Yoga Problems

Yoga is supposed to help open up and ease my joints... and with my hips I find that it helps... but oh man do my shoulder blades suffer.
The arthritis in my shoulder blades (especially the left) is so much more pronounced, and so it makes even the simplest poses absolutely unbearable.
Somehow I have to find a way to practice without fully engaging my shoulder blades. I may have to keep my arms down in various poses and skip any positions that include weight bearing on those particular joints.
Poses like Downward Dog, Chair Pose, even Lunges with raised arms cause my shoulders to scream out in agony - which then lasts for weeks before settling down.

I will need to find some modified positions if I want to continue practicing.
:(