I have a 70% lateral tear in my posterior tibias tendon, must surgery be my only option for repair?

Posterior Tibialis : Without seeing an MRI, or doing a history and physical exam, this is difficult to answer. Occasionally immobilization can be a remedy. However, depending on the extent of the 'tear' several factors come into play. It's imperative to have a physical exam, and evaluation by a specialist: Orthopedics, well trained podiatrist. Ie., diabetes? Clotting disorders? Many factors involved. See your doc?