Partial versus Total Knee Replacement

The knee can undergo arthritic degeneration in one or more of the three sections (compartments).  These compartments are the inner (medial), outer (lateral) and front (patellofemoral).  The most common compartment affected by painful arthritic degeneration is the medial compartment.  This leads to joint narrowing (“bone on bone”) resulting from cartilage loss of the medial compartment and pain.

Partial knee replacement involves an isolated resurfacing (replacement) of the involved medial compartment.  While replacements of the lateral and patellofemoral compartments have been described, they are less frequently performed in a very small subset of patients due to concerns about survivorship and deformity that cannot be corrected.

Partial Knee Replacement from Thomas Obermeyer, MD on Vimeo.

The debate of optimal candidacy for partial replacement is ongoing.  Generally, as arthritis is worse, the joint is stiff or deformed (crooked), the ligaments are imbalanced, or the patient is heavier or highly active, there may be reason to avoid partial replacement.  If a patient has normal motion in the knee with intact ligaments and isolated (unicompartmental) arthritis, he or she may be a candidate for partial replacement, however there are risks with the longevity of the implant which should be carefully considered prior to having this procedure.

Patients younger and more active are more likely to have an early failure of the implant requiring revision to a total knee replacement in their lifetime(1).  While a group of these partial replacements will require revision in 7 to 10 years, some will last 15 years or more(1).  Could the second revision procedure have been avoided had the knee just been replaced with a total the first time?  We know that most modern total knee replacements will never require revision(1).

The longevity of the partial replacement may also depend on surgical factors.  Any subtle imperfection of the alignment of the partial replacement may shift the load of impact to the other sections of the knee, leading to accelerated arthritis progression in those other sections(2).

An important factor about the decision to have a partial replacement is whether future surgery to revise the partial to a total is more complex and fraught with inferior outcomes than a well done primary total knee replacement.  This does seem to be the case(2,3).  Converting a partial to a total knee replacement comes with loss of bone and sometimes ligaments, often requiring constrained implants, cones, augments, or stems which adds complexity to the procedure(3).  For these reasons, revision of a partial replacement has a higher rate of complications and inferior outcomes compared with primary total knee replacement(3). Some evidence suggests revising a partial replacement may be equally fraught with as much uncertainty and complexity as revising a total knee replacement which is considered a major operation(2).

Living with a failed or failing partial replacement can be a challenge.  Sometimes by the time that happens the patient is older, may have comorbidities that make a major surgery risky or impossible.  Some of these older patients have to struggle with a decision to revise a failed knee implant or live with the problem.

Predicting your results with any surgery requires nuance and individualizing the decision to your unique factors.  Partial knee replacement can be a great option in some patients with a comparatively easy recovery.  It may not be the best decision for everyone however.