Orthopaedic Implants Flashcards
The distance of bone traversed by a screw is defined as?
working length
What things can increase a screws pullout strength?
1) Larger diameter screw
2) parallel placement to the trabecular pattern
3) purchase in bone with a higher density
4) augment with PMMA
Define plate working distance?
length from the fracture to the closest screw on either side of the fracture
Decrease in working distance increases the stiffness of the construct
What is true of comminuted fractures and bridge plating in regards to strain?
Bridging the fracture reduces strain as the force is dissipated amongst all fragments
Compression plating and lag screws also reduce strain is there is no fracture gap (strain defined as change in length/original length)
What are indications for locked plates?
1) Short fracture segments
2) Osteoporotic metaphyseal bone
3) Indirect reduction
4) Bridging comminuted fractures
What is increased the most with placement of bicortical locking screws vs unicortical?
Resistance to torsion
Bending and torsional rigidity are proportional to what?
torsional rigidity-radius to the 4th power of the nail
bending rigidity- radius to the 3rd power of the nail
What are factors that increase the rigidity of external fixator?
1) contact of ends of fracture
2) larger diameter pins (most important)
3) additional pins
4) decreased bone to rod distance
5) pins in different planes
6) increasing size or stacking rods
7) rods in different planes
8) increased spacing between pins
9) near pins closer to the fracture
What is the most important factor that increase the rigidity of external fixator?
Larger diameter pins
Where is the dynamic portion of the oval slot in proximal and distal locking of intramedullary nails?
Further from the fracture site
What is true regarding rigid locking plate constructs in fracture fixation?
Locking plates reduce interfragmentary strain more than conventional plating
What defines the working distance of a plate in a plate/screw fracture fixation construct?
Length between the 2 screws closest to the fracture on each end of the fracture
The greatest biomechanical difference between unicortical and bicortical locking screws is seen when what force is applied?
torsion
Limited contact dynamic compression (LCDC) plates have what advantage over standard dynamic compression plates?
Less implant-bone contact induced osteopenia
