“Do I Have The Wrong Size Knee?”
Unfortunately, there are a significant number of people who have undergone total knee replacement (TKR) and are not happy with the result. Some studies estimate 20% or more fall into this category. Defining success or a “satisfactory result” can mean two very different things to an individual who had the total knee versus the surgeon who implanted it. At the end of the day, it’s most important that the person who had the TKR is happy.
Defining “success” for the TKR begins even prior to surgery, during a frank discussion between patient and surgeon, with regard to what that patient’s goals are and if the surgeon thinks those goals are achievable and appropriate. Some goals simply are not reasonable, such as long distance running or team soccer, particularly if the new joint is to enjoy longevity.
When a patient is not happy with the result after surgery, it is very important that we understand why. Many times, the specific complaints give clues as to the underlying problem. Ultimately, if the situation is to be rectified and the problem corrected, then the specific etiology must be clearly delineated. TKRs work wonderfully, but only when a host of important variables comes together. I call these “the surgeon’s goals” which is what I try to achieve during an operation and, when accomplished, helps assure a stable, pain-free knee with which the patient is happy.
- Recreating limb alignment and a neutral mechanical axis so that weight-bearing forces are nearly equalized between the inner and outer compartments of the knee. This also has the effect of straightening any pre-TKR bowlegged or knock-kneed angulation.
- Balancing the soft tissue sleeve that surrounds the knee so that as the knee moves from extension to flexion, equal and physiologic tensions or pressures are experienced in the medial and lateral ligaments.
- Creating normal knee movement or “kinematics” as the knee ranges from extension into flexion and then back to extension. Normally, as the knee flexes, the tibia internally rotates and the femur pivots on the inner or medial compartment. During extension, this normal rotation or pivot reverses and the tibia externally rotates. The cruciate ligaments as well as other soft tissues and the shape of the bones help to control this normal complex movement.
The components implanted during TKR do a wonderful job of re-surfacing the end of the bones that make up the knee and this prevents the bones from rubbing. “Bone on bone” pain is one of the main reasons why arthritic knees are painful and a major trigger for patients deciding it’s time to choose knee replacement. For the result to be optimal after TKR, these three conditions need to be met. This can prove very difficult to accomplish consistently, especially with some patients’ particular deformities or underlying conditions.
The search for the etiology as to why a specific patient is not happy begins with a careful history. I inquire regarding what the original diagnosis was prior to the knee replacement and try and get an idea of how disabling the condition was. Was the person barely able to get up from a seated position and walk or did they have just a little discomfort after 54 holes of golf? I also ask if there were any problems with the incision after surgery or any need to return to the OR, which might increase the suspicion of an underlying infection. Were antibiotics extended after surgery or initiated after discharge? Many infections are subtle and difficult to diagnose.
What is the main complaint? Possibly pain, stiffness, poor range of motion, or feeling like the knee is not stable and that the patient can’t trust the new knee. Some patients have subtler complaints such as the “new” knee simply is not comfortable or “doesn’t feel natural.” If they’re experiencing pain, is the pain only with activity such as walking, or is it present all the time, even at rest? Does the discomfort awaken them from sleep? Can they do something that improves or relieves the pain such as assuming a particular position with the leg, using ice or pain meds, etc.?
I need to understand if there was ever a period where the patient seemed to be doing well or at least improving and then the circumstances changed. Are their symptoms now slowly improving, stable or worsening? Are the complaints tolerable or are they bad enough that the patient wants more tests, hoping to learn the specific etiology and would even consider more surgery if the condition could potentially be improved? Of course, these are just a sampling of questions that must be explored if the underlying problem is to be diagnosed and corrected.
Examining the knee is equally as important. Is knee alignment acceptable? Did the incision heal satisfactorily? Does the skin overlying the knee appear red and hot? Is the knee tender? If so, where? Is there any drainage? Does the knee appear swollen? Is there an effusion (fluid in the joint)? Does the knee fully extend? Can the person actively maintain that extended position? How much does it flex? Is this range of motion associated with pain or is it painful only in a particular position? Is the kneecap tracking or does it slide off to the side during flexion? When stressing the knee, is there more laxity on one side compared to the other?
It is important to test for stability with the knee in extension and various degrees of flexion. The anterior (front) / posterior (backward) stability also needs to be established. Other conditions that can cause knee pain must also be considered and ruled out, such as spinal disease and hip disease with referred pain to the knee. Once again, this is just a sampling of information that the physical exam can provide, giving clues as to why the result is not acceptable and to help determine the next steps to correct the problem or problems.
Good quality X-rays, including an X-ray taken with the person standing (which physiologically loads to the joint) and occasionally also including a full length X-ray that includes the hip and ankle are important. These X-ray images give important information regarding component alignment, sizing and if the joint appears stable or loose. X-rays also reveal what type of method was used to fix the components to the bone. Were the components cemented or press-fit with the hope that stability would be achieved with bone ingrowth? Do the interfaces where the bone contacts either the cement or prosthesis appear acceptable or is there a suggestion of loosening or osteolysis (bone destruction)? Accessing the equality of the inside and outside prosthetic joint space might give clues regarding soft tissue balance. Does the patella appear to be tracking centrally or is it pulled to one side? Are there residual bone spurs, which could be causing irritation or inhibit motion? As with a thorough history and physical exam, very important information can be gleaned from good quality X-rays. Similarly, a review of the X-rays that were taken prior to TKR also gives clues regarding the knee’s pre-operative deformity, appearance and underlying anatomy. Reviewing X-rays that were taken very soon after the index surgery and comparing to the most recent X-rays allows me to compare and look for subtle changes.
At this point, many times the surgeon will have a pretty good idea of what is causing the problem. Further X-rays might be necessary as well as other studies such as blood work, including an ESR and CRP.
The surgeon might suggest aspirating the knee to look for evidence of infection. Joint infection is often a difficult diagnosis to make. A new test has recently become available called the Synovasure™ test. Synovial fluid is aspirated and sent to a special laboratory where specific tests are performed. These include measuring a biomarker called alpha defensins. Biomarkers are proteins that act as the body’s natural antibiotics and are present when the body is fighting infection but not present in other conditions that can mimic infection. The Synovasure test has greatly improved our ability to diagnose infection and helps differentiate inflammation and other causes of knee pain from infection. Occasionally, the surgeon will request a bone scan, radioactive WBC labeled scan, or MARS MRI. In my experience, these studies have not been that useful. In special circumstances a CT is ordered to help better understand component positioning.
A review of the surgeon’s operative report is important. This report could give clues regarding specific difficulties or peculiarities that were encountered during the operation. The surgeon also needs to review the “implant record.” Like the operative report, the “implant record” is also a permanent part of the medical record and contains labels provided by the manufacturer naming the company that manufactured the implant, the implant brand name, size, FDA number, and expiration date. This information becomes critical if more surgery is being considered and also might give a clue as to why the knee is not performing satisfactorily. Particular bands or types may have known problems and a poorer track record than others.
If a specific etiology for the dissatisfaction can be defined, then a specific plan can be developed to address it. Depending on the diagnosis, this plan may be surgical or nonsurgical.
In my next article, I will discuss revising a TKR and a powerful new intra-operative tool which I use that can help me diagnose and treat subtle component positioning and balancing problems and then direct very specific soft tissue releases, bone resection and component changes to correct these problems.
As an update to this article I have written a blog article for those who are specifically concerned about symptoms of a wrong size knee.
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