Function and Application of Bone Screws Flashcards
What are the 3 applications of bone screws?
- Combination with plate/interlocking nail
- # reconstruction combined with plate, EDF, IM pin
- # of cancellous bone
Oblique diaphyseal fractures or fractures with large bone fragments can be reduced and stabilised with lag screws. Why are Lag screws are preferable to cerclage wires?
Placed to give more compression
True or false
Bone screws should never be used in isolation for diaphyseal #?
TRUE
What are the 3 basic screw types?
- cortical
- cancellous
- locking screw
Generally speaking, which bone type are cortical screws used in? (3)
- Diaphyseal
- Cortical
- Metaphyseal (dogs + cats)
Cortical screw:
A) Thread?
B) Pitch steepness?
C) threads per unit length compared to cancellous?
A) Fully threaded
B) Less steep
C) More
Is a cancellous or cortical screw stronger for the same size?
Cortical
True or false:
Cortical bone is only 2-3 mm thick in places
True
True or false:
With cortical screws the priority is to have a minimum number of threads engaging
FALSE
What are cancellous screws used for? (2)
To compress fragments of epiphyseal or metaphyseal bone
Cancellous screws:
A) Thread?
B) Thread depth?
C) Number of threads per unit length?
D) Pitch?
A) Partial or full
B) Deep
C) Relatively few
D) High (steep angle)
What surface area of cancellous screws thread in contact with the relatively low-density bone?
Large
Why are cancellous screws weaker?
Narrower ore diameter
Locking screws:
A) Head?
B) Pitch?
C) Core diameter?
A) Threaded
B) Narrow
C) Large
What are locking screws used in combination with?
Locking plates (in which the locking head engages with the threaded locking plate hole.)
Too much pressure needs to be avoided when using a drill - why? (2)
- Prevent damage to transcortex
- Avoid too larger hole
After using a depth gauge; what size screw is used?
Insert a screw 1-2mm longer than the measurement obtained with the depth gauge to ensure a full thread engages the transcortex.
If a screw strips accidentally; how can this be rectified?
screw one size larger can be used as ‘rescue screw’.
What equipment is needed to place screws? (7)
- Screwdriver
- Drill guide
- Tap
- Measure
- Drill
- Screws
- Bone plate
Which screws can be used alone for # repair? (2)
Positional screw
Lag screw
Can positional or lag screws achieve interfragmentary compression?
Lag
To accomplish interfragmentary compression, lag screw threads:
A) should NOT engage where?
B) SHOULD engage where?
A) Near (cis) cortex
B) Far (trans) cortex
Define lag screw
The term lag screw applies to the way a screw is functioning. A fully threaded cortical screw can be used as a lag screw if the thread is prevented from engaging with the near cortex but engages with the far cortex.
what drill bits for a 3.5mm lag screw?
2.5 mm and 3.5mm
What is the effect of failing to overdrill the near cortex when placing a lag screw?
The screw will act as a positional screw and not compress the fracture fragments
How can a partially threaded cancellous screw be used as a lag screw?
The threads engage the far cortex and as the screw is tightened the near fragment glides on the smooth shank of the screw
What is the negative of using a partially threaded cancellous screw as a lag screw?
A weak screw with a risk of screw failure at the junction of the smooth shank to threaded part of the screw
How is a fully threaded lag screw placed?
- Near fragment must be ‘overdrilled’ with a hole equal in size to the diameter of the screw threads – the glide hole - so the threads do not engage the near cortex.
- A hole equal in diameter to the screw core is then drilled in the far cortex – the thread hole.
- An insert guide is placed in the glide hole prior to drilling the thread hole to ensure that the hole is drilled at the correct angle.
Why would a countersink instrument be used?
Countersinking is performed when the screw head is directly in contact with cortical bone.
What does countersinking result in?
increased contact area between the screw head and the bone.
Which bones is countersinking NOT used in (2) and why?
Metaphyseal
Epiphyseal
As cortex too thin
What can be used in areas where countersinking is not possible to allow load distribution?
Flat washer
What can lead to loss of reduction? (3)
- Malposition of screw
- Not counter sinking
- # comminution
To maximise interfragmentary compression, lag screws should be inserted:
A) Where on the bone?
B) What angle?
A) In the middle of the fragment equidistant from the fracture edges
B) At right angle to # plane
Lag- If the screw is not inserted perpendicular to the fracture plane, then as it is tightened, what force is introduced?
Shearing
What do lag screws need to be used with? (3)
- Another lag screw
- Neutralisation plate
- Other fixation device
Positional screws - what drill diamater is used?
Almost the same
Ture or false
For positional screws, both cortices are drilled in one step?
True
Why might a positional scre be recommended?
If the insertion of a lag screw would cause an unsupported fragment to collapse into the medullary cavity.
Using a depth gauge for positional screws:
When using non self-tapping screw measurements are rounded to what mark?
up to nearest 2mm
Using a depth gauge for positional screws:
When using self-tapping screw measurements are rounded to what mark?
up to nearest 4mm mark
What is the core diameter of a 3.5mm cortical bone screw?
2.4mm
The resistance to cyclic failure or bending of a screw is related to its area moment of inertia and is directly related to..?
Core diameter to the power of four.
Small increases in screw diameter results in what change of stiffness?
Increase
What size screw should be chosen?
The largest screw possible, that does not compromise the mechanical properties of the bone in which it is to be inserted, should be chosen.
Experimental work in bone has shown that holes made within the bone will not significantly affect its mechanical properties providing that the hole diameter does not exceed ? % the diameter of the bone.
20
What size drill bit for a 3.5mm screw?
2.7mm
For conventional (non-self-tapping) screws, the hole needs to be tapped with a tap of what size?
Same size
