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Implants for Fracture Surgery

BONE SCREWS

There are two types of screws = Machine screws & Wood screws.

Bone screws are machine screws.

  • A wood screw is inserted into a small pilot hole. The screw threads compress the wood, which is less stiff than the screw, resulting in an elastic force.

  • A machine screw is inserted into a pre-drilled & pre-tapped hole. The screw itself deforms plastically when inserted into metal.

Screw Head

  • = attachment for screwdriver

  • Countersink = conical area under head

  • Hexagonal head recess design is most popular because:

    1.  it avoids slippage of screwdriver & thus head distortion

    2. it allows for better directional control during screw insertion

    3. the torque is spread between 6 points of contact

Screw Shaft

  • = smooth link betw. head & thread.

  • The 'Run out' is the transitional area between shaft & thread. This is the area screws break.

Screw Thread

  • The standard orthopaedic screw has a single thread (more threads increase the rate of advancement, but produces less compression for the same energy)

  • Core/root diameter = the narrowest diameter. 

    • The cube of the root diameter is proportional to the torsional strength of the screw.

  • Outer/thread diameter = across the maximum thread width. 

    • The larger the outer diameter the greater the resistance to screw pullout. 

  • Pitch= the distance between adjacent threads. 

    • Cortical screws have small pitch & cancellous screws have large pitch

    • The stronger the bone the smaller the pitch

  • Lead= the distance the screw advances with each turn.

    • The smaller the lead the greater the mechanical advantage of the screw.

    • Cortical screws have a smaller lead than cancellous screws

  • Pitch & lead = incline of a ramp. A barrel travels a shorter distance on a steeper incline before it gets to the top, but it is harder to push it up the ramp.

  • Thread design:

    • 'V' profile - produces shear + compression forces

    • Buttress profile - produces compression forces only

    • shear forces promote bone resorption, reducing pullout strength.

  • Thread length:

    • Partially threaded screws are designed for lagging cancellous bone.

    • 80% of the screw's grip is determined by the thread on the near cortex & 20% on the purchase at the far cortex.

Screw TipBone Screw Tips (Thakur)

  1. Blunt tip of self-tapping screw - cortical

    • fluted to act as a cutting edge & transport bone chips away.

    • the sharpness, number & geometry of flutes determines its effectiveness.

  2. Blunt tip of non-self-tapping screw - cortical

    • the rounded tip allows for more accuracy & direction into a pre-tapped hole.

    • More 'effective torque' is obtained from pre-tapping -> increased interfragmentary compression.

  3. Corkscrew tip - cancellous screw

    • compresses trabecular bone & produces compression by overshooting the pre-drilled hole.

  4. Trocar tip - 

    • doesn't have a flute, thus displaces bone as it advances.



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