Can phlebitis occur in tributaries? And can phlebitis in a small saphenous tributary cause swelling in the posterior calf?

Phlebitis. Phlebitis can occur in any vessel although typically medium size vessels. Inflammation of a major vessel would have very serious signs and very small vessels just don't seem to have this problem. Swelling of phlebitis usually follows the course of the vessel and does not cause generalized swelling of the calf. If you had MRI of leg you should have pretty good idea of problem.