Imp Tech U6b Flashcards
How does a screw provide compression between 2 objects?
- Has no thread along the section in contact with the first object so pulls it into the second object
- Block has a pre drilled hole which is wider than screw thread so pulls objects together
How is the holding strength of a screw determined?
By the strength of the screw material, the strength of the object material, design of the screw thread
What is the function of a washer?
To increase the surface area and spread the load of the screw head over a larger area
Why is the undersurface of bone screws rounded?
To allow maximum contact between screw head and bone after sinking
Reduce excessive stress which can crack bone
What are the 3 diameters of a screw?
Core diameter; smallest diameter of threaded section
Shaft diameter; unthreaded portion
Thread diameter; widest part of threaded section
Why do most screw threads have an asymmetrical shape?
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)
What are the depth and pitch of a screw?
Depth; difference between thread and core diameter
Pitch; linear distance of 360º revolution
What is tapping?
The process of cutting a thread (can be self tapping or pre tapped)
What type of bone can be threaded without a tapping instrument? Why
Cancellous (softer bone)
Has a corkscrew tip - no flute as bone can grow into it making removal difficult
What features do self tapping screws have?
A flute to allow cuttings to escape
What is lagging?
Compressing 2 objects together - ie with a screw
What are the 2 ways screws can achieve lagging?
- Designed specifically as lag screws (unthreaded top half of shaft)
- 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
Why is position of screws for lagging important?
So forces are evenly distributed
What are screws used for in orthopaedics?
- 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)
Where are screw/plate combos used?
In the forearm only for long bone fractures
To hold joints where complete reconstruction not possible
What is the bridging technique?
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
How does stress reversal of plates occurs?
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
What should take most of the load if a plate is used?
Should be shared until bone is united and bone can take all of the load (then remove the plate)
When should a plate be used?
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)
Where are plates used?
Around joints
Bones of forearm
Pelvis
Face/jaw
Which side of a fracture should a plate be placed on?
Tension side
Why should a plate be contoured?
Concave
Encourages compression of the opposite side and help with load sharing
What are the disadvantages of plating?
Can cause soft tissue damage and compromise blood supply to already damaged areas
Increases infection risk
Delayed healing
What is cerclage?
Encircling fragments of a bone with flexible wires to push them together
How are flexible wires used as a tension band? Where is it useful?
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
What are the advantages of IM nails?
Can mobilise quickly after surgery (with weight bearing)
What type of stresses are IM nails designed to withstand?
Axial + bending stress (due to sturdy + round shape)
What is the antegrade technique for insertion of an IM nail?
Modern technique
Nail inserted into bone from one end without disturbing fracture site
Use an X-ray during insertion to see the position
What is the retrograde technique for insertion of an IM nail?
Old fashioned technique
Fracture site opened using soft tissue dissection and IM nail inserted through fracture end
What is reaming? What is the alternative?
Widening of the IM canal by paring off the inner bone surface
Solid thinner nails can be inserted without damage (unreamed technique)
What are the relative advantages of steel or titanium for IM nails?
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)
What is the working length of a nail?
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)
What are the relative pros/cons of solid vs hollow nails?
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
What is compromised in nail design?
Strength and stiffness
Very stiff nails can damage the bone - needs to be strong enough to withstand forces through it
What is a simple nail designed for?
To fit the medullary canal fully
Maintain accurate anatomical alignment
Allows early weight bearing
Used in tibia + femur
What are nails with cross-screws used for?
Increase working length (longer bones can be screwed)
Good rotary control
Only in middle part of long bones
When are nails used in combination with plates?
Fractures of proximal/distal femur
Due to high bending stresses on femur is hard to realign fragments - used to allow early weight bearing
How are nails and plates used to fix femoral neck fractures?
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
What are the complications associated with IM nails?
Reamers can damage bones/get stuck
Rotary misalignment if inserted wrong orientation
Infection - almost impossible to solve
When are external fixators used following trauma?
Temporary - open fractures with significant soft tissue damage
Definitive - until soft tissue/bone healing complete (more planning in application)
What factors need to be considered in definitive placement of an external fixator?
- Placement of pins to not interfere with soft tissues
- Ensure dynamisation (sliding of one fracture segment on another to stimulate callus formation)
Where are external fixators mostly used?
Tibia (lots of soft tissue damage common - poor blood supply)
Femur
Humerus
What are the 2 main principles of external fixation frames?
Must be stable (not rigid)
Pins shouldn’t tether soft tissues/restrict wound access
Describe bilateral and unilateral external fixator frames
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
What are A or V external fixator frames?
Pins at right angles to each other
Used if unilateral frames not stable enough
Compromise between soft tissue tethering and stability
(very rare use)
What determines the stability of an external fixator frame?
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
Is rigidity the aim of external fixation?
No, aim for stability
Rigidity prevents micromovement and bone healing
What is dynamisation and when is it used in external fixation?
Modification of the frame to permit transmission of forces across the fracture site
Used when soft tissues are almost healed
What are the advantages of external fixation?
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
What are the disadvantages of external fixation?
Infection risk at bone/pin interface
High risk of pin loosening
Soft tissue tethering to skin by pins
What are the possible complications of external fixation?
Loosening of pins (once loosened need to be resited) Infection Old infected pin sites Tethering from poor pin insertion Joint stiffness if cross joints
How should pins be inserted for external fixation?
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)
