Fracture fixation

Question 1

Stress in a distal femoral locking plate =

Answer 1

Weight of patient/cross sectional area of the plate

Question 2

Calculate strain in fracture patterns =

Answer 2

Change in length/original length = Degree of movement at the fracture site/ The surface contact area of broken fragments

Question 3

Strain experienced at comminuted vs transverse fracture?

Answer 3

Comminuted fracture = less strain - larger surface area

Question 4

3 factors that need to be present in order for primary healing to occur?

Answer 4

Absolute stability
Perfect reduction (<500micrometres)
Viable bone ends

Question 5

How is cancellous bone healed?

Answer 5

Vascular ingrowth

Subsequent cell migration into the trabecular scaffold

Question 6

4 components of a screw?

Answer 6

1. Head
2. Shaft
3. Thread
4. Tip

Question 7

What determines the torsional strength of a screw?

Answer 7

The core diameter

Question 8

What is the pitch in a screw?

Answer 8

Distance between each thread

Question 9

What improves pull out strength in screws? 3

Answer 9

Finer pitch
Longer length
Greater thread depth

Question 10

What is the lead of a screw?

Answer 10

Distance travelled with a 360 degree turn

Question 11

Optimum pilot hole size?

Answer 11

85-90% of screw

Question 12

Forces at at a non-locking screw-plate-bone interface?

Answer 12

Tension at the screw head/plate and the screw thread/bone

Shear forces at the screw thread/bone interface

Question 13

Forces at at a LOCKING screw-plate-bone interface?

Answer 13

Sits neutrally in bone until loaded

Then shear and compressive forces from screw-bone interface is transmitted to plate

Question 14

Plates are used in ‘absolute stability’ mode in what 2 areas?

Answer 14

1. Intra-articular fractures

2. Diaphyseal/metaphyseal fractures

Question 15

Function of a neutralisation plate>

Answer 15

Prevents fracture rotation around compression screw

Neutralises shear forces

Question 16

Placing plate on the tension side of the fracture site increases strength by what factor

Answer 16

200x

Question 17

How does a non locked plate work?

Answer 17

Tightening of the screws compresses the plate onto the bone and creates a FRICTIONAL FORCE

Question 18

The ability of a non locked plate to resist loading is directly proportional to?

Answer 18

How much the screws can be tightened

Question 19

What confers stability to a locked plate construct?

Answer 19

Angle stable screws ‘trap’ large sections of bone between them
Internal EXFIX - separation of screws

Question 20

What are the two factors that influence plate strength?

Answer 20

Cross sectional area (WH^3)

Material

Question 21

The stiffness of a nail in torsion is governed by? 1

Answer 21

Polar moment of inertia

Question 22

How does a tension band work

Answer 22

Converts tension into compression by translating the centre of rotation from the fracture to the tension band

Question 23

Factors that increase rigidity in a EX FIX construct

Answer 23

1. ANATOMICAL REDUCTION
2. pin size
3. decreased bone to rod distance
4. pin spread
5. number of pins
6. thickness of bar/ amount of bars
7. More planes