Fracture in osteoporosis. Coloured X-ray of a fractured humerus (upper arm bone) in the shoulder of a patient with the bone disease osteoporosis. The head of the humerus (upper right, round) has broken off the bone's shaft (lower right) and become dislocated to the right. The collar bone appears in the upper left and the ribs in the lower left. Osteoporosis is caused by loss of the protein matrix in bones, and is a natural part of ageing. The condition is especially severe in post-menopausal women due to loss of the hormone oestrogen, which helps maintain bone mass. The bones become brittle, weak and liable to fracture.