University of Southampton, School of Medicine

Your skeleton is very important to you. It supports your body, protects your vital organs, and, [with your muscles, tendons and ligaments] allows you to move. It also stores calcium and other minerals for your body to use when necessary, and it contains marrow which produces red and white blood cells.

Bones are named after their shapes:

• Long (leg, arms) – hollow; main store for bone marrow; they support your weight and are crucial for the way you move
• Short (wrist, ankle) - grouped together to allow you to turn your hands and feet many different ways
• Flat (skull, sternum) – these are separate when you are born; they are glued together by special ligaments before you are an adult. They protect the brain and other organs
• Irregular (spine, hips, shoulder) These bones which have changed during human evolution to provide for particular needs, e.g. support, protection, and/or easier movement
• Sesamoid (e.g. knee) – act as fulcrum for ligaments and tendons to pass over. They have a smooth surface so that the tendons can slide over them when you bend your knee or elbow etc

Bones are joined to each other by joints. All of your bones, except for one in your neck [the hyoid bone] join with another bone. Joints allow your rigid skeleton to move. Most of your joints are 'synovial'. This means that they have a strong sac [bag] around them which contains a lubricating liquid called synovial fluid. The two main examples of synovial joints are:

• Hinge joints: [knee, elbow, wrist, ankle]. Hinge joints are like the hinges on a door. They let you move your leg one way only. In the wrist and ankle there are hinge joints and gliding joints [see below], which is why you can move these in more than one direction.
• Ball and socket joints: [shoulder, hip]. The long bones [femur or humerus] have a ball at one the end, which can move easily in the socket which is part of the hip or shoulder joint. That is why you can move your arms and legs in so many directions.

Other synovial joints are:

• Ellipsoidal [finger joints]. These allow you to bend and straighten your fingers, but you cannot make them turn [limited rotation].
• Gliding. Theseare found between the surfaces of two flat bones that are held together by ligaments. Some of the bones in your wrists and ankles move by gliding against each other.
• Pivot joint [axis and atlas vertebrae]. These joints allow you to nod your head up and down and shake it from side to side.

Joints that are allow less [or no] movement are:

• Sliding joint: [vertebra]. One bone is able to slide easily against another. There is a pad of cartilage [vertebral disc] between each vertebra, and although each joint can only move a little, you can move your back quite a lot because there are 26 vertebra working independently. The 7 neck [cervical] and 5 lower back [lumbar] vertebrae allow the most movement.
• Fixed joints: [skull]. These joints do not allow any movement. The bones are held together with fibrous connective tissue. When a baby is born, the bones are not held so tightly, so that the baby’s head can squeeze through the birth canal. By the time that the baby is two, all the joints are ‘glued’ together. They are sometimes known as ‘suture’ joints.

How is bone made?

Although there are different shaped bones, their structure is similar. They are made of living material [Osteocytes and protein] mixed with nonliving minerals [e.g. calcium and phosphorus] which make the bone hard. A protective membrane called the Periosteum covers the bones and helps to connect them to muscles and provides a rich blood supply to nourish the bone. The middle part of a long bone, called the shaft, is a tube made of hard material called compact bone. This tube is filled with yellow coloured marrow which stores fat Small holes, known as Haversian canals, run through this bone so that blood vessels can nourish the osteocytes

There are 3 main steps in bone formation:

1. Production of matrix [substance from which the bone is made]
2. Mineralization of the matrix to form bone
3. Bone remodeling – replacing old bone with new bone

This work is done by three types of cells which live within the bone (osteoblasts, osteoclasts and osteocytes). They are important for both bone formation and bone remodeling.

During bone remodeling, old bone is ‘eaten up’ by osteoclasts, and new bone is laid down by osteoblasts. This keeps the bone alive, healthy and flexible.

How do our bones grow? <4>Bones can grow in two ways. Your skull, for example was formed by intramembranous ossification. The membrane that covers the bone becomes hard [ossifies], and another membrane grows. That becomes hard as well, and is covered by new membrane … and another … and another … That is why a baby’s head is relatively soft, and why an adult has a very hard skull!

The second method is endochondral ossification. This happens in long bones, such as the thigh bone [femur]. At both ends of the bone are Epiphyseal plates. First the cartilage increases, and then it is replaced by bone cells. This process is called ossification. When all the cartilage is gone, the bone stops growing lengthways [usually well before a person is 25 years old].

At the end of a bone, there is an area of thin hard material called hyaline cartilage. In flat bones and the ends of long bones this is very thin. Underneath is spongy bone [cancellous bone] which acts as a shock absorber. In some bones this spongy material contains red marrow in which red and white blood cells are produced.

What can go wrong?

Fractures

Although bone is very strong it can snap under strain. When a bone breaks it is called a fracture. Children often have ‘greenstick’ fractures where their bones bend but the bone does not break all the way through. This is because their bones are still relatively soft.A hairline fracture is a thin break in the bone, and usually heals quickly.

If a bone breaks completely into two or more pieces it is more complicated, and sometimes has to be mended surgically using metal pins and plates.

Simple fractures usually take about 6-8 weeks to heal, although larger or elderly bones take longer.

• When a bone breaks, blood vessels rupture and cause a haematoma [blood filled swelling] at the fracture site.
• A callus [lump made of fibrous tissue, calcium, and cartilage forms in place of the haematoma. It holds the broken bone in place for the permanent repair.
• The callus becomes bonier, as the cartilage is replaced by spongy bone.
• This bony callus remodels depending on the stress placed on it, making a strong, permanent patch at the fracture site. This part of the process can take as long as a year, but the bone is often is stronger after the break than it was before.

Osteoarthritis

Osteoarthritis (OA) [inflammation of the joint] is very common in older people and causes pain and stiffness. The most commonly affected joints are the knees, hips, hands, feet and spine. Normally, cartilage covering the ends of the bone acts like a smooth cushion between the bones. In the joint there is also a thick liquid called synovial fluid that allows the joint to move smoothly. When OA affects the joint, cartilage is damaged and becomes thinner. As the cartilage tries to repair itself the bone underneath becomes thicker, changes shape and hardens. Also, as the cartilage breaks down, the bones rub against each other causing pain, stiffness, and loss of movement. There is no cure for OA but some treatments relieve the pain and allow the patient to remain active. Some joints, such as the hip and knee, can be replaced by metal and plastic ‘prostheses’, which allows the patient to regain their mobility and independence, but many joints [e.g. vertebrae] cannot be changed. One area of our research is to discover ways to improve treatment for this disabling disease.

Osteoporosis

This is a disease that usually affects older people, because bone turnover is regulated by hormones which decrease as we age. As old bone is absorbed by osteoclasts, new bone is made by osteoblasts to replace it, so that about 10% of bone in the body is changed every year. If more bone is absorbed than is made, the bones become thinner and more brittle (loss of bone mass). Younger people can also be affected, if they have certain diseases or medicines [e.g. long term steroids]. More women are affected than men, and it can be made worse by smoking or drinking too much. It is also seen in young people with eating disorders [Anorexia Nervosa, Bulimea etc].

People with osteoporosis break their bones more easily – especially their hips [head of femur]. They also become shorter as they get older as their vertebrae are affected, and they often develop other problems because they are less able to move around.

It is important to build up your bone mass whilst you are young by eating a balanced diet and exercising, so that this disease does not affect you when you are older.

What do we do?

The Bone and Joint group in Southampton is working on a number of projects.

• Some of our scientists are asking the question ‘why do people get arthritis’, and how can we help them.
• We are looking at ways to make help bones mend more quickly using the patients’ stem cells. Stem cells are found in bone marrow, and recently scientists and doctors have become very excited about them. We want to find out more about adult stem cells and the ways that we can use them to help patients. We are also looking at ways to instruct these cells to make cartilage, bone, muscle, tendon, ligaments and fat, so that damaged or diseased tissue can be replaced with the patient’s own cells.
• We are also developing scaffolds from materials found in nature, so that the cells can be grown into specific tissue types and shapes.