Cemented vs. Cementless Knee Replacement
There are two ways to hold a knee replacement to bone. Cement is the long-term gold standard: it grips immediately and works in any bone quality. Cementless relies on your own bone growing into a porous surface over the following weeks. Long-term results are essentially equal — what differs is who each one suits, and that comes down to your bone.
Key takeaways
- Cement is the long-term gold standard — durable, predictable, and it works in any bone quality.
- Bone cement is PMMA, which interlocks with the cancellous bone rather than gluing to it.
- Cementless implants have a porous titanium surface your bone grows into over the following weeks.
- A cementless knee is still solidly fixed at the end of the operation — the press-fit tolerance is very tight.
- Long-term survivorship and patient-reported outcomes are essentially equal between the two.
- Cementless suits younger patients with good bone; there is some evidence higher-BMI patients do well with it.
- Osteoporosis, soft bone, or bone defects mean cement — it relies on the cement, not on your bone.
- Some of this decision is only made once I can see and feel the bone during the operation.
The short answer
A knee replacement has to be held to your bone somehow, and there are two ways to do it. Cement is the long-term gold standard and works in any bone quality. Cementless relies on your own bone growing into the implant.
Long-term results between them are essentially equal. Both have a role in my practice, and I don't think either is better in the abstract. The choice is about your bone, not about which sounds more advanced.
How cemented fixation works
Bone cement is PMMA — polymethyl methacrylate. The name is misleading, because it doesn't work like glue. It's pressed into the cancellous bone, the porous inner bone exposed at the cut surface, where it flows into the spaces and hardens.
What holds the implant is that mechanical interlock — the cement and the bone structure are physically interdigitated. And because it sets during the operation, fixation is solid from the moment I finish. Nothing has to happen afterwards for the implant to be held.
Why cement is still the standard
Cemented knee replacement has decades of outstanding results behind it. It is durable, it is predictable, and critically it works in every patient — including bone that is thin, soft, or osteoporotic. For most patients there is a high likelihood of a cemented knee lasting a lifetime.
How cementless fixation works
A cementless implant has a porous titanium surface on the side that faces bone — on the femoral component, the tibial component, and the patella if one is used — along with pegs that are press-fit into prepared bone.
There are two stages to how it holds:
- Immediately — the press-fit tolerance is deliberately very tight, so the implant is mechanically stable the moment it is seated
- Over the following weeks — your own bone grows into and interdigitates with the porous titanium, creating a true biologic bond
This is worth being clear about, because patients often assume a cementless knee is somehow loose until the bone grows in. It isn't. It is firmly fixed at the end of the operation. What develops over time is a second, biologic layer of fixation on top of that.
What the results actually show
When you compare the two across large numbers of patients — both survivorship (how long implants last before needing revision) and patient-reported outcomes (how people say their knee actually feels and functions) — the results are considerably equal.
There is no meaningful functional difference. A patient with a cemented knee and a patient with a cementless knee should expect the same recovery and the same result. If someone tells you one type of fixation will make your knee feel better, that claim isn't supported.
When I use cementless
The profile that leans cementless:
- Under roughly 65 years of age
- Good bone stock with a healthy cortical index on imaging
- Higher BMI with good bone — there is some evidence these patients do well with cementless, likely because greater load through the implant stimulates the bone to remodel around it
Bone quality is the real criterion. Because bone density tends to be higher in men on average, men more often meet it — but it's the bone I'm assessing, not the patient's sex.
Being honest about the young-patient argument
The reason cementless is attractive in younger patients is a theoretical one: a living bond between bone and implant might be expected to outlast a cement mantle across several decades. That advantage has not been shown in the literature. It is a reasonable argument and it is why I consider cementless in younger patients — but it is not a proven benefit, and I'd rather tell you that than sell it to you.
When I cement
Cementless fixation depends entirely on your bone being able to grow into and hold the implant. When the bone can't be relied on for that, cement is the answer — because cement relies on itself for fixation, not on the quality of your bone.
- Osteoporosis or clearly soft bone
- Older patients — past about 65 to 70, cementless is not wrong, but cement offers more durable fixation with a slightly lower revision rate
- Bone defects, or a compromised metaphysis where a press-fit would not hold
- Any situation where I have a concern about achieving reliable fixation
If your bone density is already a known issue, it is worth reading bone density and joint replacement — it affects more of the plan than just fixation.
The part of this decided during surgery
Some of this can be planned in advance from your imaging — cortical thickness, any history of low bone density, a DEXA scan if you have had one. But part of the assessment can only happen once the bone is exposed.
Two things I look at directly:
- The trabeculae — the internal bone struts. If they are widely spaced, the bone will not grip a press-fit well
- Whether the bone is compressible under my finger. Bone that gives is bone that will not hold an implant by press-fit alone
If I find that, I cement — regardless of what the plan was. That is not a complication or a change of mind; it is the assessment being finished with better information than an X-ray can give.
Cost and operating time
These differ slightly, and neither should drive your decision — but you may hear about them, so here is the honest picture.
- A cementless implant typically costs more than its cemented equivalent
- But cementless uses no cement, and the operation is somewhat faster because there is no waiting for cement to cure
- A cemented knee takes a little longer in the operating room
These roughly offset, and the differences are small relative to the overall cost of the procedure. More detail on what drives the total is in how much a knee replacement costs.
What about hips?
Hips work differently, and it is worth not carrying assumptions across.
A modern hip replacement is usually cementless — a press-fit triple-taper titanium stem in the femur with a porous-coated socket in the pelvis. The shape of the femoral canal suits press-fit fixation particularly well, so it is the default rather than the exception. Cemented stems are reserved mainly for patients with very low bone density.
More on how those components are selected and sized in how hip replacement implants are chosen and sized.
Frequently asked questions
Is a cemented or cementless knee replacement better?
Neither is better. They are two ways of solving the same problem, and long-term survivorship and patient-reported outcomes are essentially equal between them. Both have a role in my practice. What differs is who each one suits — cement works in any bone quality, while cementless depends on your bone being healthy enough to grow into the implant. The honest answer is that the right choice is the one matched to your bone, not the one that sounds more modern.
What is bone cement made of?
PMMA — polymethyl methacrylate. It is worth understanding that it does not work like glue. It is pressed into the cancellous bone, the porous inner bone at the cut surface, where it interlocks mechanically with the bone structure. That interlock is what holds the implant, and it is solid from the moment the operation is finished.
How does a cementless knee replacement stay in place?
The back of the implant has a porous titanium surface, and pegs that are press-fit into prepared bone. The fit is deliberately very tight, so the implant is stable straight away. Over the following weeks your own bone grows into and interdigitates with that porous surface, creating a biologic bond. So there are two phases: an immediate mechanical press-fit, then true bone ingrowth that develops over time.
Who is a good candidate for a cementless knee replacement?
Broadly: younger patients — under about 65 — with good bone stock and a healthy cortical index. Bone quality is the real criterion, and because bone density tends to be higher in men on average, men more often meet it. There is also some evidence that patients with a higher BMI do well with cementless fixation, likely because greater load through the implant stimulates the bone to remodel around it.
Does osteoporosis mean I need a cemented knee replacement?
In almost all cases, yes. Cementless fixation depends on your bone being able to grow into the implant and hold it, so osteoporosis, soft bone, bone defects, or a compromised metaphysis all point clearly to cement. Cement relies on the cement itself for fixation rather than on the quality of your bone, which is exactly why it is the right answer when bone quality is the problem.
Do cementless knee replacements last longer?
There is a theoretical argument that a biologic bond between bone and implant should outlast a cement mantle over several decades, which is why it is attractive in younger patients. I want to be straight with you: that advantage has not been demonstrated in the literature. Cemented knees have an outstanding long-term record, and for most patients there is a high likelihood of the implant lasting a lifetime.
Is a hip replacement cemented or cementless?
Hips are different. A modern hip replacement is usually cementless — a press-fit triple-taper titanium stem with a porous-coated socket — because the shape of the femoral canal lends itself well to press-fit fixation. A cemented stem is reserved mainly for patients with very low bone density, where the bone cannot be relied on for initial stability.
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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