A spiral fracture is a bone fracture

Bone fracture

A bone fracture is a medical condition in which there is a break in the continuity of the bone...

 occurring when torque

Torque

Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....

 is applied along the axis of a bone

Bone

Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

. While torsional forces are being applied along the parallel axis of a bone, planes perpendicular to this axis are not affected. Tension is exerted upon one part of the bone, while compressive forces are exerted upon the other. When these forces have exceeded the limit tolerable by the bone, fracture occurs.