X-rays of a knee before and after unicompartmental knee replacement
Unicompartmental Knee replacement
AKA: Partial Knee Replacement
Many patients with knee arthritis have “asymmetry” of the cartilage wear involving only one side of the knee joint and this type of arthritis is more common in younger (50 yrs old) patients. The asymmetric wear is evident on x-rays and patients typically complain of pain on one side of the joint, but not globally around the knee. In the office I will obtain standing x-rays of the knee which which helps determine the status of the cartilage in both compartments of the knee. Take the x-ray below as an example, the space between the two bones on the left side of the joint is essentially zero but the space on the right side of the joint is well preserved. This tells me that the articular cartilage on the inside portion (left side) of the knee is completely gone and the patient has bone on bone (end stage) arthtis. I dont need to get any additional studies such as an MRI or CT scans because they won't change my treatment recomendation for my patients.
Standing x-ray of the knee showing bone on bone contact on one side of the joint
The components of the partial knee replacement are very similar to those for a total knee replacement but smaller. There is a polished cobalt-chrome component that is cemented on to the end of the femur (thigh bone) and a titanium tray that is cemented onto the top of the tibia. A high molecular weight polyethylene bearing surface sits in the tibial component and contacts the end of the femur. The bearing surface is the contact between the femoral component and polyethylene component on the tibial tray.
Diagram showing the components of the partial knee replacement
I currently use two types of partial knee implants. One is made by a Zimmer and is a fixed bearing partial knee in which the plastic tray is "fixed" to the tibial component. The other type of implant is made by Biomet and has a mobile tibial tray in which the plastic component can slide forward and back on the titanium tibial baseplate. I use the mobile bearing component in patients with significant deformity of the knee or subluxation of the patella as it allow me more flexibility in the placement of the tibial component.
Zimmer uni knee
Oxford Uni Knee
The current polyethylene manufacture uses and oxygen free environment and forms highly cross linked polyethylene which is able to resist shear wear better than non-cross linked plastic. The change in plastic processing was discovered about 10 years ago and has led to a dramatic improvement in the longevity of the polyethylene component. I have included a link to a report about highly cross linked polyethylene and the benefits over standard, non-cross linked plastic.
Uni-knee components
Highly Cross linked polyethylene
One of the more challenging parts of the partial knee replacement is proper sizing and placement of the tibial component. Unlike total knee replacement where the entire upper end of the tibia is exposed, the visualization of the tibia is somewhat limited. It is critical to make sure the tibial tray is placed perpendicular to the axis of the tibia and doesnt overhang the edge of the bone in front or in back. I usually check an x-ray during the operation the make sure that I am satisfied with the position of the components.
Intra-operative x-ray to confirm size and position of the tibial tray component
When examining patients before surgery I localize the pain and pay special attention to irritation around the knee-cap or patella. In patients that have significant, persistent pain around the knee-cap (patella) or a fixed contracture (inability to fully straighten the knee) I recommend a total knee arthroplasty. In most patients with "asymmetric" wear of the cartilage, the pain under their knee cap is a mild irritation rather than severe pain. When doing a partial knee replacement I focus on restoring the mechanical alignment of the leg ( most patients have a genu varum or "bow-legged" deformity prior to surgery) and replacing the worn out protion of the joint with a metal plastic bearing surface. Genu varum is the medical term for the bow-legged alignment. In children genu varum is caused by development problems whereas in adults the condition is almost always due to osteoarthritis of the knee. The x-rays below show the correction a "bow-legged" deformity with a partial knee replacement.
Correction of genu varum or "bow-legged" deformity
The advantage of a “uni” knee is that involves a much smaller surgery (about 1 hour) in which only the arthritic portion of the joint is replaced. The magnitude of the surgery is much smaller and the incision and amount of surgical dissection is concomitantly smaller. I caution my patients that if they have significant amounts of pain underneath the patella (knee cap) then the partial knee replacement might not be the best choice. Most patients have some component of patellar pain, but they characterize it as mild and in these cases I adivse patients that the sharp pain on the inside part of their knee will essentially go away but the mild irritation under the patella with continue after surgery.
After surgery I start patients on a medication called Lovenox which is a low molecular weght heparin to minimize the risk of developin blood clots in the leg. I have included a link to the medication below. Without this medication, the risk of developing a blood clot (deep venous thrombosis or DVT) is about 50%. Since many people have swelling in their leg after surgery, its very difficult to diagnose blood clots after surgery. This complication can be fatal if the blood clot dislodges and travels to the heart, this condition is called pulmonary embolism and I am very careful to minimize my patients risk of developing this condition. Thankfully the risk of developing a PE or pulmonary embolism is about 0.2% with Lovenox.
Lovenox link
CPM machine for the knee after surgery
I have patients stand the night of surgery and then physical therapy starts more aggressively on the first day after surgery. I routinely use a CPM (continuous passive motion) machine after surgery. Most patients are comfortable and go home with a walker on the 3rd day after surgery. I allow my patients to shower after the 5th day and encourage them to walk as much as possible after they return home. Most patients are able to walk comfortably with a cane after about 2 weeks and without a cane after 6 weeks.Most patients are able to leave the hospital within 2 days of the surgery and total recovery takes 8-10 weeks. Risks (blood clots, infection) are similar but less likely with the partial knee replacements.
Most total knee implants are designed to last somewhere between 15-25 years. The factors that influence the rate of wear of the polyethylene liner (the plastic portion of the total knee that wears out over time) include activity level (running and impact activities are discouraged as they increase wear), weight (obesity is one of the primary risk factors for partial knee arthroplasty mechanical failure) and manufacturing characteristics of the polyethylene. I do not offer partial knee arthroplasty in pateints with a BMI (Body Mass Index) of greater than 40 ( this is the objective definition of morbidly obese). The reasoning is that the relative risk of complications including infection, wound problems, soft tissue balancing problems, patellar tracking problems and component malpositioning are 2-3 times higher in morbidly obese patients. I would recommend patients seek help to get their BMI down to a safe level before proceeding with a total knee replacement under less than ideal circumstances. I would suggest the article link below helpful in understanding the scientific basis for my decision.
Obesity and Knee replacement, JBJS 2004
Most patients are anxious about having surgery to replace the knee joint which is understandable. I tell my patients first 3-4 weeks after surgery is difficult and they may get discouraged because the progress with rehabilitation is slow in the first month. Your knee stays swollen and stiff for the first 4 weeks or so, but the recovery is much faster than a standard total knee replacement. By 6 weeks most patients are able to walk comfortably without a cane and by 3 months most patients have minimal pain.
As a rule, I have my patients come in about once year for x-rays of their knee. I check to make sure the polyethylene insert is not wearing out and I can detect this early by looking at the x-rays over time. When the plastic insert becomes less than 5 mm thick, I recommend revising the knee by replacing the polyethylene insert. If done early, I can replace the plastic insert without having to change out the rest of the knee replacement which means much less surgery for my patients. If patients wait too long, the particles from the worn out plastic can cause loosening of the remaining implants (reactive osteolysis) and this necessitates revision of all components of the total knee. This is a much larger surgery with longer recovery time. The link below describes the annealing process for highly crosslinked polyethylene and the benefits of this processing over standard polyethylene.
After Surgery
Patients arrive at the hospital the day of surgery and check in with admissions at the hospital. Prior to surgery you will have had your medical clearance and blood work done. You will meet your anesthesiologist to discuss your anesthesia preferences. The surgery takes about 1 hour and after surgery you spend another 45 minutes to an hour in the recovery room. In the evening after surgery you have a physical therapy session and stand at the edge of the bed. For the first day you will have a knee immobilizer on to minimize your risk of falling. The second day you start using the CPM machine and it will gently bend and straighten your knee. I have patients use the CPM for about 6 hours a day. In addition, the physical therapist will work with you twice a day until you leave the hospital for roughly 45 minutes each time. Most people are discharged home on the 3rd day after surgery. While in the hospital, the discharge planner will help you set up home physical therapy, CPM machine rental and other DME (durable medical equipment) such as a walker and elevated toilet seat. You may arrange some of these items prior to your admission. For additional information about what to expect after surgery, click on the link below to visit my post-op instruction sheet for knee replacement :
POST OP KNEE REPLACEMENT INSTRUCTIONS