There’s no warning-sign list just for the knee. It isn’t one of the bony spots where pressure sores usually form, or one of the places venous or arterial leg ulcers usually appear. So what decides which signs apply to a knee wound is not where it is, but how it happened.
If it is a surgical incision that was closed, contact your surgeon for more redness or pain; swelling or bleeding; the wound is larger or deeper; or the wound looks dried out or dark. Also call if the drainage increases or becomes thick, tan, green, or yellow, or smells bad (pus), or if your temperature is above 100°F (37.7°C) for more than 4 hours.
If it is a surgical wound being allowed to heal open, there is a separate list. Contact your provider if there is more redness, pain, swelling, or bleeding at the wound site; if the wound is larger or deeper, or it looks dried out or dark; if drainage increases or becomes thick, tan, green, or yellow, or smells bad (which indicates pus); or if your temperature is 100.5°F (38°C) or higher. The two temperature thresholds are not the same, and which applies depends on your wound, not on your preference.
If the wound is not surgical — a fall, a graze, a cut — the general signs apply. Redness, increased pain, or yellow or green pus, or excessive clear fluid around the injury are signs of infection. Black edges around the injury are a sign of dead tissue.
What’s special about a knee is that the wound sits across a joint you can’t stop using. Two things follow from that. Do not wear tight clothing that rubs against the incision while it is healing, and do not pull at your stitches or try to remove them on your own. And one reason to contact your provider right away matters wherever a wound crosses a joint: a wound that has come open or the stitches or staples have come out too soon.
On washing, showers are better than baths because the wound doesn’t soak in the water. Soaking the wound could cause it to reopen or become infected.