Internal fracture fixation Flashcards
What are the major forces acting on a fracture?
What are the principles of biological osteosynthesis?
- Indirect reduction techniques
- Fracture stabilization using bridging implants
- Limited reliance on secondary implants
- Limited, if any, use of bone grafts
Describe the various gauge diameters and relative tensile strengths of orthopedic wire
What is tensile strength of a wire dependent on?
The cross sectional area (pi x radius squared). Therefore small increase in diameter has a significant effect on tensile strength
In how many directions will a hemicerclage counteract forces?
One direction only - must decide on direction in which to wrap the wire based on this
How many loops are required for a stable twist knot cerclage?
1 - 1.5
What are the general principles of cerclage application?
2 or more cerclage, 0.5 a diameter of the bone apart, perfectly reconstructed bony column. If to be used as sole fixation with IM pin must have long oblique fracture 2.5-3 times the diameter of the bone.
The resting tension of cerclage drops below 30N after how much of a collapse in the bony column?
1% collapse
How much does pushing a twist knot flat after twisting reduce tension?
45-90%
What is the function of a skewer pin?
Prevent shearing of short oblique fractures when secured with a cerclage.
Describe the differences in initial tension and load before loosening for the twist, single loop and double loop cerclage
What is the size of the three available K-wires?
0.035, 0.045, 0.065 inch corresponding to 0.9, 1.1, 1.6mm
How does the area moment inertia of interlocking nails and orthopedic plates differ?
The AMI of interlocking nails is calculated by radius to the fourth power, plates are calculated by thickness to the third power
How much of the medullary canal is ideally filled by an intramedullary pin?
70%
What is stacked pinning, and does it have any biomechanical benefit?
Use of multiple small pins rather than a single large pin. Has been suggested to improve rotational stability, but biomechanical evaluation has not shown any significant advantage.
Is there a biomechanical advantage to Rush pinning over cross pinning?
No, stability is greater with the cross pinning technique.
How much of the medullary cavity is ideally filled with an interlocking nail device?
75-80% (avoid going larger than 90% to prevent iatrogenic fracture)
What are the three types of interlocking nail?
Regular, angle stable, inverse
In which direction of bending are interlocking nails weakest?
Parallel to the long axis of the nail cannulation.
Label the following image.
What is the benefit of using bolts over screws in an interlocking nail device?
Bolts are better able to resist bending (increased core diameter due to absence of threads and higher AMI).
Is the AMI of a 8mm ILN greater than a 3.5mm DCP?
Yes - 6.8 times the AMI of a regular 3.5 DCP, and 3.5 times the AMI of a broad DCP.
What are the main biomechanical differences between the AS-ILN and the regular ILN?
AS-ILN eliminates the slack that was experienced with the regular ILN (particularly severe in torsion [33 degrees of slack]). Hourglass shape of the AS-ILN also increases the AMI of the implant and reduces stress risers at the implant/bolt interface
How does the torsional stability of an inverse ILN compare to a 2.4mm LCP?
Reduced torsional stability, with evidence of slippage of the locking mechanism of the bolts.