So after two different vets and a set of x-rays, we have determined that Snape fractured the neck of his left femur. On Monday the surgeons here will performing a femoral head and neck ostectomy, which as far as I can tell involves shaving his hind end, cutting him open, removing the now-severed ball of his femur from the joint, sawing off the neck of his femur, and then stitching him up and sending him home the next day. This is apparently a better procedure than just putting the bones back together because it avoids the risk of developing arthritis in the hip joint. It will cost about $3000.
We still don't know how this happened, but I am looking for new housekeeping services, so if you have any recs for maids in the D.C. area, please let me know.