Knee replacements fall into several distinct categories based on how much of the joint is resurfaced, how the implant is designed, and how it is anchored to bone. The broadest distinction is between a total knee replacement, which resurfaces all three compartments of the joint, and a partial replacement, which resurfaces only the damaged compartment. Within those categories, implant design choices like whether to keep or sacrifice a key ligament, whether the bearing surface is fixed or mobile, and whether the components are cemented or press-fit all shape how the new knee moves and how long it lasts. Understanding these options matters because the “best” knee replacement is not a single device but a combination of choices tailored to each person’s anatomy, age, and activity level.
Total Knee Replacement
A total knee arthroplasty (TKA) is the most common type. The surgeon resurfaces the ends of both the femur (thighbone) and the tibia (shinbone), and usually the underside of the kneecap as well. A plastic spacer sits between the metal components to provide a smooth gliding surface. TKA is suited for people whose arthritis has spread across the entire joint or who have significant deformity or ligament damage that rules out a less invasive option.
Because the whole joint surface is replaced, TKA tends to have excellent long-term survivorship. Registry data consistently show that modern total knee implants last well beyond a decade for the large majority of patients. When a TKA does eventually fail, the most common reasons are infection, loosening of the implant from bone, and instability of the joint.1PubMed Central. Analysis of Total Knee Arthroplasty revision causes Revision surgery, which replaces part or all of the original implant with a new one, is itself a distinct category and tends to be more complex, sometimes requiring specialized constrained implants and bone grafts.2PubMed Central. Outcomes in revision total knee arthroplasty
Partial (Unicompartmental) Knee Replacement
The knee has three compartments: the medial (inner), lateral (outer), and patellofemoral (behind the kneecap). When arthritis is confined to just one of these, a partial replacement resurfaces only that compartment, leaving the healthy bone, cartilage, and ligaments elsewhere in the joint intact. The most common version targets the medial compartment, since that is where wear most often concentrates.
Compared to a total replacement, a partial knee replacement (also called unicompartmental knee arthroplasty, or UKA) preserves more natural tissue, which translates into a few practical advantages. Hospital stays tend to be about one to two days shorter. A systematic review and meta-analysis found better functional scores for UKA than TKA, along with lower rates of serious complications like blood clots and cardiac events in large registry studies.3BMJ. Patient relevant outcomes of unicompartmental versus total knee replacement: systematic review and meta-analysis Early recovery is also faster: one study found that patients walked measurably farther at six weeks and three months after a partial replacement than after a total, though the difference disappeared by one to two years.4Journal of Bone and Joint Surgery. Comparison of Functional Recovery Between Unicompartmental and Total Knee Arthroplasty
The trade-off is durability. Revision rates at five years are consistently higher for partial replacements than for total replacements across trials, cohort studies, and registry data.3BMJ. Patient relevant outcomes of unicompartmental versus total knee replacement: systematic review and meta-analysis A large randomized trial (the TOPKAT study) found that five-year patient-reported outcomes were similar between the two procedures, but reoperation rates were comparable as well in that specific trial setting, which speaks to how much surgeon expertise and patient selection matter.5PubMed Central. Total versus partial knee replacement in patients with medial compartment knee osteoarthritis: the TOPKAT RCT In short, a well-chosen partial replacement gets you back on your feet faster, but the risk that you may need a second surgery down the road is somewhat higher than with a total.
Patellofemoral Arthroplasty
Patellofemoral arthroplasty (PFA) is a less commonly performed partial replacement that targets only the groove behind the kneecap, where the kneecap slides against the thighbone. It is designed for people whose arthritis is confined to this compartment. The success of PFA depends heavily on patient selection: ideal candidates have isolated patellofemoral arthritis, often with a shallow trochlear groove (the bony channel the kneecap tracks in), and no significant malalignment or early arthritis in the other compartments.6PubMed. Patello-femoral arthroplasty- indications and contraindications
The downside is survivorship. Population-based data from England’s National Joint Registry found that PFA had roughly 85% survival at ten years, compared to about 95% for TKA.7PubMed. Outcomes of patellofemoral joint arthroplasty compared with total knee arthroplasty for osteoarthritis Within the first two years, the most frequent reason for revising a PFA was persistent pain, while infection was the more common early trigger for TKA revision. That gap in survivorship is why some surgeons skip PFA altogether and proceed directly to a total replacement, even when only the patellofemoral compartment is affected. The debate is ongoing, and PFA remains a reasonable option for the right patient who wants to preserve as much natural knee as possible.
Cruciate-Retaining Versus Posterior-Stabilized Implants
Within total knee replacement, one of the biggest design decisions involves the posterior cruciate ligament (PCL), the thick band at the back of the knee that helps control how the shinbone moves relative to the thighbone during bending. Two main approaches exist:
- Cruciate-retaining (CR): The surgeon keeps the PCL intact. The implant relies on the natural ligament to help guide motion during flexion.
- Posterior-stabilized (PS): The surgeon removes the PCL and the implant has a built-in post-and-cam mechanism that mimics its job, guiding the femur to roll back on the tibia during deep bending.
You might expect one to clearly outperform the other, but decades of research have found remarkably little difference in patient-reported outcomes. A review from the orthopedic literature concluded there was no notable difference in functional outcome, range of motion, or survival rate between the two designs in most studies.8PubMed Central. What to Know for Selecting Cruciate-Retaining or Posterior-Stabilized Total Knee Arthroplasty A meta-analysis confirmed that final range of motion in bending and straightening does not differ significantly between the two.9PubMed Central. Cruciate-Retaining Versus Cruciate-Substituting Total Knee Arthroplasty: A Meta-Analysis Where differences do show up is in how the components move at a detailed level: CR knees allow more front-to-back sliding of the contact point, while PS knees produce more controlled rollback. One long-term follow-up study found that PS knees had slightly faster timed physical function tests, but clinical outcome scores between the groups were statistically identical.10PubMed. Comparison of Contact Kinematics in Posterior-Stabilized and Cruciate-Retaining Total Knee Arthroplasty at Long-Term Follow-Up
In practice, the choice often comes down to surgeon preference and the condition of the patient’s PCL. If the ligament is healthy and functional, a CR design avoids removing it. If the ligament is damaged or the surgeon needs to make larger bony cuts (for example, to correct a deformity), a PS design is the more predictable option.
Fixed-Bearing Versus Mobile-Bearing Implants
Another design fork involves how the plastic spacer between the metal components is attached. In a fixed-bearing design, the spacer is locked into the tibial tray. In a mobile-bearing design, the spacer can rotate or slide slightly on the tray. The theory behind mobile bearings is that allowing some movement could distribute stress more evenly, reducing wear over time.
In practice, this theoretical advantage has been hard to demonstrate. A randomized trial with nine years of follow-up found no significant clinical differences between fixed and mobile bearings in range of motion, Oxford Knee Scores, or other patient-reported outcomes.11PubMed. Mobile-bearing versus fixed-bearing total knee implants. Results of a series of 100 randomised cases after 9 years follow-up The fixed-bearing group in that trial did show a higher rate of a radiographic finding called osteolysis (tiny areas of bone loss around the implant), but the difference was not clinically meaningful. Mobile bearings carry a small additional risk of the spacer dislocating, which is not possible with a fixed design. Most modern TKAs use fixed bearings.
Constrained and Hinged Implants
Standard CR and PS implants rely on the patient’s own ligaments (or a cam mechanism) for stability. But some knees are too damaged for that to work. People with severe deformities, gross ligament insufficiency, massive bone loss, or conditions like Charcot arthropathy may need implants that provide their own built-in stability. These are called constrained or hinged implants.
A rotating-hinge design, for example, physically links the femoral and tibial components with a hinge mechanism that allows bending and some rotation but prevents the knee from buckling sideways. These are used in both primary and revision surgeries when softer-constraint implants would be unstable.12PubMed Central. Total knee arthroplasty using hinge joints: Indications and results Studies of rotating-hinge prostheses in severely affected knees show substantial improvement in function and pain reduction, though outcomes depend on using them for the right indications.13PubMed. Primary total knee arthroplasty using rotating-hinge prosthesis in severely affected knees The more stability an implant provides, the more stress gets transferred to the bone-implant interface, which can affect longevity. That is why surgeons use the least amount of constraint necessary for a given knee.
Cemented Versus Cementless Fixation
Knee implants are attached to bone in one of two ways. Cemented fixation uses a fast-setting polymer (polymethylmethacrylate) to glue the components in place. Cementless fixation relies on textured or coated surfaces that encourage bone to grow directly into the implant over time, holding it firmly without glue.
Cemented TKA has been the gold standard for decades and has the longest track record. A 15-year randomized trial found no significant difference in revision rates between cemented and cementless knees: survival was around 81% for cemented and 75% for cementless, a gap that was not statistically meaningful.14PubMed. A randomised controlled trial of cemented versus cementless press-fit condylar total knee replacement: 15-year survival analysis A systematic review and meta-analysis echoed that finding, concluding there was no difference in revision rates, though cementless implants showed a slightly higher rate of radiolucent lines (thin gaps visible on X-ray between the implant and bone) that could hint at future loosening.15PubMed Central. Cemented vs. cementless fixation in primary total knee arthroplasty: a systematic review and meta-analysis
Cementless designs are attractive in theory because they avoid cement debris and may preserve more bone stock for a future revision. They have become more popular in younger patients who are expected to outlive their implant and may eventually need a second surgery. But the evidence has not yet shown them to be clearly superior, and cemented fixation remains the default choice in most practices worldwide.
Gender-Specific Implants
The end of the thighbone is shaped differently in men and women on average. Women’s knees tend to be narrower relative to their front-to-back dimension. When a standard implant is sized to fit the front-to-back measurement in a female knee, the metal component may overhang the sides of the bone. One study found that nearly half of the women measured had more than two millimeters of overhang at the front of the femur with a standard implant, compared to just 2% of men.16PubMed. Gender differences in distal femoral morphology and the role of gender specific implants in total knee replacement: a prospective clinical study Overhang can irritate surrounding soft tissue and cause pain. Gender-specific implants are narrower to reduce this risk. Whether they lead to meaningfully better outcomes at the population level is still debated, but they represent one more way the procedure can be tailored to individual anatomy.
Mechanical Versus Kinematic Alignment
The way an implant is positioned in the leg matters as much as the implant itself. Two alignment philosophies dominate the conversation:
- Mechanical alignment: The traditional approach. The surgeon aims to create a straight line from the hip through the knee to the ankle, positioning components at right angles to that line. The goal is to distribute load evenly across the implant.
- Kinematic alignment: A newer philosophy. The surgeon positions the components to match the patient’s own pre-arthritic joint anatomy rather than forcing a neutral mechanical axis. The idea is that restoring the knee’s natural alignment produces more natural-feeling motion.
Early comparative data favor kinematic alignment in terms of patient satisfaction. One study found that kinematically aligned knees scored meaningfully better on multiple outcome measures at six months than mechanically aligned ones, and achieved about five degrees more flexion.17PubMed. Kinematically versus mechanically aligned total knee arthroplasty A broader review found that kinematic alignment has not been associated with higher complication rates or worse implant survival, and outcomes tend to be at least as good as mechanical alignment.18PubMed Central. Kinematic alignment in total knee arthroplasty Longer-term data are still accumulating, but the trend is clear enough that kinematic alignment has moved from experimental curiosity to mainstream practice in many centers.
Robotic-Assisted and Computer-Navigated Surgery
How the bone cuts are made represents yet another layer of variation. Traditional TKA uses manual cutting jigs, essentially metal guides that the surgeon pins to the bone before sawing. Computer-assisted navigation tracks the position of instruments in real time and gives the surgeon feedback on alignment during the procedure. Robotic systems go a step further by physically guiding or constraining the saw to execute a pre-planned cut path.
Robotic-assisted TKA consistently achieves more precise implant positioning and reduces the number of alignment outliers compared to conventional jig-based surgery.19PubMed Central. Robotic technology in total knee arthroplasty: a systematic review One real-world study found that femoral and tibial components deviated by less than one degree on average from the planned position.20PubMed Central. Real-world accuracy of robotic-assisted total knee arthroplasty and its impact on expedited recovery The technology appears especially valuable in difficult cases: in knees with severe varus or valgus deformity, robotic-assisted surgery produced significantly fewer alignment outliers, less pain, and better early functional scores than conventional surgery.21PubMed Central. Robotic-assisted total knee arthroplasty improves implant position and early functional recovery for the knee with severe varus/valgus deformity
Patient-specific cutting guides (PSCGs), made from preoperative CT or MRI scans and 3D-printed for each patient, represent a different technological path. They were designed to improve alignment without the cost of a robotic system. However, a Cochrane review found that PSCGs resulted in little to no difference in alignment precision compared to conventional instruments, and may actually perform worse than computer-assisted navigation for reducing alignment outliers.22PubMed Central. Patient-specific cutting guides for total knee arthroplasty A separate randomized trial confirmed that any alignment advantages of PSCGs over conventional instruments were very small and unlikely to be clinically meaningful.23PubMed. A prospective randomised controlled study of patient-specific cutting guides compared with conventional instrumentation in total knee replacement So while the idea is appealing, PSCGs have not lived up to their initial promise the way robotic platforms have.
Custom-Made Implants
Beyond patient-specific cutting guides, some companies now manufacture entirely custom implants designed from a scan of the individual patient’s knee. Rather than selecting the closest fit from a range of standard sizes, the implant itself is built to match the patient’s bone geometry. These have been commercially available in the United States since 2011 and in the United Kingdom since 2012. Published data associate custom implants with higher patient satisfaction, more accurate alignment, less bone removal, and better long-term survivorship compared to standard off-the-shelf components.24Orthopaedics and Trauma. Knee arthroplasty Off-the-shelf knee replacements versus robotics versus customized implants: the reality versus the hype The cost is higher, and availability is still limited to certain centers. Whether the benefits justify that cost for the average patient remains an open question, but for people with unusual anatomy or those who had a poor experience with a standard implant, customization is an increasingly real option.
Same-Day Discharge
The setting of the surgery itself has become another variable. Knee replacement was once a guaranteed multi-day hospital stay. Now, same-day (outpatient) joint replacement is growing rapidly. A systematic review found that roughly 95% of patients selected for same-day discharge went home as planned, with failures mostly due to pain, low blood pressure, or nausea. There were no deaths and only about 2% required reoperation within 90 days.25The Journal of Arthroplasty. The Shift to Same-Day Outpatient Joint Arthroplasty: A Systematic Review A meta-analysis comparing outpatient and inpatient joint replacement found no significant differences in complications, readmission rates, or patient-reported outcomes.26PubMed. Safety and efficacy of outpatient hip and knee arthroplasty: a systematic review with meta-analysis
Same-day discharge is not for everyone. Patient selection is critical: good candidates tend to be relatively healthy, motivated, have adequate support at home, and have no major medical comorbidities. One urban hospital’s outpatient program found that about 4% of patients needed to be converted to an inpatient stay, mostly because they did not clear physical therapy milestones on the day of surgery.27PubMed Central. Institution of same-day total joint replacement at an urban safety net hospital during the COVID-19 pandemic Still, the option exists and is safe for carefully selected patients, representing a significant shift in how knee replacements are delivered.
Knee Replacement in Younger Patients
Most knee replacements are performed on people in their sixties and seventies, but a growing number of younger patients are receiving them as well. The assumption has long been that younger, more active patients will wear out their implant faster. The evidence is more nuanced. A systematic review found that TKA provides clear functional improvements in younger patients but does see a moderate increase in implant failures into the second decade.28PubMed Central. What is the evidence for total knee arthroplasty in young patients?: a systematic review of the literature Thin polyethylene bearings (less than nine millimeters) have been flagged as a risk factor for failure in younger patients specifically.29PubMed Central. Total knee arthroplasty in young patients: Factors predictive of aseptic failure in the 2nd–4th decade
Interestingly, the link between youth and implant failure may not be as straightforward as “young people are more active.” A study that actually measured activity levels in younger TKA patients found that sustained high activity was not likely to be the main cause of revision in this group, and that using age as a stand-in for activity level is misleading.30PubMed Central. Are younger patients undergoing TKAs appropriately characterized as active? For younger patients, implant selection, bearing thickness, and surgical precision may matter more than activity restrictions.
Metal Allergy and Implant Materials
Standard knee implants are made from cobalt-chromium alloys, which contain trace amounts of nickel. For patients with known or suspected metal hypersensitivity, hypoallergenic alternatives exist. Oxidized zirconium (often marketed as Oxinium) is one such option, designed to reduce exposure to nickel and cobalt. However, one study found an unexpected result: synovial fluid in knees with Oxinium hypoallergenic implants actually had nickel levels more than three times higher than knees with standard cobalt-chromium implants, despite having much lower cobalt and chromium levels. Patient-reported outcomes were similar between the two groups.31Orthopaedic Proceedings. “NICKEL-FREE” OXINIUM HYPOALLERGENIC VERSUS STANDARD COBALT-CHROME CONTAINING TOTAL KNEE ARTHROPLASTY: IS THERE A DIFFERENCE IN SYNOVIAL METAL IONS AT MINIMUM TWO-YEAR FOLLOW-UP? This finding complicates the picture for patients choosing a hypoallergenic implant specifically because of nickel sensitivity, and the researchers explicitly cautioned against relying on that particular implant for nickel-allergic patients when used with manual instrumentation. The broader lesson is that “hypoallergenic” does not automatically mean free of all sensitizing metals, and patients with confirmed allergies should discuss the specific alloy composition with their surgeon.