Biomechanics of Screws, Nails and External fixators

Courtesy: Amr Abdelgawad, Maimonaides Medical Centre, NY, USA

BIOMECHANICS OF SCREWS :

• Inner diameter of the screw is the same as the diameter of the drill bit for the threaded hole.
• Pitch is distance between two threads.

Screw factors associated with higher pullout Strength (bone quality is the most important factor related to pullout strength):

o Smaller inner diameter.
o Larger outer diameter.
o Fine pitch between two threads.

BIOMECHANICS OF NAILS :

• Increasing the diameter of solid nail increases bending stiffness to the fourth power.
• This means that doubling the radius of the nail results in an increase in the bending stiffness by a factor of 16.
• Both torsion and bending rigidity of nail is proportional to the 4th power of the diameter.
• The working distance of the nail is the distance between the locking screws closest to the fracture on both sides of the fracture.
• The longer the working length, the less rigid is the nail (the more motion at fracture side.)
• The shorter the effective working length of the nail fixation, the stiffer the device.
• Causes of less rigid nail : titanium nails, less thickness ( less diameter), longer working length of the nail.
• Dynamic locking screw : placed away from fracture in the oblong hole (to allow dynamization).

BIOMECHANICS OF EXTERAL FIXATION :

Methods to increase the rigidity of an external fixator :

o Pins : increasing pin diameter, increase pin number, increased pin spread within each segment, closer pin to the fracture.
o Rods : increasing the number of connecting rods (stacking), decrease the bone-rod distance, thicker rods and multi planar rods and pins (multiple planes of fixation).
o The single most important factor is the pin diameter because it has an exponential effect.
o Increased pin diameter (despite increase the stability) , may contribute to stress riser formation as bigger hole is now drilled in the bone.

Methods to increase the rigidity of a ring fixator :

• Increasing the number of wires, tensioning the wires , lower ring size (closer to body), wires closer to fracture , using two rings (stack) on each side, increasing the number of connecting rods between the rings on opposite sides of fracture.
• Coating with hydrox-apatite (HA) increase the fixation of pin to the bone and increase the torque needed for extraction.
• The pin’s weakest point is the thread-shank junction.
• Burying the thread shank junction in the proximal cortex can double the pin’s stiffness.