Fracture Mechanisms and Healing Flashcards
What factors determine the mode and location of a fracture?
geometry and structure of the bone loading mode (compression, bending, torsion) loading rate
What quantity takes into account the cross-sectional area of the bone and its distribution of bone tissue in bending?
second moment of area
What quantity takes into account the cross-sectional area of the bone and its distribution of bone tissue in torsion?
polar moment of inertia
How are long bones structured so as to increase their resistance to fracture?
much of the bone tissue is distributed at a distance from the neutral axis and thus they have a much larger second moment of area than a tube of the same shape that is solid
How does strength change as the polar moment of inertia increases?
increases
Where in the tibia does torsional fracture commonly occur?
distally
Where is the weakest point of the fibula?
proximal 1/3
Which type of bone is weakest?
cancellous
more likely to fail under axial compression causing supracondylar and tibial plateau fractures
Describe the loading on opposite sides of bone in bending
one side is loaded in tension and the other in compression
Why does the side of the bone in tension fail first in bending?
bone is weaker in tension than compression
What fracture patter results from bending?
transverse
In pure compression, what fracture pattern occurs?
oblique
When bending is superimposed on axial compression, a combination of two fracture patterns occurs, which two?
what pattern is this?
transverse
oblique
butterfly segment
In pure torsion, what fracture pattern occurs?
spiral
True or False: Most fractures occur due to a single method of loading
False - most are a combination
How does bone strength vary with loading rate?
Higher loading rate = higher strength
What happens in a high energy fracture?
the high energy is suddenly released and the bone fails - resulting in a comminuted fracture with severe soft tissue damage
i.e. high velocity gun shot fracture
Name an example of a low-velocity fracture
spiral fracture due to a skiing injury
Describe the fracture process (5 steps)
energy is delivered to the limb
the energy is transferred via the soft tissue to the bone which absorbs the energy
the bone breaks and energy is released back to the soft tissues
the broken bone and damaged tissues bleed causing a haematoma
an acute inflammatory response occurs which causes pain and commences the healing process
Which type of bone healing is natural bone healing?
secondary
What is the relationship between fracture energy and healing time?
increased energy = longer healing time
What is a pseudoarthrosis?
excessive movement causes a false joint to form between rapidly proliferating cartilage cells at either end
Describe the secondary fracture healing process
Weeks 0-2 - The haematoma is invaded by macrophages in surrounding tissue which are responsible for “mopping up” dead and damaged tissue. The haematoma and dead cells are absorbed into the macrophages.
Weeks 2-6 - New capillaries grow into the fracture haematoma bringing with them cells of healing and repair including fibroblasts, which form fibrin (scar tissue) and also other cells including bone forming osteoblasts. At the same time the surviving periosteum begins to regenerate and grow between the bone fragments.
Weeks 6-12 - New bone tissue is laid down in the endosteal space from the residual living bone and eventually the two ends are reunited as a ball of “provisional callus” which appears as a dense area on an X-ray.
Up to about 12 months - If circumstances are correct the provisional callus continues to form woven bone which gradually remodels to form a cortex.
Up to 2 years - The callus matures so that the trabecular pattern is reformed and the bone remodels to accommodate the stresses that the bone experiences in that anatomical region.
What is primary bone healing?
if no movement takes place between fracture fragments it heals without callus formation
How does primary bone healing take place?
New Haversian systems grow directly across the fracture gap
What is an advantage and a disadvantage of primary bone healing
A - radiologically the fracture resolves quicker
D - the bone takes longer to regain its original strength
What causes a bone to heal by primary healing
rigid fixation which removes all loading - thus the bone becomes weaker from stress shielding
What properties does the callus give to the bone as it heals?
callus increases cross-sectional area and therefore second moment of area
which increases the structural support to compensate for the lower strength and rigidity of the callus material
What is the equation for rigidity?
R= EI
Young’s Modulus x Second moment of area
Name 3 possible factor which may explain why movement at the fracture site influences bone healing
electrical effects by moving hydroxyapatite crystals (HAp is piezoelectric i.e. develops a charge when it is loaded)
hormonal factors - substance P produced at fracture sites
electro-magnetic effects produced through electron flow away from the fracture site
