lag screw

1. LAG SCREW PRINCIPLES

2. • LAG SCREW IS A TECHNIQUE OF INSERTION
• A SCREW IS USED TO COMPRESS FRACTURE
FRAGMENTS TOGETHER(INTER-FRAGMENTARY
COMPRESSION)
• ITS PULL THE FRAGMENT TOGETHER PRODUCING
PRESSURE ACROSS THE FRACTURE LINE
• COMPRESSION BETWEEN FRAGMENTS INCREASES THE
FRICTION FORCE SO THAT INTER FRAGMENTARY
MOTION IS LESS LIKELY AND THEREFORE STRENGTHENS
THE FRAC
• EFFECT OF MAXIMAL INTERFRAGMENTARY
COMPRESSION
• SCREW MUST BE INSERTED INTO CENTRE OF
FRAGMENT
• RIGHT ANGLE TO FRACTURE LINE
• 2 SCREW IS BETTER THEN ONE
• it produces 2500-3000 newtons of force

3. PLACE TO APPLY THIS PRINCIPLE
• neck of femur fracture
• condyles of femur
• simple undisplaced medial or lateral tibia plateau
• oblique, spiral, or butterfly type pattern fracture in long bones
• phalanx or metacarpal

4. types of screws
• cortical screw for diaphyseal fracture
• for 3.5mm screw (near cortex 3.5mm, far cortex
2.5mm, tap with 3.5)
• for 4.5mm screw(near cortex 4.5mm, far cortex
3.2mm, tap with 4.5mm)
• cancellous screws
• 4.0/6.5 mm
• for screw 6.5 (drill bit 3.2, tapped with 6.5mm)
• for screw 4.0( drill bit 2.5, tapped with 4.0mm)
• FULL/PARTIAL (partial type- 16MM OR 32MM)

5. lag screw
A. independent primary lag screw
B. using a compression plate as
the primary loading device
with the addition of a
subsequent lag screw through
the plate
C. protected by a plate

6. • FRAC CAN BE FIX SUCCESSFULLY WITH SCREW ALONE IN SITUATION
WHERE HIGH BENDING LOADS ARE NOT ANTICIPATED OR CAN BE
PROTECTED AGAINT
• GOAL IS TO CREATE A COMPOSITE STRUCTURE THAT WILL BEAR A
LOAD
• ADDITION OF A PLATE TO THE STRUCTURE SIMPLY ADD MECHANICAL
SUPPORT AND GENERATE FORCE THAT RESIST BENDING
• SCREW FIXATION ALONE-ROVIDE STABILITY IF THE FRACTURE LENGTH
IS AT LEAST 2-3 TIMES DIAMETER OF THE BONE- MIN 2 SCREWS

7. • INSERTING SCREW RIGHT ANGLES TO THE FRACTURE PLANE -
PROVIDES MAX INTERFRAGMENTARY COMPRESSION BUT MINIMAL
AXIAL STABILITY
• IF RIGHT ANGLES TO THE LONG AXIS OF THE BONE- PROVIDES MAX
AXIAL STABILITY
• IF WE FIX SPIRAL FRAC- 1 SCREW RIGHT ANGLE TO LONG AXIS AT
CENTRE AND OTHERS RIGHT ANGLE TO FRACTURE PATTERN

8. • A PLATE FIXATION SCREW THAT CROSSES A FRACTURE LINE SHOULD
BE APPLIED AS LAG SCREW
• USING IT WITHIN A PLATE AS A LAG SCREW RESULTS IN 50% LOSS IN
COMPRESSION EFFECT BECAUSE OF INDENTATION OF THE SCREW THREADS
WITHIN THE CORTICAL BONE OF THE GLIDING HOLE
• WHEN USE SHAFT SCREW AS LAG SCREW, IT DOES NOT ENGAGE THE NEAR
CORTEX AND ITS USE SIGNIFICANTLY IMPROVES THE STATIC COMPRESSION
FORCE EXERTED BY THE LAG SCREW
• AS SCREW TINGHTENED, THE SLIDING FIT OF THE SHANK SCREW IN THE
GLIDING HOLE AVOIDS THE LOCKING EFFECT SECONDARY TO MOVEMENT OF
THE SCREW IN THE A DIRECTION OTHER THAN ALONG ITS LONGITUDINAL
AXIS

9. Failure to perform
proper countersinking
causes an eccentric
loading and lessens the
degree of compression.
In very thin cortex it
might also lead to slight
displacement of the
fragments because of
the eccentric force.