Imp Tech U6b

Question 1

How does a screw provide compression between 2 objects?

Answer 1

1. Has no thread along the section in contact with the first object so pulls it into the second object
2. Block has a pre drilled hole which is wider than screw thread so pulls objects together

Question 2

How is the holding strength of a screw determined?

Answer 2

By the strength of the screw material, the strength of the object material, design of the screw thread

Question 3

What is the function of a washer?

Answer 3

To increase the surface area and spread the load of the screw head over a larger area

Question 4

Why is the undersurface of bone screws rounded?

Answer 4

To allow maximum contact between screw head and bone after sinking
Reduce excessive stress which can crack bone

Question 5

What are the 3 diameters of a screw?

Answer 5

Core diameter; smallest diameter of threaded section
Shaft diameter; unthreaded portion
Thread diameter; widest part of threaded section

Question 6

Why do most screw threads have an asymmetrical shape?

Answer 6

Flat on upper surface and rounded underneath

Means have wider surface for pulling and little friction on the underside (most of torque used for pulling rather than overcoming friction during insertion)

Question 7

What are the depth and pitch of a screw?

Answer 7

Depth; difference between thread and core diameter

Pitch; linear distance of 360º revolution

Question 8

What is tapping?

Answer 8

The process of cutting a thread (can be self tapping or pre tapped)

Question 9

What type of bone can be threaded without a tapping instrument? Why

Answer 9

Cancellous (softer bone)

Has a corkscrew tip - no flute as bone can grow into it making removal difficult

Question 10

What features do self tapping screws have?

Answer 10

A flute to allow cuttings to escape

Question 11

What is lagging?

Answer 11

Compressing 2 objects together - ie with a screw

Question 12

What are the 2 ways screws can achieve lagging?

Answer 12

1. Designed specifically as lag screws (unthreaded top half of shaft)
2. Drill a gliding hole in one fragment so screw can slip though

Screw then screws into second fragment so it compresses the 2 objects together

Question 13

Why is position of screws for lagging important?

Answer 13

So forces are evenly distributed

Question 14

What are screws used for in orthopaedics?

Answer 14

• Prevent sideways displacement of fragments
• Hold a plate against bone
• To increase grip of an IM nail on bone
• Part of external fixator assembly
• To permit displacement in an axial direction (ie dynamic hip screw/condylar screw)

Question 15

Where are screw/plate combos used?

Answer 15

In the forearm only for long bone fractures

To hold joints where complete reconstruction not possible

Question 16

What is the bridging technique?

Answer 16

Used in complex fracture with lots of fragments (complete restoration would risk damage to blood supply)
2 main fragments linked with a plate to maintain bone length/alignment, small fragments left unfixed in between

Question 17

How does stress reversal of plates occurs?

Answer 17

If there is a gap at the fracture site and the plate takes most of the load, cycles backwards and forwards as loaded
Causes fatigue failure

Question 18

What should take most of the load if a plate is used?

Answer 18

Should be shared until bone is united and bone can take all of the load (then remove the plate)

Question 19

When should a plate be used?

Answer 19

When anatomical alignment needs accurate restoration
when screws are inadequate (large bending forces)
When load sharing can be guaranteed (if can’t be guaranteed but plate at risk of fatigue failure)

Question 20

Where are plates used?

Answer 20

Around joints
Bones of forearm
Pelvis
Face/jaw

Question 21

Which side of a fracture should a plate be placed on?

Answer 21

Tension side

Question 22

Why should a plate be contoured?

Answer 22

Concave

Encourages compression of the opposite side and help with load sharing

Question 23

What are the disadvantages of plating?

Answer 23

Can cause soft tissue damage and compromise blood supply to already damaged areas
Increases infection risk
Delayed healing

Question 24

What is cerclage?

Answer 24

Encircling fragments of a bone with flexible wires to push them together

Question 25

How are flexible wires used as a tension band? Where is it useful?

Answer 25

Uses power of surrounding muscles to provide compression (from muscle contractions under the wire providing greater tension in the wire) Useful around damaged joints as it promotes stability/recovery

Question 26

What are the advantages of IM nails?

Answer 26

Can mobilise quickly after surgery (with weight bearing)

Question 27

What type of stresses are IM nails designed to withstand?

Answer 27

Axial + bending stress (due to sturdy + round shape)

Question 28

What is the antegrade technique for insertion of an IM nail?

Answer 28

Modern technique Nail inserted into bone from one end without disturbing fracture site Use an X-ray during insertion to see the position

Question 29

What is the retrograde technique for insertion of an IM nail?

Answer 29

Old fashioned technique | Fracture site opened using soft tissue dissection and IM nail inserted through fracture end

Question 30

What is reaming? What is the alternative?

Answer 30

Widening of the IM canal by paring off the inner bone surface Solid thinner nails can be inserted without damage (unreamed technique)

Question 31

What are the relative advantages of steel or titanium for IM nails?

Answer 31

Stainless steel: strong + stiff, well tolerated by body Titanium: very low toxicity, slightly less stiff than steel (notch sensitivity - if a hold drilled in/abraded, more sensitive to weakening)

Question 32

What is the working length of a nail?

Answer 32

The length of the nail that transmits load from one fragment to the other (in a simple fracture is very short, in a complex fracture is fixed to the 2 main fragments either side so can be quite long through the fragments)

Question 33

What are the relative pros/cons of solid vs hollow nails?

Answer 33

Solid: - stronger than a hollow nail of same volume - can be thinner than hollow nails (requires cross screws to grip hollow bone) Hollow - can select the best side to grip the reamed bone - curved to match the shape of the bone they are designed for - less stiff in bending than solid nails

Question 34

What is compromised in nail design?

Answer 34

Strength and stiffness | Very stiff nails can damage the bone - needs to be strong enough to withstand forces through it

Question 35

What is a simple nail designed for?

Answer 35

To fit the medullary canal fully Maintain accurate anatomical alignment Allows early weight bearing Used in tibia + femur

Question 36

What are nails with cross-screws used for?

Answer 36

Increase working length (longer bones can be screwed) Good rotary control Only in middle part of long bones

Question 37

When are nails used in combination with plates?

Answer 37

Fractures of proximal/distal femur | Due to high bending stresses on femur is hard to realign fragments - used to allow early weight bearing

Question 38

How are nails and plates used to fix femoral neck fractures?

Answer 38

Provides extra support to lateral surface with plate Nail from plate into femoral neck Nail acts as a cantilever against plate to maintain normal alignment

Question 39

What are the complications associated with IM nails?

Answer 39

Reamers can damage bones/get stuck Rotary misalignment if inserted wrong orientation Infection - almost impossible to solve

Question 40

When are external fixators used following trauma?

Answer 40

Temporary - open fractures with significant soft tissue damage Definitive - until soft tissue/bone healing complete (more planning in application)

Question 41

What factors need to be considered in definitive placement of an external fixator?

Answer 41

- Placement of pins to not interfere with soft tissues | - Ensure dynamisation (sliding of one fracture segment on another to stimulate callus formation)

Question 42

Where are external fixators mostly used?

Answer 42

Tibia (lots of soft tissue damage common - poor blood supply) Femur Humerus

Question 43

What are the 2 main principles of external fixation frames?

Answer 43

Must be stable (not rigid) | Pins shouldn't tether soft tissues/restrict wound access

Question 44

Describe bilateral and unilateral external fixator frames

Answer 44

Bilateral: - (more old fashioned) - Bone pins across both cortices and through skin/soft tissues on both sides - stable but soft tissue tethering and limited motion Unilateral: - (more modern) - Pass through skin + soft tissues on one side only (stop on opposite side of cortex) - Allows stability with minimal soft tissue tethering and good mobilisation

Question 45

What are A or V external fixator frames?

Answer 45

Pins at right angles to each other Used if unilateral frames not stable enough Compromise between soft tissue tethering and stability (very rare use)

Question 46

What determines the stability of an external fixator frame?

Answer 46

``` Configuration of the frame Degree of contact between bone ends Extent of soft tissue damage Quality of bone/pin interface How tight clamps are Total number of pins ```

Question 47

Is rigidity the aim of external fixation?

Answer 47

No, aim for stability | Rigidity prevents micromovement and bone healing

Question 48

What is dynamisation and when is it used in external fixation?

Answer 48

Modification of the frame to permit transmission of forces across the fracture site Used when soft tissues are almost healed

Question 49

What are the advantages of external fixation?

Answer 49

``` Quickly assembled/fitted in emergencies Can be adjusted later if not a good position first time Beam can be removed to take X-ray Versatile frame for different sites Good soft tissue access ```

Question 50

What are the disadvantages of external fixation?

Answer 50

Infection risk at bone/pin interface High risk of pin loosening Soft tissue tethering to skin by pins

Question 51

What are the possible complications of external fixation?

Answer 51

``` Loosening of pins (once loosened need to be resited) Infection Old infected pin sites Tethering from poor pin insertion Joint stiffness if cross joints ```

Question 52

How should pins be inserted for external fixation?

Answer 52

Cross as little muscle as possible (causes pain and limited movement = joint stiffness) Consider the position of the joint (ie ankle in plantigrade position to avoid equines - very difficult to correct with physio)