 In this video, I will describe the process of bone remodeling and define osteoporosis, differentiate among the different types of fractures, and finally describe the process of bone regeneration to repair a fracture. Bone remodeling is a process that occurs throughout the lifespan where new bone is formed by the activity of osteoblasts and bone is also simultaneously being broken down and reabsorbed by osteoclasts. This occurs in order to maintain the proper proportions of bone as we grow, as our bones become longer to remodel the proportions of the bone throughout adolescence, throughout growth, but also in order to repair bone that is damaged if we have a fracture, or to strengthen bone in response to mechanical stresses that are occurring throughout our life. Mechanical stress will stimulate remodeling of bone in order to make bone strong enough to adapt to the load which it experiences. This is a fundamental principle of bone remodeling known as Wolff's law that under normal circumstances bone can adapt to strengthen areas that experience the most mechanical stress. Bone remodeling is also important to maintain blood calcium homeostasis as parathyroid hormone stimulates osteoclasts. More calcium will be released from bone and bone will become weaker in the area where calcium has been released. Osteoporosis is the reduction in bone mass that results in a decreased bone density and bones that are light and fragile, that are more prone to fracturing. Osteoporosis occurs when the osteoclast activity that breaks down the matrix of bone to release calcium occurs at a higher rate than osteoblast activity that deposits calcium in the bone to strengthen bone. This leads to bones that are fragile and light. We can see the example here of the effects of osteoporosis on the thoracic vertebrae where there is normally a curvature to the vertebral column in the thoracic region to help support the weight of the upper body. That weight can lead to compression of the vertebral column and exaggerate the curvature of the thoracic region, a condition known as kyphosis or hunchback, which is more common in individuals with osteoporosis. While both men and women are at risk of osteoporosis, there is a higher risk of osteoporosis in women after menopause. This is because estrogen normally reduces the activity of osteoclasts and as estrogen levels fall during menopause, osteoclast activity increases. There's normally an increase in bone density throughout the early years of life that peaks around 30 years of age and then declines afterward. A variety of factors can contribute to the rate of the increase or decrease in bone density. For example, dietary intake of calcium and vitamin D levels that are available sufficient enough to absorb calcium from our diet so we can get vitamin D that we can synthesize from the sun or we could also get vitamin D from food, especially if it's been added as is the case with fortified milk. Vitamin D is important for the absorption of calcium, so if we don't have enough vitamin D we can become calcium deficient because we can't absorb the calcium in our diet. However, a poor diet with a low calcium level of the diet could also lead to calcium deficiency and decreased bone density. Losses is an important factor because putting a load on our bones, providing some mechanical stress to our bones stimulates bone remodeling, stimulates the activity of osteoblasts to strengthen the regions of the bone that are experiencing the most mechanical stress. Other lifestyle factors include smoking and drinking alcohol that can reduce calcium absorption and genetics is also an important factor. The tendency to accumulate bone mass is something that runs in families and there are mutations in certain genes that can increase our risk for osteoporosis. Nutrition for osteoporosis includes nutritional supplementation with calcium and vitamin D and when possible, exercise to promote remodeling and strength to help maintain bone. However, it's also important to be careful to avoid fractures in osteoporosis, not to over exert or to lose your balance and fall. There are also drugs that are effective against osteoporosis, including hormone replacement therapy in women especially, replacement of estrogen and progesterone can be a treatment in order to decrease the activity of osteoclasts. There are drugs called bisphosphonates, a couple of examples of bisphosphonates are Phosamax and Boneva. So bisphosphonate is a broad class of drugs that inhibit osteoclast activity, but it's a specific chemical classification of drugs. There are other drugs that have been more recently developed in order to stimulate the activity of osteoblasts. Interestingly, Forteo is a form of parathyroid hormone that has a paradoxical effect when injected. This form of parathyroid hormone can stimulate the activity of osteoblasts even though the normal effect of parathyroid hormone released by the parathyroid gland in our body is to stimulate the activity of osteoclasts. In this situation when it is injected it has the opposite effect and stimulates osteoblast activity. Vitamin D is an important factor for the maintenance of bone density and the development of bone because vitamin D is necessary for the absorption of calcium from the diet. We can produce vitamin D if we have sufficient exposure to the sun, UVB radiation is necessary for a step in the synthesis of vitamin D. Vitamin D is synthesized from cholesterol and so it is considered a steroid hormone. We could also get vitamin D from the food or from supplements although there are not many foods that naturally contain vitamin D. Fatty fish is an example that contains vitamin D and some mushrooms contain vitamin D. And then you could also take supplements of vitamin D. You could take vitamin D pills and vitamin D has been fortified in whole milk and so the vitamin D has been added to whole milk as an effort to increase the calcium absorption from that milk and reduce the risk of calcium deficiency and a disorder that results in children called rickets where bones become weak and bend under the weight of the body. If there is not enough calcium available to strengthen the bones during childhood it results in this condition known as rickets. After the precursor form of vitamin D is absorbed from the diet or synthesized in the skin it is further activated in the liver through a process of hydroxylation. So hydroxylation is when a OH oxygen and hydrogen hydroxyl group is added to the chemical and there are multiple steps of hydroxylation that are involved in activating vitamin D. The first step occurs in the liver and then it's subsequently activated in the kidney and this activation is under the regulation of parathyroid hormone so that parathyroid hormone stimulates the activation of vitamin D and increases calcium absorption from the diet. A fracture in which the skin remains intact is known as a closed or simple fracture in contrast. An open or compound fracture is when an end of a bone carries through the skin. This carries a high risk of infection so an open or compound fracture is very serious to make sure that both the bones are reset so that they can heal properly and also so that the wounds are cleaned thoroughly and closed to prevent infection. A transverse fracture is a fracture where the bone breaks straight across the long axis of the bone. In contrast an oblique fracture is when the bones break at an angle that is not a straight 90 degrees relative to the long axis but rather at an oblique angle such as 45 degrees or 30 degrees. A spiral fracture occurs when bone segments are pulled apart as a result of a twisting motion. A comminuted fracture is when several fragments are formed from one bone that breaks into numerous small pieces. An impacted fracture is when one bone fragment is driven into the other. This is also known as a compression fracture from a strong compression force could cause one bone to be forced into another. Last we see a green stick fracture. A green stick fracture is a partial fracture in which only one side of the bone is broken. Green stick fractures are more common during adolescence because the bones are more flexible with a relatively higher amount of collagen and a lower concentration of hydroxyapatite making the bones less brittle and more flexible. In order to heal after a fracture the first stage in fracture repair is the formation of a fracture hematoma commonly referred to as a blood clot which occurs when ruptured blood vessels are detected by platelets and clotting factors which leads to activation of the platelets and clotting factors. Platelets will stick together and form a platelet plug and the clotting factors will lead to the formation of an insoluble fibrous network of protein which helps to reinforce the platelet plug in order to stop bleeding. Then the formation of an internal and external callus occurs where osteogenic cells from the bone membranes will differentiate into osteoblasts and osteoblasts will fill in spongy bone in order to help hold together the bone fragments. There will also be chondroblasts that migrate in and create cartilage to help stabilize the bone fragments. The internal and external callus are a mixture of both spongy bone and cartilage. In the next step the ossification of the cartilage is a mechanism of endocondral ossification so the cartilage is replaced with osseous tissue. Now the entire callus is filled with spongy bone. The last stage in fracture repair is remodeling of that callus where the osteoclasts will remove damaged bone fragments and remove spongy bone from the surface and then osteoblasts will lay down a new layer of compact bone around the surface and gradually the bone will be remodeled to have the same overall shape as the original bone.