Posterior Spine Fixation
Ghazwan A. Hasan
Kindi Teaching Hospital / Arab Board Council
3rd year trainee
• For Spine fusion either Instrumental or Non-
Instrumental in both use bone graft.
• Segmental pedicle screw fixation is rapidly becoming
a widely used method of spinal instrumentation.
• Pedicle screw instrumentation enables a rigid
construct to promote stability and fusion for
numerous spinal pathologies including:
trauma, tumours, deformity and degenerative
• Eliminating motion between the affected segments
increases the likelihood of fusion and may reduce the
degree of pain the patient experiences. Properly
applied, spinal instrumentation maintains alignment
and shares spinal loads until a solid, consolidated
fusion is achieved.
• Spinal instrumentation systems are described
in terms of where the hardware is attached:
• The anterior devices such as anterior plates
and screw systems usually are classified as
those systems that are designed to attach to
the anterior or anterolateral aspect of the
vertebral body. Typically, the plate or rod
construct is transfixed to the involved vertebral
segments by screws that pierce one or both
cortices as well as gain purchase in the
cancellous bone of the vertebral body.
• The posterior systems are
affixed to the elements
situated posterior to the
vertebral body, the spinous
processes, pedicles, facets
, or laminae. These
use laminar hooks, pedicle
screw systems, facet
screws and wiring
• Interbody fusion systems promote fusion between the
vertebral bodies by the incorporation of a device or
graft that spans the disc space. Although allograft and
autograft spacers are routinely used in combination
with other anterior or posterior instrumentation, a
variety of devices are now available and approved for
implantation. Usually, the interbody systems are
further classified by the surgical approach used during
device implantation. The comingling of system and
approach has given rise to such contemporary
terminology as Anterior lumbar interbody fusion
(ALIF), transforaminal lumbar interbody fusion
(TLIF), posterior lumbar interbody fusion
(PLIF), and Xtreme Lateral lumber interbody
• Spinal stability is defined as the ability of the
spine under physiologic loads to maintain
normal relationships between vertebrae, in
such a way that there is neither damage nor
subsequent irritation to the spinal cord or
• Adjacent Segment Disease(ASD):Abnormal
loading and increased mobility in adjacent
segments may explain the development of
ASD, but it is still unclear whether it is caused
by fusion sequelae or is the result of natural
• Internal fixation for the posterior aspect of the
spine was first reported in 1891 by Hadra
when he documented the use of wires around
the spinous processes for treating Pott’s
• In 1911, Hibbs described an operation for
treating progressive spinal deformities in three
patients in which he proposed utilizing a “bony
bridge” to prevent kyphosis and to provide a
cantilever support to the deformed spine.
• In 1948, King reported the use of facet screws
in the lumbo-sacral spine and indicated that
he had been using them for eight years.
• In 1949, Thompson and Ralston reported a
pseudarthrosis rate of 55.1% following spinal
fusion in a group of patients in which some type
of internal fixation (i.e., a stainless steel machine
screw) was utilized across each of the facet
• Holdsworth and Hardy reported in 1953 the use
of posterior plates attached to one or more
spinous processes to treat fracture dislocations in
the thoraco lumbar spine.
• In the early 1980s, Dr. Steffee made the use of
pedicle screws popular in the United States with
his design of segmental spine plates fixed with
• Posterior bony elements of a lumbar vertebra
include the spinous process, laminae, pars
interarticularis, facets, transverse processes, and
• Posterior soft tissue structures include the
supraspinous ligament, interspinous
ligament, ligamentum flavum, and facet capsules.
• The pedicles are cylindrical bony projections off
the posterosuperolateral aspect of the vertebral
body that connect the vertebral body with the
posterior bony arch.
• The transverse pedicle diameters range from 4.5
mm at T5 to 18 mm at L5, and the sagittal
diameter is generally slightly larger than the
transverse slant diameter.
• The angle at which the pedicle emerges from
the vertebral body in the transverse plane also
varies with craniocaudal location; being less
than 10" in the thoracic spine (with a slight
anterolateral angulation at T 12), it
progressively increases in the lumbar spine to
a maximum of almost 30"medial angulation
from posterolaterally to anteromedially at L5
• The pedicles also exhibit varying angles in the
sagittal plane. The pedicles are directed
approximately 15"-17" cephalad for the
majority of the thoracic spine, and neutral (90")
for the majority of the lumbar spine with the
exception of L5, which angles caudally an
average of 18"
• The distance to the anterior vertebral cortex as
measured from the posterior aspect of the
pedicle, through the pedicle, is approximately 40
to 45 mm in the thoracic spine and 50 mm in the
• A pedicle screw should ideally be placed along
the axis of the pedicle, incorporating the largest
available transverse and sagittal pedicle
• Biomechanical in vitro studies of axial pull-out
strength and load to failure have helped in
determining the optimal depth of pedicle screw
penetration, approximately 60% of the fixation
strength of the thoracic and lumbar pedicles is in
the pedicle itself.
• The cancellous bone in the vertebral body adds
another 15 to 20% of strength, whereas
purchase in the anterior cortex offers another 20
to 25% increase, Thus, it is generally thought
that it is not necessary to routinely engage the
anterior cortex, which also avoids the potential
danger of injuring the anterior vascular
structures, including the aorta.’
• Usually, the risks associated with penetration of
the anterior cortex exceed the benefits
gained, unless the pedicle and body are
extremely osteoporotic, weakened, or
fractured, or maximum available fixation
strength (eg., reducing a spondylolisthesis) is
CLINICAL ANATOMYO OF SURROUNDING
NEURAL AND VASCULAR STRUCTURES
Coroneal section through the
pedicles of L 1 and L2 at the level
of the ventral cauda equina roots
and the anterior portion of the
Axial section of L4 through the
midportion of the pedicle.
New Technology in Spine instrumentation
• Silver Oxide Coated Implant as antibacterial.
• Hydroxiapetide Coated screws for
• Canulated scews for cement injection in
• Facet Joint Replacement.
• Expandable cages ( Anatomical).
• Balloon Kyphoplasty.
Properties of Graft Materials
• Graft Osteogenic Oseto- Osteo-
Materials Potential induction conduction
• Autogenous bone o o o
• Bone marrow cells o ? x
• Allograft Bone x ? o
• Xenograft bone x x o
• DBM x o o
• BMPs x o x
• Ceramics x x o
• DBM = Demineralized bone matrix; BMP = Bone morphogenetic proteins
• A fundamental objective of preoperative planning is to
confirm the correct vertebral level or levels of fusion and
to localize the incision. Anteroposterior (AP) and lateral
radiographs, as well as axial computerized tomography
(CT) or magnetic resonance imaging are used for the
purpose of identifying the injured vertebra or vertebrae
and to estimate the position, orientation, and width of the
vertebral pedicle; the latter pedicle information is
important for later instrumentation of the vertebrae.
• Using flexion and extension radiographs, translational
motion of 3.5 mm or angular motion of 11 degrees of one
vertebra over another is indicative of segmental instability.
• Patient Positioning( Prone or Knee chest)
• The intra-abdominal pressure must be minimized to avoid
venous congestion and excess intraoperative bleeding, while
allowing adequate ventilation of the anesthetized patient.
• Position the hips in neutral or slight extension.This helps
maintain lumbar lordosis, which is imperative.
• All pressure points should be carefully padded. The abdomen
and genitalia (in males) should hang free. The arms must be
placed in a 90/90 position to avoid brachial plexus traction
injury. The ulnar nerve should be protected.
• The lower extremities are placed in anti-embolic stockings and
• Spinal cord monitoring may be used.
• X-rays and fluoroscopy should be available for verification of
the appropriate level and in the case of instrumentation, to
verify appropriate instrumentation placement
• Surgical exposure of the lumbar spine:
• Midline incision extended to an additional level
Screw Hole Preparation
• Exposure of the junction
between the pars
• Pedicle entrance point is
at the crossing of two lines
• Vertical line: 2-3 mm lateral
from the pars and slants slightly
from L4 to S1.
• Horizontal line passes through
the middle of the insertion of
the transverse processes or 1-2
mm below the joint line.
• 1-2 mm lateral from the center
of the pedicle to insert the
screw without disturbing the
facet joint above and to
medialize the screw for better
Preparation of Fusion Bed and Grafting
• Marking screw holes
Screw Selection and Insertion
• Screw Diameter:
• Perforation of the pedicle into the medial or inferior side
has higher chance of nerve root injury.
• Screw Length:
• Long enough to pass the half of the vertebral body but
• Short enough not to penetrate the anterior cortex
- Rod length must not be too long so that the
proximal tip of the rod do not touch the inferior
facet of the upper vertebra.
Tightening the nuts and set screws.
Spinal Implant Materials
• 316L Stainless steel:
• Strong and stiff
• Poor imaging compatibility: artifact to CT and
• Titanium Alloy (Ti6Al4V ELI):
• No artifacts during CT and MRI
• Excellent fatigue strength, high strength, high
• High resistance to fretting corrosion and wear
Bone-Metal Interface Strength
• Pedicle screws are known to provide the
strongest bone purchase compared to
wires, hooks, and vertebral screws.
• Screw Pullout Strength:
• Affected by major diameter and bone quality, but
not by minor diameter, thread type, and thread
• Insertion depth is not critical.
• Screw insertion torque was known to have
relationship with screw pullout strength.
• Conical screws showed similar pullout strength to
that of the cylindrical screws.
Surgical Construct Stability
• Construct stability varies depending on the size of the screws
and rods (plates).
• Recommended rod diameter is 6 mm or ¼ inch in adult spine surgery.
• Preservation of more than 70% of the disc or meticulous
anterior grafting is critical to obtain stable construct with no
hardware failure (screw or rod breakage).
• Modern spinal fixation systems, regardless of anterior or
posterior fixation, similarly significant stability in
flexion, extension, and lateral bending, but not effective in
preventing axial rotational (AR) motion.
• Use of a crosslink (DDT) is recommended to improve the AR
stability, particularly in the fixation of long segments (more than 2
Implant Assembly Profile
• Anterior Instrumentation:
• Critical in anterior plating of the cervical spine, and
the profile must be less than 3 mm.
• Lower profile is recommended in the anterior fixation
of the thoracolumbar spine.
• Posterior Instrumentation:
• Assembly profile is not as critical as in anterior
fixation, but lower profile is recommended because a
high profile may cause a surgery for implant removal
due to patients’ uncomfortness
Ease of System Assembly
• Screw Insertion:
• Screw insertion according to the best possible
anatomic orientation and location
• Adjustment in Screw-Rod Assembly:
• Rod bending
• Angular adjustment
• Medial-lateral adjustment
• Polyaxial screw head vs. Connector
• All assembly procedures can be made from the top.
Complication & Difficulties
• ADJACET SEGMENT DISEASE.
• ANATOMICAL VARIATION: may preclude
placing a screw in a particular pedicle, or
necessitate using a smaller screw than
• OSTEOPOROSIS: may prevent adequate
screw fixation In such situations, the options
are either to remove the screw or to place it
with polymethylmethacrylate (PMMA)
augmentation or Hydroxiapetide coated.
Complication & Difficulties
• TECHNIQUE-RELATED FACTORS: Poor
fixation could be the result of inadequate surgical
technique, such as placing the screw outside the
pedicle, fracturing of the pedicle, or placing screws
of inadequate circumference, thread size, or length.
• NEURAL DAMAGE: Risks of neural damage can
be minimized by thorough knowledge of the spinal
anatomy as well as practicing on anatomic
• VASCULAR INJURY: Perforation of the anterior
cortex of the vertebral bodies or sacrum can cause
significant injury to the adjacent anterior
structures, most notably the aorta, at levels at or
Complication & Difficulties
• POSTOPERATIV EPIDURAL HEMATOMA: Because
of the increased dissection and exposure necessitated
by pedicle screw placement, postoperative epidural
hematoma is a potential risk.
• OVERCORRECTION: With increased fixation
strength, afforded by pedicle screws, overcorrection of
a deformity, especially in cases of degenerative
scoliosis in which distraction is used, is a potential
• PSEUDARTHROSIS: It is a potential concern that
increased fixation of the lumbosacral spine may lead to
stress shielding and, therefore, an increased incidence
Complication & Difficulties
• Loss OF CORRECTION/STABILIZATION: Loss
of correction or stabilization or both can be the
result of poor fixation attributable to a number of
factors, including osteoporosis or improperly
placed pedicle screws.
• INFECTION: There are several sources for
infection, such as an extensive dissection, a
large number of instruments, and the use of an
image intensifier, which involves more operating
Tips and Pearls
1. Preoperative planning is paramount in determining the
extent of fusion and the site for bone graft harvesting.
2. If instrumentation is to be used, preoperative planning
allows determination of the levels of instrumentation and
the site for purchase (pedicle, lamina, facet). If pedicle
screws are planned, the sagittal and transverse orientation
of the pedicles and their width, length, and height should
3. Imaging studies should be evaluated for the presence of
any spinal anomalies.
4. The patient should be positioned in spinal lordosis. This is
particularly important when long lumbar fusions are
planned; it diminishes the possibility of postoperative flat
back syndrome and spinal dysfunction.
5. Intravenous antibiotics are administered preoperatively. In
the case of instrumentation, broad gram positive and
gram-negative spectrum antibiotics are administered.
Tips and Pearls
6. Paravertebral soft tissue dissection should proceed in a
caudal-cephalad direction due to the caudalcephalad
orientation of the paravertebral muscle attachments.
7. The facet joint immediately cephalad to the fusion must be
preserved to avoid iatrogenic instability.
8. Dissection should extend to the tips of the spinous
processes bilaterally and to the lateral aspect of the facet
joints. All soft tissue should be denuded to allow maximal
surface area for bony fusion.
9. The majority of the decortication is carried out using sharp
instruments such as osteotomes, curettes, and rongeurs.
10. Facet joints should be thoroughly cleared of all soft tissues
and the intra-articular portion of the joint denuded of
11. If a lumbar decompression has been performed prior to the
fusion, the exposed dura should be protected with
cottonoids to diminish the likelihood of an iatrogenic dural
What To Avoid
1. Avoid prolonged muscle retraction. Self-retaining
retractors should be relaxed every hour and the
wound re-irrigated. This allows re-perfusion of the
paravertebral muscles and diminishes the possibility of
infection. If there is excessive tension on the
muscles, the incision can be lengthened.
2. Avoid using an inadequate amount of bone graft;
occasionally harvesting of both iliac crests may be
necessary and/or augmentation with allograft bone
may be needed.
3. Avoid blood pooling, which occurs if the lordotic
cephalad segments are decorticated first.
4. Avoid bony fragment extrusion into the spinal canal.
5. Avoid injury to the facet immediately cephalad to the
6. Avoid postoperative administration of nonsteroidal
anti-inflammatories for 3 months(increase the use of
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