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					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
  connective tissue:
•   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:
1- Wires:
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.
2- Screws:
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
    muscular force.
 if fracture is subjected to bending, plate will resist
    the tensile forces and there will be compressive
    forces across the fracture pressing the two
    fragments together.
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
   part only.
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
   than normal
As healing occurs, the callus becomes more
   mineralized and increases the material strength
   of the bone.

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