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Knee Replacements: Philosophies

So you've decided you're having a knee replacement, you now have a brief idea of what it's all about but then you're hearing all those buzz words about rapid-recovery, day-surgery, robots, patient specific guides, kinematic alignment etc. etc. and want to know what the deal is. Here is a brief summary of the buzz words:


Recovery:

Generally speaking a knee replacement is a painful procedure. We can reduce the pain with a number of things, like local anesthetic, blocks, infusions etc. Most of us use a combination of the above: we try and do the operation under spinal anesthetic and we try and put lots of local anesthetic in and around your knee or sometimes the anesthetist does something similar using nerve blocks.


Unfortunately all of these things usually wear off after 24-48 hours and then you start feeling the operation regardless of how comfortable you were just after the operation.


This is why most patients end up staying in hospital a few days after surgery. However there is a push these days to minimize how long you stay in hospital, and this is mainly due to costs. The quicker you're out of hospital, the cheaper it is for the insurers. Having said that, the quicker you get out of hospital the better you tend to feel as well.


The whole rapid recovery thing is designed to make you really comfortable straight after the operation, then get you home whilst you are comfortable and hope that when the pain hits you, you'll be well settled at home and not inclined to come back into hospital.


I agree it's a good idea to be at home, but you definitely need the supports in place to make it a pleasant experience.


Alignment Philosophies:

There are 2 broad alignment philosophies: Kinematic and mechanical.


In kinematic knee replacements (iKA below), only the amount of bone that is to be replaced by metal is taken out (red and blue dotted lines). This essentially means that your overall alignment (bow-legged or knocked knee) remains that way, albeit a little better. And that the knee is placed in a slightly angulated way when compared to the horizon.


Pros: The knee is meant to feel more natural, as it is what you are used to and more matched to your anatomy. And no soft tissue releases are required as no particular alignment is being aimed at.


Cons: It is unclear whether having a knee that isn't fully straight may cause the prosthesis to wear out quicker, because of point loading. And the arthritic knee is not the same as a natural knee. Nor is one that has been cut open and operated on. So the whole argument of making the knee feel natural may be moot.



In mechanical alignment (aMA above) the bone cuts (red and blue dotted lines) are made in such a fashion as to make your leg straight and correct your bowed-legs/knocked knees. The knee replacement is placed pretty much parallel to the horizon.


Pros: The knee looks straight. The knee is loaded evenly, hence there is an assumption that it will wear out less quickly.

Cons: Your knee is not used to being straight so may feel unnatural. Often some soft-tissue releases are required to make your knee straight.


Overall, the research doesn't convincingly support one philosophy over the other, so I wouldn't worry about it too much at the moment.


Robot vs instrumented vs patient specific vs computer navigated:

I'll talk about this one in a separate blog.


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