How Your Knee Replacement Implant Is Chosen
Most people picture the knee as a hinge. It is not — as the knee bends it also rotates, around a pivot point on the inner side of the joint. That single fact drives almost every implant decision I make. This is why I use a medial pivot design, why I keep the PCL rather than removing it, and how size, rotation and ligament balance are planned for your particular knee.
Key takeaways
- The knee is not a simple hinge — it rotates around a pivot point on the inner (medial) side as it bends.
- A medial pivot implant is shaped to reproduce that motion, cupping the inner femoral condyle for stability.
- It is a cruciate-retaining design, meaning the PCL is kept rather than removed.
- The PCL contributes to proprioception — your sense of where your knee is in space.
- A posterior-stabilized knee removes the PCL and requires cutting a box out of the end of the femur.
- No study proves a medial pivot knee feels better — but it removes less bone and produces more consistent rollback.
- More constraint is added only when the ligaments genuinely require it.
- Size, rotation, alignment and plastic thickness are planned before surgery and confirmed against ligament tension during it.
The short answer
Most knee implant decisions come down to one question: how closely can this implant reproduce the way your knee already moves? That is what kinematics means, and it is the thing I care about most.
For the large majority of my patients that means a medial pivot design — an implant shaped around the rotation your knee actually performs, which keeps your PCL and removes less bone than the alternative.
Your knee is not a hinge
Almost everyone — including a lot of diagrams in waiting rooms — pictures the knee as a hinge that opens and closes in one plane. That is not how it works.
As your knee bends, it also rotates. The rotation is not centerd in the middle of the joint; it happens around a pivot point on the inner (medial) side. The outer side travels further, the inner side stays relatively fixed, and the whole joint turns slightly as it flexes.
This matters because an implant that only hinges is reproducing about half of what your knee does. Everything below follows from taking that rotation seriously.
What a medial pivot implant does
A medial pivot knee is built around that motion. The plastic surface on the inner side is deeper and cupped — it holds the inner femoral condyle in a stable, contained position — while the outer side is shaped to let the femur travel and rotate through the arc.
- The cupped medial side gives the knee a stable center to rotate around
- The lateral side is free to move, reproducing the rotation your knee already performs
- The result is a more consistent, more predictable pattern of motion through flexion
- That stability is what patients tend to describe when they say a knee feels dependable under load
In my experience that combination — stable on the inside, free to rotate on the outside — is the best all-around design for people who want to stay active. It is what I use for my athletes and for the weekend warriors who intend to get back to everything.
What we know, and what we don't
I would rather be straight with you about the evidence than sell you something.
What is not proven
There is no study that proves a medial pivot knee feels better to patients than a posterior-stabilized knee. How a knee feels is genuinely difficult to measure, and anyone who tells you the question is settled is going beyond the evidence.
What we do know is more concrete:
- It removes less bone — there is no box cut out of the end of the femur
- It preserves the PCL rather than sacrificing it
- Fluoroscopic motion studies show more consistent femoral rollback — the knee moves the same way, reliably, through the arc
- The cupped medial surface provides more inherent stability than a flatter plastic insert
Those are design facts, not marketing claims, and they are the honest basis for the choice. Patients do frequently tell me the knee feels natural and that they feel secure being athletic on it — but I offer that as what I see in clinic, not as proof.
Why I keep the PCL
The posterior cruciate ligament sits inside your knee, and it does more than restrain motion. It contains nerve endings that contribute to proprioception — your sense of where your knee is and what it is doing without looking at it.
A medial pivot knee is a cruciate-retaining design, so that ligament stays. The alternative, a posterior-stabilized knee, removes the PCL and replaces its function with a mechanical post and cam built into the implant. Fitting that mechanism requires cutting a box out of the end of the femur.
If the ligament is there and working, I would rather keep it than remove it and rebuild its job out of plastic and metal. That is the whole argument, and it is why a posterior-stabilized knee is not my routine choice.
Where standard cruciate-retaining fits in
A conventional cruciate-retaining knee was trying to do what a medial pivot knee now does — keep the PCL and let the knee rotate. But its plastic surface is flatter, so it gives the joint less inherent stability. A medial pivot design is the next generation of that idea: the same ligament-preserving principle, with a shape that actually guides the rotation instead of just permitting it.
When more constraint is needed
Retaining the PCL only works if the ligaments can do their job. When they cannot, the implant has to take over more of the stability, and there is a ladder for that.
- Medial pivot (cruciate-retaining) — my default, for knees with functioning ligaments
- Constrained posterior-stabilized — for significant laxity, hypermobility, or instability a retaining design cannot control
- Constrained condylar — for more severe ligament insufficiency
- Rotating hinge — reserved for severe revision situations, not primary knee replacement
The principle is to use the least constraint that will give you a stable knee. More constraint transfers more force into the bond between implant and bone, so it is not something to add for its own sake. Where a knee has already failed and needs redoing, the calculation changes — I have written about that in revision knee replacement.
Alignment and balance
There is a long-running debate in knee replacement between mechanical alignment — restoring a neutral, textbook axis — and kinematic alignment, which aims to reproduce the alignment you personally had before arthritis.
I do not think either extreme is right. I use a hybrid: mechanical resection as the framework, with kinematic adjustment built into it. If a knee sits in significant varus, I may accept some of that rather than forcing it fully neutral — how much depends on how the gaps measure and how the soft tissues tension.
Which brings up the part that decides how the knee actually feels:
Gap balancing
A knee replacement has to be balanced both straight and bent. The space between the bones — the gap the implant fills — needs to be even in extension and in flexion, with the ligaments under appropriate tension in both. That comes from the combination of where the bone cuts are made, what size components are used, and how they are rotated. I measure those gaps and check the tension rather than assuming the cuts alone got it right.
A knee that is tight in flexion feels stiff on stairs. A knee that is loose feels unreliable. Balance is what sits between those, and it is surgical work, not something an implant provides on its own.
What is planned before surgery
As with the hip, the operation is planned on a screen before it happens. Computer templating on your X-rays works out:
- Tibial component size, rotation, and position on the bone
- Femoral component size — including whether a narrow or standard shape fits your anatomy better
- Femoral rotation, which drives how even the flexion gap will be
- The thickness of the polyethylene insert that sets the final tension
- Whether the patella needs resurfacing, and what size if so
The point of planning it in advance is that everything is verified against your own anatomy in the operating room. The plan tells me what to expect; the ligament tension tells me whether it was right.
The same principles applied to the hip are covered in how hip replacement implants are chosen and sized, and where robotic assistance does and does not change this is in robotic knee replacement.
What to ask your surgeon
If you are being offered a knee replacement, this is a reasonable question to ask, and any surgeon should be glad to answer it:
The question
Are you planning a posterior-stabilized knee for me — and if so, why?
There are legitimate answers. Some knees genuinely need that stability, and a good surgeon will be able to tell you exactly which feature of your knee led to the decision. What you are really finding out is whether the choice was made for your knee specifically, or by default.
If you would like a second view on what is being planned for you, a second opinion is a normal and reasonable thing to seek before an elective operation. Send your X-rays ahead and the conversation will be far more useful.
Frequently asked questions
What is a medial pivot knee replacement?
A medial pivot knee is an implant design shaped to reproduce how a natural knee actually moves. Your knee does not simply hinge — as it bends, it rotates around a pivot point on the inner side of the joint. A medial pivot implant has a deeper, cupped surface on that inner side that holds the femur in a stable position while allowing the outer side to rotate through the arc. The intended result is a knee that moves and feels closer to a natural one.
Is a medial pivot knee better than a posterior-stabilized knee?
I want to be honest about the evidence here. There is no study proving that a medial pivot knee feels better to patients than a posterior-stabilized knee — feel is very difficult to measure. What we do know is that a medial pivot design removes less bone from the femur, keeps the PCL rather than sacrificing it, and produces more consistent femoral rollback on fluoroscopic motion studies. Those are the reasons I use it, and I think it is fair to say that rather than overstate the case.
What is a posterior-stabilized (PS) knee?
A posterior-stabilized knee removes the PCL and replaces its function with a mechanical post-and-cam built into the implant. To fit that mechanism, a box is cut out of the end of the femur, which removes more bone than a cruciate-retaining design. PS knees have a long track record and are still widely used. I reserve that level of built-in stability for knees whose ligaments genuinely need it rather than using it routinely.
Why does keeping the PCL matter?
The posterior cruciate ligament sits inside the knee and contains nerve endings that contribute to proprioception — your sense of where your joint is in space and what it is doing. If the ligament is intact and working, my preference is to keep it and choose an implant designed to work with it, rather than remove it and rebuild its job out of plastic and metal.
How do you decide how much stability my implant needs?
It is assessed for each knee. Most knees do well with a medial pivot design, which retains the PCL. If a patient has significant ligament laxity, hypermobility, or instability that a retaining design cannot control, I move up to a constrained implant — a constrained posterior-stabilized or constrained condylar design. A rotating hinge is reserved for severe revision situations, not primary knee replacement.
What alignment technique do you use for knee replacement?
A hybrid. I use mechanical resection as the framework, with kinematic adjustment built into it. If a knee sits in significant varus, I may accept a small amount of that alignment rather than force it fully neutral — the decision depends on how the gaps measure and how the ligaments tension. I measure the flexion and extension gaps and check tension rather than relying on the cuts alone.
Can you review my knee X-rays before I come in?
Yes. Sending X-rays in advance lets us do preliminary planning and makes a first appointment — or a second opinion from out of town — considerably more useful than a conversation without images. It also lets you find out what design is being planned for you and why, which is worth knowing before you commit to an operation.
This article is for general education and is not a substitute for personalized medical advice. Recovery timelines vary by patient, procedure, medical history, and surgeon-specific protocol. Please consult Matthew Harb, M.D. about your specific condition.
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