Oh goodness sorry I missed this Christine, yes it's very worrying and you do feel guilty when something like this happens but it does happen fairly frequently...so don't beat yourself up.
It sounds like Spangle has come through absolutely fine. Rupert did the same on a bully stick many years ago.
It shows that normally she is well managed for pain as in the past mine have not wanted to chew with SM/CM symptoms when it's not so well controlled.
Tania sent us a wonderful hamper from http://www.venisonforpets.co.uk/product_range.php they love the marrow bones, they seem to soften when chewed too and are not rock hard like some treats.
Desrae's advice is very sensible, never leave a dog unattended with anything like this - especially not rawhide - so that you can be there if they choke or if they are chewing rather too enthusiastically!
It also makes things more interesting if you restrict access! We keep the marrow bones in an air tight container with those food grade silica sachets - they keep for ages. When they've been chewing, we leave the bones out to dry overnight and then put them away.
Thank goodness Spangle is ok now - gentle hugs for her xx