Bone Plates Flashcards
How are bone plates differentiated? (2)
- System of coupling between plate and screw (lock vs non)
- Function
What plate is described:
The design of the screw hole infers axial compression across the fracture as the screw is inserted. The screw hole has a specific oval geometry, meaning that when a hole is drilled eccentrically away from the fracture, the bone fragment moves horizontally towards the fracture site as the screw is tightened.
Compression
What plate is described:
The fracture is anatomically reconstructed with screws, pins or wires and the plate is applied to help protect the reconstruction by resisting bending forces.
Neutralisation
What plate is described:
A plate is applied across a non- reconstructed fracture and, as such, is required to resist all the load-bearing forces (plate bears all of the forces until bone healing is well under way). The plate is subject to high bending forces, so additional fixation is often used (i.e. an intramedullary pin or orthogonal plate).
Bridging
Compression plates - The screw hole has a specific oval geometry meaning that when a hole is drilled eccentrically away from the fracture, what happens as the screw tightens?
The bone fragment moves horizontally towards the fracture site
In neutralisation mode, the fracture is anatomically reconstructed with screws, pins or wires and the plate is applied to help protect the reconstruction by resisting what?
Bending forces
In bridging mode, a plate is applied across a non- reconstructed fracture and, as such, is required to resist what forces?
All load bearing forces
Additional fixation is often needed with bridging, why is this?
Plate is subject to high bending forces
What are the 2 systems of coupling between plate and screws?
Non locking
Locking
How are non locking plates named?
By the size of the primary screw used to secure them
What forces do non locking plates rely on to transfer load? Where are these forces?
Frictional forces between plate and bone
Non locking plate:
How is friction caused?
Friction is created by the screw lagging (or squeezing) the plate to the bone and, as such, the plate has to be accurately contoured to the bone.
With a non locking plate; how much friction is between the metal and bone?
Low
When placing a non locking plate, how can disruptive forces be decreased? (3)
By anatomically reducing the fracture;
By increasing the bending stiffness of the plate (i.e. using a bigger plate, but the size of the bone limits this);
And placing the plate on the tension aspect of the bone.
What is the correct tension side of the humerus?
Cranial or lateral
What is the correct tension side of the tibia?
Cranial or medial
What is the correct tension side of the femur?
Lateral
What is the correct tension side of the radius?
Craniomedial or cranial
In bridging mode, axial load (the largest force) is transferred from bone to plate to bone by means of.?
The screws
Non locking plate:
What can happen with the screws in the holes?
the screw is able to move in the screw hole (i.e. it can pivot or toggle)
With non locking plates; what is the only thing responsible for stability?
Friction of plate-bone interface
With non locking plates what is important for active thread-holding power?
Screw purchase in the trans-cortex
What screws often fail with a non-locking plate?
monocortical
What interface with non locking plates and monocortical screws is the weak link?
Screw-bone interface
When prepping for surgery, what can be done to fatigue the muscle and assist # reduction?
Hang the animal from affected limb whilst prepping
If the proximal radius is needed, what muscles can be incised? (2)
- Pronator
- Supinator
During elevation of pronator and supinator muscles take care to protect what? (2)
Median n
Radial n
What increases compression at site of # with a DCP?
Overcountering of the plate
True or false:
Over bending the plate ensures that optimal contact will be obtained at the far cortical surface when axial compression is applied.
True
Which part of a DCP goes over the #?
Centre of plate
Where is first screw placed with DCP?
Distal fragment, hole closed to #
Where is the 2nd screw placed with DCP?
Hole closest to # - proximal
How to cause compression with screws with DCP?
Alternatively tighten the 2 closet to #
How many compression screws with DCP?
2 - but can be 4
What is the maximum amount of compression that can be achieved with a single 3.5 mm screw applied in a loaded position in a LC-DCP?
1mm
What does the oval screw hole allow with DCP plate?
The creation of axial compression.
Where is compression to be applied with DCP?
Toward centre of plate (where there is a slightly extended middle section)
Which # types can be compressed together? (2)
Transverse
Short oblique
Compression results in load sharing - of what?
Bone and plate share axial force
What are the 2 ways screws can be applied with DCP?
- Neutral
- Load position
How many cortices should be engaged by screw threads either side of the #?
6
DCP:
With the drill guide in load position, the screw is placed eccentrically and the screw head interacts with screw hole geometry, which permits which movement? Resulting in?
simultaneous downward and horizontal movement of the screw as it is tightened to the plate, which moves the bone fragment with it.
What is the minimum distance from # site to first screw either side with DCP?
4-5mm
Which side of the bone area DCP applied? Why
Tension - resist bending force
DCP plate screws:
A) Angled longitudinally?
B) Angled transversely?
A) 25 degrees
B) 7 degrees
When contouring a DCP to apply compression to a fracture, the plate should be..?
Pre-stressed
Limited contact DCP:
Has a scalloped under surface; what does this result in? (2)
more even force distribution and less periosteal vascular compromise,
Limited contact DCP:
A) Stiffness compared to DCP?
B) Strength compared to DCP?
A) Less
B) Less
Limited contact DCP :
A) Longitudinal angle for screws?
B) Screw hole placing is..?
C) Which direction is compression?
A) 40
B) Uniform
C) both
Veterinary Cuttable Plate:
A) Length?
B) Hole shape?
C) Where are screws placed?
A) Cut to size
B) Round
C) Close together (more screw holes per unit length)
Veterinary Cuttable Plate are useful in which # and why?
Juxta-articular as the screws can be placed cloe together
Veterinary Cuttable Plate:
A) Strength cf to DCP?
B) Stiffness cf to DCP?
C) What property do they have which increases advantages?
A) Low
B) Low
C) Can be stacked
Reconstruction plates:
What is the design, and what does this allow for?
Soft steel with notches between screw holes
- Allows contouring in 6 planes
Reconstruction plate
What is the stiffness/strength?
Low
Where can reconstruction plates NOT be used?
Weight bearing areas
What are the specially shaped plates/indications? (7)
Acetabular plate
Supracondylar plate
TPLO plate
T plate
Pancarpal arthrodesis plate
Pantarsal arthrodesis plate
Partial tarsal arthrodesis plate.
Monocrotical or bi-cortical screws have been shown to resist torsion more effectively and can confer a greater bending stiffness to construct?
Bi-cortical
Which screws are recommended locking plates?
Bi-cortical
If they do not engage with the bone, what do locking screws do to the bone?
Push the bone away from the plate
Locking plate constructs fail in different ways to non- locking plates. Name these (3)
Screw breakage or loosening;
Compressive failure of the bone around the screws;
Inadequate stiffness of the construct (related to the material properties of the implant and number of screws).
With LCP where is the compression directed?
Middle of the plate
What is the difference between drill guide for locking screw vs non locking?
Locking - dedicated drill guide
Non locking - universal
Other than DCP, name other locking implants? (4)
String of pearl (SOP) plate
Advanced locking system (ALPS)
Fixin system
Polyaxial locking plates.
String of pearl plate:
A) What does the ridge towards the top of the pearl produce?
B) What mechanism does this allow?
C) Drill hole into the pearl to ensure..?
A) Produces an interference fit with the screw head when it is fully seated
B) Double locking
C) Drill hole is perpendicular to plate.
Which of the following statements about SOP plates is true?
A) They are relatively expensive.
B) They use standard cortical screws that have smaller core diameter than locking screws.
C) They use standard cortical screws that have larger core diameter than locking screws.
D) They cannot be contoured in three planes.
B)
They use standard cortical screws that have smaller core diameter than locking screws.
Advantages of SOP plate? (3)
Relatively inexpensive.
Use standard cortical screws.
Can be contoured in three planes.
Disadvantages of SOP plates? (2)
Standard cortical screws have a smaller core diameter than locking screws and thus a relatively weaker shear strength.
The plates are bulky.
What are the Advanced Locking System (ALPS) made of? (2)
Titanium
Titanium alloy
The Advanced Locking System (ALPS)
A) What screw type?
B) How many places can it be bent in?
A) locking or non locking
B) 2 planes
What is the shape of the underside of the Advanced Locking System (ALPS)?
Sculpted
The Advanced Locking System (ALPS)
Dedicated drill guides are used to allow cortical screws to be placed in A) or B) mode and locking screws to be placed C) to the plate.
A) neutral
B) compression
C) perpendicular
What do fixin systems use?
The Fixin system uses a threaded ‘bushing’ that couples with the plate.
Fixin system:
The bushing is threaded as is the hole in the plate, so what can happen if it is inadvertently cross-threaded.
the bushing screws into the plate and can be removed
What locks the screw with fixin system?
The cylinder of the screw head is slightly more tapered than the screw hole in the bushing, and so a ‘Morse taper’ effect locks the screw into the plate.
What angle can be used in Polyaxial locking plates include the PAX, VetLOX and Evolox?
10 degrees from perpendicular in any direction
Steps to apply a plate in compression mode
- An appropriate-sized hole is drilled in a neutral position.
- The hole is measured and tapped (if necessary).
- The screw is inserted and tightened.
- Ensure that the plate is aligned on bone, especially the proximal and distal ends.
- Drill the screw hole on opposite side of fracture.
- Drill guide in load position.
- The hole is measured and tapped.
- The screw is inserted and tightened.
- This compresses the fracture.
- If appropriate you can insert a second screw (per side) in compression.
- Repeat.
- Tighten both screws
- Insert neutral screws and progressively move away from #. Alternative sides
- Check all for tightness.
True or false, when compressing:
Before you tighten the second screw, the first screw on the same side must be loosened.
True
What happens if you over compress a #?
Fissures form
An LC-DCP plate applied as a neutralisation plate following placement of two lag screws is most appropriate for which of the following features?
A non re-constructible femoral diaphyseal fracture
A long oblique diaphyseal humeral fracture
A simple transverse diaphyseal tibial fracture
A tibial crest avulsion fracture
A long oblique diaphyseal humeral fracture
Steps to place a neutralisation plate?
- reconstruct fragments
- Interfragmentary compression applied (Lag screws – take care not to interfere with plate - can be placed through plate if appropriate).
- Contour plate to bone
- Drill guide used in neutral position.
- Plate screws applied.
- Order of screw insertion not as important.
- Ensure plate is aligned to bone.
What order to place screws for neutralisation plate
Not important
With application of a plate in bridging mode there is no attempt to reconstruct fracture, but what is the aim? (2)
Restore:
- Bone length
- Bone alignment
The adductor magnus brevis inserts along the linea aspera. How is this found?
This is a distinctive ridge found along the caudolateral aspect of the femur.
Bridging plate:
If the drill contacts the intramedullary pin it can be slightly redirected to allow clear passage to the trans cortex. If there is inadequate space what may be required?
Monocortical screw
How to confirm allignment of hole when placing bridging plate?
K wire
Which fragments is bridging plate applied to?
Major
What can help pre op with plate preparation? (bridging)
Pre contour (opposing leg)
When applying an DCP as a bridging plate for stabilisation of a comminuted fracture which of the following statements is correct?
A) Secondary or biological bone healing occurs
B) Anatomical reduction is achieved
C) The application of cerclage wire increases the stability of the repair
D) Apposition of fracture fragments should be near perfect
A) Secondary or biological bone healing occurs
Bridging plate:
Drill guide in which position?
Neutral
Bridging plate:
How many screws
3 screws in each fragment (6)
Bridging pate must resist what forces?
All forces - load bearing
Where are screws NOT placed with bridging plate?
Over communuted #
Rules for locking and non-locking hybrid fixation (3)
- Plate needs to be contoured accurately
- Place standard screws first
- Place locking screws afterwards
Hybrid fixation:
Contouring for:
A) Conventional screw?
B) Locking screw?
A) Contact (friction)
B) Gap smaller than 2mm
Whya re locking screws placed after with hybrid fixation? (2)
- Lags (squeezes) the plate to the bone
- Improves the stability of the construct.
