A portal scar is a small scarred area related to an arthroscopic portal - the small hole the surgeon makes to inserts the arthroscopy instruments.
It can take a month or more after surgery for arthroscopy portals to heal, during which time they can remain very tender. An unhealed portal remains a point of vulnerability to infection, which has the potential to spread inside the knee joint itself.
Even after healing, the scar can remain an area of irritation, and in some people the scar tissue can heap up into an ugly bump.
There will always be several small (1cm) scars after an arthroscopic procedure, but the nature of the scar depends upon the skin type of the patient. With a skin that heals well they will be fine whiter lines, but in a person who scars badly the edges of the would may seem to heal and then start to get angry and heap up into raised red edges. A kheloid is an extreme version of this. Usually, however, they heal without other problems.
Superficial portal infections are quite common, but always need to be taken seriously. Swabs should be taken and sent to the laboratory to be cultured, so that the right antiseptics and antibiotics can be prescribed. The important thing is to heal the infection as quickly as possibly so that infection does not track through the portal into the joint cavity itself.