Nearly every patient will experience some degree of permanent numbness on the lateral (outside) side of the knee after knee replacement surgery. This is anticipated. It is so common, most surgeons do not discuss this with their patients prior to surgery. It is not a complication, but a necessary side-effect of achieving a safe exposure to perform knee replacement surgery.
What is going on?
There are cutaneous (skin) nerves that cross the front of the knee from the inside (medial) to the outside (lateral). A knee replacement incision is made longitudinally over the front (anterior) of the knee. These small nerves must be cut to allow deeper exposure. Other than a numb patch on the outside of the knee, there is generally no other negative effect.
Once rehabilitated, patients rarely even mention this finding. Most are just thrilled their arthritic pain is gone and they are back to their desired activities again.
Orthopedic Surgeon focused on the entire patient, not just a single joint.