The traditional approach: mechanical alignment
For decades, the standard technique for total knee replacement has been mechanical alignment. This method positions the implant so that the leg's mechanical axis, an imaginary line from the centre of the hip to the centre of the ankle, runs straight through the centre of the knee, regardless of what the patient's natural alignment looked like before arthritis set in.
Mechanical alignment has a long track record and remains a well-supported, widely used technique.
The kinematic alignment approach
Kinematic alignment instead aims to recreate the position and orientation of the joint line, and the natural axes the knee rotated and bent around, before arthritis changed its shape. In practice, this means the surgeon plans the bone cuts around the patient's own pre-arthritic anatomy rather than a fixed universal target.
The reasoning behind this approach is that every knee is naturally slightly different, most people don't have a perfectly "neutral" leg alignment to begin with, so a replacement that respects that individual anatomy may feel more natural to move through.
Professor Rodda explains kinematic alignment in detail.
What the evidence currently shows
Kinematic alignment is a genuinely evolving area of knee replacement, not a settled question. Some studies report comparable or improved patient-reported comfort and a more natural feel with kinematic alignment; other studies show outcomes broadly similar to mechanical alignment. Long-term data, particularly on implant survival over 15 to 20+ years, is still accumulating, since mechanical alignment has a much longer track record to compare against.
Neither approach is right for every patient. Significant deformity, ligament laxity, or other individual factors can affect which alignment strategy is appropriate, and that decision is made on a case-by-case basis after examination and imaging.
Detailed clinical information
A longer discussion of the two alignment philosophies, the trial evidence and where the long-term data runs out. Not needed to understand the approach above.
Two ways to line up a knee replacement.
Almost all of the public conversation about knee replacement is about the implant and the robot. The decision that arguably matters more is where the implant is put, and there are two philosophies about that.
Mechanical alignment is the traditional approach and it has been the default since the 1970s. Every knee is cut so that the hip, the knee and the ankle line up on a straight vertical axis, whatever shape the leg was before. It is reproducible, it is easy to teach, and the implant survivorship data behind it runs for decades.
Kinematic alignment starts from a different premise: that the knee had a shape before it wore out, and that restoring that shape gives a joint that moves the way the patient's brain and muscles already expect. The cuts are planned to reproduce the pre-arthritic joint surfaces rather than to produce a standard axis.
Why this is not a trivial distinction.
Most naturally occurring knees are not neutral. A substantial proportion of people who have never had knee trouble in their lives are two or three degrees away from a straight mechanical axis, and always have been. Mechanical alignment corrects those people to neutral, which means the operation changes the shape of a leg that was working perfectly well until the cartilage wore out.
That correction has to be paid for somewhere, usually by releasing ligaments to make the knee balance in its new position. Kinematic alignment aims to avoid that trade by leaving the ligaments the length they already are and putting the joint surfaces back where they were.
The clinical argument is that this is part of why a proportion of knee replacements feel mechanically sound and still do not feel like the patient's own knee. It is not the whole answer to that problem and it should not be sold as one.
What the evidence actually shows.
This is where the marketing tends to outrun the data, so it is worth stating carefully.
Randomised trials comparing kinematic with mechanical alignment have generally found equivalent or modestly better patient-reported outcomes for kinematic alignment in the first two to five years. Differences, where they reach significance, tend to be small and to sit around or just above the threshold at which a patient would notice them.
What does not yet exist is mature long-term survivorship data. Mechanical alignment has registry follow-up measured in decades. Kinematic alignment has been in widespread use for a much shorter period, and the theoretical concern raised against it, that leaving a knee in more varus loads the implant differently over twenty years, has not been resolved by data either way.1
The honest position, and the one taken here, is that kinematic alignment is a reasonable, evidence-supported approach with good early results and incomplete long-term data. It is not a proven superior operation, and any surgeon telling you it is has gone past what is known.
Restricted kinematic alignment.
The compromise most surgeons using this approach actually practise. The plan reproduces your own anatomy, but within defined safe boundaries: if reproducing your native alignment would put the implant outside a range the evidence supports, the plan is pulled back to the edge of that range rather than followed off the end of it.
It matters because the knees where pure kinematic alignment is least appropriate are exactly the ones with the worst deformity, and those are common.
Where the robot comes in.
Kinematic alignment asks for something manual instruments are poor at: reproducing one specific person's joint lines to within a degree. This is the practical reason robotic assistance and kinematic alignment are usually discussed together.
The robot does not decide the alignment. It measures your anatomy, builds the plan around it, shows the surgeon what the knee will do through its range before any bone is cut, and then holds the execution to that plan. The judgement about what the plan should be is the surgeon's. What the robotic evidence actually shows covers the separate question of whether the robot improves outcomes.
What this means for your operation.
You do not need to choose between these approaches yourself, and a surgeon who asks you to has handed you a decision you have no basis for making. What is reasonable is to ask which approach is planned for you and why, and to expect an answer that refers to your particular knee rather than to a philosophy.
In this practice, knee replacement is planned most often with Medacta MyKnee patient-matched instrumentation, with robotic assistance also used where it suits the case. Either way the plan is aligned to reproduce the patient's own anatomy within safe boundaries, and the operation is most often performed through the subvastus, quadriceps-sparing approach. Where a knee is not suitable for that, it is aligned differently and you are told so.
Common questions.
What is kinematic alignment in a knee replacement?
An alignment philosophy that aims to restore the knee's own pre-arthritic joint surfaces and alignment, rather than cutting every knee to a standard neutral axis. The implant is positioned to match your anatomy instead of your anatomy being corrected to match a standard position.
How is it different from mechanical alignment?
Mechanical alignment, the traditional approach, aims for a neutral straight leg in every patient regardless of the shape they started with. Kinematic alignment reproduces the individual's own joint lines. Most naturally occurring knees are not neutral, so the two approaches produce different implant positions in the same person.
Is kinematic alignment better?
Randomised trials comparing the two have generally found equivalent or modestly better patient-reported outcomes for kinematic alignment in the first few years, and long-term implant survivorship data is still maturing. It is a reasonable, evidence-supported approach rather than a proven superior one, and that is how it is described here.
Do I need the robot for kinematic alignment?
Not strictly, but it is what makes the plan reproducible. Reproducing a specific patient's joint lines to within a degree is difficult with manual instruments and straightforward with robotic assistance, which is why the two are used together in this practice.
Can any knee be kinematically aligned?
No. Where the deformity is severe, where the ligaments are not balanced, or where there has been previous fracture or osteotomy, reproducing the original anatomy is either not possible or not desirable. Those knees are aligned within safe boundaries instead, which is sometimes called restricted kinematic alignment.
References.
- Australian Orthopaedic Association National Joint Replacement Registry. Hip, Knee & Shoulder Arthroplasty Annual Report. Adelaide: AOA.
- Evans JT, Walker RW, Evans JP, Blom AW, Sayers A, Whitehouse MR. How long does a knee replacement last? A systematic review and meta-analysis of case series and national registry reports with more than 15 years of follow-up. Lancet. 2019;393(10172):655-663.
This page provides general information about a medical condition and a surgical procedure. It is not individual medical advice and does not replace consultation with a qualified practitioner. Outcomes vary between individuals and all surgery carries risk. A referral is needed for a consultation with Professor Daevyd Rodda; without one, you can start with the practice nurse practitioner, who can provide it.