So why do some patients fracture bones from simple falls or knocks whereas others con be involved in a high impact accident and come out unscathed? Osteoporosis is a common risk factor especially amongst the elderly that predisposes them to fractures. Osteoporosis is defined as a reduced bone mass with normal mineralization.
Bones not only provide support to allow us to move and maintain our shape, they are vital for producing blood cells from the stem cells found in the bone marrow and also for storage of lipids and minerals. These minerals give bone its highly specialised extracellular matix (ECM), differentiating it from other forms of connective tissue. In addition to the collagen and associated proteins in the ECM of connective tissue, bones are impregnated with calcium hydroxyapatite crystals. These associate with the collagen fibres making bone strong and hard. The ECM is organised into thin layers known as lamellae.
Bones are constantly being broken down and reformed by osteoclast (resorb bone) and osteoblasts (create new bone). The osteoblasts produce the crystals of calcium and phosphate with are laid down in the ECM. This process is very delicate and is influenced by a number of different factors. Hormones play a major role; oestrogen, parathyroid hormone and testosterone to a lesser extent.
Osteoporosis is a condition most common in postmenopausal women, this is due to the decline in oestrogen. This hormone when presents binds to osteoblasts via oestrogen receptors and promotes their activity. Therefore a decline in oestrogen would correlate with a lower level of active osteoblasts leading to reduced bone mass.
Parathyroid hormone (PTH) has the opposite effect in that it stimulates osteoclasts to resorb the bone so that there is a release in calcium into the circulation, therefore hyperparathyroidism can also lead to osteoporosis.
Patients that might be at risk of developing osteoporosis are usually investigated, or if a patient presents with a fracture and the mechanism of injury doesn’t fit the extent of the injury. A bone mineral density scan can be before that detects a patient’s bone mass and rates it against the average population to obtain a T score (<2.5 is diagnostic of osteoporosis).
Simple lifestyle modifications can reduce the risk of osteoporosis such as regular exercise and smoking cessation. For those with diagnosed osteoporosis lifestyle advice is very important along with Bisphosphonates which act by inhibiting osteoclast activity therefore decreasing bone reabsorption and increasing bone mass.