Parts of a screw and cortical , cancellous screw differences described.

1. PRESENTATION ON PARTS OF A BONE
SCREW, DIFFERENCE BETWEEN WOOD
SCREW AND BONE SCREW, DIFFERENCE
BETWEEN CORTICAL AND CANCELLOUS
SCREWS.
Dr. Sourav Bhattacharjee
DNB Trainee, Deptt. Of Orthopaedics
LF Hospital & Research Centre,
Angamaly

2. ANATOMY OF A SCREW
A Screw is a device that turns rotatory motion into
linear motion. A simple machine that converts small
amount of applied torque into a large internal linear
tension along its axis while producing compression
between the two surfaces it holds together.
Fig: A schematic diagram showing the function of a screw.

3. The bone-screw is the most commonly used
implant in Orthopaedics. It is used to hold two or more
objects together and also to have ‘LAG’ effect i.e
compressing two ends or surfaces.
The different parts of a screw are:1.Head
2.Countersink
3.Shaft or Shank
4. Run-out
5. Thread
6. Core
7. Tip

4. Fig. Different parts of a Bone Screw.

5. 1.HEAD
Serves as an attachment for the screwdriver. A
important feature in the screw head is the screw
drive which allows the screw to be turned by the
screwdriver so that linear motion is produced. Firm
purchase of the screwdriver on the screw head is
needed as loose purchase may cause damage to the
surrounding tissues and may cause difficulty in
insertion or removal of the screw.
5 Designs of screw head are in use, these are -

6. a)Single Slot: Conventional but inefficient, there is one single
slot running across the screw head. Disparity between the slot
and screwdriver tip may cause slipping.Needs direct visualiztion
while aligning and the torque is applied only at 2 points.

7. Cruciate Head:
Has 2 slots at right angles which provide a better hold and better
torque transmission and a wider contact area than the single slot
screws. Also called Fearson Screw.

8. Phillips Head:
Resembles the cruciate head but are specially designed so that
the screwdriver cams out as the slots are recessed. The recessed
head provides a secure grip and a better torque transmission,
but axial thrust also has to be adequate.There is chance of
corrosion at the depth of the head due to lesser oxygen levels
which makes the screw prone for breakage during removal.Also
compromises initial unstable reduction of fracture fragments.

9. Recessed Hexagonal Head:
Currently the most popular design in use, also called hex head.Allows a strong
and ‘Alignment-Insensitive’ connection between the head and the driver and
offers good lateral guidance allowing blind insertion and removal. The torque
transmission is independent of axial thrust and doesn’t compromise initial
unstable reduction of fracture fragments.
It is bit difficult to manufacture and if a worn-out screwdriver is used then it
may cause expansion of the screw head by application of excessive torque
because the flats of the hex head driver and the hex head recess are
connected in a direction tangential to the axis of applied force.

10. Stardrive:
Provides all the advantages of HEX. Has better hold and
resistance to stripping as the flats are oriented more
perpendicularly to the axis of the applied force.

11. 2.Countersink
Countersink or the under surface of the screw is of two types Conical
and Hemispherical.
Conical countersink screws need to be inserted perpendicularly in a
platehole. It can be applied centrally to the plate hole while a sidewise
insertion provides a compression effect.
If not inserted perpendicularly to the plate hole the screws with conical
countersinks doesn’t adapt well with the plate hole and create undesirable
and high forces.
On the other hand screws with Hemispherical countersink can be
inserted at any angulation through a washer or a platehole maintaining
concentric contact between the screw and the side of the plate.
Its only disadvantage is its undue prominence when used without a plate.

12. Fig: Different types of countersinks and their effect on plate-Screw construct.

13. Since 2000s a new type of screw with threaded
countersinks and steeper side walls is being used. It is used only
with a Locked Internal Fixator plate hole and is called locking
head screw.
The pitch on the countersink of that screw is identical to
the pitch on its shaft. It provides axial and angular stability and
need about one third of the torque required in other screws for
insertion.

14. 3.Shaft
It is the part between the head and the thread in an screw and it shouldn’t be
confused with the Root or Core of the screw. Its length is variable.
A B C D
Fig. Variable Shaft Length in different types of screws.
A.Fully threaded cortical screw, B.Partial threaded cortical screw, C.Fully Threaded
Cancellous Screw, D.Partially Threaded Cancellous Screw.

15. 4.Run-Out
It is the area between the Shaft and the Thread where the thread
ends. This part of the screw is subjected to extreme stress due to abrupt
changes in the diameter and presence of sharp corners.Incorrect insertion of
a screw may cause breakage at the ‘Run out’ mostly in a spiral fashion.
Fig: Run-out in a screw.
In an insufficiently tightened plate screw construct ; shearing forces
can develop which can break the screw at the Run Out. Now there is
undercutting of the Run Out which eliminates the expansion stress in the
bone hole.

16. 5.Thread
The Thread can be visualized as a long inclined plane or a wedge
encircling the Root or Core of the screw.
Fig. Screw thread compared as an inclined plane
Encircling around the core of the screw.

17. The Screw thread can be of the following type:
a)V type Thread: Has a slanted profile on both sides, produces both shear and
compression force at bone thread interface.
b)Buttress Thread : Buttress threads widen at the base to form a buttress that
resists bending of thread under load. It is slanted only on the leading edge, on
the trailing edge it is perpendicular to the shaft and faces the screw head. It
provides compression force at bone thread junction & offers a higher pull-out
strength in the long run.
Fig: Different types of Screw Threads
c) The locking screw threads are symmetrical, shallower and coarser( having a
wider base) than a conventional screw. It is designed to resist shear load.

18. The Screws can be fully or partially threaded used in
different principles of fixation.
The Thread Diameter is the maximum diameter of the
screw measured from thread tip to thread tip across the shaft.
Upto a certain level the more the thread diameter the more
holding power and resistance to pullout. Excessive thread
diameter on the otherhand may produce excessive load at the
bone-thread interface causing thread bending.
Fig : Thread Diameter of a bone screw.

19. 6.Core
Core is the solid stem of the screw from which the thread protrudes. This
central part of the screw provides strength to the screw.
The core diameter represents the smallest diameter of the screw across the
base of thread, also known as the root diameter.
The torsional strength of the screw varies to the cube of the core diameter
and the tensile strength varies to the square of the core diameter.
Core diameter determines the size of the drill-bit used for the pilot hole. As
the 4.5mm cortical screw and the 6.5mm cancellous screw both have a core
diameter of 3.0 mm , the same 3.2mm drill-bit is used to drill pilot holes in
these screws .
Fig: Core of a Screw

20. 7.Pitch and Lead
Pitch is the distance between the base of two threads in a screw, and
the Lead means the distance the screw travels after a single complete torsion
or turn.
A fine pitched screw having a shorter lead travels a small linear
distance after a complete rotational movement and has a greater mechanical
advantage, produces greater compression and has more advantage than that
of a coarsely pitched screw.
Fig. Pitch of a screw

21. 8.Tip
The Tip is the part of the screw opposite to the head. The screw
length is measured from the top of the Head to the screw Tip.
Fig: Length measurement of a Screw
There are 5 common types of screw tips, as follows…

22. A.Self Tapping: The self tapping screw has a thread cutting device called the
flute. This mechanism having a cutting edge with a positive rake angle cuts
threads into the bone over which the screw advances. The bone chips are cut
from the bone face and are directed away so that they can be packed by the
oncoming threads.
The self tapping tip has no purchase at the distal cortex , so the
length should be so chosen that the entire fluted region protrudes beyond
the distal cortex.These tips reduce the instrument load and lessen surgery
time as the screw cuts its own threads.
Fig: A self tapping locking head screw with flute.

23. B. Self Drilling Self Tapping: Recently developed , used only in locked external
fixator plate hole.
The sharpened tip drills a hole efficiently and a threads are also
being crafted by a tap at the tip that trails the drillbit section. It is exclusively
used for monocortical insertions the sharp tip may cause injury to the soft
tissues beyond the distal cortex. Mainly used in diaphyseal regions.
Fig: Self drilling self tapping Locking Head Screw

24. C.Non Self Tapping Tip: Lacks the Flute the threads extend all the way to the
tip. As the threads are tapered effective torque can be producedwhile
inserting the screw which ensures greater inter-fragmentary compression.
Fig: A non self tapping screw tip.
D.Corkscrew Tip: Used in cancellous screws where
the tip cleans the predrilled hole.Suitable only in the
Cancellous bone.

25. E.Trocar Tip: In the trocar tipped screw the trocar displaces the bone as it
advances.
EX: Schanz Screws
Fig: A schanz Screw with a trocar tip.

26. WOOD SCREW Vs MACHINE SCREW
To understand the difference between a Wood Screw and a
Machine Screw we need to remember the third law of motion by
Newton which says “Every force has equal and opposite reaction
force.”
After a screw is inserted the compression force needed to
hold the two fragments together has to originate from a
deforming force of the screw or the surrounding material.

27. WOOD SCREW:
The Wood Screw has larger tapered threads and are put into wood through a
pilot hole smaller than its core diameter. The threads of the screw form their
own mating threads by compressing the wood around. As the screw inserted
is much more stiffer than the surrounding wood, the wood deforms and this
plastic deformity of the surrounding material lends the compressive force
necessary for holding the 2 wood pieces together.

28. MACHINE SCREWS
In case of machine screws the hole in which the screw is to be
inserted in drilled and tapped i.e the threads are pre-made , the pilot hole is
of the same diameter as its core diameter. When the machine screw is
inserted the shank of the screw undergoes a plastic deformity compared to
the larger chunk of the metal in which the screw is inserted in. This plastic
deformity of the screw shaft gives off the required compression force to hold
the two metal pieces together.

29. Cortical Screw Vs Cancellous Screw
• CORTICAL SREWS
The cortical bone screw is a machine-type screw. The threads are
smaller (in diameter) and are closely placed (lower pitch). The core
diameter is relatively large and provides the necessary strength. The
smaller pitch increases the holding power of the screw. Threads cut
in the pilot hole before this screw is inserted; this is achieved by a
separate tool (tap) or by the self-tapping tip of the screw. The elastic
reaction, vital to hold the bone surfaces together, comes from elastic
deformation of the bone and not the screw. This happens because
the screw is stiffer than the cortical bone. The modulus of elasticity
screw is more than 10 times that of bone; therefore, much of the
elastic deformation occurs in the bone.

30. A) B)
Fig. A.2.7mm Cortical Screw B. 3.5 mm Cortical Screw

31. CANCELLOUS SCREW: A cancellous bone screw is a modified wood-type
screw. Its tip is not tapered. It has larger threads and a higher pitch as
compared to the cortical screw. The core diameter, which is smaller than that
of the shaft, provides a greater surface area for purchase of the screw threads
on bone. An increase in the thread diameter of a cancellous screw increases
its pull-out strength. A cancellous screw is inserted into an untapped pilot
hole; the size of the pilot hole equals the core diameter of the screw. Its large
threads form companion threads in the bone by compression and by
deforming the bone trabeculae. The spring reaction comes from the
cancellous bone as it is deformed during the thread forming process.