Failure with bending: In bending the bone is loaded in compression on one side (the concave side) and in tension on the other side (the convex side). The bone thus shears out a “butterfly” fragment on the compression side and develops a transverse fracture on the tension side.
The amount of axial tension or compression that is present with the bending load determines the size of the “butterfly” fragment. The quality of the bone also influences this. Bone in children is weak in compression but strong in tension (as it has good collagen but less mineral), and thus bending loads may “buckle” the compression cortex (also known as a “torus fracture”) rather than shearing out the fragment.
Failure with torsion: Torsional loads produce spiral fractures. These fractures hinge open on the straight longitudinal portion with the spiral side “pulling apart.” A piece of chalk or a brittle bread stick make great examples, twist one ‘til it breaks and play with the spiral to see how it fits together.