Internal fracture fixation

Question 1

What are the principles of biological osteosynthesis?

Answer 1

1. Indirect reduction techniques
2. Fracture stabilization using bridging implants
3. Limited reliance on secondary implants
4. Limited, if any, use of bone grafts

Question 2

Describe the various gauge diameters and relative tensile strengths of orthopedic wire

Answer 2

Question 3

What is tensile strength of a wire dependent on?

Answer 3

The cross sectional area (pi x radius squared). Therefore small increase in diameter has a significant effect on tensile strength

Question 4

In how many directions will a hemicerclage counteract forces?

Answer 4

One direction only - must decide on direction in which to wrap the wire based on this

Question 5

How many loops are required for a stable twist knot cerclage?

Answer 5

1 - 1.5

Question 6

What are the general principles of cerclage application?

Answer 6

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.

Question 7

The resting tension of cerclage drops below 30N after how much of a collapse in the bony column?

Answer 7

1% collapse

Question 8

How much does pushing a twist knot flat after twisting reduce tension?

Answer 8

45-90%

Question 9

Describe the differences in initial tension and load before loosening for the twist, single loop and double loop cerclage

Answer 9

Question 10

What is the size of the three available K-wires?

Answer 10

0.035, 0.045, 0.065 inch corresponding to 0.9, 1.1, 1.6mm

Question 11

How does the area moment inertia of interlocking nails and orthopedic plates differ?

Answer 11

The AMI of interlocking nails is calculated by radius to the fourth power, plates are calculated by thickness to the third power

Question 12

How much of the medullary cavity is ideally filled with an interlocking nail device?

Answer 12

75-80% (avoid going larger than 90% to prevent iatrogenic fracture)

Question 13

What are the three types of interlocking nail?

Answer 13

Regular, angle stable, inverse

Question 14

What are the main biomechanical differences between the AS-ILN and the regular ILN?

Answer 14

AS-ILN eliminates the slack that was experienced with the regular ILN (particularly severe in rotation). Hourglass shape of the AS-ILN also increases the AMI of the implant and reduces stress risers at the implant/bolt interface

Question 15

What are the primary differences between cortical and cancellous screws?

Answer 15

Cortical screws have less pitch (distance between threads) and less depth to threads compared to cancellous screws. This is to increase core diameter to better resist bending forces (less risk of screw pull-out with locking systems)

Question 16

What determines screw pull-out strength?

Answer 16

The diameter of the screw and the strength of the bone

Question 17

What determines the bending strength of a screw?

Answer 17

The core diameter

Question 18

Ideally how long is an oblique fracture to allow lag screw placement?

Answer 18

1.5 times the diameter of the bone

Question 19

What is the optimal tightness of a screw?

Answer 19

70% of stripping strength

Question 20

Identify common screw types

Answer 20

Question 21

What is generally the weakest point of a conventional plate/screw construct?

Answer 21

The shear strength of the bone screw interface

Question 22

What are the different methods of plate application?

Answer 22

Compression, neutralization, bridging, buttress (specifically for transcortical defects in metaphyseal regions), elastic plate osteosynthesis

Question 23

How do you calculate the AMI of a screw?

Answer 23

Radius to the fourth power

Question 24

How much of the medullary canal should be filled by a IM pin in a plate rod construct?

Answer 24

35-40%. One study found a 20% decrease in plate strain with each increase of 10% of canal filling. Greater than 50% canal filling may result in a too-stiff construct that minimizes axial micromotion.

Question 25

What is the minimum number of screws for a locking construct?

Answer 25

1 bicortical, 1 monocortical. Third screw if placed close to fracture gap will increase axial stiffness. Fourth screw will increase torsional stiffness regardless of location.