It may be that you have “cold sores” on that knee.
Herpes simplex is capable of colonizing sites other than
the area around our mouth.
Your doctor can send out a culture if you want to find out. Longer term treatment with oral antivirals such as
Valacyclovir daily has been used safely for prevention . If you don’t
want orals topical antiviral meds will help when it occurs.
Have you noticed any triggers for your outbreaks?