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Ask The Doctor: Total Knee vs Partial Knee

Published: 15:24 pm, Tue May 11, 2021

Dear Dr. Zartman,

     I have been told that I have arthritis of the knee and it is causing me to limp.  It mostly bothers me with an ache when walking or going up and down stairs. Lately I even have some pain while sitting and it sometimes bothers me at night now. I ’ve tried medication and the exercises that my family doctor has prescribed but it’s not helping much. I know I’m overweight and have started a weight loss program with the help of my doctor but the pain just won’t stop.

    My doctor sent me to see an orthopedic surgeon and he took x-rays of my knee while I was standing.  He told me that gives a better look at the cartilage space (or lack of it) in my knee and can help guide treatment.  I found out I have no cartilage left on the inner side of my knee, - the medial side – and the bones are touching.  I think that explains why I sometimes feel a sharp pain and some grinding.   We talked about injections of steroid or a gel; but I know, and the surgeon agreed, that injections are only a temporary solution.  I am tired of living with this pain and asked him about a more lasting solution.

     He told me about partial knee replacement vs. total knee replacement.  I’m not quite sure what the difference is between the two.  Does one work better than the other?  I’m not that old, so I wonder how long knee replacements last.  Will I need another replacement when I get older?

Answer:

     Both of the questions you ask are good and it is important for both you and your surgeon to be “on the same page” regarding both of your expectations of the surgery and its results.  First of all, I find many patients, when they hear Total Knee Replacement, think the surgeon removes all the bones of the knee – the top of the tibia, the lower end of the femur (thigh bone) and the patella (kneecap), and replace them with metal.  I have often thought that we really should describe the operation as a RESURFACING.

     After you are under anesthesia the surgeon makes an incision down the middle of your knee in the front.  The patella and surrounding tendons are them moved off to the side so a good view of the knee joint is visible. T hen, using your pre-operative x-rays, and sometimes a computer assistant as a guide, careful cuts are made on the end of the femur and tibia. The surgeon then removes only the damaged or missing cartilage and about 10mm (3/8”) of bone on each.  Additional drilling and cutting is performed to shape the bone based on the pre-operative xray measurements and measurements of your bones’ size and shape taken during surgery.  Appropriately sized metal components are then chosen and, in most cases, cemented to the bones.  The worn out cartilage on your knee cap is shaved away and replaced with a heavy duty plastic surface which is also cemented to the remaining patella bone.  After the cement has hardened a plastic spacer between the tibia and femur is locked in place on the tibia component and that become your new knee joint.  All the worn out cartilage and bone spurs are removed and replaced with highly polished metal and very durable plastic.  You now have a new Surface on the bones, but almost all of your own bones and other tissues remain intact.

     Many patients ask what will happen to their meniscus cartilage.  Some patients have had an MRI which reveals one or more meniscus tears as well as the arthritis.  Almost all patients whose knees have deteriorated to the point that they need knee replacement also have meniscus tears. I t is part of the overall degeneration of the knee.  Whether your meniscus is torn or not, the meniscus cartilages, as well as the ACL, and usually the PCL, are removed at the time of knee replacement.  This is necessary, as modern knee replacements are designed to work without these structures.

     What the Total in Total Knee replacement means is that all the cartilage of the femur, tibia, and patella is removed and replaced.  If, as in your case, the cartilage is only damaged on one part of the knee – the medial side in your case - then a Partial Knee Replacement may be a good alternative to a Total Knee Replacement.  A Partial, or Unicompartment, knee replacement only replaces the cartilage in the area where the wear is the worst.  This can be on the medial (inner) side, the lateral (outer) side, or just the kneecap.  The most common of these scenarios is what you have, medial compartment arthritis.  Depending on your situation and age, this may be an attractive alternative.    

     Studies show that patients undergoing partial knee replacement recover faster and have less pain after surgery than patients undergoing total knee replacement. In addition, partial knee replacement often gives a more “natural” feeling knee. This is because it is a smaller, shorter operation with less cutting of bone. In addition, the major internal ligaments of the knee, the ACL and PCL, are kept in place instead of being removed with a total knee replacement.  For the right patient, a partial knee replacement can give excellent pain relief and function.

     There are some down sides to partial knee replacement, though.  It does not relieve any pain coming from the areas of the knee not resurfaced. Your surgeon will ask you where on your knee you feel the pain and if this area does not match where the x-rays show the arthritis to be severe, a partial knee replacement may not take away all your pain.  For some patients this may be acceptable.  For someone who wants a fast recovery and a more natural felling knee, a 90% reduction in pain instead of 100% may be an acceptable bargain.  In addition, some elderly patients who may be at higher risk may be better suited for a smaller operation with fewer possible complications.

     The question of how long a knee replacement lasts is always relevant.  In general, most total knee and hip replacements have a 1% per year failure rate, meaning the joint is not working satisfactorily and has to be replaced again.  That means that at ten years from now 90 out of a 100 knee replacements will still be doing a good job for their new owners.  A recent study published in the March edition of The Journal of Arthroplasty, the major medical journal related to total joint replacement, reports on the results of partial knee replacements in patients under the age of 55.  These younger patients are thought to be the highest risk for failure due the increased wear and tear on the knee in this relatively young and active population.  This study showed that 90.4% of partial knees were still in place and had not failed at 10 years, and 75.1% were still functioning well at 19 years.  This gives us reason to have faith that, in the appropriate patient, a partial knee replacement can be a good and long-lasting solution to the pain of knee arthritis.

     If the partial knee replacement fails, it is most often due to continued pain in the knee or worsening of arthritis in the parts of the knee not replaced. The treatment then is to convert the partial knee replacement to a total knee replacement. Although this is a second operation, it is usually effective at relieving the pain.

     It is important to discuss your desires and goals with your surgeon, including your desire for quick recovery, a more natural feeling knee, and your willingness to accept a slightly higher risk of a second operation. Your surgeon will give you his or her judgement as to whether your arthritis is localized enough for you to benefit from a partial knee replacement. Do some reading, but don’t trust everything you read on the internet!

     - Dr. Z

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