Principles Of Stable Frame Biomechanics

Question 0

What is the major advantage of external fixators?

Answer 0

It can be used in uni planar mode thus minimizing the transfixion of soft tissues and bulk of fixator

Question 1

Conventional external fixator characteristics

Answer 1

Rely on large diameter stuff pins (4-6mm) for stability
Pins act as cantilevers
Pins threaded and fix to bone

Question 2

When applying a frame, what are the 4 questions you should ask?

Answer 2

What kind of fracture?
What are treatment goals?
How stable does frame need to be?
Where can frame go to maximize stability?

Question 3

What kind of treatment goals can be achieved with a frame?

Answer 3

DCO, lengthening, fusion

Question 4

Why are bars for the tibia placed anteriorly?

Answer 4

Bc Sagittal bending moments are 2-5x stronger than coronal

Question 5

When applying half pins, how should they be placed?

Answer 5

1. At least 2 pins per bone fragment
2. At least 2cm gap b/w the fracture and the nearest pin
3. Maximize pin spread (near near-far far)

Question 6

Good pin interface minimizes:

Answer 6

Bone resorption
Pin tract infections
Pin loosening

Question 7

How do you increase stability in half pin spread

Answer 7

Increase spread between pins on each fragment and minimize the spread between the interior pins

Question 8

Clamps are placed inside the inner half pins resulting in:

Answer 8

A reduced lever arm

Question 9

What should the bar to bone proximity be?

Answer 9

1. Place bar as close as possible to limb to reduce stress/flexing of half pins and bar this producing a stiffer frame
2. Typically there should be two finger breadths between hardware and skin

Question 10

What are steerage pins?

Answer 10

Aligns interior pins with plane of fracture to aid in reduction and force fragments together during weight bearing. Helps eliminate shear

Question 11

What are the five methods of increasing stability ?

Answer 11

Controlling both near and far ends of each bone fragment

Decreasing the distance between the rod and bone

Increasing number of fixation rods

Increasing the diameter of pins

Using multiplayer fixation

Question 12

Characteristics of circular fixation:

Answer 12

Small diameter tensioned wires
Not cantilever system
Modularity
Control

Question 13

What are the disadvantages of circular fixation

Answer 13

Potentially transfixing tissue soce wires pass through both sides
Bulk
Femur/humerus

Question 14

What are the three goals when placing Ilizarov frames

Answer 14

Maximize stability to prevent shear forces while allowing axial micromotion and promote fracture healing

Gain stable fixation to create the proper environment for bone formation

Allow for early weight bearing

Question 15

What are ilizarovs principles of bone healing/osteogenesis?

Answer 15

Preservation of blood supply

Preservation if osteogenic tissue

Stable fixation

Functional activity
Early mobilization

Question 16

When are wires indicated in frames?

Answer 16

Metaphyseal or short bone fragments
Allows axial motion during loading (trampoline)
Simplifies ring/remove in office

Question 17

What are the factors what increase stability in a circular fixation?

Answer 17

Type of wire (smooth or olive)
Diameter of wire (1.8mm in adults and 1.5 in pediatric pts)
Number of wires
Tension of wires

Question 18

What is the most stable angle of fixation for wires?

Answer 18

90 degrees

Question 19

What increases stability in a circular frame?

Answer 19

Increased number of rings-use smallest size possible

Rigidity of connecting elements between rings

Question 20

Mounting strategies for rings

Answer 20

2 rings per limb segment
2 points of fixation per ring
2x2 points of fixation 1 ring/segment

Question 21

Intrinsic factors

Answer 21

Configuration of bone ends
Modulus of elasticity of tissue between bone ends
Greater tension of soft tissues surrounding bone the greater the stability