How does a knee replacement get aligned, and does it matter?

A knee replacement takes out worn surfaces and puts new ones in. Where exactly those new surfaces sit is not obvious, and it is not automatic. Somebody decides.

That decision is called alignment. It is made before the operation, not discovered during it. It is also something you will never be able to check yourself. You cannot feel a degree, and no X-ray you are handed will tell you whether the plan was the right one.

So it is worth understanding. It is one of the few genuinely important things about your operation that you cannot see for yourself.

What is actually being decided?

Two things, mostly.

How straight your leg is, seen from the front. Will it end up perfectly straight, or keep some of the bow or knock-knee it had before? Most people know their own legs are not perfectly straight. Most people are right.

How the parts are tilted, seen from the side. How far forward the thigh-bone piece is tilted, and how much the top of the shin bone slopes backward. Patients almost never hear about this one. It matters a great deal, because it changes how your ligaments tighten and loosen as the knee bends.

Alignment strategies are just different answers to what those numbers should be, and where the answer should come from.

What is mechanical alignment?

Setting every leg perfectly straight.

The target is a straight line from the center of the hip, through the center of the knee, to the center of the ankle. Every knee gets the same target, no matter what that leg looked like before arthritis.

The appeal is durability and consistency. Load spreads evenly across the implant, which is what the parts were designed for. The technique is also teachable and repeatable. The long track records held in national joint registries were built on knees aligned this way.

That record deserves one honest qualification. Registries do not record which alignment strategy was used. So what they show is a large body of successful knee replacements from an era when mechanical alignment was the norm. It is a strong starting point, not a head-to-head result.

The known limitation is simple. Many people never had a straight leg to begin with. Straightening them changes the shape their ligaments grew up around, so those ligaments then have to be released and balanced to fit the new shape.

What is restricted kinematic alignment?

Restoring the shape your knee had before the arthritis, within safe limits.

Rather than making every leg straight, this strategy aims to rebuild the knee’s own original geometry. The reasoning is that your ligaments developed around that shape and should balance more naturally against it.

The important word is restricted. Following the original shape wherever it led could put the implant under loads it was never designed for. So limits are set, usually a few degrees either side of straight. If a knee’s natural shape falls outside those limits, it is brought back inside, or aligned mechanically instead.

It is a rule, in other words, and it is based on measurements taken from imaging. That means it can be done by hand with conventional instruments. Dr. Patel does it that way.

What is personalized alignment?

Building the plan around your specific knee rather than around a rule.

Instead of working to a set boundary, the surgeon plans from your knee’s own geometry and from how your soft tissues actually behave when tested during the operation.

Both restricted kinematic and personalized alignment are individualized. The difference is how much of the plan comes from a rule, and how much comes from the knee in front of the surgeon.

Dr. Patel performs personalized alignment with robotic-arm assistance on the Mako System. His reason is the side-to-side geometry described earlier: how far the thigh-bone piece is tilted forward, and how much the shin bone slopes. Those cuts can be made by hand, and routinely are.

His clinical judgment is that hand-made cuts vary more than he is willing to accept when the whole plan depends on them. A CT-based plan and robotic-arm assistance hold those cuts closer to the target. That is his stated reason for using the technology here. It is his view, not settled consensus, and it should be read that way.

The robotic arm does not perform the operation. He builds the plan, controls the arm, changes the plan when the knee calls for it, and sets the assistance aside when it is not the right tool. More on that: what the robot actually does in a knee replacement.

Which one is better?

Nobody knows yet. Here is what the studies actually found.

A 2025 analysis combined 6 randomized trials covering 444 patients, comparing restricted kinematic against mechanical alignment. Two function scores came out higher in the kinematic group. Three other patient-rated scores showed no difference, and neither did the two X-ray measurements of alignment. Revision rates showed no significant difference either.

Six trials and 444 patients is a small evidence base for a question this important. A result that shows up on two measures and not on five others is a result to hold loosely.

A 2024 randomized trial of 98 patients found something more interesting than an overall winner. It asked how often patients simply forgot they had an artificial knee. At one year, the kinematic group scored about 10 points higher on a 100-point scale.

But then the researchers separated patients by the natural shape of their knee. Almost the entire difference came from the naturally bow-legged patients. Among patients whose knees were already close to straight, the two strategies produced no meaningful difference at all.

If that holds up, it changes the question. It stops being “which strategy is better” and becomes “which knees, if any, are affected by the choice.” That trial used a single implant design, and 98 patients is not enough to settle a question about a subgroup. It needs to be repeated before anyone builds a practice on it.

On durability, a 2025 study followed 104 kinematically aligned knees for an average of about eleven years. Ninety-nine out of every 100 were still in place, and about 95 in 100 patients were satisfied or very satisfied. That is reassuring against the specific worry that a non-straight alignment might wear out sooner.

It is also a study with no comparison group, which its own authors classify as the weakest tier of evidence. It can tell you those knees did well. It cannot tell you they did better than anything.

What does this mean for you?

No alignment strategy is offered here as better than another, because the evidence does not support that claim in either direction.

What can be said is narrower and more useful. The alignment for your knee is chosen deliberately, from your imaging and your anatomy, rather than defaulting to whatever the instruments make easiest.

When is mechanical alignment the right choice?

Often. It remains the right call for a large share of knees.

A leg that has bowed or angled well outside the safe limits points toward it. So does significant deformity, previous surgery that changed the anatomy, or ligaments that will not balance to the knee’s original shape. Choosing mechanical alignment is not a downgrade.

The plan also gives way when the knee disagrees with it. If the joint does not behave as planned during the operation, the plan changes. That is judgment being used, not a setback.

Worth asking at the visit

• Which alignment strategy are you planning for my knee, and what about my anatomy led you there?
• Would my knee fall inside or outside the limits for restricted kinematic alignment?
• Does my plan need robotic assistance, and what would change without it?
• What would make you change the alignment plan during the operation?
• What is actually known about long-term results for the strategy you are proposing?

More depth on how these strategies are used here: kinematic and personalized alignment. And on the operation itself: total knee replacement.