Can you move your thumb out, not touching the safety and let it rest on the base of the weak hand thumb? If not you may need to cross thumbs like a revolver grip. If you cross thumbs be careful during recoil to not engage the safety. Neither is as good a grip as putting the thumb on top of the safety, but if you are in pain you won't have much fun shooting.
My thumb has some strength but has no sensation except for some on the outside. I can place it under the safety and parallel to the slide comfortably; however, due to joint fusions, grafts, etc. there is not sufficient mobility to get in on the safety.
I guess my real question should be whether my thumb is to be involved with the grip or should I let it play a neutral role? Do some people not ride the safety?