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)
What are the characteristics of a LC-DC plate?
oLimited contact dynamic compression plate
oDCP refers to the oval inclined holes which allows insertion of screws in neutral or compression mode as well as for angulation of the screw
oLC refers to the undercuts between screw holes which reduces the area of contact between the plate and bone to preserve blood supply; undercuts also distribute the strength evenly across the plate (eliminates stress risers); undercuts also allow for greater angulation of screws
What are the characteristics of a locking plate?
oThreads in screw head engage threads in plate to create a fixed angle device
oConventional plates rely on the frictional forces between the plate and bone to resist applied load; locking plates rely on the plate-screw interface for construct stability
oPreserves blood supply as plate-bone contact not essential
oConventional plates fail because of loss of bone purchase of the screw and sequential pull out of the screws; locking plates fail when all screws fail simultaneously
oLocking screws must be applied after reduction and compression have been achieved
oIndications:
- Osteoporotic bone
- Bridge plating (internal fixator)
- Metaphyseal fractures with a short articular block
- Periprosthetic fractures (often require monocortical screws)
- Plating of fractures where anatomical constraints prevent plating on the tension side of the bone (e.g. short segment fixation)
oLocking screws
- Larger core diameter and finer pitch as bending strength more important than pullout resistance
oStability increases with:
- bicortical locking screws
- increased number of screws
- screw divergence from screw hole < 5 degrees
- longer plate
What are the characteristics of an intramedullary nail?
- Fixation stability = relative
- Bone healing = secondary
- Typically a load sharing device (except in comminuted fractures it becomes load bearing)
- Goal is to restore length, alignment and rotation (anatomic reduction is not the goal)
- Nails resist bending moments but not compression or torsion
- Interlocking screws function to resist compression and torsion
- dynamic locking–>axially and rotationally stable fractures
- static locking–>axially and rotationally unstable fractures
- secondary dynamization for nonunion
- remove proximal interlocking screw or move proximal interlocking screw from the static to dynamic slot
•Nail biomechanics
- Stiffness depends on:
- Diameter – stiffness is proportional to the radius^4 (increased diameter = increased stiffness)
- Material – stainless steel is stiffer than titanium
- Wall thickness
- Slotted vs. non-slotted – slotted nail is less stiff
•Advantages
- Preservation of soft tissue and blood supply at fracture site (due to indirect reduction and percutaneous insertion distant from fracture site)
- Reaming provides autogenous bone graft
•Disadvantages
- Disruption of endosteal blood supply
- Reductions can be difficult in segmental and comminuted fracture patterns
What is strain and how does it correlate to fracture healing?
[JAAOS 2016;24:711-719]
Strain = movement at a fracture gap divided by the original distance of the gap (expressed as a %)
- A. Strain ≤2% = primary bone healing (heals with haversian remodeling)
- B. Strain 2-10% = secondary bone healing (heals with endochondral ossification and callus)
- C. Strain >10% = fibrous tissue healing
What is the consequence of a plate construct that is too stiff or too flexible?
[JAAOS 2016;24:711-719]
- Too stiff = insufficient callus, nonunion, plate failure
- Too flexible = hypertrophic nonunion, plastic deformation of plate, malunion
What factors influence stiffness of a plate construct?
[JAAOS 2016;24:711-719]
1.Plate length
- A. Plate length should be 8 times the length for simple fractures
- B. Plate length should be 3 times the length for comminuted fractures
- C. Longer plates decrease stress concentration around the fracture and allows well spaced fixation
2.Plate material
- A. Steel is stiffer than titanium and titanium alloy
- B. Titanium most closely approximated the (Young’s) modulus of elasticity of cortical bone
3.Working length
- A. Working length of the plate = distance between the proximal and distal screw in closest proximity to the fracture
- B. Longer working length reduces stiffness
- C. Shorter working length predisposes to nonunion
4.Screw density
- A. Increased screw density increases stiffness
- B. Screw fill should not exceed 50%
- C.More than 4 diaphyseal screws are not required (even in osteoporotic bone)
- Screw length
* A. Bicortical vs. unicortical screws have a longer working length with greater bending and torsional resistance and ultimately greater stiffness - Screw type
- A. Nonlocking, locking, hybrid
- B. Hybrid constructs reduce plate to bone distance and increase strength
What is recommended for bridge plating?
[JAAOS 2016;24:711-719]
- Opt for titanium when possible
- Longer plates preferred
- A. Longest plate that is anatomically feasible
- B. Minimum 2-3x the length of comminuted fractures
- Less than 50% screw fill
- ≤4 diaphyseal screws
- Increase working length
