest patellar prosthesis that could fit a prepared female kneecap averaged about 31 mm in diameter, compared to about 35 mm in males. These are substantial differences that have led researchers to argue that sex-specific patellar component designs are needed.
Shape differences go beyond simple size. Male patellae tend to have a more posteriorly curving medial facet and a taller, narrower facet shape compared to female patellae.23PubMed. Sex differences in patellar facet shape among healthy and osteoarthritic cohorts There are also differences between right and left sides and across age groups, with patellar height, thickness, and lateral facet width changing as people age.24Journal of the Anatomical Society of India. A Guide to Patellar Implant Design: Radiologic Investigation on Gender and Age-related Morphological Differences and Surgical Characteristics of the Patella When a patellar implant does not match the patient’s anatomy well, it can overhang the edges of the bone, understuff or overstuff the joint, or track improperly, all of which contribute to pain and early failure. This is an area where one-size-fits-all implants fall short, and it is an active area of implant development.
Robotic Assistance and the Future of Patellar Surgery
Robotic-assisted knee replacement has grown rapidly over the past decade, and its effect on the patellofemoral compartment is an emerging area of study. Early research has looked at whether robotic guidance can achieve more consistent patellar alignment. One study defined “safe zones” for patellar tilt, rotation, and overstuffing in robotic-assisted total knee replacements and found that while patients outside these zones did not have worse clinical outcomes, those with certain alignment parameters (patellar tilt below zero, overstuffing above zero, or rotation within a narrow range) had significantly higher rates of needing secondary patellar resurfacing. In other words, robotic precision may help surgeons predict and prevent the need for additional patellar work, even if it has not yet shown a clear advantage in patient-reported outcomes.
Three-dimensional printing and patient-specific instrumentation are also being explored for patellar components. Given the wide variation in patellar anatomy between sexes, ages, and individuals, custom-matched implants are an appealing concept. Whether they prove cost-effective and clinically superior enough to justify the added complexity remains an open question, but the anatomic data supporting the need is already compelling.