Fracture fixation Flashcards
Stress in a distal femoral locking plate =
Weight of patient/cross sectional area of the plate
Calculate strain in fracture patterns =
Change in length/original length = Degree of movement at the fracture site/ The surface contact area of broken fragments
Strain experienced at comminuted vs transverse fracture?
Comminuted fracture = less strain - larger surface area
3 factors that need to be present in order for primary healing to occur?
Absolute stability
Perfect reduction (<500micrometres)
Viable bone ends
How is cancellous bone healed?
Vascular ingrowth
Subsequent cell migration into the trabecular scaffold
4 components of a screw?
- Head
- Shaft
- Thread
- Tip
What determines the torsional strength of a screw?
The core diameter
What is the pitch in a screw?
Distance between each thread
What improves pull out strength in screws? 3
Finer pitch
Longer length
Greater thread depth
What is the lead of a screw?
Distance travelled with a 360 degree turn
Optimum pilot hole size?
85-90% of screw
Forces at at a non-locking screw-plate-bone interface?
Tension at the screw head/plate and the screw thread/bone
Shear forces at the screw thread/bone interface
Forces at at a LOCKING screw-plate-bone interface?
Sits neutrally in bone until loaded
Then shear and compressive forces from screw-bone interface is transmitted to plate
Plates are used in ‘absolute stability’ mode in what 2 areas?
- Intra-articular fractures
2. Diaphyseal/metaphyseal fractures
Function of a neutralisation plate>
Prevents fracture rotation around compression screw
Neutralises shear forces
Placing plate on the tension side of the fracture site increases strength by what factor
200x
How does a non locked plate work?
Tightening of the screws compresses the plate onto the bone and creates a FRICTIONAL FORCE
The ability of a non locked plate to resist loading is directly proportional to?
How much the screws can be tightened
What confers stability to a locked plate construct?
Angle stable screws ‘trap’ large sections of bone between them
Internal EXFIX - separation of screws
What are the two factors that influence plate strength?
Cross sectional area (WH^3)
Material
The stiffness of a nail in torsion is governed by? 1
Polar moment of inertia
How does a tension band work
Converts tension into compression by translating the centre of rotation from the fracture to the tension band
Factors that increase rigidity in a EX FIX construct
- ANATOMICAL REDUCTION
- pin size
- decreased bone to rod distance
- pin spread
- number of pins
- thickness of bar/ amount of bars
- More planes
