Chalkstick fractures are fractures, typically of long bones, in which the fracture is transverse to the long axis of the bone, like a broken stick of chalk. A healthy long bone typically breaks like a hard woody stick as the collagen

Collagen

Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

 in the matrix adds remarkable flexibility to the mineral and the energy can run up and down the growth rings of bone. The bones of children will even follow a greenstick fracture

Greenstick fracture

A greenstick, buckle or torus fracture is a fracture in a young, soft bone in which the bone bends and partially breaks. A person's bones become harder and more brittle with age. Greenstick fractures usually occur most often during infancy and childhood when bones are soft...

 pattern.

Chalkstick fractures are particularly common in Paget's disease of bone

Paget's disease of bone

Paget's disease is a chronic disorder that can result in enlarged and misshapen bones. The excessive breakdown and formation of bone tissue causes affected bone to weaken, resulting in pain, misshapen bones, fractures, and arthritis in the joints near the affected bones...

.