I recently heard from someone who struggles with poses such as Upward-Facing Dog because of a fused ankle. The foot no longer goes into plantar flexion, which is the position you take after rolling over your toes and resting the top of feet on the mat with the soles facing upward.
It's a common dilemma for yoga students who have sustained injury to the ankle, which either limits their ability to point the foot or to flex the ankle back in the opposite direction (called dorsiflexion). When surgery is required, it improves the stability of the ankle for standing and walking, but doesn’t usually allow much range of motion beyond that.
I learned a useful substitute for this plantar flexionI was studying yoga at the Krishnamacharya Yoga Mandiram in Chennai, India, we were instructed in Surya Namaskar, or Sun Salutations, we were encouraged to transition from Downward-Facing Dog into Upward-Facing Dog without doing Chaturanga Dandasana. And because we were inhaling into Up Dog and exhaling right back into Down Dog, we were told to keep the toes turned under and the ball of the foot on the floor, such as in the back foot in a High Lunge. (See photo below.) It was just so efficient!
I came to appreciate this variation so much that I did it more and more in my home practice and started teaching it to my students as well. In the case of a fused ankle, this variation works beautifully! You keep both ankles, feet and toes in the lunge position. And of course you can hold this variation for several breaths, just as you would the top-of-the-foot version. I would recommend that you also press the heels strongly back while in the pose to keep the legs more active as a support for the overall shape of Up Dog. You can even set up with your feet near a wall and press your heels into the wall to train the feet and legs to maintain this sort of directional action when you are doing the pose in the middle of the room.
I have also found that using the yoga wedges as a lift and support can also be helpful, especially under the ball of the front foot in poses like Triangle, where the front ankle, again, needs to able to point, or plantar flex, to get the entire foot grounded. This way of supporting the limited range of motion of the ankle joint could also be helpful for those suffering from the much more common ankle sprain, which requires limiting movement in the recovery period until the swelling subsides.