Spinal traction with metal tongs in the skull is used to treat a fracture of the

The answer to this question would be: vertebrae 

In vertebrae fracture, traction is important to assure immobilization of the vertebrae. Movement can cause the fragment of bone in the fracture damaging the neural tissue in the vertebrae.
To ensure no movement in the neck, metal tongs can be put on the skull. The tongs will help fixate the skull with the shoulder.