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What is a Total Knee Replacement?

A knee replacement, also known as a Total Knee Arthroplasty, is a surgical procedure that replaces the bone in a painful arthritic knee with a metal and plastic replacement. 

Here is how we get to needing a knee replacement...Our joints are all lined with cartilage.  The cartilage is so slippery that its co-efficient of friction is less than ice gliding on ice.  This smooth gliding is essential for our daily motion.  Cartilage is great, except for one not so tiny detail.  Cartilage is incredibly sensitive to stress, and it cannot grow back.  So when cartilage is injured it dies and doesnt come back, this is post-traumatic arthritis.  When cartilage wears out with age, this is called osteoarthritis.  When cartilage wears out from the stress of inflammation, this is called rheumatoid arthritis, or some other type of reactive arthritis.  

Without cartilage the knee cannot glide smoothly, it generates a lot of friction, and this causes inflammation. Arthritis is the inflammation of a joint, typically caused by the loss of cartilage.   

The inability to restore normal cartilage is arguably the biggest problem facing orthopedics today.  

What is the long term outcome?  

There is a lot of variability in how long a knee replacement will last.  The metal and plastic has a certain lifespan inside your body.  However, this lifespan is also affected by a persons activity level (the plastic wears out faster in more active people), and by the skill of your surgeon.  The prosthetic parts need to be inserted in great alignment so that it wears out evenly, and that too much stress isnt placed on one part of the prosthetic.  The replacement is like a chain: only as good as its weakest link.  If one part wears out too fast, the whole thing will become painful and require a replacement of the replacement.  

The average lifespan of models placed within the last two or three decades is about 15-20 years.  If everything was done correctly.  

The good news is that things are only getting better.  Newer materials are being developed that has a longer shelf life.  The plastic that is used to make the hip socket has been reinforced in a process called "cross-linking" which makes it more resistant to wearing out.  Most companies that make these implants say that the average knee replacement should last 30 years.  

Surgeons are also becoming more skilled at performing this surgery, and technologies like computer-navigation, help align each of the implants to achieve a near perfect match with a persons normal anatomy.  Other technologies have been developed to create custom implants based on a 3D model of your bone taken from an MRI (or CAT scan).   

One of the biggest issues that orthopedic surgeons are now facing is that people are asking for replacements at a much younger age than just a decade ago.  This procedure has been so successful at alleviating pain and allowing people to return to an active lifestyle, that many people dont want to wait until they are in their 60s to get a knee replacement.  This younger age group (people in their 50s) pose a whole new challenge to surgeons and engineers.  People at this age are considerably more active and put greater stresses on their new hips.  People are also living longer, they are outliving their hip replacements.  Now I know, its a good thing that people are living long healthy lives, but it means that even when a very successful hip replacement lasts 30 years (which is a long time), the replacement will need replacement.  This is called a revision surgery and is considerably more complex then putting in the first replacement.  


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