Many patients with knee arthritis have asymmetrical cartilage wear. This type of arthritis is more common in younger patients. People typically complain of pain on one side of the joint as opposed to globally around the knee in more involved arthritis. Standing knee x-rays help determine the status of the cartilage in both the medial and lateral compartments of the knee. In the x-ray the space between the two bones on the left side of the joint is minimal but the space on the right side of the joint is well preserved.
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 onto 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.
When examining patients before surgery special attention is paid to irritation around the knee-cap or patella. In patients that have significant, persistent pain around the patella or a fixed contracture (inability to fully straighten the knee) total knee arthroplasty is a more appropriate operation. In most patients with asymmetric wear of the cartilage, the pain under their knee cap is a mild irritation rather than severe pain.
The advantage of a “uni” knee is a smaller surgery in which only the arthritic portion of the joint is replaced.
After surgery, patients start on a medication called Lovenox which is a low molecular weight heparin used to minimize the risk of developing DVTs (blood clots) in the leg. Without this medication, the risk of developing a deep venous thrombosis or DVT is about 50%. This complication can be fatal if the blood clot dislodges and travels to the heart. This condition is called pulmonary embolism. The risk of developing a PE or pulmonary embolism is about 0.2% with Lovenox.
Spinal anesthetic is ideal if possible because it makes pain control after surgery is much easier. In addition, many patients receive an additional femoral nerve block after the surgery. This numbs the area over the front of the thigh and knee for about 18-24 hours after surgery. Patients begin physical therapy by standing the night of surgery and more vigorous therapy begins on the second hospital day. Most patients use a continuous passive motion machine (CPM) the day after surgery and continuing after discharge from the hospital. Patients put their full weight on the leg right after surgery, albeit with some help from the physical therapists. Most patients are comfortable and go home with a walker on the 2nd day after surgery. Patients shower after the 5th day and are encouraged 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. Total recovery takes 8-10 weeks. Risks are similar to total knee replacement but less likely with partial knee replacements.
Most 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 implant 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 total knee arthroplasty mechanical failure) and manufacturing characteristics of the polyethylene. We do not offer total or partial knee arthroplasty in patients 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 is 2-3 times higher in morbidly obese patients. We recommend patients seek help to get their BMI down to a safe level before proceeding with a total or partial knee replacement under less than ideal circumstances. The article below is helpful in understanding the scientific basis for this decision.
Obesity and Knee replacement, JBJS 2004
Patients come in about once a year for x-rays of their knee to make sure the polyethylene insert is not wearing out. This is detectable early by looking at the x-rays over time. When the plastic insert becomes less than 5 mm thick, revising the knee by replacing the polyethylene insert is advised. If done early, we can replace the plastic insert without having to change out the rest of the knee replacement. 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.