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Update on Partial Knee Replacement

Posted Nov 08 2009 10:00pm

This is a guest post by Kim Wright, PT and her husband James (Jay) Butler, MD. Kim is a physical therapist who specializes in orthopedics and her husband is an orthopedic surgeon. They both practice in Houston, Texas. I asked Kim and Jay if they would share their knowledge and experience with knee osteoarthritis and knee replacement and they generously agreed.


The knee joint is composed of three compartments.  They are the inside(medial), outside (lateral) and the kneecap (patellofemoral). Wear and tear on our joint surfaces occurs over time depending on activities we perform and injuries that occur.  Some of us will have more wear and tear due to the alignment or posture of our legs.  Knee alignment refers to the angle of your thigh bone from your hip to your knee and the angle of your shin bone from your knee to your ankle.  Your foot alignment is an angle formed from your heel to your forefoot. Terms such as knockneed, bowlegged and pronation refer to these angles. The angles influence how the force from the weight of your body and activities like running or hiking is distributed through your joints.

If you tend to be knockneed, bowlegged or if you tear a meniscus (shock absorbing pad between the bones) or suffer a fracture, the forces can be increased in one of the compartments. The extra force is transmitted to the bone and the overlying cartilage (articular surface).  Given enough time, this surface wears away and the bone can become exposed and painful.  This wearing away of the articular surface is called chondromalacia (arthritis). It is usually graded I-IV with I being the mildest degree of damage.  When the damage progresses to stages III and IV the bone is actually becoming exposed and this is when pain and swelling occur and simple life activities such as walking, squatting and stairs become difficult.   
Model_oxford_partial_knee
When any of the three compartments suffer significant wearing of the articular surface causing Grade IV chondromalacia, the doctor may suggest a partial joint replacement.  Partial joint  replacements are ideal for patients with isolated Grade IV chondromalacia in only one compartment. This usually occurs in younger patients 40-60. Patients with a lot of arthritis in one compartment and some in the others may still have a certain degree of pain and swelling after surgery from the remaining arthritis. This is why a careful examination and selection process is critical for a good outcome.  There are unicompartmental replacements ( ”Uni”) for the medial (ins IMG_0347 ide), lateral (outside) and patellofemoral (kneecap) compartments. In contrast, a total knee replacement replaces all three compartments.

The surgery involves replacing the two sides of the bone of the damaged compartment with metal and plastic components. If you have medial or lateral compartment arthritis, and this has made your leg a litt IMG_0350 le knockneed or bowlegged, the placement of the components will correct the alignment. The surgery involves an incision on the front of the knee.

The recovery is similar to a total knee replacement except it is usually with less pain and is faster. It involves a short stay in the hospital (2-3 days), one-two weeks using crutches or a walker and then 4-6 weeks of physical  therapy. Someone with a unicompartmental replacement should expect to regain full straightening and almost full bend of the knee as compared to their normal knee during the first 4-6 weeks after surgery.

Sports are allowed with a unicompartmental replacement. Sports not involving repetitive high impact activities such as jumping, running and cutting are the best. For example, many patients return to walking, biking, hiking, light doubles tennis, weekly game of softball or baseball, dancing, gym workouts, exercise class. Younger patients may choose to return to running and other more strenuous sports, but this may lead to early deterioration of the plastic (polyethylene) component.  Like all joint replacement and arthritis patients, individuals with a unicompartment replacement should maintain a healthy weight and perform exercises that promote good joint health regularly. This will help prevent wearing of the plastic component and reduce progression of less severe arthritis that may exist in other parts of the joint.

A unicompartment replacement should last about 10-15 years before needing a tune up. Sometimes the unicompartment replacement is just a time grabber for someone who may be destined to a total joint replacement.  For other patients that actually fall into the “isolated” arthritis category it can be the last replacement they need.  All joint replacements have a similar life expectancy before revisions are needed and this is usually stated as 10-15 years on average. There are two reasons the unicompartment replacement may need to be revised over time. Pre-existing arthritis in other parts of the knee can progress and the plastic component can wear out.  If other parts of the knee progress to Grade IV chondromalacia (arthritis) the unicompartment replacement can be converted to a bicompartmental replacement for the onset of kneecap arthritis or a total knee joint for generalized arthritis development in the remaining two compartments.   As with any surgery, but especially one that is performed on an isolated group of patients, ask your doctor why you qualify and ask how many of these procedures they have performed and their specific outcomes.   

For more information on unicompartment replacements you can go to the website for the brand of prosthesis your physician uses or as an example go to www.biomet.com and choose patient and caregiver section, joint replacement, knee pain, knee joint replacement or knee joint products. You will read both good and bad reports from actual patients as well as research articles.  Just remember that there is a very select group of patients that are appropriate for this procedure that will do well over time. Talk with your physician about your qualifications.

For outcome information on custom patellofemoral replacements you can reference the article from James E Butler, MD; Robert Shannon, MD Orthopedics 2009, 32:81 or www.orthosupersite.com.

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