Differences in Results After Partial Versus Total Knee Implants

By far more patients have a total knee replacement (TKR) over a unicompartmental knee arthroplasty (UKA). The risk of having additional surgery after a unicompartmental implant may be the driving factor behind this decision. This study was done to compare results after both procedures.

The difference between a total knee replacement and unicompartmental knee arthroplasty is more than just the implant itself but let's review that first. As the name suggests, with a total knee replacement (TKR), the surgeon removes the entire damaged knee joint and replaces both the upper and lower sides of the joint.

The unicompartmental arthroplasty (UKA) involves just replacement of the inner or outer half of the joint. Most patients having a UKA have problems with knee alignment and have worn out just the medial joint surface. Medial refers to the side closest to the other knee.

The idea behind a UKA is that there's no need to replace the entire joint when only one-half is compromised. Other advantages of the UKA include a shorter hospital stay, fewer infections, and faster recovery. Studies show patients having a UKA are at lower risk for blood clots. They also have less pain after surgery compared with a total knee replacement (TKR).

The major stumbling block with a unicompartmental knee arthroplasty (UKA) is the revision rate (second surgery). It is twice as high for UKA compared with a total knee replacement (TKR). There are several reasons for the higher failure rate of the UKA. The unicompartmental implant is more likely to come loose. Bone fractures around the implant are also more common in this group.

The authors of this study from Norway compared outcomes for the unicompartmental versus the total knee replacement. Norway has an established registry for arthroplasties (implants) making this type of comparison possible with a large number of patients.

All patients having any artificial joint implants in Norway are registered. Information about their diagnosis, age, joint involved, and surgery performed is included in the database. Baseline pain and function are not part of the registry information, so patients selected to be in the study were sent a survey of questions by mail.

Patient selection was based on current age (less than 85 years) and when the surgery was done (at least two years ago). Differences in results were compared by implant type and brand, patient age, sex (male versus female), and time since the operation.

As shown in previous studies, the unicompartmental knee arthroplasty did have more favorable results with less pain and better function at all time points. But the differences weren't all statistically significant. And when asked about quality of life, patient ratings were equal between the two groups.

The authors took a closer look at various implant brands (i.e., made by different manfacturers). Again, there weren't significant differences among the three prostheses included.

One difference that did stand out was based on sex. Men with unicompartmental arthroplasties (UKAs) had less pain and better knee motion and function compared with women who had the same implants. The men also had better scores for function based on daily activities, sports, and recreational activities.

In summary, there are only small differences in results to favor unicompartmental arthroplasty (UKA) over total knee replacement (TKR). Better motion leads to better function and performance. The most likely reason for the increase in quantity and quality of motion with UKA is the preservation of knee ligaments and overall knee anatomy made possible by the UKA.

The lack of a big difference between UKA and TKR came down to two main variables. One was the fact that there were more UKA implants that came loose. And secondly, more arthritis developed in the knees of patients with a UKA requiring further surgery (usually conversion from a UKA to a TKR).

The authors concluded by saying their Norwegian Arthroplasty Register with its large number of patients makes this type of research possible. They had over 1300 participants and plenty of patients with each implant design to make comparisons possible. Equal results among the different brands suggest it may not make a difference which one the patient gets. More study is needed to verify this idea.

With equal results but higher revision rates with the unicompartmental arthroplasty (UKA), it may be better to go with a total knee replacement (TKR). Only patients needing more knee flexion might find the UKA a better choice.

Reference: Stein Håkon Låstad Lygre, MSc, PhD, et al. Pain and Function in Patients After Primary Unicompartmental and Total Knee Arthroplasty. In The Journal of Bone and Joint Surgery. December 15, 2010. Vol. 92-A. No. 18. Pp. 2890-2897.

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