screws and plate

1. IMPLANTS –SCREWS AND PLATE

2. A screw is a device which converts rotational forces into linear
motion
Screws

3. Screws
The two main thread
types of surgical
screws are for cortical
bone and for
cancellous bone.
Each screw type is
available in fully
threaded and partially
threaded format.

4. PARTS-
HEAD,SHAFT,THREAD,TIP

5. HEAD OF
SCREW
Firm purchase of screw driver
during insertion and removal
of screw.
FOUR DESIGNS OF SCREW
HEAD RECESS:
Single slot head-
inefficient,slot across
diameter of head
Cruciate head-two slots at
right angles,better efficient
than sigle slot.
Phillips head-resembles
cruciate head ,but the slots
stop short of the periphery
and are recessed.
Recessed hexagonal head[Hex
head]-currently most
used,strong and alignment
insensitive connection ,good
lateral guidance,does not slip
the new socket and driver tip
now introduced by synthes-
maintains advantages of
hexbut offers better resistace
to striiping.

7. countersink
• Undersurface of head.
• Either-conical,hemispherical.

8. Function of
screw head
Provides the means of applying
torque that is twisting force
It acts as a stop.

9. Shaft/shank
Smooth link between head and
thread

10. Run out
• Between area between shaft and thread represents a location of
significant stress concentration by presence of sharp corners .

11. Pitch
Distance between adjacent threads .
A cortical screw with affine thread has small pitch whereas
cancellous screw with a coarse thread has a large pitch.
The stronger the bone the smaller the pitch and weaker the
bone ,larger the pitch.
Cortical -1.75 mm
Cancellous-2.75 mm

12. The tip
Self tapping
Non self tapping tap
Cork screw tip
Trocar tip
Self drilling self tapping tip

13. Screws
Diameter
• Diameter of the core determines the
minimal hole size for the screw to be
accommodated in the bone and determines
the drill used to create the pilot hole for the
screw.
• In other words the drill to be used will be
the same (approximately the same)
diameter as the core of the screw.

14. Screw
Length
• The nominal length of the screw is from the top
of the head to the screw tip.
• An appropriate length of screw needs to be
chosen.

15. Screws
• Too short - it will not gain full purchase in
the bone.
• Too long - it may cause problems by
irritating the soft tissues, or protruding
subcutaneously.

16. Screws
Pitch
• The pitch of the screw is the length travelled by
the screw with each 360° turn of the spiral.

17. Screws
• Shorter the distance - “finer” the pitch.
• Longer the distance - “coarser” the pitch.
• Cortical bone screws have a fine pitch.
• Cancellous bone screws have a coarse pitch.

18. Screws
Pitch
• distance between threads
Lead
• distance advanced with one revolution
Screw working distance (length)
• defined as the length of bone traversed by the screw
Outer diameter-diameter across the maximum thread width
.
Root (inner) diameter

19. Types of screws:

20. Types of screws
Cortical screws
• The threads are smaller (in
diameter) and are closely
placed (lower pitch).
• smaller pitch increases the
holding power of the screw.
• The modulus of elasticity
screw is more than 10 times
that of bone; therefore, much
of the elastic deformation
occurs in the bone.

21. Cancellous screws
• Its tip is not tapered.
• It has larger threads and a higher pitch as
compared to the cortical screw.
• An increase in the thread diameter of a
cancellous screw increases its pull-out
strength.
• The spring reaction comes from the
cancellous bone as it is deformed during
the thread forming process.

22. Self tapping screw
• Refers to a screw which is inserted directly into a pre-drilled hole
without first tapping a thread.
• Self-tapping screw may further be subdivided into thread-forming and
thread-cutting screws. The thread-forming type moulds (i.e. forms) its
own elastic-plastic deformation or by local destruction of the bone.
The thread-cutting screw cuts its threads through the bone over
which it advances.
• The cancellous bone screw is a thread-forming, self-tapping screw.
The screw thread forms its own mating bone thread by compressing
the soft cancellous bone.

23. NON SELF TAPPING SCREW
• A non-self-tapping screw allows precision placement in hard cortical
bone, particularly if one is trying to insert a screw obliquely into the
bone to lag two bone fragments together.
• The NST screw is incapable of cutting a channel in cortical bone and
can be removed and reinserted without the fear of inadvertent
damage.

24. FULLY AND PARTIALLY THREADED SCREWS
• A fully threaded cortical screw can function as lag screw only when
the near cortex is over-drilled. A fully threaded cortical screw may be
self-tapping or non-self-tapping.
• A partially threaded cortical screw is called a shaft screw. The shaft
diameter corresponds to the outer diameter of the thread. This screw
has better strength and stiffness than a fully threaded screw which is
an advantage when it is used as a lag screw and as an axial
compression screw. It is a non-self-tapping screw

25. CANNULATED SCREW
• Precise insertion in metaphyseal or epiphyseal site over a guide wire
reducing the problem of having to remove and reposition an
incorrectly placed screw.
• A guide wire accurately visualizes the path of the screw. If guide wire
position must be changed, it can be done without enlarging the hole
and sacrificing purchase strength of the bone. Final placement of the
screw requires use of cannulated drill and occasional use of a
cannulated tap. The screwdriver is also cannulated. Cancellous
cannulated screws come in large and small sizes.

26. HERBERT SCREW
• For interfragmentary compression.
• no head and threads are present at both ends of the screw, with a
pitch differential between the leading and trailing threads. intention
is for the screw to be buried beneath a bony surface.

27. PLATES

28. 1). Neutralization plates
2. Compression plates
3. Buttress plates
4. Condylar plates

29. NEUTRALIZATION PLATE
• acts as a ‘bridge’.
• It transmits various forces from one end of the bone to
the other, bypassing the area of the fracture.
• Its main function is to act as a mechanical link between
the healthy segments of bone above and below the
fracture. Such a plate does not produce any compression
at the fracture site.
• A plate used in combination with a lag screw is also a
neutralization plate merely protects the lag screw,
allowing mobilization of the extremity.

30. COMPRESSION PLATE
• produces a locking force across a fracture site to which it is applied.
• The effect occurs according to Newton’s Third Law (action and
reaction are equal and opposite). The plate is attached to a bone
fragment. It is then pulled across the fracture site by a device,
producing tension in the plate. As a reaction to this tension,
compression is produced at the fracture site across which the plate is
fixed with the screws. The direction of the compression force is
parallel to the plate.

31. Role of
compression
What does a
compression
plate achieve?
a. Compaction of the
fracture to force
together the
interdigitating
spicules of bone and
increase the stability
of the construct.
b. Reduction of the
space between the
bone fragments to
decrease the gap to
be bridged by the
new bone.
c. Protection of the
blood supply through
enhanced fracture
stability.
d. Friction, which at
the fracture surfaces
resists the tendency
of the fragments to
slide under torsion
or shear.

32. Methods of achieving compression
• SELF COMPRESSION PLATE:
This is a device that converts the torque (turning force) applied to the
screw head to a longitudinal force which compresses the fractured
bone ends. The screws and plates are designed to facilitate this
conversion. As the screw advances in a self-compressing plate it slides
down on an inclined plane that is part of the plate’s screw hole

33. Tensioning device
A special tensioning device
can be attached between
the bone plate .A bolt is
then tightened to pull the
plate across the fracture
site. This produces tension
in the plate and large
compressive forces-across
the fracture.

34. Eccentric screw placement
• Eccentric placement of a screw in a plate hole creates considerable shear
stress in the screw. The same force is transmitted to the plate and can
occasionally be used to produce interfragmental compression. To achieve
this, a screw is eccentrically placed in the hole of a plate.

35. Compression plates
• Work by placing a cortical screw
eccentrically into an oval hole in the plate.
• During the final tightening of the screw, the
screw head will be forced into the center of
the hole squeezing the fracture together
beneath the plate.
• Creates compression at the fracture site.

36. BUTTRESS PLATE
• is to strengthen (buttress) a weakened area of cortex.
The plate prevents the bone from collapsing during the
healing process. It is usually designed with a large
surface area to facilitate wider distribution of the load.
• Such a plate acts as a buttress or retaining wall. A
buttress plate applies a force to the bone which is
perpendicular (normal) to the flat surface of the plate. A
buttress plate must be firmly anchored to the main
fragment
• It is commonly used in fixing epiphyseal and
metaphyseal fractures

37. CONDYLAR PLATE
• Has been in the treatment of intra-articular distal femoral
fractures.
• It has two mechanical functions. It maintains the reduction
of the major intra-articular fragments, hence restoring the
anatomy of the joint surface. It also rigidly fixes the
metaphyseal components to the diaphyseal shaft,
permitting early movement of the extremity.
• This plate functions both as a neutralization plate and as a
buttress plate. Since the plate can be attached to a
tensioning device and has specially designed screw holes, it
also functions as a compression plate.
• A condylar plate is used to fix a proximal femoral
osteotomy and intercondylar fracture of the femur

38. SOURCE

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