Plate/Screw Principles (Complete) Flashcards
What is the definition of a screw?
- Mechanical device that converts torque into compression (between plate and bone or two bone fragments)
- Mechanical device converts rotation into linear motion
What are the components of a screw?
oCentral core (core/inner/root diameter)
- Bending strength is proportionate to inner diameter^3
oOuter/major diameter
- Pullout strength is proportionate to outer diameter^2
oThread - engages the bone and is responsible for the function and purchase
oTip - can be blunt, sharp, self-tapping
oHead - engages the bone or plate
oRecess in the head
oPitch – distance between screw threads (a screw will advance a distance in the bone equivalent to the pitch)
oLead - distance advanced with one revolution
oScrew working distance (length) - the length of the bone traversed by the screw
Why is tapping a screw important?
oNecessary for cortical bone (and dense cancellous bone) so that the torque is converted to compression rather than overcoming friction between screw and bone
oFailure to tap can result in shear failure at the head shaft junction
What is screw pullout?
oDefinition – the maximum force that a screw can withstand along its axis
oPullout force increases with larger outer diameter, smaller inner diameter, finer pitch, more engaged threads and greater density of bone
- Pullout strength is proportionate to outer diameter^2
What is the difference between locking and non-locking screws?
oLocking screws large core diameter with shallower threads which maximize fatigue strength
oNonlocking screws have smaller core diameters with deeper threads which maximizes purchase power
What are the different screw designs?
oFully threaded, partially threaded, cannulated, self-tapping
oCortical, cancellous, locking
What are the different screw functions?
- Lag screw
- •Provides interfragmentary compression and absolute stability (poor rotation, bending and shear stability)
- Plate screw
- Nonlocking screw that fixes plate to bone via compression
- Positioning screw
- Fully threaded screw that holds two bone fragments at a fixed distance without compression (eg. Syndesmosis screw)
- Push-pull screw
- Temporary point of fixation used to reduce a fracture by distraction and/or compression
- Poller (blocking) screw
- Used as a fulcrum to redirect an IM nail
- Interlocking screw
- Couples an IM nail to bone to maintain length, alignment and rotation
What is a lag screw?
- Provides interfragmentary compression and absolute stability (poor rotation, bending and shear stability)
- By technique – near cortex is drilled slightly larger than the outer diameter and the far cortex is drilled to correspond with the inner diameter; the thread engages the far cortex and the head engages the near cortex
- By design – partially threaded screw inserted so that threads engage the far cortex and not the near cortex
- Ideally directed perpendicular to the fracture line
What are the different functions of a plate?
- Neutralization
- Compression
- Buttress (antiglide)
- Tension band
- Bridge
What are the characteristics of a neutralization plate?
oPlate neutralizes shear, bending and rotational forces to protect the lag screw which provides interfragmentary compression
oFracture orientations = oblique or spiral
oFixation stability = absolute
oBone healing = primary
What are the characterstics of a compression plate?
oPlate maintains compression at the fracture site
oFracture orientations = transverse or oblique
- Tranverse fractures require slight prebending to prevent gapping at the opposite cortex of the plate
- Oblique fractures require plate fixation to the fragment with the obtuse angle to create an axilla for the opposite fragment to lock into
oFixation stability = absolute
oCompression is generated by two mechanisms:
- External tension/compression device and push-pull screw
- Plate design with oval holes and eccentric screw placement
oBone healing = primary
What are the characteristics of a buttress plate?
oPlate functions to resist shear forces and displacement when a fragment is axially loaded
- Buttress plate = when applied to an intra-articular fractures
- Antiglide plate = when applied to diaphyseal fractures
oFracture orientations = intra-articular fractures that extend to the metaphyseal region (buttress) or oblique (antiglide)
oFixation stability = absolute (often combined with lag screws)
oBone healing = primary
What are the characteristics of a tension band plate?
oPlate applied to the tension (convex) side of an eccentrically loaded bone (eg. Femur, olecranon) converts bending moment into compression at the fracture site (functions as a door hinge)
oFracture orientation = transverse or short oblique
- The compression side must be anatomically reduced without gapping
oFixation stability = absolute
oBone healing = primary
What are the characteristics of a bridge plate?
oPlate bridges/bypasses an area of comminution with fixation proximal and distal to fracture site
- Functions to maintain length, alignment and rotation while preserving soft tissue and blood supply at the fracture site
- Recommended to choose a plate 3x as long as the fracture zone
oFracture ortientation = diaphyseal or metaphyseal comminution
oFixation stability = relative
oBone healing = secondary
What are the different plate types/designs?
- LC-DCP (Low contact dynamic compression plate)
- Locking plate
- 1/3 tubular
- LISS (Less invasive stabilization system)