Biomechanics of implants

Cerclage Wire:

  • Tensile Strength: Proportional to wire diameter. The optimal number of twists is 4-8.
  • Notch Sensitivity: A 1% notch in the wire can reduce tensile strength by 60%.
  • Cable: Has higher fatigue resistance than wire.
  • Alternatives: Number 5 Ethibond has 30% tensile strength of an 18g wire, and Mersilene tape has 50% of an 18g wire.

Screws:

  • Purpose: Convert rotational motion into linear motion, creating compressive force.

Screw Pullout Factors:

  • Increases With:
    • Larger screw diameter
    • Longer embedded length
    • High-quality bone
    • More surface area and cortical purchase
    • Increased thread depth and pitch
  • Cannulated Screws: Lower pullout strength (15% less than non-cannulated screws), as the core diameter is increased, reducing thread depth and rigidity.

Components of a Screw:

  • Core Diameter: Diameter of the screw’s core.
  • Outer Diameter: Total diameter of the screw.
  • Effective Thread Depth: The difference between core and outer diameter (larger depth increases pullout strength).
  • Pitch: Distance traveled per screw turn (increased pitch lowers pullout strength; decreased pitch increases it).
  • Head: Allows driver attachment, halts motion.
  • Tap: Creates a female helix for screw threads. Self-tapping screws cut threads automatically.

Nails:

  • Strength: Bending and torsional rigidity increase with the 4th power of radius (doubling the radius increases rigidity by 16 times).
  • Slotted Nails: Similar calculations but take into account the inner diameter, making them weaker than solid nails.
  • Advantage: Nails act as load-sharing devices but are vulnerable in metaphyseal fractures due to increased stress on the locking screws and bolts.

Design Improvements:

  • Material Around Locking Holes: Increased to prevent failure.
  • Screws Locking into Nails: Enhance stability.
  • Working Length: The unsupported length, longer lengths result in less stiffness and greater stress.

Plates:

  • Weaknesses: More prone to failure due to low bending rigidity. They rely on the fracture reduction for stability, functioning as load-sharing devices.
  • Positioning: Plates should be placed on the tensile side of the bone to prevent fracture gapping and reduce bending moments.
  • Pre-bending: Helps avoid gapping on the far cortex, improving the load distribution.

Locking Plates:

  • Fixed Angle Construct: Near the mechanical axis, eliminating weak points between screws and the plate.
  • Benefits: Avoids periosteal blood supply disruption and enables minimally invasive application.
  • Challenges: Excessive stiffness and lack of compression can cause non-union, and cross-threading can reduce stiffness.

External Fixation:

  • Working Length: As in nails and plates, working length is critical for construct stiffness. Placing bars closer to the skin shortens working length, increasing stiffness.
  • Stiffness Factors:
    • Increasing the size of pins or bars.
    • Shortening the working length from the fracture.
    • Adding more pins and bars.
    • Reducing the distance from the bars to the skin.

Forces and Effects on Orthopedic Implants:

  • Strength: Depends on material properties and structure shape. Tubular structures are more vulnerable to torsion.
  • Stiffness: A material’s ability to resist deformation. Stress divided by strain.
  • Rigidity: A structural property that refers to the ability to resist deformation.

Plates:

  • Rigidity: Increases with the 3rd power of thickness (doubling thickness increases rigidity 8 times).

Tubular Structures:

  • Torsional Rigidity: Proportional to the 4th power of radius (doubling radius increases torsional rigidity 16 times).
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