-
8/8/2019 Biological and Mechanical Properties of External Fixation
1/38
-
8/8/2019 Biological and Mechanical Properties of External Fixation
2/38
Why do we have bones
?
-
8/8/2019 Biological and Mechanical Properties of External Fixation
3/38
Why do fractures heal
?
-
8/8/2019 Biological and Mechanical Properties of External Fixation
4/38
Evolution
endoskeleton
genetic pool
reproduction
intramembranous v endochrondralossification
-
8/8/2019 Biological and Mechanical Properties of External Fixation
5/38
Intramembranous Bone
Formation
occurs at periosteal surface
pelvis, scapula, clavicle, skull
osteoblasts form a calcified matrix withincollagen framework
everyday stress fractures
~ primitive process
-
8/8/2019 Biological and Mechanical Properties of External Fixation
6/38
Endochondral Bone Formation
occurs at growth plates and within #callus
osteoblasts form osteoid on cartilagenousframework
fractures
~ mature process
-
8/8/2019 Biological and Mechanical Properties of External Fixation
7/38
Fracture Treatment
reduce HoldRehabilitate
-
8/8/2019 Biological and Mechanical Properties of External Fixation
8/38
Fracture Stability
impaction of bone ends
adjacent ligaments soft tissue damage
-
8/8/2019 Biological and Mechanical Properties of External Fixation
9/38
Fracture Immobilisation
nothing
plaster
cast brace
internal
plate
intramedullary nail
external fixator
mixture
-
8/8/2019 Biological and Mechanical Properties of External Fixation
10/38
Fracture Behaviour
UNCERTAINTY?
TRUST ?
-
8/8/2019 Biological and Mechanical Properties of External Fixation
11/38
Fracture Behaviour
STABILITYversus INSTABILITY
STIFFNESS versus STRENGTH
-
8/8/2019 Biological and Mechanical Properties of External Fixation
12/38
Why do bones fracture ?
related to crystal structure and collagenorientation
cortical bone is weakest in tension andshear
low energy # follow stress lines
-
8/8/2019 Biological and Mechanical Properties of External Fixation
13/38
What types of bone healing ?
primary = union without callus
secondary = union with callus
-
8/8/2019 Biological and Mechanical Properties of External Fixation
14/38
Compression Plating
PRIMARYBONE HEALING
rigid plating inhibits # callus
haversian remodelling
contact healing - contact areas
non-contact healing - non contact areas
-
8/8/2019 Biological and Mechanical Properties of External Fixation
15/38
Haversian Remodelling
PRIMARYBONE HEALING
revascularise necrotic # ends
reconstitution of intercortical union
-
8/8/2019 Biological and Mechanical Properties of External Fixation
16/38
Haversian Remodelling
REQUIRES
exact reduction
stable fixation
existence of a blood supply
-
8/8/2019 Biological and Mechanical Properties of External Fixation
17/38
DCP Plates
compression decays with time
50% within 2 months
same decay occurs with intact bone
due to haversian remodelling
-
8/8/2019 Biological and Mechanical Properties of External Fixation
18/38
Gap Healing
incongruencies
fill with woven bone
secondary osteons scaffold the gaps
-
8/8/2019 Biological and Mechanical Properties of External Fixation
19/38
Primary Bone Healing
oversimplification
often see small amount of callus opposite
a plate
#s healing without callus but have 2ndryhealing
scaphoid and talus = little periosteum or softtissues
-
8/8/2019 Biological and Mechanical Properties of External Fixation
20/38
Healing under Plates stress shielding
vascular disturbance
number of cortices
biological fixation
-
8/8/2019 Biological and Mechanical Properties of External Fixation
21/38
IM Nails
reamed versus non-reamed
extramedullary callus
slowed collapse and endochondral repair
-
8/8/2019 Biological and Mechanical Properties of External Fixation
22/38
External Fixation
primary bone healing can be seen withex-fix
less rigid fixation - controlled motion
-
8/8/2019 Biological and Mechanical Properties of External Fixation
23/38
Mechanical Strength of Ex-
Fix
NOTBASEDSOLEYON FIXATOR
composite structure
load sharing device
strength of the bone
HOWMUCH WEIGHTBEARING ?
-
8/8/2019 Biological and Mechanical Properties of External Fixation
24/38
How Rigid Should an
Ex-Fix be ?
Controlled micro-movement
Non-union - prevent periosteal new
callus
-
8/8/2019 Biological and Mechanical Properties of External Fixation
25/38
Ex-Fix Disassembly ?
Controlled removal
initial stiff frame then more elastic ?
-
8/8/2019 Biological and Mechanical Properties of External Fixation
26/38
Ex-Fix Pins
pinholes < 30% of bone diameter
5mm pin 144% stronger than 4mmpin
separate pins greater than 45
-
8/8/2019 Biological and Mechanical Properties of External Fixation
27/38
Ex-Fix Frames
11mm tubular rod x2 stiffer than solid
rods of 8mm diameter increasing pin spread increases bending
stiffness
decreasing bone to rod distance increasesbending stiffness
-
8/8/2019 Biological and Mechanical Properties of External Fixation
28/38
Rigidity of Ex-Fix
Factor
1 pin diameter2 pin number
3 bone rod distance4 pin group separation
5 Half pin separate > 45
-
8/8/2019 Biological and Mechanical Properties of External Fixation
29/38
How to improve pin bone
interface ?
apply half pins rather than wires
wash pins at time of surgery
avoid thermal necrosis - hand drill
pre-loading
dynamization
-
8/8/2019 Biological and Mechanical Properties of External Fixation
30/38
Axial Stimulation at the
fracture
PASSIVEDYNAMISATION
due to pin bending
remove side bars and pins
-
8/8/2019 Biological and Mechanical Properties of External Fixation
31/38
Axial Stimulation at the
fracture
ACTIVEAXIALDYNAMISATION
weightbearing - without pin bending
relaxation of axial constraint
-
8/8/2019 Biological and Mechanical Properties of External Fixation
32/38
Axial Stimulation at the
fracture
CONTROLLEDAXIALDYNAMISATION
controlled force/displacement actuator
cam device
-
8/8/2019 Biological and Mechanical Properties of External Fixation
33/38
Circular Ex-Fix
recent innovation
1.8 to 2mm wires
circular rings
anatomic considerations
bending stiffness independent ofloading direction
-
8/8/2019 Biological and Mechanical Properties of External Fixation
34/38
Half frame versus
Ilizarov
Ilizarov half as rigid in compression
A-P and torsion comparable
M-L half frame x10 stiffer thanIlizarov
-
8/8/2019 Biological and Mechanical Properties of External Fixation
35/38
Ilizarov Frame Stability
larger wire diameter
decrease ring size
use of olive wires
more wires/half pins
cross wires at 90
increase wire tension
centre rings closer to fracture
closer adjacent rings
-
8/8/2019 Biological and Mechanical Properties of External Fixation
36/38
Frame Stiffness ?
mono-planar
biplanar circular
THEULTIMATEQUESTION !
-
8/8/2019 Biological and Mechanical Properties of External Fixation
37/38
Ex-Fix Summary
ideal construct for external fixation is notknown
can allow for a change incompression/alignment
modulate bone healing between minimal
and maximal callus formation
-
8/8/2019 Biological and Mechanical Properties of External Fixation
38/38