What is a unicompartmental (partial) knee replacement?
There are three “compartments” of the knee joint: medial (inside), lateral (outside), and patellofemoral (joint between the kneecap and the thighbone).
The most common kind of knee replacement is a “total knee”, in which all three compartments are resurfaced. Some people have knee arthritis that is isolated to one compartment (usually the medial). In a “Uni” or partial knee replacement, only the damaged portion of the knee is resurfaced, and the remainder of the knee is left intact.
Who is a candidate for a “Uni”?
The best candidate for a “uni” knee has pain only on one side of the knee, and mainly has pain just with weight bearing (not at rest). The X-ray should confirm that the arthritis is confined to one part of the knee. In addition, patients with unstable knees due to absent or non-functioning ligaments (the ACL, for instance), and those with inflammatory arthritis (rheumatoid, lupus, psoriasis, etc) are not candidates for unicompartmental replacement. In general, about 30% of patients with knee arthritis are candidates for the “uni”.
What are the differences in a “uni” and a “total” knee?
The most important thing to remember is that both types of knee replacement work very well. There are several areas where unis differ from total knees.
In Unicompartmental knees,
usually:
- The incision is smaller.
- There is less blood loss. Less than 1% of partial knee replacements require a blood transfusion.
- Risk of infection is lower.
- Stiffness is very uncommon and the ability to flex the knee is usually a little greater.
- Early recovery is faster. Most patients can go home on the day they have surgery. Patients get off their walker faster (usually 7-10 days) and physical therapy is not usually needed. By one year, function is about the same with partial and total knee replacement.
- Less bone is removed, so the knee feels a little more normal. It is generally easier to squat, kneel and use stairs with a unicompartmental compared to a total knee replacement. Most patients with a “uni” on one side and a “total” on the other prefer the “uni” knee.
- Revision is much easier. Because the “uni” surgery removes less bone and retains more of the native knee, if the implant fails, it is simpler to “re-do” or revise. Revision of a total knee replacement, while usually successful, has many potential pitfalls and the results are usually not as good as those of a primary total knee replacement. Results of revision of unicompartmental knee replacement to total knees are similar to those of primary or first time, total knee replacements.
The downside to unicompartmental knee replacement:
If unicompartmental knee replacement didn’t have some drawbacks, everyone would do them. While the long-term results of uni knees are almost as good as those of total knees, they are not equal. Unicompartmental knees have a slightly higher failure rate than total knees. In the first year, about 3% of uni knees will fail because of implant loosening (compared to 1% of total knees). As time goes on, the native cartilage left behind can wear out, requiring revision to a total knee. As with a total knee, the implant is made of metal and plastic, and it can wear out also.
Unicompartmental knees are also much more operator dependent. While total knees are done using a variety of cutting guides, uni knees are usually done “free hand”, which requires the surgeon to make accurate cuts for optimal implant placement. In addition, not all knees are clear cut. While some knees clearly have arthritis involving only one compartment, many knees have advanced arthritis in one area, and mild arthritis in others. These knees require the surgeon to make a judgment call on whether a “Uni” is the appropriate implant. Factors such as patient age, size, and activity level also play a role. For these reasons, many surgeons choose not to do unicompartmental knee replacement and do exclusively total knee replacement.
What if the “uni” fails?
Depending on the reason for failure, the unicompartmental knee replacement can be revised to another “uni” or to a total knee. Because the “uni” removes so little bone, revising it to a total knee is similar to a primary, or first time, knee replacement.