The document discusses the basic principles and parts of screws used in orthopedic surgery. It describes the four main parts of a screw - the head, shaft, thread, and tip. It discusses different types of screws including cortical, cancellous, and special screws like Herbert and variable angle locked screws. It also covers principles like lag screws and the benefits of far cortex locking screws which provide flexible fixation while promoting interfragmentary motion and secondary bone healing.

1. Abdelaziz A Ibrahim
Ortho. Specialist
HSH

2. In 1958, The AO formulated four basic principles, which
have become the guidelines for internal fixation.

3. Diffintion:
Screw is a device which converts
rotational Forces into linear motion.

4. A screw has a core about which is wrapped a spiral
surface
Most surgical screws are made of Titanium or
Stainless Steel or Bioabsorbable

5. PARTS OF SCREW
Head
Shaft
 Thread
Tip

6. HEAD:
 Functions:
Attachment of a screwdriver – to apply torque
Arrest forward motion
A) Slotted
B) Cruciate
C) Philips
D)Hexagonal /
Allen
E) Torx Stardrive

8. SHAFT:
 Smooth Link.
 Not present in standard cortex
screw.
 Present in cortical shaft screws
Or Cancellous screws.

9. RUN OUT:
 Transition between shaft and
thread
 Site of most stress riser
 Screw Break:
- Incorrectly centered hole.
- Hole not perpendicular to
the plate.

10. THREAD:

11. DIAMETER:
 Thread diameter: Diameter across the maximum
thread width.
- Cancellous have larger thread diameter.
- Affects the pull out strength.

12.  The Maximum width of the
Thread.
 Narrowest diameter of the
screw across base of Thread.
 Thread depth more in
Cancellous screws Less in
Cortical screws

13. TIP:
 There are five types of tips of bone screws.
 Non-self-tapping tip.
 Self-tapping tip.
 Corkscrew tip.
 Trocar tip.
 Self drilling Self Tapping.

14. TYPES OF SCREWS:
Cortical Cancellous
Machine type.
 Smaller thread.
 Lower pitch.
 Larger core
diameter.
 Smaller pitch higher
holding power.
 Higher surface area.
 Better hold in
cortical bone
 Wood type.
 Core diameter is less – Larger
threads.
 Higher Pitch.
 Greater surface area for
Purchase.
 No need for tap.
 Pilot hole equals core diameter.
 Better fixation in Soft cancellous
Bone
Fully threaded
screws
Fully or Partial
threaded

15. Lag screw principle:
Lag screws / the lag technique compresses the fracture
fragments together.
The cancellous bone screw and the cortical screw
through a gliding hole

17. Special Screws:
Herbert Screws:
Specialized implant to achieve interfragmentary
compression.
No head and threads on both ends with a pitch differential.
In small bones, such as the scaphoid, self compressing,
double pitch screws can be Used.

19. VA locked screws head are spherical rather
than straight.
Be Careful - Look similar
Variable angle locking screws Standard locking head
Variable Angle Locked Screws:

21. AcuTrak Screw System:
Fully threaded Screw has a
variable thread pitch.
Self tapping
Pitch differential causes
compression at fracture site.

23.  STANDARD LCP : (bridge plating) depends on
secondary bone healing rather than primary bone
healing traditionally achieved by external fixation
constructs more flexible inter fragmentary motion ( 10
times more than LCP / DCP).
 Can supresses interfragmentary motion ( HIGH
STIFFNESS), insufficient for secondary # healing. 1.
 Rigid locking screws - uneven stress distribution
stress shielding under the plate bone resorption.
 To enable flexible fixation with LCP’s / to reduce the
stiffness FAR CORTEX LOCKING SCREWS WERE
DEVELOPED
Far Cortical Locking Screws:

25. Parts:
1- HEAD – locking thread.
2. MOTION CONTROL COLLAR.
3. FLEXIBLE SHAFT reduced diameter
elastically deflect ( within - motion control
collar of near cortex).
4. FAR CORTEX LOCKING THREADS
resembling an monolateral external fixator

26. Flexible Fixation : Flexibility is directly proportional -
working length of the screw.
Stiffness is reduced by screws – fixed in plate & far cortex, while retaining a
controlled motion envelope at near cortex.
Thereby promoting inter-fragmentary motion [ IDEAL: 0.2 – 1 mm ] to
produce secondary callus

28. It has 4 Key Features:
1.Flexible fixation. ( reduces stiffness by 80- 88%)
actively promotes callus formation similar to
external fixator.
2.Uniform load distribution – mitigate stress
raisers.
3.Progressive stiffening – on increased loading.
4.Parallel inter-fragmentary motion – by ‘S’ shaped
flexion. [ FUNCTIONS AS EXTERNAL FIXATORS IN
BIOMECHANICAL BEHAVIOUR AND BIOLOGIC
HEALING ]

30. Interfragmentary motion Due to flexible shaft
produces ..... ‘S’ SHAPED FLEXION.... symmetric
callus – formed....... PREVENTING PARTIAL NON
UNION
RESEMBLES internal fixation by construct but replicates the
biomechanical function of external fixators.

34. Thank You