Custom means two different things and the difference matters
When a patient asks me for a custom knee replacement, my first job is to establish which of two very different things they mean, because they are usually not aware there are two.
One customizes the implant itself, manufactured to your anatomy weeks before the operation. The other customizes the cutting guides, which are made to your anatomy while the implant remains a standard component chosen in the room. I use the second more commonly, in selected patients, and I do so deliberately.
The reason is not cost and it is not convenient. It is that I want to be able to change the size of the implant after I have balanced the knee, and a custom implant cannot be changed.
One kind of custom: a customised implant. The femoral component on the right was manufactured to match one patient’s anatomy, shown beside a three-dimensional printed model of the bone. This is the version whose size is fixed before the operation — the version I generally do not use.
What actually gets made for you
We take a magnetic resonance imaging scan of the leg. From that scan a three-dimensional model of your knee is built, and cutting guides are manufactured that fit your bone in one position and one position only, because they are shaped to your particular surface.
In the operating room those guides sit onto the bone and indicate where the cuts go, based on a plan built from your own anatomy rather than from a standard instrument set referenced to average landmarks.
The implant that goes in afterwards is a standard, well-proven component. What was customized was the route to it.
The plan built from your scan. Before the operation a three-dimensional model of your knee is used to position a standard implant and set each bone cut — size, rotation and depth of resection. The guides are then manufactured from this plan.
Customised templates and cutting guides manufactured for a single patient from their own scan, laid out before the operation. Look at how each piece is shaped to fit one specific bone surface. These are the guides; the implant that follows is a standard component.
My own imaging study. Anonymised and the file information removed.
How the guide holds its place. An anterior ridge on the femoral guide and a posterior hook on the tibial guide let each piece seat on the bone in one position only and stop it rotating. That is how a guide made from your scan passes its plan to the bone.
Manufacturer illustration.
Why a guide can only go on one way. The green shape is the guide and its undersurface is a negative of your own bone. Look at the right-hand panel, where the same guide offered at a slightly different position no longer sits flush.
Why I keep the implant decision in the room
This is the whole argument and it is worth setting out properly.
A knee replacement is not simply a matter of cutting bone accurately. The knee has to be balanced, meaning that the soft tissues on the inner and outer sides hold it evenly through its range of movement. In a knee with a significant deformity, achieving that requires releasing tissue on the tight side.
Releasing tissue changes the space I have to fill. A knee that needed one size before the release may need another after it.
If the implant was manufactured to your scan weeks ago, it is what it is. I can no longer respond to what I have in surgery.. If instead the guides were customised and the implant is a standard component with a full range of sizes open in the room, I balance the knee first and then choose.
So in the knees where I expect to need releases, and where I therefore expect to want the freedom to change size intraoperatively, the customised guide is the version of customisation that helps and the customised implant is the version that constrains me.
Why keeping the decision in the room matters. A standard implant comes with a range of insert types — posterior-stabilised, constrained and condylar constrained — and a full range of sizes. Because the implant is standard, I can choose among these after the knee is balanced rather than commit weeks earlier.
Who I use these guides for
I use them routinely. The situations where I think they especially earn their place are unusual anatomy, significant deformity, and a knee containing retained hardware from previous surgery where the standard instruments that pass down the middle of the bone cannot be used.
That last one is the clearest indication. If there is a rod or a plate in the femur, an instrument designed to go down the canal has nowhere to go. A guide referenced to the outside surface of your bone does not have that problem.
The published work on patient-specific guides shows improvement in some alignment measures and no consistent advantage in how patients actually feel afterwards.
What the scan costs you
An additional appointment and an additional imaging study weeks before your operation, plus the manufacturing time for the guides, which means the operation cannot be scheduled at short notice.
What custom guides add to the calendar. Look at the gap between the scan and the operation, which is the reason a case using guides cannot be scheduled at short notice.
What I will not combine
Patient-specific guides, computer-assisted navigation and robotic assistance are three different solutions to the same problem, which is where to put the cuts. They are alternatives rather than additions.
You cannot meaningfully stack them. A guide made from your scan and a robot working from its own registration are two different plans, and running both does not produce a better answer. It produces two answers and a decision about which to believe.
So I pick the approach that suits the knee and I use it properly.
What actually decides whether you like your knee
Not the technology used to make the cuts. In my experience it is three things.Whether the knee is balanced through its whole range, which is assessed by hand with trial components in.
Whether the kneecap tracks correctly, which depends on recognising whether it sits high or low and on placing the patellar component towards the inner side.
And whether the patient does the work in the first six weeks, because a knee that does not regain its range early is difficult to get back later.
Every one of those is judgement and effort rather than equipment. That is not an argument against the technology. It is an argument for keeping it in proportion.
The three things holding up a satisfactory knee. Look at where the technology sits.
Knees I have replaced using custom guides
These are my own patients, each published as an individual case study with the full films.
Several of these cases also have a surgical planning video published alongside them.
From the case Custom Knee Replacement Surgical Plan.
From the case Knee Arthroplasty Implants.
From the case Left Custom Total Knee Arthroplasty in a 68-year-old male.
Preoperative X-ray of the left knee showing anteroposterior and lateral views, from the case Left Custom Knee Replacement in a 70-year-old female.
From the case Custom Left Knee Replacement in a 71-year-old male.
From the case Bilateral Customized Total Knee Replacement
From the case Bilateral Customized Total Knee Replacement in a 72-year-old female.
From the case Right Total Knee Replacement with Custom Instruments in a 67-year-old female.
Preoperative anteroposterior and Lateral X-Ray views of the Right Knee, from the case Custom Unilateral Knee Replacement in a 74-year-old Female.
Preoperative X-ray of the right knee showing anteroposterior and lateral views, from the case Customized Right Knee Replacement in a 71-year-old patient.
Postoperative X-ray showing anteroposterior and lateral images of the left knee, from the case Case study: Left Customized Knee Replacement in an 80-year-old male. The x-rays are rotated and the lateral should have been better.
Postoperative x-ray images showing anteroposterior and lateral views of the right knee, from the case Customized Right Knee Replacement in a 72-year-old patient. The x-rays are rotated and the lateral should have been better.
Every custom knee case I have published
Age and sex are changed in all my x-rays for de-identified.
I have no financial relationship with any manufacturer of patient-specific guides, planning software or implants.
Custom Knee Replacement FAQ’s
- Do all patients get a customized knee replacement, if not why not?
- Why does my surgeon not do a customized knee replacement?
- How long does the swelling last after knee replacement surgery?
Find out the answers to these and other questions here.
Why choose Dr. Karkare?
As a top joint replacement specialist in New York City, Dr. Karkare has a stellar reputation, extensive experience, ensuring patients have the broadest array of safe and effective treatment options to relieve pain and other symptoms.
This enables every patient to make an informed, educated decision so they can feel confident in their care every step of the way.
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