If you can find a PT well versed in osteoporosis, that would be great for a re-education on how to move and exercise safely. They are hard to find! Online resources such as Sherri Betz and Margaret Martin are both excellent and have lots of you tube video demonstrations for you to follow. The main movement adjustments you might want to incorporate are avoidance of crunches or forward bending in a spinal curve as well as twisting to your end range. Forward bending should be done with a flat back, bending your knees and hingeing at the hips. So in your exercise class (or anywhere else), it's really best to avoid those crunches. Once you get used to using your back that way, you'll find that you incorporate it into all aspects of life; loading the dishwasher, taking things out of the oven, picking something up off the floor. Bending that way requires a squat so you're also using your quads and glutes which is helpful for building strength, stability and balance.