Osteoporosis is a lessening of the bone material (bone density) and therefore weakening of the bones meaning that patients with osteoporosis are more prone to bone fracture. Bone is a living material with cells called osteoclasts responsible for removing worn material and osteoblasts are the bone building cells. From age 16 and 18 bones start to stop growing in length, but bone density continues to increase slowly until mid 20s. The balance between worn bone removal and new bone construction stays stable. As part of natural ageing after the age of 35, bone loss increases very gradually. However, bone loss becomes more rapid in women for several years following the menopause and can lead to osteoporosis and an increased risk of fractures.