Informasi singkat mengenai screw

1. SCREW

2. Introduction
Screws are commonly used
in orthopedic surgery for
various purposes, such as
interfragmentary fixation,
fastening soft tissue to
bone, or holding plates or
nails to bone.
They play a crucial role in
orthopedic fixation by
assisting in bone healing,
reducing stress, and
decreasing fracture gaps.

3. Introduction
There are different types of
orthopedic screws, including
cancellous bone screws,
cortex screws, and
cannulated screws, each
designed for specific
applications.
Additionally, screws can be
used to fix soft tissue to
bone and to achieve
interfragmental
compression, with specific
designs and characteristics
tailored to their intended
applications

4. A. Design and Function
• What is screws?
• A screw is a powerful mechanical device that converts rotation into linear motion.
Since the introduction of locking head screws, all other screws are called
conventional screws. A screw can be used to compress a fracture surface, fix a
plate to the bone, or attach a fixator to the bone.
• Position screws hold fragments together without compression. Lag screws can be
inserted through a plate or independently. The term lag screw refers to
compressing two fragments together.

5. • Central core that provides strength
• Thread that engages the bone and
converts rotation into linear motion
• Tip that may be blunt or sharp
• Head that engages either the bone
or the plate
• Recess to attach a screwdriver

6. • Design (eg, cannulated,
locking head)
• Dimension (eg, 4.5 mm) •
Characteristics (eg, self-
tapping, self-drilling)
• Area of application (cortex,
cancellous, monocortical, or
bicortical)
• Function or mechanism

7. The types and functions of screws differ, however
all screws have a head that has a recess for the
screwdriver attachments.
The core diameter of the screw corresponds to the
size of the
drill bit used to create a tract for the screw. The thread
diameter of
the screw corresponds to the screw size, ie, 3.5 mm or
4.5 mm. The
tip of the screw can be rounded or pointed depending
upon whether
it is self-tapping or non self-tapping in function. The
pitch (distance
between the thread) of the screw differs according to
the screw being
cortical or cancellous or locking in nature.
a Cortex screw.
b Partially threaded cortex screw.
c Cancellous bone screw.
d Partially threaded cancellous bone screw.
e Locking head screw.

8. The rotation of a screw creates linear
motion through screw threads. As the
screw moves forward, it presses
against the bone cortex and further
advancement compresses the head,
creating preload. This preload
compresses the fracture and prevents
separation, while friction opposes
displacement by shear. Lag screws
provide absolute stability. In contrast,
LHS have a head with a matching
thread in the plate hole.
A screw is an efficient tool for the fixation of a
fracture by interfragmentary compression or
for fixing a splint, such as a plate,
intramedullary nail or fixator, to the bone.

9. 2. Biomechanics
• Axial force in a screw is produced by rotating it clockwise.
• Screw pitch must be small enough to prevent unwinding.
• Screw design and insertion method influence heat generation.
• Thermal necrosis can be caused by blunt drill bits or large wires.
• Locking head screws provide minimal axial force and act as fixators.
• Countersink can reduce contact stress and risk of microfracture.
• Torque applied to screw thread is minimal in LHS.
• Uncontrolled torque during tightening can cause failures.
It is important that the surgeon checks the quality of the screwdriver tip regularly
before use.

11. • The countersink device is used to
prepare the bone to receive the head of
the screw so that the contact area
between the screw head and the bone is
increased (a-b).
• With-out countersinking, forces would
concentrate at a small area (c–d).

12. 3. Screw thread
There are three types of screw threads used for the AO technique. The cortex screw
thread is designed for diaphyseal bone and comes in different sizes. The cancellous
bone screws have a deeper thread, larger pitch, and larger outer diameter than the
cortex screws. The LHS used with the locking compression plate (LCP) have a larger
core diameter and a shallow thread with blunt edges. This results in increased
strength and a larger interface with the bone compared to conventional screws. The
head of the screw is also threaded. The shallow thread requires precise insertion
technique with the power drive to prevent toggling and reduce purchase at the
screw thread – bone interface.
When inserting LHS, the last revolutions must always be done by hand using a
torque-limiting screwdriver.

13. 4. Screw Tip
Different screw tip designs are available, such as smooth conical tips, self-tapping
tips, and self-tapping/self-drilling tips. A tap is an instrument used to cut a channel
for a screw thread. The original screws with smooth conical tips were designed to be
inserted after tapping the drill hole. Tapping may weaken screw pull-out strength
due to inadvertent toggling of the tap, which enlarges the hole. In osteoporotic and
cancellous bone, it is often better to avoid tapping or use it only for the near cortex.
Tapping with a power drive reduces toggling but makes controlling deep penetration
more difficult and dangerous.
To avoid problems with screw removal the surgeon should try to disengage bone in
the flutes by first slightly tightening the screw, thereby shearing off the ingrown
bone from the cutting flutes. Thus, the first turn should be clockwise followed by
anticlockwise removal of the screw.

15. • (a-b) - When using self-tapping screws in bones with a thin cortex, it
is essential to make sure that the tapping flutes pass the cortex to
increase the pullout strength of the screw.
• (c–d) - In thick cortical bone, the pullout strength may not be
affected significantly when a proper size self-tapping screw is used
in comparison to a nonself-tapping screw.

16. 4. Screw Function
A screw can be used to perform different functions depending upon the surgical
plan.
• If the screw is introduced only a short
distance inside the bone, there is risk of
loosening of the screw with increased
loads (a).
• When using these screws, it is noted
that the tip is sharp and it can injure
soft tissue beyond the cortex if it
protrudes through the far cortex. This
screw type should only be introduced
deep enough to engage into the far
cortex (b).

17. Function of Screws

18. 4. Screw Function
A glide hole is drilled into the near cortex with a larger diameter than the screw
thread. In the far cortex, a smaller hole is drilled using a guide. This is the pilot hole.
A self-tapping screw or tap can be used to create a threaded hole. A fully threaded
cortex lag screw creates preload and compresses bone fragments as it is tightened.

19. B. The Lag Screw
• Interfragmentary compression
1.1 Application of a fully threaded screw as a lag screw
A screw with complete threading can serve as a lag screw, as long as the threading
does not come into contact with the outermost layer of the bone near the screw
head (referred to as the near cortex). A glide hole is made in the near cortex to
achieve the technique. A smaller hole, called a pilot hole, is drilled in the opposite
cortex. The pilot hole is the same diameter as the screw. A screw can be inserted into
the pilot hole or a tap can be used to cut a channel for the screw threads. When a
fully threaded lag screw is used, it only engages in the threaded hole. Preload is
created as the screw head is pressed against the cortex, compressing the bone
fragments.

20. B. The Lag Screw
• Interfragmentary compression
1.1 Application of a fully threaded screw as a lag screw
The principle technique is demonstrated in cancellous bone by drilling a pilot hole
across the fracture. The partially threaded cancellous screw's smooth shaft serves as
the gliding hole, engaging the far cancellous and cortical bone. Preload is created
when the head is compressed against the near cortex, resulting in interfragmentary
compression.

22. B. The Lag Screw
2. Screw-tightening and torque-limiting screwdrivers
When an experienced surgeon tightens a screw optimally, they achieve torque close
to the thread-stripping torque. It is not logical to tighten screws to this limit due to
the high axial force they generate. When a screw achieves its holding force through
preload, there is little force left to sustain additional load. Historically, surgeons
aimed for maximum axial force, including repeated retightening. Nowadays,
surgeons are advised to apply lag screws (and plate screws) at approximately 2/3 of
the maximum torque. Surgeons should also be aware that titanium screws provide
less tactile feedback compared to stainless steel, requiring extra care during
insertion.

23. B. The Lag Screw
Lag screw technique. The glide hole in the
near cortex is wider than the diameter of
the thread. The thread hole in the far cortex
is the same as the core diameter of the
screw and has been tapped.

24. B. The Lag Screw Compression of a partial articular fracture
using a partially threaded 6.5 mm
cancellous bone screw. The thread pulls the
opposite bone fragment toward the head
of the screw. The shaft of the screw does
not transmit any great axial force between
the shaft and the surrounding bone. The
length of the screw shaft must be chosen
so that the threaded part of the screw lies
fully within the opposite bone frag?ment.
To prevent the screw head from sinking
into the thin cortex, a washer is used.

25. B. The Lag Screw
3. Compression
Tests conducted on skilled surgeons have indicated that screws measuring 4.5 mm were
regularly tightened to a torque resulting in 2,000-3,000 N of axial compression.
Measurements taken in vivo revealed that the initially applied compression gradually
decreases over several months. However, this compression persists beyond the time
needed for the osteons to bridge the fracture gap and facilitate primary bone healing.
If the strain produced by micromotion is greater than the strain tolerance of bone, the
screw will become loose and then place additional strain on the adjacent screw. There
will be progressive loosening of the implant. This is a particular problem in osteoporotic
bone that has low strain tolerance.

26. B. The Lag Screw
4. Screw insertion
The screws in conventional plating can be inclined to optimize the lag screw position
or bypass comminution or a fracture line. The inclination is locked by the purchase of
the screw in the far cortex. Monocortical screws rely on the design of the screw head-
to-plate locking process for stability. This type of construction is referred to as LHS.
Monocortical screws are only indicated in certain cases, as their hold is not as strong
as bicortical screws. They are commonly used for periprosthetic fractures when
bicortical screws cannot be inserted.
Locking head screws cannot be used as lag screws.

27. B. The Lag Screw
• a. Histological section of a well-fixed screw.
There is close contact between bone and
adjacent screw with bone remodeling.
• b. The appearance of the “thread” where the
screw had been undergoing movement
within a range of micrometers. Bone has
been resorbed and replaced by fibrous
tissue that no longer has holding power.
Mechanical loosening of a screw
—biological reaction.

28. B. The Lag Screw
The screw hole in the limited?contact dynamic
compression plate (LC-DCP) allows inclination of
screws and optimal lag screw placement.

29. B. The Lag Screw
4. Modes of failure
Screws can fail due to axial pull out, bending forces, torque, or a combination of
these factors. Surgeons may cause screws to fail during insertion by applying
maximum torque. Most conventional screws have poor resistance to bending and
torque because of their small core diameter. Increasing the core diameter of a screw
can increase resistance to bending by threefold with only a 30% increase in core size.
The best implants can withstand intermittent peak loads without irreversible damage
to the bone-implant interface. Plates, nails, and fixators can spring back into shape
after giving way to peak load, but screws are less tolerant. Overload can cause the
bony thread to strip and permanently reduce the screw's holding power.

30. B. The Lag Screw
4. Modes of failure
Screws can fail due to axial pull out, bending forces, torque, or a combination of
these factors. Surgeons may cause screws to fail during insertion by applying
maximum torque. Most conventional screws have poor resistance to bending and
torque because of their small core diameter. Increasing the core diameter of a screw
can increase resistance to bending by threefold with only a 30% increase in core size.
The best implants can withstand intermittent peak loads without irreversible damage
to the bone-implant interface. Plates, nails, and fixators can spring back into shape
after giving way to peak load, but screws are less tolerant. Overload can cause the
bony thread to strip and permanently reduce the screw's holding power.

31. B. The Lag Screw
• a. The lag screw is oriented perpendicular to the fracture plane. This is an ideal
inclination in the absence of forces along the bone axis.
• b. An inclination halfway between the perpendiculars to the fracture plane and
to the long axis of the bone is better suited to resist com?pressive functional
load along the bone’s long axis.
Optimal inclination of the screw in relation to a simple fracture plane

32. C. Clinical applications of lag screws
1. Positioning of lag screw
Lag screws are most efficient when placed perpendicular to the fracture plane or in
an inclination between the perpendiculars to the fracture plane and the long axis of
the bone. The choice depends on the presence or absence of forces along the bone
axis. The perpendicular position is easy to achieve and provides optimal function. In
long spiral fractures, multiple lag screws can be used. The direction should follow the
spiral plane, but this can damage soft tissues and periosteum, so preservation of the
periosteal blood supply is important. Before inserting an inclined lag screw in
diaphyseal bone, a countersink for the screw head should be prepared.

33. C. Clinical applications of lag screws
2. Lag screws in metaphyseal and epiphyseal regions
Articular and periarticular fractures require precise alignment and stability to
maintain joint congruity. Lag screw fixation is commonly used in this area,
sometimes requiring a washer to prevent sinking into the bone. Postoperative
treatment often includes a protection or buttress plate. Locking head screws and
plates can provide stability in cases with multiple metaphyseal fragments. Lag screws
cannot be replaced by these screws.

34. B. The Lag Screw
Partially threaded cancellous bone screw used
to lag a tibial plateau fracture

35. Keypoint
• Implants must tolerate peak load without irreversible loss of bone-
implant interface.
• Screws are less tolerant of peak load compared to plates and nails.
• Friction generated by turning screws within bone creates heat.
• Heat generated during screw insertion can cause thermal necrosis of
bone.
• Locking head screws provide fixation based on the locked screw head.
• Different designs available for the tip of the screw include smooth, self-
tapping, and self-drilling.

36. Conclusion
• Different designs of screw tips are available for insertion.
• Locking head screws cannot be used as lag screws.
• Removal of self-tapping screws may be difficult in certain cases.
• Surgeons should try to disengage bone in the flutes before screw
removal.
• Two forces are active during screw insertion: tangential and axial.
• Most self-drilling screws are also cannulated screws.
• A guide wire is used to determine the direction and assist with
measurement.

37. Thank you