Yoga is safe for people with chronic pain and/or injuries. But it must be taught according to principles of proper alignment. Instructors must have experience and a good understanding of these principles and their application.
In some cases, a private lesson may be the way to begin. Yoga is known for it's therapeutic nature and all students are trained with the notion that yoga postures should never create pain, which is differentiated from the sensation of stretching muscles. We regard pain as a signal that an action is being performed in such a way that we are out of alignment. The alignment principles of Yoga have helped thousands of people strengthen and open their bodies intelligently, without injury, and have helped thousands more heal injuries through therapeutic application of yoga poses.