some biomechanics explanation on implants especially lag screw and tension band principle - done for basic ortho skills course

1. BIOMECHANICS OF COMMON
IMPLANTS

2. OUTLINE
 LAG SCREW CONCEPT
 TENSION BAND PRINCIPLE

3. LAG SCREW CONCEPT
 Aim – to achieve compression across fracture
 2 types
 Cortical screw through gliding hole
 Cancellous bone screw

4. LAG SCREW CONCEPT
 Prerequisites:
Screw must be
placed
perpendicular
to fracture site

5. LAG SCREW CONCEPT
 Prerequisites:
NOT in comminuted
fractures
With cortical screws,
requires over drilling

6. LAG SCREW CONCEPT
 Cortical screw through gliding hole:
Drill near cortex with drill
bit = outer screw diameter
Drill far cortex with drill
bit = inner screw diameter

7. LAG SCREW CONCEPT
 Cortical screw through gliding hole:
Countersink for what?
To create a platform on the
near cortex for the screw
head – ensures full contact

8. LAG SCREW CONCEPT
 Cortical screw through gliding hole:
Improper countersink – eccentric loading when
compress, might displace #

9. LAG SCREW CONCEPT
 Cortical screw through gliding hole:
If countersink too deep – remove almost all cortical
bone on near cortex, screw may break through and end
up in medullary canal

10. LAG SCREW CONCEPT
 Cortical screw through gliding hole:
Tap the far cortex, must push
it thru the near cortex – will
form false track
Final screw placement

11. LAG SCREW CONCEPT
 Cancellous bone screw:
Drill near cortex with 3.2mm
drill bit for 6.5 screw, do not
perforate far cortex
Measure and tap with
6.5mm cancellous bone
tap

12. LAG SCREW CONCEPT
 Cancellous bone screw:
Place screw, being cancellous bone, doesn’t require gliding
hole and countersink, but requires washer as cortex is thin

13. TENSION BAND PRINCIPLE
 Concept introduced by Pauwels (an engineer)
An eccentrically loaded bone will always
have a tension side and compression side

14. TENSION BAND PRINCIPLE
A tension band converts a tension force into a
compressive force across the whole width of bone

15. TENSION BAND PRINCIPLE
Even when put on tension side, an intact buttress still
needed in compression side for the tension band to work

16. TENSION BAND PRINCIPLE
Dynamic tension band – produces increased
compression with motion

17. TENSION BAND PRINCIPLE
Static tension band – already producing compression
at moment of reduction – remains constant

18. TENSION BAND PRINCIPLE
Placement of wires ideally 5mm deep to the anterior
surface, will be close to joint– but will not impair
tension band principle if not.

19. TENSION BAND PRINCIPLE
 Which one?
More stable against
torsional forces
Less likely to cut thru the
medial and lateral retinacula

20. TENSION BAND PRINCIPLE
 What causes this?
How to prevent it?

21. TENSION BAND PRINCIPLE
Know any other
techniques?