Partial Knee Replacement - Procedure and Downside

What is it? Who is it for?

X-ray image of a partial knee replacement
X-ray image of a partial knee replacement. SHEILA TERRY / Getty Images

The knee jointt is composed of 3 parts: medial (or inside), lateral (outside), and patellofemoral (joint between kneecap and the femur). A partial knee replacement, as known as a unicondylar knee arthroplasty (UKA) is a surgical procedures that replaces only one of those three parts. Traditionally a UKA replaces the medial compartment of the knee. More recently UKAs have been designed for the lateral aspect of the knee.

The Procedure

This procedure is limited to patients with arthritis that effects only the middle compartment of the knee, less commonly it is used for patients with arthritis of only the lateral compartment. A reasonable question is ‘if we have the technology to replace the whole knee, why bother just replacing part of it?’ The advantages of a UKA are that it preserves the cruciate ligaments of the knee (ACL and PCL), and thus can lead to more natural knee motion. If a patient does not have well functioning ACL/PCL, then they won’t benefit from this procedure. Another benefit is that this is a faster, less morbid procedure. UKAs are associated with less blood loss and faster recovery. A study that looked at patients with a UKA on one side and a traditional total knee on the other, found that patients preferred the UKA side.

The Downside

So what are the downsides? The main downside comes from the possible need to convert a partial knee replacement to a total one.

This happens of the UKA causes severe pain, fails, or the adjacent parts of the knee develop arthritis. Converting a UKA to a total knee replacement is associated with worse outcomes comparing to doing a total knee replacement on a native knee. Most surgeons would compare converting a UKA to total knee, with revision knee replacement (performing total knee for a second time on a joint that already had a total knee replacement prior).

The other downside is that a fairly small number of patients are candidates for partial knee replacement. The traditional indications for this procedure, developed in the late 1980s, suggest that  patients with the following characteristics do best with partial knee replacement:

  • Arthritis of a single compartment of the knee
  • Age greater than 60
  • Weight less than 181 pounds
  • Low Demand
  • Minimal rest pain
  • Range of motion over 90 degrees
  • Patients without significant bowlegged or knock-kneed deformity

More recently these indications have began to expand, and newer studies have shown that perhaps younger, and slightly more overweight patients may do well with partial knee replacement.

Closing Thoughts

This all sounds like a great idea, right? But how do these do? Well, it turns out that partial knee replacements do fairly well! Most studies of UKAs done in the late 90s or early 2000s show that the 10-year survival of these implants is about 90% to 95%. An important aspect to keep in mind that patients that were included in these studies are very carefully selected.

A safe assumption is that as the indications for this procedure are stretched, the outcome will likely decline. Overall this appears to be a great option for a very select group of patients. Patients that do not fit the strict criteria for a partial knee replacement, will not do well with one.

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