Basic science of Fracture Management

Clinical image

Plate fixation of an ankle fracture. Source: Wikimedia Commons — গীতাশ্ৰী গগৈ আপ্তে; CC BY 4.0. Image binary is embedded locally in this package; original source and licence are retained.

• Plates

Key points

• What plate you will use?

• I use the xxxx plate because it is perfect/ I like it/ My hospital supplies it/ my rep says

• it is the best

Q: Design the perfect Plate/IM Nail?/

• what plate you prefer?

• Choice of implant ?

• I use the xxxx plate because it is perfect/ I like it/ My hospital supplies it/ my rep says

• it is the best

Summary

• don't jump to implants
• Question is about your understanding of concepts and their application
• Structure your answer

Q: Design the perfect Plate/IM Nail?/

• what plate you prefer?

• Choice of implant ?

• The choice/design of implant require consideration of the following factors

• Material Properties

• Structural properties

• Interface of fixation

• Modality of use

Ideal material

• My ideal material

  • Should be Bio-inert, reliable, easily manufactured and sterilisable. It should be prize and corrosion resistant, ductile, tough and hard

• Example:

• Stainless steel 316 L

• Titanium Alloy 6Al4V (T 89%, Al 6%, V 4%)

• Cobalt Chromium Alloy

• (iron 62%, Chromium 18%, M 3% N16% Carbon <= 0.03, low)

Metals

• Crystalline structure

• Metal ions are +ve, they are packed by

  • Hexagonal close pack 12
  • Face centred cubes 12
  • Body centred cubes 8

• Coordination number: no of neighbours

• Grain boundaries: where one crystal meet the next

• Dislocations: irregularities in the crystal

Why metals are ductile?

• Due to movement of dislocations through lattices
• Slipping of +ve ions over each other to form new bonds with delocalised electrons
• Grain boundaries resist slipping and decreases ductility
• Annealing increases ductility (decrease dislocations)
• Alloying, quenching and cold working decreases ductility

Material Property

• Youngs Modulus : stiffness (GPa)
• Elasticity/Plasticity
• Yield Point
• Brittle/Ductile
• Toughness
• Hardness
• Endurance limit

Key points

?

• Brittle: exhibits elastic property up to point of failure
• Ductile: undergo permanent deformation before complete failure
• Hardness: ability to resist scratch
• Toughness: energy per unit area absorbed before breakage
• Endurance limit: max stress below which material will not fail regardless of number of loading cycle(10 million in ortho)

SS 316 L

• Chromium: Corrosion
• Nickel: Stabilise austenite to steel
• Molybdenum: Protective mould from exposure to acid
• No ferrate element: non magnetic
• No inclusion: means no sulphur, less corrosion

Key points

Structural Properties

• Cantilever bending (Axial stiffness)
• Bending stiffness
• torsional stiffness

Cross sectional shape

• Stiffness depends on the radius or height and breadth of material
• Increasing this will increase second moment area and increases stiffness
• rod r4
• Beam: wh3

Key points

Girders

Cantilever

• strain is proportional to the square of distance of the force from the fixation point

Interface fixation

• Conventional screws
  • Compression
  • Plate friction
• Locking screws
  • Fixed angle device
• combination

Screw

• Definitions
  • pitch
    • distance between threads
  • lead
    • distance advanced with one revolution
  • screw working distance (length)
    • defined as the length of bone traversed by the screw
  • outer diameter
  • root (inner) diameter
  • bending strength is proportionate to inner (minor) diameter3
  • pullout strength is proportionate to outer (major) diameter2
    • maximized by
      • large outer diameter difference
      • fine pitch
    • pedicle screw pullout most affected by quality of bone (degree of osteoporosis)
• Types of screws
  • cortical screws
  • cancellous screws
  • locking screws

Conventional screw

• Depends on compression on to the bone to resist shear forces
• Individual screw fails sequentially
• Only distal tip resists pull out
• Only proximal part in contact with bone resist shear

Key points

Locking screw

• enblock pull out
• entire screw resist shear force
• Angular stability

Key points

Blood supply

• High pressure nutrient artery system
• Low pressure periosteal system
• M-E system (Dale and Harris A and B)

Preservation of blood supply

• Plate design = Low contact Application: Internal Ex Fix

Key points

Combi Holes Plate

• Intra operative flexibility
• Less number of plates in the shelf

Pre contoured and fracture specific plate

• Variable angle plate
• Fixed angle plate

Modality of use

• what are my intentions

• how i can achieve those intentions

• How i hold my intentions until fracture unites

• Primary vs secondary healing

• Anatomical vs alignment

• Rigid vs relative

• Load sharing vs bearing

• combination

• straight vs variable locking

Key points

• What plate you will use?

One way

• Assuming that this is an isolated closed injury and the limb is neurovasularly intact, after adequate consent, my positioning of the patient is
• My intention of fixation is
• I will achieve the goal by
• I will hold the fixation till the fracture heals byfixation

What is working length?

• Un supported length of construct
• How to increase rigidity?
• How does that increase rigidity?

Increase rigidity

• Material properties: Stiffer material
• Structural properties: Increase pin and rod diameter
• Interface fixation: decrease working length of pin and fixator
• Modality of fixation: Load sharing by anatomical reduction

Key points

• What is working length?

• What is working length?

There are four ways of obtaining interfragmentary compression with a plate:

• compression with the dynamic compression unit in a plate (LC-DCP);
• compression by contouring (overbending) the plate;
• compression by additional lag screws through plate holes;
• compression with the articulated tension device.

Buttress Plate

• To prevent any sliding of the plate, the screw is placed as proximal as possible in the hole.