Ortho implants Flashcards
What are the AO principles?
fracture reduction to restore anatomical relationships
fracture fixation providing absolute or relative stability as the fracture requires
preservation of blood supply to soft tissues and bone
early and safe mobilisation of the injured part and the patient as a whole
What do we mean by small and large fragment?
Small fragment
- 2.7/3.5mm screws and implants
- smaller long bones - radius/ ulnar/ ankle/ clavicle fractures
Large fragment
- 4.5/5.5 - screws and implants
- larger bones - femur, tibia, pelvis, humerus
What does AO say about small frag/large frag minimum cortical holds?
- small fragment plates - need 6 cortices either side of the fracture
- large fragment plates - need 8 cortices either side of the fracture
what is the working length of a nail?
- the unsupported portion of the bone
- between the most distal prox locking and most prox distal locking
- reaming allows greater bone contact at the isthmus - working length is shortened from the locking screw to isthmus
- only for bending stiffness not torsional
are cannulated or solid nails stiffer?
for the same diameter of nail, then solid nails are stiffer
for the same volume of material used, then cannulated nails as a larger diameter is created
tell me about load sharing for intramedullary nails?
im nails load share for fractures with close bone contact
load bear with comminuted fractures where there’s poor bone contact
what factors affect the stiffness of an intramedullary nail construct?
1) young’s modulus of material
2) radius of material - SMA and PMA
3) working length
4) distal locking
a) torsional stability
b) reduces shortening
c) axial loading
what happens when you double the thickness of an im nail? and a plate?
doubling the thickness, increases the bending rigidity by:
4th power of the multiplier for a nail - e.g. 16
3rd power of the multiplier for a plate e.g. 8
what is the second moment area and the polar moment of inertia?
second moment area is a measure of how the material is distributed in the cross-section of an object relative to the load applied.
polar moment of inertia - applies to the ability of a cylindrical structure to resist torsion
how do you improve the stiffness of an external fixator?
PINS
- increase the diameter of the pins - this increases the bending rigidty by the power of 3
- increase the number of pins
- pins in multiple planes
- HA coated pins
BARS
- increase the number of rods
- double up the bars
what is this? and how does it work?
- compression screw
- variable pitch
What is this? How does it work?
Where would you put your screw holes?
What type of fracture healing occurs?
What are the modes of plate fixation?
What determines the bending rigidity of plates?
Modes of plate fixation:
- bridging
- buttress
- compression
- tension band
- protection/ neutral - when used with a lag screw
Tell me about locking screw options
- dynamic - screw is implanted in an oblong hole -allows the nail to move relative to the screw - small micromotion - compression at the fracture site
- static - fixes the fracture in place - improved stability
how does balanced traction work?
Hamilton-Russell skin traction
- used for femoral shaft fractures
- if traction required for more than 24hrs
components
- padded sling placed under the slightly flexed knee
- skin traction applied to the lower leg
- traction cord and pulley system
how does it work?
- upward force of the sling
- longitudinal pull of the skin traction
- creates a resolution force in line of the femur
risks of skin traction
- loosening
- constriction
- friction
- allergies
- poor control
