Can you break a hip without falling?
Regarding hip fractures in the elderly, the majority of them occur from falls. I would have to agree with your uncle's doctor here. If he is an experienced orthopedist, he knows that hip fractures from a fall would appear totally different from a spontaneous fracture from walking.
Furthermore, a hip fracture that occured just from walking is very rare. It would mean that your uncle was severely osteoporitic, which would be rare in a man. Also, he would have fractured many other bones before this one, as the femur is one of the strongest bones in a human.
About your uncle- he may want to believe he broke his hip first, because that may make him feel less embarrassed. It may also help him feel that the fall wasn't his fault.
Please don't feel you need to defend him and make people think he is not "mixed up". The best thing you can do is support him by letting him know that you will be there for him during his recovery, as this will be a difficult process. He needs to participate in physical therapy so that he can get stronger and walk again.
