Foam Roller Extensions
The two basic movements of the spine (in the sagittal plane) are flexion and extension. When you are doing crunches, you are doing spine flexion. The opposite of flexion is extension. A body of research by M. Sinaki going back to 1984 (Archives of Physical Medicine and Rehabilitation 1984) has shown that back extension exercises are protective against vertebrae fractures in post menopausal ladies. This research has been woefully overlooked.
It is difficult to find good back (or spine) extension exercises. Foam Roller Extensions are one option.