Rockwood book reading

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Rockwood book reading

• Chapter 1

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External fixator

• An external fixator is an assembly of
• pins attached to bone fragments
• clamps and sidebars that couple the pins.

3
Loosening of External Fixator Pins

• Three mechanism
• (1) Thermal necrosis around the pinhole site
  
• (2) High local stresses can occur in the
  pins and
• bone if the hole through which the pin
  is
• inserted is undersized
• (3) Micromotion, which induces bone
  resorption
• at the pin/bone interface if the pin is
  a loose fit
• in the hole.
• A If the pin and bone hole are the same diameter
• micromotion can occur with bone
  resorption.
• B If the pin is lt 0.3 mm in diameter than
  the hole
• in bone, microfracture may occur during
  insertion.
• C If the bone hole diameter is about 0.1 mm
  smaller
• than the pin diameter, the bone is
  prestressed but
• does not fracture, micromotion is
  eliminated, and pin
• stability is maintained

4
Optimal stiffness??

• The optimal stiffness of a fixator is not
  specifically known
• Optimal the stiffness
• (1)stabilize the fracture and create healing
  changes as the
• fracture consolidates
• (2)rigid enough to initially support the
  forces applied by the
• patient during ambulation without causing
  malalignment of
• the fracture.
• (3)Stiff ? ? that the fracture is shielded
  from the stresses
• required to stimulate healing.
• Wolff law ? X

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Diameter of a pin or sidebar ?
stiffness and strength ? 4X Length
(distance between bone ?
stiffness and strength ? 3X
surface and sidebar) Distance (between pins and
fracture)? stiffness and strength ?
Number of a pin ?
stiffness and strength ?
Diameter of a pin or sidebar ?
stiffness and strength ? Threaded pin
?
stiffness and strength ?
6

• Stiffness ?
• Transfix gt half pin
• 2-plane gt 1-plane
• Bilateral gt unilateral
• Number and diameter of the
• transfixing wires

box type was the stiffest (two rings above and
two below the fracture, along with anterior half
pins, two connecting rods, and a
unilateral bar)
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Fixation in Osteoporotic Bone
DHS, Angular plate, and Buttress plate
V.S Cannulated screw condylar screw

Impaction wide buttress long splintage
8
Bone augmentation

• Enhancement of local bone density using either
  polymethylmethacrylate (PMMA) or absorbable
  hydroxyapatite cement, has been studied,
  particularly in relation to fixation of femoral
  and vertebral osteoporotic fractures.
• Biomechanical studies
• PMMA/ hydroxyapatite cement
• in femoral neck fractures
  ? 170 strength
• in osteoporotic compression fracture
  ? 125 strength

unstable intertrochanteric fractures fixed with
cement DHS
? strength
Posteromedial and Lateral wall involvement
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BIOMECHANICAL ASPECTS OF FRACTURE FIXATION IN
SPECIFIC LOCATIONSProximal femur
Principal compressive group
Wards triangle
Secondary compressive group
Principal tensile group
during normal activities compressive force
acting through the femoral head ? 4X-8X body
weight
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BIOMECHANICAL ASPECTS OF FRACTURE FIXATION IN
SPECIFIC LOCATIONS proximal femur
The joint reaction force can be divided into two
components One is perpendicular to the axis of
the sliding screw and causes shearing of the
fracture surfaces along the fracture line, which
results in inferior displacement and varus
angulation of the femoral head, and increases the
resistance of the screw to sliding. The other
component is parallel to the screw, driving the
surfaces together, enhancing stability by
frictional and mechanical interlocking of the
fracture.
? parallel component of the joint force to allow
the surfaces to slide together.
higher-angle hip screw
Ensure that the screw can slide freely in the
barrel of the side plate or the hole in the nail.
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The edge of the barrel is the fulcrum The screw
should be engaged as deeply as possible within
the barrel. Fh X Le Fb X Lb Lb ? ,
Fb ? the screw contacts the barrel, causes a
greater frictional resistance force that requires
more force to overcome in order to permit
sliding
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Factors that do increase the strength
The most important factor has been found to be
the quality of the reduction due to the
importance cortical buttressing in of reducing
fracture displacement
If no washers are used to distribute the screw
load against bone, the fixation screw heads have
been found to pull through cortex near the
greater trochanter, when the cortex is thin.
Finally, if the screws are not well supported
inferiorly where they cross the fracture, they
may rotate inferiorly carrying the femoral head
into a varus orientation
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BIOMECHANICAL ASPECTS OF FRACTURE FIXATION IN
SPECIFIC LOCATIONSAround the Metaphyseal Region
of the Knee
Construct stiffnesses 1.condylar plates
2.plates with lag screws
retrograde IM supracondylar nail across
the fracture site gt (14 less stiff in axial
compression 3.blade plates.
17 less stiff in torsion ) The most
important factor identified was maintaining
contact at the cortex opposite that on which the
fixation device was applied. Fixation constructs
without cortical contact were only about 20 as
stiff as those with cortical buttressing
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BIOMECHANICAL ASPECTS OF FRACTURE FIXATION IN
SPECIFIC LOCATIONSAround the Metaphyseal Region
of the Knee
Fixation with T-plates and screws has been found
to provide the greatest resistance to an axial
compressive load
With the addition of a plate, not only is the
load distributed to the plate, but additional
screws can be placed in the stronger cortical
bone distal to the metaphysis of the tibia. The
disadvantage of adding a buttressing plate is the
greater invasiveness that it requires for
installation and poor soft tissue coverage.
multiple Kirschner wires, the construct's
stiffness is most increased by adding more wires,
regardless of their specific orientations.
15
Fixation of Patellar Fractures
16
Fixation of Patellar Fractures

• Large tensile and bending forces generated by
  contracting the quadriceps muscles.
• The tensile force causes significant bending in
  the patella with the knee flexed, which tends to
  open the anterior surface of the fracture.
• Screw fixation can generate greater compression
  across the fracture site, but wire can withstand
  higher tensile forces

17
Tile classification of pelvic fracture
Rockwood Green's Fractures in Adults 6th
P1606, AO tech. chapter 11 p.264-266
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Young and Burgess classification of pelvic
injuries.
A- C lateral compression injuries. Type I
stable, Type II hemipelvic instability, Type
III bilateral instability D- F
anteroposterior compression injuries and injury
to the ligaments of the pelvis. Type I stable
Type II rotational instability (sacrospinous
and sacrotuberous ligaments rupture) Type III
completely unstable hemipelvis (posterior
sacroiliac ligaments fail) G shows a vertical
shear injury, which ruptures the pelvis
anteriorly and posteriorly
OKU 7-8
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Fixation of Pelvic Fractures

• Incompletely sacroiliac (SI) joint disrupted
• anterior plating of the symphysis pubis alone
  provides stability similar to anterior plating
  combined with posterior fixation using as an SI
  plate or iliac screws
• Complete disruption of the SI joint or with
  fracture through the sacrum
• using a single-legged stance model, the use
  of two anterior plates combined with a single SI
  screw was most effective at reducing SI joint
  gapping, rotation, and pubis symphysis gapping.

20
Fixation of the Spine-external fixation
halo apparatus is an external fixation device for
cervical spine injuries, it stabilize the injured
cervical spine against bending, Although
stiffening the vest enhances its ability to
stabilize the injury, this property must be
balanced with enough flexibility to provide
reasonable comfort for the wearer and to
accommodate chest expansion and contraction.
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Fixation of the Spine -internal fixation

• Wire can resist only tension
• Screw can resist forces in all directions
  (tension, compression, bending transverse
• to the axis of the screw), except for
  rotation about its longitudinal axis.
• Hook only resists forces that drive its surface
  against bone and depends also on its
• shape and the bone surface it rests against.
• screws are biomechanically superior to other
  forms of vertebral
• attachments.

22
Fixation of the Spine -internal fixation

• Pullout strength increases with
• - increased density of the bone
• - a greater insertion depth (at least gt 50) -
  engagement of the anterior cortex
• - a larger screw diameter.

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Fixation of the Spine -internal fixation

• The screw tends to toggle about the base of the
• pedicle, which is the stiffest region and is
  mainly
• composed of cortical bone.
• Toggling tends to open the screw hole in a
• windshield wiper fashion.
• Toggling can be reduced if the screw head is
• locked to the plate or rod and if the plate or
  rod
• contacts the vertebra over a wide area.
• Longer fixation, attached to a greater number of
  
• vertebrae reduces forces acting on the screws,
• because of the effect of the greater lever arm
  of
• a longer plate or rod.

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Fixation of the Spine-internal fixation
The major differences between these approaches
relate to the location of the fixation (either
anterior, lateral, or posterior) and to the
method by which the fixation is attached to bone.
Generally, the most rigid fixation is the one
with the longest moment arm from the center of
rotation of the injured segment. For a specific
applied moment (e.g., flexion), posterior
fixation, being located further from the center
of rotation, results in greater rigidity. Left
figure shows the approximate locations of the
centers of rotation at different cervical spine
levels when the posterior elements have been
disrupted.
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Fixation of the Spine-internal fixation after
corpectomy

• Biomechanical testing has shown that
• the least stability anterior plating
  alone
• the greatest stability posterior rods /-
  anterior plate
• Another biomechanical test showed that in
  sagittal plane
• the least rigid strut grafting alone
• less rigid anterior plate alone
• more rigid anterior plate strut grafting
• the most rigid lateral mass plates

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Fixation of the Spine-internal fixation

• Constrained or semiconstrained plate?
• compressive load a transmitted through the
  graft
• about 40 in constrained
• about 80 when a semiconstrained
• ? semiconstrained devices prevent fatigue
  loosening

• Coupler bars, which connect the fixation rods
  to form an H configuration, prevent the rods from
  rotating medial and lateral when torsion is
  applied to the motion segment
• This construct significantly enhances the
  implant's torsional and lateral bending stability.

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Fixation of the Spine-internal fixation

• Interbody fusion?
• (1) Graft or non-graft?
• ?significant effect of the graft in
  sagittal plane
• stability of the fixation.
• (2) Cage or non-cage?
• -screw designs with a horizontal
  cylinder and
• external threads
• - box shapes
• - vertical cylinders.
• ? all cage designs increased flexion
  stiffness
• by 130 to 180
• ? it buttresses a posterior fixation
  system against
• flexion moments, reducing forces in
  the fixation
• ? Fixation with cages alone did not
  significantly
• increase lumbar motion segment
  stability, so
• augmentation with posterior
  fixation in cases of
• motion segment instability is
  necessary.