The external frame is anchored to the pelvis. Two surgical pins are driven into the bone of the pelvis and another into the femur (leg bone). The pelvic pins create a fixed point of attachment relative to the acetabulum (the socket part of the joint) to which a third pin in the femur can be joined. In this way the femoral head (the ball part of the joint) can be held in position in the acetabulum while healing occurs. The use of a special hinge in the external frame allows the dog to use the leg while maintaining reduction (i.e. keeping the joint together). The X-ray on the left was taken to check the alignment of the first pelvic pin. Because this area of the pelvis has very thin bone the pin should ideally go right through into the sacrum to get more grip. The sacrum is part of the spine and the pin must be carefully aimed to avoid the spinal canal (where the nerves are). Move the mouse over the image to highlight the sacrum (in green) and the spinal canal (in red). The optimal zone for the pin is within the solid green area. This pin is nicely positioned in the pelvis and can now safely be driven onwards into the lower, safe, part of the sacrum.