How do the bones work?

Our bones’ outer walls are referred to as the outer bone layer (compacta). This layer is hard and especially strong. Inside bones there is a supporting structure with interconnecting bony plates and rods called trabeculae. It is called spongy bone because of its sponge-like structure, but is sometimes also referred to as trabecular or cancellous bone. Long bones like the arms and legs also have a bone marrow cavity.

The bones have good blood circulation: Numerous veins pass through them. The spongy bone and the marrow cavities contain red bone marrow, which produces blood cells and what is known as fatty marrow from fat tissue. In childhood, many of our bones contain red bone marrow, but in adulthood red bone marrow is only found in certain bones like the ribs and vertebrae, the sternum (breastbone) and the pelvis.

Some bones develop from connective tissue, like the skull. Others start off as cartilage that is later replaced by bone tissue. The arm and leg bones are examples of this. But they do not completely turn into bone for as long as we are still growing: A small amount of cartilage remains at the ends of the bones. These parts are called growth or epiphyseal plates. New cartilage is constantly being produced there, which later becomes bone. That allows the bones to keep growing longer. Towards the end of puberty, the growth plates gradually turn into bone as well, and our bones are fully grown.

The mineral content of our bones is crucial for their strength. Calcium phosphate is the most important of these minerals. It makes the bones harder and stronger. The more minerals a bone has in it, the harder and denser it is. But the strength of a bone also depends on the shape and density of the spongy bone inside.

Two kinds of living cells that make our bones a kind of permanent construction site are walled inside the bone tissue: The osteoblasts build layer upon layer of new bone material all around themselves and make the bone grow. The osteoclasts do the opposite: They break down old bone tissue. Together, these two types of cells make sure that every bone can adapt to the strains it is put under and can repair itself after a fracture.

As long as the same amount of tissue is produced as is broken down, the bone gradually regenerates without getting weaker. That means all of the bone tissue in the skeleton is completely replaced about every eight years. Various hormones such as sex, growth and thyroid hormones control this regeneration process.

Adult skeletons weigh about seven to nine kilograms. Bone cells make up about one percent of the bone, the rest is bone tissue. It is made up of 25% water and 25% protein such as collagen. Minerals such as calcium phosphate make up 50% of the bone.

Bones are still growing in children and teenagers: They grow longer, heavier, and more dense. Our bones are most stable around the age of 30. After that, our bodies slowly start to break down more bone than they produce. That process speeds up at around the age of 50, especially in women. Up until menopause, the female sex hormone estrogen protects the bones by slowing down bone loss. Bone tissue is then broken down more quickly when estrogen levels gradually go down after menopause.

The risk of fractures can increase in older age and osteoporosis can develop. But there are lots of things you can do to protect and strengthen your bones, even in older age. Exercise is most important, because if enough strain is put on the bones their metabolism also remains active and new bone tissue is produced.

The ends of the bones in our joints are covered in a layer of cartilage. If that cartilage is very worn, perhaps due to old age, the bone also changes. This kind of wear (degenerative changes) of the bones (osteo) and cartilage (chondro) is called osteochondrosis.

If a vertebral disc between two vertebrae gets thinner, bone growths often develop on the edges of the vertebra (osteophytes). That can also happen on worn joints, like on the knee or hip. These growths can be like bulges or bumps. They develop because the body tries to make the joint bigger to spread the pressure across a larger area. Depending on their size, osteophytes can go unnoticed, but they can also cause pain and restrict mobility in the joint. They can cause narrowing (stenoses) of the spinal canal where the bone marrow is found.

Especially in children and teenagers, a fragment of bone and cartilage can become loose under a joint surface because of bone wear. This is called “osteochondrosis dissecans.” If the loose fragment stays where it is, it only causes mild pain or might not even be noticed. But it can also become completely detached and move into the joint. A “joint mouse” like this can cause painful swelling or block the joint. Joint mice are usually broken down over time. But because the area in the joint where the fragment broke off never heals completely, there is an increased risk of osteoarthritis.

Experts think that along with hereditary predisposition, severe joint strain, metabolic disorders, and hormones can also be a risk factor for developing a joint mouse. Osteochondrosis dissecans is most common in the knee joint, but it can affect any joint.