Stress shielding refers to the reduction in bone density (osteopenia) as a result of removal of typical stress from the bone by an implant (for instance, the femoral component of a hip prosthesis). This also occurs underneath bone plates and intra medullary rods. How does stress shielding happen? The science of it is that bone growth is stimulated by loading or weight bearing. Consequently anything that relieves the bone of weight bearing is likely to incur osteopenia. Non-traumatic examples of this are 1. long term bed rest which is common in pre-twentieth century ailments when patients could be confined to bed for months with conditions such as pulmonary tuberculosis, heart conditions such as heart attacks or heart failure. Conditions where nowadays, patients are encouraged to be up and ambulant within a day or two2. having a leg in a plaster cast or requiring non-weight bearing for prolonged periods of time, implying a matter of 3-6 months3. the most well known case of this problem was when man started to explore space. It’s well-known that in space, the human body is weightless and therefore osteopenia will be more than likely to occur. After the first trip around the moon, it was generally expected that when the astronauts took their first step out of the recovery helicopter onto the ship, in at least one of them a bone in their leg would just give way. Later journeys and in the Space Station, exercise was an important part of the daily routine for astronauts aboard the station to prevent bone and muscle loss. On average, astronauts exercise two hours per day. The equipment they use is different than what is used on Earth. Lifting 200lbs on Earth may be a lot of work. But lifting that same object in space would be much easier. Because of microgravity, it would weigh much less than 200 pounds there. That means exercise equipment needs to be specially designed for use in space so astronauts will receive the workout needed. How does bone grow/heal The pathology of bone growth is that stem cells, the bone equivalent to a foetus, are developed in quantity within the medullary cavity by small organisms call pre-osteoblasts. Just like it mammals, not all these cells go on to develop but most do and develop into immature bone cells, osteoblasts, which rest in the cancellous material in the centre surface of the hard bone. Over a few days, these grow and develop and gradually move into the hard bone. One little known fact about bone is that it is continually growing from within out, just like skin. Over the next few weeks, the baby bone cells mature and migrate towards the outer surface of the bone where they die and are absorbed by other organisms called osteoclasts. This is how a bone grows or heals after injury. Interesting note: did you know that bone is the only tissue in the body to NOT leave a scar when it sustains an injury? This obviously excludes very major fractures.