Patient Information - International
What is bone?
Bone is the hard connective tissue that makes up the majority of the skeleton. It has many roles within the human body, including:
- Support of body structures: The skeleton gives our body its recognizable shape and form
- Protection of vital organs: Such as the skull protecting the brain, and the ribs protecting the heart and lungs
- Facilitate movement: Bones form almost all of our joints such as the knee and hip, and allow us to move around effectively
- Production of red blood cells: Large long bones for example, the thigh bone (femur), are part of our body's system for making red blood cells, which takes place in the bone marrow. Red blood cells carry oxygen around our body
- Storage of vital minerals: Minerals such as calcium, phosphorous, silicon and multiple trace elements are stored in our bones
The Skeleton
As you can see above, and feel on your own body, all of our bones are different shapes. They are shaped the way they are for specific reasons; our bodies shape them while we are growing and developing, into the perfect shape required for the function each one of them must perform.
So, we have long bones such as our femur, irregular bones such as our knee-cap (or patella) and we have flat bones such as our shoulder blade (or scapula).
Bone itself is of two types.
- Cortical bone, which is flat and hard, arranged in sheets on the surface of our bones to give a hard outer layer that resists impact forces.
- Cancellous bone is a softer bone, which forms a honeycomb-like lattice of struts inside our bones called trabeculae. The structure of these trabeculae gives bone great strength against bending forces; we use the same process ourselves to reinforce many everyday structures such as corrugated cardboar.
The bones in our body exist in a dynamic equilibrium – there is constant remodelling of existing bone. Old bone is eaten up by specially designed cells called osteoclasts, a process complemented by the laying down of new bone by, other specialist cells called, osteoblasts. This keeps the bone healthy, alive and flexible. TOP
What are bones made of?
A femur (thigh) bone
Our bones contain certain substances that are vital to our skeleton's healthy growth, function and repair: (roll over the red text to read more detail)
- Collagen - a fibrous protein that gives bones flexibility
- Calcium Phosphate - vital to our bones and skeleton because this mineral gives bones their strength (the form of Calcium Phosphate found in bone is known as Hydroxyapatite)
- Growth factors – chemical signals that are vital to bone formation, bone repair and bone remodelling
- Specialized cells –
- Osteocytes are mature bone cells, and make up 90% of the cells in bone. They are actively involved in the routine turnover of bone through mechanosensory mechanisms.
- Osteoblasts are responsible for the formation of new bone (osteogenesis).
- Osteoclasts are responsible for breaking down bone, and releasing its stored minerals (osteolysis).
- Trace Elements – such as silicon, fluoride and zinc, essential for the normal growth and development of the human skeleton. Although they represent a very small portion of bones by weight, they play an important role in bone metabolism, remodelling, and repair.
Collagen is a protein that gives bones flexibility. If our bones were simply made of minerals such as Calcium, they would be very brittle and powdery like chalk. Collagen also provides bones with resilience, so that when we do repetitive or strenuous activities our bones can cope. Finally, collagen provides our bones with a framework on which minerals can be laid down, most importantly Calcium Phosphate – known as Hydroxyapatite within bone
Calcium Phosphate; don't be put off by the complicated formula: (Ca10(PO4)6(OH)2). This simply tells us that this mineral contains Calcium (Ca), Phosphorous (P), Oxygen (O) and Hydrogen (H).
The form of Calcium Phosphate found in bone is known as Hydroxyapatite or 'HA' as it is more commonly known is. It is vital to our bones and skeleton because this mineral gives bones their strength. HA is an extremely hard substance and ensures that bone can bear heavy loads - HA gives us weight-bearing capabilities. One other function that HA performs is that it 'glues' together the collagen fibres in our bone providing a strong and stable matrix or framework.
Growth Factors are vital to bone formation, bone repair and bone remodelling (which we will talk about later in this section). Sometimes a process or reaction requires a signal to 'trigger' it into action or to encourage the process to take place quickly and efficiently. In a similar way, growth factors initiate or stimulate bone growth, repair and remodelling.
Growth factors are substances found in blood and bone which are vital for the proper function of bone - they are part of the messaging or communication system which tells our bone what to do - grow, change shape, dissolve away or repair. Some examples of growth factors you may hear are bone morphogenetic protein (BMP), transforming growth factor (TGF) and platelet derived growth factor (PDGF).
Specialized cells are the working part of bone. Our bone is dynamic living tissue and is continually renewing itself, so the normal state for our bone is a continual balance between old bone being broken down and new bone being laid down.
Trace Elements - Extensive research on dietary silicon has shown that silicon is a critical factor in normal bone development. The mineral phase of normal bone is hydroxyapatite with varying amounts of trace elements such as silicon and zinc. X-ray analysis has identified silicon in growth areas in the developing bones of young people. In adults, silicon is localized in active osteoblasts (bone forming cells) and is a major ion of bone forming cells (on a par with calcium, phosphorous and magnesium). Silicon is localized in those specific portions of tissues associated with calcification. Its importance in bone formation and repair is well established. TOP
Bone Remodelling
If you ask your bones to do something that they wouldn't normally do, then they will 'remodel' accordingly.
For if you were a manual worker, working physically hard all day, then your bones would be thicker and stronger than someone who spends less time with physical activity.
This is why it is important for us all to take regular exercise, to stress our skeleton and make strong healthy bones.
There are times when our bones do not repair themselves or remodel effectively. If you:
- have a disease that effects bone physiology, such as osteoporosis,
- or are involved in trauma, fracturing a bone,
- or have an operation i.e. spinal fusion, where new bone is required,
then your bones may need additional help to repair themselves.
Diseased bone
Fracture
Spinal Fusion
Bone Repair - How does bone heal?
 |  |
 |  |
Inflammation / Hematoma
First, when our bones are damaged, either through disease, accident, or surgery, the site of the damage will bleed and become inflamed, just as you would expect to happen with your skin. This causes a blood clot to form around the injured bone, known as a hematoma.
Cell differentiation
Next your body receives the message (via growth factors) that repair is required and you need more specialized cells that build bone (osteoblasts); this is called osteoblast or cell differentiation. The osteoblasts then get to work.
Provision of a framework
The osteoblasts need to have a structure on which to lay down the new bone; they need some scaffolding or a framework to work with. Your bone can grow across a small gap - 'bone to bone' for example in a minor break or fracture, but where there is a large gap or space then your bone may need help from doctors and science to repair itself. When this is the case, and help is required to support your body's natural healing mechanisms, you may need the help of a bone graft or bone graft substitute.
Remodelling
When your bone has a good framework for bone to be laid onto, and your osteoblasts have lain down new bone cells, you are left with a callous at the site of injury, in the same way as you would expect a scab on your skin when you have cut yourself. The final phase of bone repair is remodelling of this callous back to as close as possible to the bone's original shape. This task is performed by the osteoclasts.
The bone healing process can take a varying amount of time, depending on several factors:
- Age of patient – older patients heal more slowly than young patients
- Patient general well being – for example, smoking and obesity are thought to delay bone healing
- Co-morbidities i.e. bones of patients with diabetes heal slower
- Bone health – conditions such as osteoporosis may delay healing
Why do I need bone graft or bone graft substitutes?
As described earlier, bone grafts or bone graft substitutes are often used where there is a large gap or space between bones that needs help to repair itself.
While the ultimate goal of your surgery is to obtain pain relief, maintaining structure and stability is crucial during the recovery process when new bone is forming.
Common examples of where bone grafts may be used are:
Fractures
Bone grafts can be used in trauma operations with the aim to help bridge or regenerate areas of skeletal loss with viable bone while maintaining limb length, alignment and function
Spinal fusion
Bone grafts are used to enable fusion, fill voids and enhance biological repair of skeletal defects. Metal fixation devices are used to correct and stabilize the operative site, which minimizes movement and encourages bone fusion. The bone graft allows bone to grow, fussing the site to create long term support.
Other
Bone grafts can be used in numerous other procedures such as filling bone cysts or surgically made defects, e.g., following bone tumour removal
Bone grafts and bone graft substitutes are used to encourage rapid bone growth, deliver effective fusion, and provide lasting results. TOP
What are the different types of bone grafts or bone graft substitutes?
Autograft
Autograft bone is the patient’s own bone and is most commonly harvested from the top of the pelvis during an operation. Surgeons may also utilize bone they have removed while gaining access to a surgical site e.g. spine.
Although autograft is considered the gold standard in bone grafting, spinal fusion using this bone graft is unsuccessful in up to 50% of patients
Refs
Minamide A et al. Evaluation of carriers of bone morphogenetic protein for spinal fusion. Spine 2001; 26: 933-939.
4 DePalma AF, Rothman RH. The nature of pseudarthrosis. Clin Orthop 1968; 59: 113-118.
5 Steinmann JC, Herkowitz HN. Pseudarthrosis of the spine. Clin Orthop 1992; 284: 80-90.
6 Zbeblick TA. A prospective randomized study of lumbar fusion: Preliminary results. Spine 1993;18: 983-991.
The principal disadvantage of using iliac crest autograft bone is that often another incision needs to be made to harvest the bone for grafting. This requires additional surgery time and can result in the following complications:
- Chronic pain at graft harvest site (10% to 25% of the time)
- Infection
- Bleeding
- Nerve damage
- Bone fracture
The chances of having a complication increase with the size of the bone graft and patient obesity. In procedures where the surgeon opts to use iliac crest autograft, patients may find the bone graft harvest site to be more painful than the surgery site itself. TOP
Allograft bone
Allograft bone (i.e. bone donated from a dead person or another patient during an operation, e.g., primary hip replacement) eliminates the need to harvest the patient’s own bone.
The donor cancellous bone graft acts as a bone scaffold onto which the patient’s own bone grows and is eventually replaced over years. There are believed to be no living cells in the bone graft (if there are, the graft is not sterile), so there is little chance of a graft ‘rejection’ as with an organ transplant.
Demineralized Bone Matrix (DBM) is a type of allograft. One of the main issues with DBMs is that the quality of the material can be highly variable dependent on the quality of the donor bone. In addition, consistent consenting of the patient receiving the donor bone, donated from a dead person, should be undertaken by the surgeon.
There is a small risk of transmission of an infection from a donor. The risk of contracting a disease such as human immunodeficiency virus (HIV) or hepatitis from an allograft has been estimated to be between 1 in 200,000 to 1 million. Case reports document that bacterial, viral and prion (e.g. Creutzfeldt-Jakob disease [CJD]) diseases have been transmitted through implantation of contaminated allografts, although modern procurement and sterilization methods for bone tissue have lessened the risk of this occurring TOP
Synthetic Bone Graft Substitutes
Despite the benefits of autograft and allograft, the limitations and disadvantages of each have led to the development of bone graft substitutes.
Scientists have developed several alternatives, many of which are available for clinical use. These alternatives use a variety of materials, including natural and synthetic polymers, ceramics, and composites, whereas others have incorporated growth factor and cell-based strategies that are used either alone or in combination with other materials.
Key types of Synthetic Bone Graft Substitute
- Ceramic-based – calcium phosphates, calcium sulphates and calcium carbonates
- Growth factor-based – bone morphogenetic proteins or BMPs
- Collagens and Cell based grafts
The Ideal Bone Graft Substitute
The ideal Bone Graft Substitute should have an optimized scaffold structure, biologically sympathetic chemistry and surface charge, and structural stability that is consistently reproducible.
For further information please talk with your doctor about diagnosis and treatment options. TOP
