Prof. Dr. Samy Abd Elsamad
Dr .Manal Radwan Salim
Fracture fixation could be internal or
external fixation depending on the
type of fracture, severity of fracture
and age of the patient. Both
techniques influence the mechanical
stresses inside the bone and as a
result may enhance or delay the
healing process. The same is applied
to external fixation.
1- for Internal fixation: the material used
must have adequate mechanical strength
and fatigue resistance), long fatigue life.
Also must be applied on the tensile side of
the fracture.(most destructive type of stress.
2- for External fixation by cast: It is better to
use weight bearing plaster cast as it
facilitate healing of fractures 2 or 3 times
faster than the non-weight bearing plasters.
3-Relation between mechanical
stresses and metabolic activity of
• Pure tensile and pure compressive stresses
causes connective tissue to form bone and
enhance fracture healing.
• Shear stresses causes connective tissue to
form fibrous tissue. If the shear stress is
limited, fracture callus will be formed. If it
is severe, non union may result.
• Stresses from all directions causes
connective tissues to form cartilage.
Types of internal fixation:
Best used in treatment of transverse. For example
fracture of olecranon. Correct wiring convert
muscle tensile force into compressive forces
produced by tension in the wire.
The reaction force from the fragment is
compressive so when the tendon force
increases there is an increase in the
compressive forpce across the fracture surface,
By bringing the two fragments together.
Best used in cases of spiral fractures,
but two screws are needed a
transverse screw is needed to avoid
bending loads while an oblique screw is
used to avoid torsional stresses.
As the transverse
screw alone will
cause the bone to
be subjected to
Oblique screw will cause bone to be
subjected to bending at the fracture site.
3- Plate and screws:
It should best be applied at tensile surface of the
fracture so the fracture will be compressed by
if fracture is subjected to bending, plate will resist
the tensile forces and there will be compressive
forces across the fracture pressing the two
If the plate is not applied on the tensile side,
during bending of the plate resistive effect will
be cantered at the neutral axis and the fracture
will be compressed in the compressive load
The problems may be encountered due to the
usage of using plate and screw:
1- if the screw don’t tightly set:
Bending stresses will occur at screws. The plate
will bear the bending moment alone
2- if the plate is thin, fatigue failure may occur
3- in case of fracture with a gap or a comminuted
fracture, one plate and screws cant’s be used
because the plate will bend page 41
Application of screws or plates will introduce
many changes in the stress distribution in the
bones at the fracture site.
- Stresses are normal in the areas away from the
- - there is a stress concentration at each end of
the plate because this area is the intersection
between a normal stress area and a lower stress
- - stresses decrease at the areas between the
According to the principle of ‘’ bone formed when
needed and resorbed when not needed’’
So the areas of higher stresses will form bones
and those of lower sresses, bone density will
So, refracture may occur at areas of lower sress
(which is the area between the screws) if the
fixation is applied for along time with high
loading behaviou ( weight bearing load).
After removal of screws:
There will be holes inside the bone.
If the segment is loaded ( in compression fo
example) the total load is the same for any
cross section of the bone so; - stresses are
Stresses are represented by number of lines which
are proportional to the total load on the
Bone healing after the fracture:
The areas of high stresses concentration in the
bones (holes) will form new callus formation.
This callus consists of newly low mineralized
Fracture callus bridges a gap between fractured
bone ends, so the diameter will increase more
As healing occurs, the callus becomes more
mineralized and increases the material strength
of the bone.