Ch. 41 and 41 Internal and External Fixation

Question 1

How does a plate or implant used for a fracture with anatomic resconstruction aid?

Answer 1

the implant acts as a neutralization implant

Question 2

what are the principles of biological osteosynthesis?

Answer 2

1. indirect fracture reduction using limited surgical approaches with minimal disturbance of the hematoma
2. fracture stabilization using bridging implants rather than anatomic reconstruction and rigid fixation
3. limited reliance on secondary implants
4. limited, if any, use of bone grafts

Question 3

what is the relationship of tensile strength to size of wire?

Answer 3

related to the cross sectional area = pi x radius^2

Question 4

what size wire is 18 gauge? what is its relative tensile strength

Answer 4

18 gauge is 1 mm and has a relative tensile strength of 1 - it is what we use to compare things to

Question 5

what size wire is 16 gauge and what is its relative tensile strength?

Answer 5

1. 2 mm

1. 4 tensile strength

Question 6

what size wire is 20 gauge and what is its relative tensile strength?

Answer 6

0.8 mm and 0.64x

Question 7

what size is 22 gauge wire and what is its relative tensile strength?

Answer 7

0.6 mm and 0.36x

Question 8

what size is 24 gauge wire and what is its relative tensile strength?

Answer 8

0.5 mm and 0.25x tensile strength

Question 9

what is the loop tension and load resisted before loosening for a twist cerclage

Answer 9

70 N

270 N

Question 10

what is the loop tension and load resisted before loosening for a single loop cerclage?

Answer 10

165 N and 260 N

Question 11

that is the loop tension and load resisted before loosening for a double loop cerclage?

Answer 11

390 N and 662 N

Question 12

what is a Morse taper?

Answer 12

the screw-cone peg that was the locking bolt of the first generation angle stable interlocking nail

Question 13

The area moment of intertia in the solid section of an 8 mm interlocking nail is approximately ___ times that of a 3.5 mm DCP and ___ times that of a Broad 3.5 DCP

Answer 13

The area moment of intertia in the solid section of an 8 mm interlocking nail is approximately 6.8 times that of a 3.5 mm DCP and 3.5 times that of a Broad 3.5 DCP

Question 14

what are the four advantages of an ILN?

Answer 14

1. placed in neutral axis so subjected to compressive forces instead of bending
2. greater AMI for ILN than for a plate - the plate is to the third power bc it is a rectangle but the ILN is to the fourth power because it is a cylinder - so the AMI goes up much faster as you increase in implant size for an ILN than plate
3. stability in torsion and compression with the locking of the nails
4. eliminates any risk of screw pull out

Question 15

where is an ILN the weakest?

Answer 15

when bending occurs in a plane parallel to he long axis of the nail cannulation
–> decreasing cannulation size results in a stronger nail
As an example, a 6 mm nail accepting a 2.7 instead of 3.5 screw lead to a 52 fold increase in nail fatigue… but then those screws broke easily so finally someone made a bolt

Question 16

what percent of the medullary canal should you fill with an ILN?

Answer 16

70%

Question 17

what does it mean to secure a nail in dynamic mode?

Answer 17

only place bolts in one fragment

requires that a fracture be load sharing to try this

Question 18

what two factors determine pull out strength of a screw?

Answer 18

the outer diameter and the strength of the material into which it is placed

Question 19

what determines the bending strength of a crew?

Answer 19

the core diameter

Question 20

what percentage of a screws’ stripping torque is the optimal tightness of a screw?

Answer 20

70%

Question 21

how much more torque can be applied for a star head vs hexagonal head?

Answer 21

65% more

Question 22

how much angulation can be given for a locking screw in the PAX securos system?

Answer 22

10 degrees

Question 23

for bridge plating, what is the recommended plate span ratio for comminuted versus simple fractures?

Answer 23

2-3 times the length of the fracture if comminuted

8-10 times the length of the fracture if simple

Question 24

what is the best age to apply elastic plate osteosynthesis?

Answer 24

younger than 5-6 months

Question 25

what percent of the canal should be used for the IM pin in a plate rod construct?

Answer 25

35-40%
BEcause for every 10% in canal filling, plate strain is decreased by 20% but you don’t want more than 40% or there is too much rigidity

Question 26

how does the traditional positive profile transfixation pin compare to the Duraface negative profile pin in terms of stiffness, pin strength, and resistance to cyclic fatigue?

Answer 26

Duraface has 55% increase in stiffness, 54% increase in strength and 2.3-4.9 times increase in cyclic fatigue

Question 27

which will add more stability to an ex fix frame, a diagonal or articulating bar? what is the difference between the two?

Answer 27

a diagonal crosses the fracture gap
an articulating bar does not
a diagonal will add more stability than an articulating bar

Question 28

by how much will adding an ex fix to an ILN decrease torsional compliance and bending compliance?

Answer 28

adding an ex fix to an ILN decreases torsional compliance by 25% and bending compliance by 60%

Question 29

how big should the acrylic bar be in a free form ex fix?

Answer 29

acrylic bars should be 2-2.5 times the diameter of the bone

alternatively, they should be 3-4 times the size of a stainless steel bar to reach a similar stiffness

Question 30

where along the bone should you place ex fix transfixation pins in relation to the joints or fracture?

Answer 30

place pins 3/4s of the bone diameter away from adjacent joints and 1/2 of the bone diameter away from the fracture line

Question 31

what strategies can be used to strengthen the pin bone interface junction and pin pull out strength?

Answer 31

• pin type - use threaded pins
• pin size - threaded portion should be 20-30% of the bone diameter
• pin placement - central placement through the widest part of bone available
• insertion technique - low speed <300 rpm and pre drill

Question 32

what strategies can decrease the load and bending forces per pin in an ex fix?

Answer 32

• adequate pin number - 3-4 pins per fragment
• decrease pin working length - place clamp with bolt toward patient and no more that 1 cm from surface of skin
• pin configuration and frame stiffness - far-near-near-far pin placement, multiplanar frame, augmentations (articulation, diagonals, intramedullary pin)