Biomechanics of fracture fixation Flashcards
What is a screw?
A screw consists of head (with drive mechanism), shaft, thread and tip
Difference in the way locking vs non-locking plates work
A locking plate provides stability as a fixed-angle construct; fixed-angle properties obviate the need for compression and contact between plate and bone.
A non-locking plate obtains fixation stability by the frictional force between plate and bone.
Intramedullary nail
Draw how TBW works for olecranon fracture
Size of K wires use for fixation of paediatric supra condylar elbow fracture
2.0 mm
Factors leading to optimal screw biomechanics
Core diameter of screw
Outer thread diameter
Thread density
Bone density
Pilot holes
How do plates fail?
Plate bending: Stress that exceeds elastic limit on plate
Plate fracture: Cyclical bending forces above fatigue limit
Construct pullout: non locked screw loosen
Screw head failure: Screw driver become threaded when over tightened / tightened at inappropriate angle
Plate strength determine by
Cross sectional area of the plate
Material
Role of reaming
Allows slightly larger nail to be passed
1mm increase in locking bolt diameter increases strength by 2.5x
IM reaming destroy centrifugal blood flow. Paradoxical stimulatory effect on periostea blood supply, reverses blood flow to become centripedal
Create autologous bone graft at fracture site
Working length of nail
Distance between closest proximal and distal statically locked bolts
Principle of TBW
Tension band converts a tensile force into a compressive force by translating the centre of rotation of a fracture to instead sit at the tension band.
