if are you just check your friend's condition? How's your back? 등은 좀 괜찮아? Is your back better? 등은 많이 좋아졌어? question after you can say : 나는 니가 한국에 돌아간뒤 너의 상태가 많이 걱정되었어 어떻게 지내 ?

I'm not a native, just for the record. However, it seems that the question I hear in dramas would work in this situation: 몸 괜찮아요? This literally means, "Is your body okay?" I hear this expression used when someone is sick or hurt. I suppose you could be more specific and say: 등 괜찮아요? or be more literal and say 등 어때요?