Tendonitis can be caused by poor use patterns. Reducing inflamation is just as important as proper tissue nutrition.
Yucca, licorice root, ginger for starters.
Finding a teacher who uses relaxation as the basis for physical movements ie Sandor or the Russians will go a long way to also solving these issues.
In essence the problem usually develops when the "contrary action muscles" are under tension at the same time as the main movement - usually due to poor foundational development of relaxation during practice. Each movement should begin and end in full relaxation mode naturally.
A good examination of this issue from a violinist's perspective is Malva S Freymuth: "Mental Practice and Imagery for Musicians".