Improvements Over the Years Makes the Unicompartmental Knee Replacement Ideal for Some PatientsFor patients with painful arthritic changes on one side of the knee, a complete and total joint replacement may not be needed. Since 1964, surgeons have been using and perfecting the concept of a unicompartmental knee implant. Results with these devices have continued to improve greatly in the last few years. The reasons for the improved outcomes are explained in this article.
First of all, surgeons have come to understand the importance of patient selection in assuring a successful result for unicompartmental knee arthroplasty (UKA). Adults with unicompartmental (one-sided) knee pain while at rest seem to do best with the UKA. If there are major limitations to motion or significant anatomical deformities, then a total knee replacement is advised.
Patients with osteoarthritis (rather than rheumatoid arthritis) and who are not overweight or obese are the best candidates for a UKA. Studies have shown that being overweight is directly linked with the need for a revision after UKA. And, of course, if there's arthritic damage to the other side of the joint, the patient should really have a total joint replacement.
Age used to be an important factor. No one under 60 years old would be considered for a UKA. But with improved implant designs and better surgical techniques, the range of acceptable ages has expanded. Younger, more active patients between the ages of 40 and 60 are now considered for this procedure. A few studies have reported patients as young as 35. But there is a concern that the patient will need too many revision surgeries in a lifetime to start so young with even a unicompartmental joint replacement.
There is one other patient factor to consider when choosing patients for a UKA -- and that's the diagnosis. Patients with posttraumatic arthritis don't do as well as those who have osteoarthritis associated with aging. More studies are needed before firm guidelines can be made regarding this patient characteristic.
The UKA implant also lasts longer with fewer problems when the patient has a normal (intact) anterior cruciate ligament. Without this important restraint, joint deformity develops. There is uneven motion of the bones forming the knee joint as they against each other. Over time, this factor reduces the survival rate of the implant because of increased or uneven wear and then loosening of the implant.
Second, improved surgical technique has been shown to be extremely important. In the early days of UKAs, it was easy to overcorrect a knee deformity and end up with a failed surgery. Getting the right patient, using the most appropriate implant for that individual, and maintaining proper limb alignment are now understood to be a necessary part of the equation for success.
Correct limb alignment refers to the fact that it can be very easy to insert the implant with too much rotation or tilt to one side or another. The surgery can be done with an open incision, which gives the surgeon a better view of the joint and easier time of aligning the implant.
Or it can be done as a minimally invasive procedure with just a three-inch incision. It's harder for the surgeon to see what he or she is doing with minimally invasive surgery. But the fact that it can be done successfully with less disruption of the surrounding muscles makes the minimally invasive approach very attractive. The stay in the hospital is shorter and the cost is less.
Third, changes have been made in the implant design that have improved results. The polyethylene (plastic) platform that the implant sits on is thinner than it used to be. Finding the right balance of thickness has been a challenge that is yet to be overcome completely. The surgeon aims for correction of any deformities but tries to ere on the side of undercorrection instead of overcorrection.
The slope (or curvature) of the implant has been changed over the years. Surgeons were able to see that an increased slope led to a higher rate of implant loosening. And they've discovered that the slope makes a difference when the ACL isn't present or is damaged. In such cases, an implant with a neutral slope is selected.
Different types of implants have been developed. Some sit right on top of the bone. These are called resurfacing designs. Others require a portion of the bone surface to be removed to make an inset design for the implant.
There's also the fixed-bearing versus the mobile-bearing implant. This feature describes how much the implant pieces move and rotate against each other. The mobile-bearing unit seems to be winning out. It has a larger area of contact to spread out the load resulting in lower wear rates. Mobile-bearing units are more difficult to get the right balance of knee flexion and extension. This requires a perfect soft-tissue balance to achieve.
The authors conclude that the unicompartmental knee arthroplasty (UKA) has come a long way, baby. In its early days, there were high rates of failure and revision surgeries. Today studies show excellent medium-to-long-term results. There are fewer reoperations, less joint degeneration, more evenly balanced knees, and the possibility of remaining more active.
Choosing the right patient, the optimal implant, and providing the best surgical technique results in shorter hospital stays, more people discharged to home, faster recovery, and improved appearance (smaller incisions). The surgeon can reduce the amount of soft-tissue trauma using a minimally invasive approach -- but there is an increased risk of implant malalignment or malposition.
Amir A. Jamili, MD, et al. Unicompartmental Knee Arthroplasty: Past, Present, and Future. In The American Journal of Orthopedics. January 2009. Vol. 38. No. 1. Pp. 17-23.
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