Orthopaedics 1 – Bone: Structure, function and Healing
Anil Chopra
1.
Describe the structures of long and short bones including cancellous and trabecular bone
2.
Describe the mechanical properties of bone and mechanisms of bone failure
3.
List the stages of fracture healing including mal union and non union
Bone

Skeleton weighs 2 Kg

206 bones in a human
Bones are used for structure, support, movement and as a calcium store for the body.
It consists of:
» Mesenchymal cells
» Extracellular matrix
» Blood vessels
» Nerves
» Lymphatics
» Periosteum
» Marrow
There are 2 main types of bone tissue:

Cortical or compact bone
–
Long-bone diaphyses
• consist almost entirely of cortical bone.

Cancellous or trabecular bone
–
The metaphyses of long bones and most short and flat bones
• consist of relatively thin shells of cortical bone with large volumes of cancellous bone.
These different types are important in the healing of fractures and function of implants.
There are also different types according to maturity:
Woven or Immature bone
– the embryonic skeleton
– the initial fracture repair tissue which is replaced by lamellar bone as the fracture remodels
– has a more rapid rate of deposition and resorption
–
Has an irregular woven pattern of matrix collagen fibrils consistent with its name, approximately
Four times the number of osteocytes per unit volume
Periosteum : consists of an outer fibrous layer, an inner more cellular and vascular cambium layer. The periosteum is thicker in children and infants.

Osteoblasts : these are cells that line the surface of bone. Cells close to the osteoblasts but away from the surface are called preosteoblasts . Their job is to produce:
– type I collagen
–
PTH and Vitamin D receptors
–
Osteocalcin
Osteoclasts : these are large resorbtive cells that lie in pits within the bone microbiology. They show a brush border and may be active or inactive.
Bone consists of inorganic and organic material:

Organic
–
40%

Inorganic
–
Mineral (60%)
– Collagen
–
Proteolycan
–
Glycoproteins
–
–
Growth factors
Cytokines
– Calcium Hydroxy-apatite

Responsible for compressive strength

Primary Mineralization

Secondary Mineralization
–
Osteocalcium Phosphate (Bushite)
The shape and size of bones varies with age and sex:
Bone fracture results from impact that causes mechanical failure of tension and compression, loss of stability, collateral damage to the whole person as well as to blood vessels, nerves and skin.
Fractures are classified according to their anatomy or their operative condition.

Healing – There are 3 phases to healing:
» 1-3 days – osteoblasts are activated
» 1 week – osteoid is deposited on the wall of the hole, no osteoclastic reabsorption is involved and the holes are filled with lamellar bone.
» 2 weeks – larger holes are bridged by a scaffold made of woven bone resulting in intertrabecular spaces,
» Weeks later – remodelling occurs.
Damage to vascularized tissues and the resulting haemorrhage initiates a response that includes inflammation, repair and remodelling.
The response begins with inflammation and ends with remodelling
1.
Injury results in haematoma formation within the medullary canal, between the fracture ends and beneath the periosteum
2.
The osteocytes die back to the level of the collateral circulation

3.
Severely damaged periosteum, marrow and surrounding tissue all contribute to necrotic tissue to fracture site
4.
Cytokines are released that stimulate angiogenesis
5.
Fibroblasts appear
6.
Inflammatory phase persists for 24-72 hours
7.
Undifferentiated mesenchymal cells migrate to the area and repair phase begins
8.
Inflammatory mediators from platelets and other damaged cells cause vasodilatation and plasma exudation resulting in local oedema
9.
IL-6, IL-1, PGDF, TGF-B, FGF influence cell migration, proliferation and differentiation
Failures of healing include:
•
Delayed Union
• Non-union
–
Hypertrophic
–
Atrophic
– Pseudoarthrosis
–
Fibrous
Gene expression:
The expression of genes can affect the growth factors, repair proteins and hormones needed in healing.
Some local growth hormones include:

Bone Morphogenic Protein
–
Osteoinductive
• Target cell is undifferentia ted perivascular mesenchymal cell

Transforming Growth Factor Beta
–
Induces mesenchymal cells
• to produce type II collagen and proteoglycans
–
Regulates cartilage and bone formation

ILGF – insulin like growth factor F

PDGF – platelet derived growth factor F
Hormone
Anabolic steroids
Corticosteroids
Calcitonin
Thyroid/parathyroid
Growth hormone
Effect
+
-
+?
+
+ mechanism
Enhanced callus formation
Decreased
Bone remodelling
Enhanced callus formation

Management
There are a number of different treatments for fractures, the choice of which depends on a number of factors including skills, equipment, environment, time pressures, budget.
Plaster cast fixation
Traction
External fixation
Internal fixation
If there is a closed fracture, then a plaster cast is sufficient, surgery is not needed, natural healing will occur (malalignment will occur early if at all). If there is a more serious injury, then primary healing in the form of external or internal fixation is needed.
In the growth of a callus, the fracture stiffness normally rises exponentially, and the fracture strains fall.
Hydroxyapatite coated stems can be used
–
Very good indeed in cortical bone
–
No progressive radiolucency
– But continued remodelling
Extra-cortical-plates

Not intra-medullary stems

Different thicknesses

With and without slots
Joint Sparing Prosthesis

Plates, just for early fix
–
Screws of little use..

Intraosseous blades
–
And bone bridging

Growing mechanism
– Invasive or non invasive