Understanding knee replacement
There is more than one way to replace a knee.
Partial or total. Quad-sparing or parapatellar. Cruciate-retaining, posterior-stabilized, or medial-congruent. Mechanical or kinematic alignment. Robotic or conventional. None of these labels makes an operation automatically better — here is what the published evidence actually says about each choice.
The essential difference
How much of the knee needs to be resurfaced?
A knee has three compartments. A partial replacement treats one damaged compartment and preserves the healthy parts. A total replacement resurfaces the end of the thighbone and top of the shinbone across the knee; the kneecap surface may also be resurfaced.
Preserve what is still healthy
May fit when: arthritis is truly limited to one compartment, the important ligaments work, and deformity is limited and correctable.
- Less bone and soft tissue are removed.
- Recovery and return to activity may be faster.
- Motion may feel more natural and function may be slightly better.
- Early medical complications may be lower in selected patients.
- Only a minority of knees meet the requirements.
- Arthritis can progress in the untreated parts of the knee.
- Registry studies show a higher chance of later revision.
- Results depend heavily on selection and surgeon experience.
Treat arthritis across the knee
May fit when: arthritis affects more than one compartment, or there is substantial deformity, stiffness, or ligament damage.
- Works for a much wider range of arthritis patterns.
- Treats damaged surfaces throughout the knee at one operation.
- Has a long record of predictable pain relief.
- Large registries show a lower long-term revision rate.
- More bone and soft tissue are altered.
- Early recovery is usually slower than after a partial replacement.
- The knee may not feel as natural as a successful partial replacement.
- Stiffness, infection, blood clots, and other surgical risks still apply.
How common is each in the United States?
Total replacement is overwhelmingly more common.
AJRR reports that unicompartmental knee replacement—the most common type of partial replacement—has made up fewer than 5 of every 100 knee replacements since 2015. The registry analyzed 101,468 partial procedures performed from 2012 through 2024.
For scale, a recent Medicare-based analysis estimated about 1.01 million primary total knee replacements in 2022. These sources measure volume differently, so the figures should not be added together. The AJRR percentage specifically describes unicompartmental replacement and does not include every patellofemoral partial replacement.
What should you make of the evidence?
Selection and surgeon experience can change the comparison.
A 10-year randomized trial performed by experienced surgeons found similarly good clinical outcomes and similar reoperation and revision rates for partial and total replacement. In contrast, AJRR data for U.S. Medicare patients age 65 and older found higher cumulative revision after partial replacement over longer follow-up—4.55% versus 3.05% at 12 years. Both findings can be true: carefully selected patients treated by experienced partial-knee surgeons may do very well, while average registry results include broader practice patterns.
“Which compartments are damaged on my weight-bearing X-rays, are my ligaments suitable for a partial replacement, and how many partial and total knee replacements do you perform each year?”
The path into the knee
What happens to the quadriceps mechanism?
The skin incision can look similar with each approach. The important difference is the deeper opening used to move the kneecap and expose the joint. All approaches can be used to perform the same total knee replacement; the approach does not determine the implant, alignment philosophy, or technology.
Through the quadriceps tendon—beside the kneecap
What is opened: The arthrotomy begins in the medial portion of the quadriceps tendon, continues along the inner border of the kneecap, and extends into the capsule below it. The quadriceps muscle belly is not cut. The tendon and capsule are repaired at closure.
- Excellent, familiar exposure of the entire knee.
- Can be extended when the knee is stiff, deformed, or unexpectedly difficult.
- Reliable access for complex primary and revision procedures.
- Longest and broadest clinical track record.
- The quadriceps tendon must heal after being opened and repaired.
- May delay early quadriceps activation compared with some tendon-sparing approaches.
- May disturb more of the blood supply on the inner side of the kneecap.
Around or partly through the vastus medialis
The incision follows the fascia along the lower border of the vastus medialis. The muscle is lifted and retracted rather than split, preserving both its fibers and the quadriceps tendon.
The quadriceps tendon is preserved, but a short portion of the vastus medialis muscle fibers is split in line with the fibers to gain exposure.
- Preserves the quadriceps tendon.
- Subvastus also avoids splitting the quadriceps muscle fibers.
- Some trials show slightly less early pain, faster straight-leg raise, or earlier motion.
- May reduce the need for a lateral release of the kneecap.
- Exposure can be more limited and technically demanding.
- May be difficult in very muscular, obese, stiff, or severely deformed knees.
- Subvastus can take longer; forceful retraction may still stress soft tissues.
- The approach may need to be extended for safe visualization.
Where “Jiffy Knee” fits
A trademark is not a separate outcome category.
“Jiffy Knee” is a trademarked soft-tissue-preserving technique that keeps the vastus medialis and quadriceps tendon intact and uses a modified medial, subvastus-type path. Other surgeons may use different names for related muscle- or tendon-sparing modifications. Published trials generally compare anatomical approaches—subvastus, midvastus, or medial parapatellar—not marketing names. Ask exactly which tissues are opened, not only what the procedure is called.
What the literature says
No approach has shown a consistent long-term advantage.
A 2025 systematic review of 27 randomized trials found that no approach consistently improved range of motion, patient-reported outcomes, or reoperation rates. A network meta-analysis of 60 randomized trials likewise found no functional difference among approaches in the short or medium term. Subvastus and midvastus approaches may offer small early benefits in pain, straight-leg raise, or motion, but the best evidence shows that these differences generally fade. Safe exposure, accurate implant placement, and the surgeon’s experience with the chosen approach matter more than the label.
“Which anatomical approach will you use, which tendon or muscle fibers are opened, why does it fit my knee, how often do you use it, and would you extend the exposure if needed to place and balance the components safely?”
Three common articulation strategies
What keeps the knee stable as it bends?
All three use metal components with a plastic insert between them. The key difference is how the design manages front-to-back stability and guides motion after the anterior cruciate ligament has been removed.
The PCL helps guide motion
What it means: The posterior cruciate ligament—or PCL—is preserved and balanced. “Cruciate-retaining” does not mean both cruciate ligaments are retained; the ACL is removed in standard total knee replacement.
- Preserves the functioning PCL and avoids a central post-and-cam.
- Usually requires less bone removal from the femur than a traditional PS design.
- Long clinical history and favorable registry survivorship.
- Requires a healthy, usable PCL and careful ligament balancing.
- A PCL that is too tight or too loose can affect motion or stability.
- Not automatically more natural-feeling or better for every patient.
Often considered when: the PCL is healthy and the surgeon can balance it reliably.
A post-and-cam substitutes for the PCL
What it means: The PCL is removed. A raised plastic post engages a cam on the femoral component during bending to help provide stability and guide femoral rollback.
- Does not depend on a functional PCL.
- Can make balancing more predictable when the PCL is absent, damaged, or difficult to balance.
- May provide slightly greater flexion in some studies, without a clear satisfaction advantage.
- Traditional PS designs remove additional femoral bone for the cam box.
- The post-and-cam introduces design-specific risks such as post wear, fracture, or patellar clunk.
- U.S. registry data show somewhat higher revision risk than CR in older patients.
Often considered when: the PCL cannot be retained or reliable flexion stability requires substitution.
Insert shape creates a stable medial pivot
What it means: The plastic insert is more conforming on the inner side of the knee. This supplies front-to-back stability medially while allowing more motion laterally—typically without a PS post-and-cam.
- Can provide stability without cutting a PS box into the femur.
- Aims to reproduce a stable medial side with lateral rotation during bending.
- May be used with the PCL retained or sacrificed, depending on the implant and balance.
- “Medial congruent” and “medial pivot” are related ideas but not interchangeable registry categories.
- The increased conformity changes load transfer, so positioning and balance still matter.
- Long-term comparative evidence is less mature than for traditional CR and PS designs.
Often considered when: the surgeon wants medial stability without a traditional PS mechanism.
How these designs are being used in the United States
Medial-congruent use has risen quickly.
In the 2025 AJRR report, medial-congruent inserts represented 42.9% of reported primary total knee designs in 2024, posterior-stabilized 34.1%, and cruciate-retaining 6.6%. The registry reports medial-pivot designs separately at 3.9%.
Utilization describes what surgeons used—not which design is best. These categories are based on reported implant data and do not capture every design nuance.
What the evidence supports
No articulation is universally superior.
Comparative studies generally find similar pain, function, range of motion, and satisfaction across well-balanced designs. AJRR’s observational Medicare analysis found lower adjusted revision risk for CR and medial-congruent designs than PS through the available follow-up, but registry comparisons can reflect patient selection, implant mix, surgeon technique, and time in use—not only the articulation itself.
“Is my PCL healthy enough to retain? Which articulation do you recommend, what provides stability in that design, and what result do you expect it to improve for me?”
Two different targets
Should the replacement straighten every knee the same way—or recreate your knee?
Alignment describes how the surgeon positions the implants and balances the ligaments. Mechanical and kinematic alignment are different plans for doing that. Neither name describes the implant brand, incision, or whether a robot is used.
Aim for a neutral mechanical axis
The target: A straight load-bearing line from the center of the hip, through the knee, to the center of the ankle. The femoral and tibial components are generally placed perpendicular to their mechanical axes.
Bone cuts follow standardized neutral targets. Ligament releases or adjustments may be used to create stable, usually rectangular spaces as the knee bends.
- Long clinical history and the deepest body of survivorship data.
- Standardized, reproducible targets that avoid extreme implant positions.
- Familiar to most knee-replacement surgeons.
- Not every person had a naturally neutral leg before arthritis.
- Changing the native joint-line angle can alter ligament tension and knee motion.
- More soft-tissue release may be needed in some knees.
Aim for your pre-arthritic joint lines
The target: Resurface the knee so implant thickness and position reproduce the joint-line angles and rotational axes you likely had before cartilage and bone were worn away.
The surgeon uses measured resections and accounts for cartilage and bone loss, aiming to restore the knee’s native ligament tension and reduce the need for releases.
- Preserves more of the patient’s individual joint-line orientation.
- May require fewer ligament releases and can improve intraoperative balance.
- Some studies report small early gains in function or motion.
- Can place the limb or components outside traditional neutral targets.
- Extreme anatomy may raise concern about load, wear, or loosening.
- Accurate reconstruction and surgeon experience are especially important.
A middle path
Restricted kinematic alignment: personalize within guardrails
Restricted kinematic alignment tries to reproduce your anatomy when it falls within defined boundaries. If your natural or arthritic alignment is too far outside those limits, the plan is moved partway toward neutral. The exact boundaries and balancing method vary, so ask your surgeon what “restricted” means in their practice.
Do not confuse the plan with the tool
Alignment is the plan. Robotics and navigation are tools.
A robot, navigation system, sensor, or patient-specific guide can help measure and execute either philosophy. Technology may improve the accuracy of the chosen target, but it does not prove that the target itself is best for you.
What the literature says
No alignment philosophy is universally superior.
Recent randomized-trial reviews reach mixed conclusions. Some report modest early improvements in function or flexion with kinematic alignment; others find the differences too small to matter clinically or identify problems in how studies define and combine techniques. A 10-year randomized trial found no meaningful difference in patient scores, revision, or radiographic loosening. Ten-year restricted-kinematic case-series results are reassuring, but they do not replace larger randomized or registry comparisons. Mechanical alignment therefore remains a proven standard, while kinematic and restricted kinematic alignment are reasonable options in experienced hands.
“Are you recommending mechanical, kinematic, or restricted kinematic alignment for me? What anatomy are you trying to restore, what limits will you use, how will you check ligament balance, and how often do you perform this technique?”
Two very different technologies
One helps perform the operation. The other helps observe recovery.
Robotic assistance is used in the operating room to plan and guide bone preparation. A smart implant contains sensors that can transmit measurements after you go home. Neither technology decides whether you need surgery, replaces an experienced surgeon, or guarantees a better result.
A more precise way to execute the plan
What it does: Uses imaging or anatomical mapping, computer planning, and a robotic cutting guide or arm to help position the components and assess ligament balance. The surgeon remains in control.
- More accurate bone cuts and component positioning.
- Fewer radiographic alignment outliers.
- Useful measurements for personalized alignment and soft-tissue balance.
- Possible small early differences in pain or function in some studies.
- Consistently better pain relief, function, or satisfaction.
- Lower revision risk or longer implant survival.
- A clinically meaningful advantage for every patient.
Tradeoffs: Added equipment, cost, operating time or learning curve, and—depending on the system—small pin sites with uncommon pin-related complications.
Objective recovery measurements from inside the knee
What it does: An implantable sensor in a tibial stem extension records movement with accelerometers and gyroscopes. Current systems can report measures such as steps, walking speed, cadence, stride length, distance, and functional knee motion.
- Passive, day-to-day information between office visits.
- An objective view of recovery that complements how you report pain and function.
- Earlier recognition of an unusual recovery pattern may be possible.
- May help a care team personalize follow-up or rehabilitation.
- That choosing a smart implant improves pain, function, or implant survival.
- That an alert reliably diagnoses infection, loosening, stiffness, or another complication.
- Which measurements should trigger intervention and whether intervention changes the outcome.
Tradeoffs: Additional implanted electronics and a stem extension, a compatible home data system, privacy and data-sharing choices, and a much smaller evidence base than standard knee implants.
What the robotics literature says
More precise does not yet mean a better long-term result.
Randomized trials and contemporary meta-analyses consistently show that robotic assistance can reduce alignment outliers. However, improvements in patient-reported scores are absent, small, inconsistent, or below thresholds patients are likely to notice. A randomized trial with mean 13-year follow-up found no difference in function, loosening, complications, or 15-year survivorship. A 2025 survivorship meta-analysis likewise found no significant difference in implant survival, complications, or postoperative pain. Robotics is therefore a potentially useful execution tool—not evidence by itself that one surgeon or operation is better.
What the smart-implant literature says
Better measurement is promising; better outcomes remain unproven.
The FDA authorized the first implantable post-surgical knee-motion measurement device in 2021. Early studies show that it can collect daily gait measurements with high data availability. Those objective measurements correlate only weakly with patient-reported improvement, suggesting that the two describe different parts of recovery. A small case series showed how changing gait data might help identify patients with stiffness, but larger prospective trials have not yet shown that smart-implant monitoring itself improves treatment decisions or final outcomes.
“What will this technology change in my operation or recovery? Which patient outcome—not just an X-ray measurement or data point—has it been shown to improve? What extra risks, costs, follow-up, and data sharing come with it?”
Other choices your surgeon will explain
Fixation
Knee components may be cemented, cementless, or hybrid.
Bone quality, implant design, activity, and the surgeon’s results with the system should drive the choice.
Surgery location
Knee replacement is performed both in ambulatory surgery centers and in hospitals. Compare the two settings — including safety findings, candidacy, and Medicare cost differences.
Keep reading
Wondering whether joint replacement fits your situation? Take the free TJV Fit Score — a cited, methodology-first assessment.