Unit 6a: Fractures, fracture healing & non-surgical fixation Flashcards
What are the most important mechanical properties of bone?
strength and stiffness
Which type of loading is a long bone most resistant under?
best resistance to compression, then tension, and is weakest in shear.
What determines the location and type of fracture?
- geometry + structure of the bone
- type of loading (eg. tension, compression, shear)…
- rate of loading
Under an axial load, what is the stiffness of the bone proportional to?
its cross sectional area
Under a bending load, which two factors affect stiffness of a bone? what term describes this?
cross sectional area and the distribution of bone around a neutral axis
= second moment of area
Under a torsional load, which two factors affect stiffness of a bone? what term describes this?
cross sectional area and the distribution of bone around a neutral axis
= polar second moment of area
How does the geometry of the tibia affect fracture location when subjected to a torsional load?
fracture occurs distally
because distally there is more bone located close to the neutral axis so the polar second moment of area is low
whereas proximally there is less bone distributed further so the polar second moment of area is high
Is cortical or cancellous bone weaker under axial loading?
cancellous
Which parts of a long bone are made of cortical and which parts are made of cancellous bone?
mid-diaphysis = cortical metaphyses = cancellous
Which fracture pattern is the result of pure bending?
transverse
In a transverse fracture, which side is loaded in tension and which side in compression?
convex side in tension
concave side in compression
In a transverse fracture, which side fails in
a) adults ?
b) children?
a) convex/tension side as it is weaker in tension than compression
b) concave/ compression
Which fracture pattern is the result of pure compression?
oblique
Which fracture pattern is the result of bending and compression?
butterfly segment
- bending causes transverse crack on tension side of bone
- compression causes an oblique fracture
- results in fragment on compression side of bone
Which fracture pattern is the result of pure torsion?
spiral
Why does a spiral fracture pattern form?
the fracture line at about 45° to the axis about which the torque is applied
failure of the bone in tension, perpendicular to the crack
Which portion of the bone fractures under a compressive load?
close to or within the metaphyses
How does rate of loading determine bone strength?
it is stronger at a higher loading rate than at a lower loading rate
Describe the fracture process
- energy to limb
- energy soft tissues to bone
- bone absorbs energy
- bone fractures releasing energy into soft tissues
- damaged bone and soft tissues bleed causing a haematoma
- acute inflammatory response causes pain and healing
How does the size of gap affect bone healing?
If the gap is small it heals but if large it does not
What characterises fracture healing?
callus formation
What is secondary bone healing?
callus developing, around the fracture site, from mesenchymal (primitive) tissue then chondroid (cartilage) and then osseous (bone) tissue. Later, remodelling takes place and the external callus gradually disappears as the bone regains its original strength, shape and internal architecture.
How long does it take for most long bone fractures to heal?
6-12 weeks
What happens to fracture if blood supply is not re-established?
bony union will not take place. This may be described as an “atrophic” or fibrous union
What happens if there is excessive movement during fracture healing?
cartilage laid down instead of bone or pseudoarthrosis (false joint) may form
What happens if there is no relative movement (or micromovement) taking place between fracture fragments during the healing process?
primary bone healing with no callus formation
Is primary or secondary bone healing quicker?
primary gets its structure back quicker but not its strength
What is the disadvantage of primary bone healing?
rigid fixation deprives bone of its natural loads causing stress shielding and weakness
“Bone has the ability to remodel, by altering its size, shape, and structure, to meet the mechanical demands placed on it” describes which term…
Wolff’s law
How does callus formation give structural support to a healing bone?
increases the second moment of area which compensates for the lower strength and rigidity of the material of the callus
How does loading encourage bone union?
loading bone along long axis
Name three possible factors which may explain why movement at the fracture site influences bone healing.
Electrical effects caused by moving crystals of hydroxyapatite, which are the basic mineral constituents of bone. Hydroxyapatite is known to be piezoelectric, that is it develops an electric charge when loaded.
Hormonal factors (there is some evidence of a hormone “Substance P” being produced at fracture sites).
Electro-magnetic effects produced through electron flow away from the fracture site.
How much blood is lost from
a) femoral fracture?
b) pelvic fracture?
a) 1 litre
b) 3 litres
What is the first step in fracture management?
reduction
What is the second step in fracture management?
holding
What are the two types of reduction?
open
closed
What are the two types of holding?
internal and external fixation
Give 3 methods of external fixation
plaster of paris
traction
external fixator
What is the primary objective of all kind of fixation devices?
to minimise deformation (or movement) between the fracture fragments.
How do splints work? when are they suitable?
stabilise soft tissues of a stable fracture to resist bending forces after reduction but are of little use in resisting torsional and compression forces
What material is plaster of paris?
calcium sulphate
What risk is there when using a plaster of paris cast?
produces heat when the crystal sets to a solid which may damage patient
How is a plaster of paris cast made?
using cotton bandages, hard-coated with crystals of calcium sulphate hemihydrate
getting wet and putting around splint
In a plaster of paris cast what does
a) the bandage provide?
b) the starch provide?
a) vehicle
b) accelerates drying
Give the two ways that plaster functions
- hydraulic theory - rigid exoskeleton supports soft tissues which support the bone
- periosteal hinge - three point bending fixates bone
How do casts have to control the position of the bone?
- length - prevent shortening
- prevent A/P or medial/lateral tilt
- rotation about long axis of bone
In which way do externally applied splints struggle to control bone position? How is this controlled?
struggle with controlling rotation
by applying to joint above and below
Describe an advantage of a functional brace, compared to a full- leg cast.
dont immobilise the limb so muscles dont waste
How is traction as a holding device difference to using traction to reduce a fracture?
traction reduces a fracture by controlled use of force on a relaxed limb in order to position bones
Traction as a holding technique depends on altering muscle tone in order to maintain a position achieved at reduction.
What role do the muscles play in holding a fracture, in traction?
The muscles increase in tone if subject to an increased load in traction, maintaining the position of the broken bone fragments.
What is Static (fixed) traction?
load is applied to the limb and attached to a splint eg. Thomas’ splint
What is the disadvantage of using Static (fixed) traction?
immobilises the joint so there is no movement at fracture site + muscle weakness
What is dynamic traction?
pulleys allow joint to move but with the net force along the axis of the bone
Why is balanced traction used?
to offset the pressure effects caused by splints
Give some general disadvantages of traction
prolonged hospitalisation + immobilisation
Bed sores
chest and urinary infections
disuse atrophy of muscles and bone
Name the factors taken into consideration when deciding on the method that will be used to hold a fracture.
the patient’s age and functional requirements
the method and character of injury: open/closed, high/low energy, soft tissue injuries
available facilities
skills of the operator
