Parts
A standard orthopaedic twist drill bit has:
- shank
- body
- flutes
- margins or lands
- cutting lips
- point
The flutes allow removal of bone debris and provide a path for coolant. The point geometry influences centring, penetration and heat generation.
Helix angle
A larger helix angle generally improves chip clearance but may reduce stiffness. A smaller helix angle gives a more rigid drill with slower chip evacuation. Orthopaedic drill design balances cutting efficiency, control and thermal injury.
Point angle
The drill point consists of two cutting lips meeting at the chisel edge. Point geometry influences the force required to penetrate bone and the tendency of the drill to wander.
Drilling parameters
Important determinants of drilling performance include:
- drill diameter
- drill sharpness
- rotational speed
- axial force
- feed rate
- cooling and irrigation
- cortical thickness
Thermal injury
Excessive heat may cause thermal osteonecrosis and impair fixation. Heat generation increases with a blunt drill, prolonged drilling and poor cooling. Intermittent drilling, sharp bits and irrigation reduce thermal injury.
Practical points
- Use the correct drill diameter for the screw system.
- Protect soft tissues with a drill sleeve.
- Avoid uncontrolled plunge beyond the far cortex.
- Measure after drilling with an appropriate depth gauge.
- Replace damaged or blunt drill bits.
Diagram
