You may want to read
this thread on the forum about a student who has spinal stenosis. In general we can say that students "who have been diagnosed with advanced spinal stenosis should avoid extreme extension of the spine, such as back bends in yoga." Of course, in Yin Yoga, we do not do extreme backbends: Sphinx should be fine, but everyone is different and you have to pay attention to the sensations while in the pose, when coming out and in the next day as well.
Let me know if you still have questions after reading the thread.
Cheers
Bernie
Quote is from
Dr Ananda Ghandi