
Poses that strengthen bones include Down Dog, Plank, Upward-Facing Dog and Staff Pose. Here's a link to other yoga poses that help build bone safely:
http://www.fitbie.com/get-fit/build-strong-bones-yoga
There are conflicting opinions about spinal twists as to whether they help strengthen the spine safely or whether they may heighten fracture risk for those with low bone density in the spine. Find a yoga instructor who is knowledgeable about bone issues and work with him/her to find modifications to the poses that are safe for you.