What does loosening the screw mean on retainers?

Horse Fetlock has a Screw in it? What does this mean?

• There is a horse for sale (I have owned horses all my life). I was looking at this horse and the vet said there was nothing to worry about (had it vet checked). However the owner said the horse has a screw in it's fetlock. I re-asked my vet and once again he said it shouldn't be a problem. But was not detailed on what it is and I cant find much info. Can someone tell me more please? What is it? Is the horse ok?

• Answer:

  The horse had surgery for some sort of bone injury in the fetlock, and it now has a screw as a result of it. The horse might go its entire life without any sign of the injury. It also might show complications from the injury as it grows older, such as arthritis, etc. I would have that fetlock x-rayed to see if everything is ok. I would also get a second opinion from a different vet.