SPINAL FUSION AND INSTRUMENTATION

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SPINAL FUSION AND INSTRUMENTATION

• Tae-Hong Lim, Ph.D.
• Department of Biomedical Engineering
• The University of Iowa
• Iowa City, Iowa

2
Normal Function of the Spine

• Protect spinal cord and nerves
• Support the body weight and external load
• Stability
• Allow motion of the body for various activities
• Flexibility

3
Spinal Disorders

• Trauma
• Fractures, Whiplash injury, etc.
• Tumor
• Infection Inflammatory Disease
• Deformity
• Scoliosis, spondylolisthesis, degenerative lumbar
  kyphosis, etc.
• Cervical Low-back Pain
• Degenerative disease, such as disc herniation,
  stenosis, spondylolisthesis, etc.

4
Treatment of Spinal Disorders

• Conservative Treatment
• Degenerative disease
• Stable fracture
• Mild deformity
• Surgical Treatment
• Failed conservative treatment
• Unstable fracture (dislocation)
• Progressive deformity

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Goals of Spine Surgery

• Relieve pain by eliminating the source of
  problems (decompression)
• Stabilize the spinal segments after decompression
• Restore the structural integrity of the spine
  (almost normal mechanical function of the spine)
• Maintain the correction
• Prevent the progression of deformity of the spine

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Spinal Fusion

• Elimination of segmental movement across an
  intervertebral segment by bone union
• One of the most commonly performed, yet
  incompletely understood procedures in spine
  surgery
• Non-union rate 5 to 35

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Types of Fusion
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Factors for Considerationin Spine Fusion

• Biologic Factors
• Local Factors
• Soft tissue bed, Graft recipient site
  preparation, Radiation, Tumor and bone disease,
  Growth factors, Electrical or ultrasonic
  stimulation
• Systematic Factors
• Osteoporosis, Hormones, Nutrition, Drugs, Smoking
• Graft Factors
• Material, Mechanical strength, Size, Location
• Biomechanical Factors
• Stability, Loading

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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
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Spinal Instrumentation

• Goals of Spinal Instrumentation
• Correction of deformities or misaligned segments
• Enhancement of solid fusion
• Maintain anatomic alignment until a solid fusion
  takes place and
• Allow early mobilization of patients
• by providing an immediate stability

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Spinal Instrumentation Types

• Implantation Method
• Wiring, Hooks, Screws
• Rods vs. Plates
• Spinal Level
• Cervical, Thoracolumbar
• Position
• Anterior vs. Posterior Instrumentation

Vertebra
Graft
Vertebra
Pedicle screw instrumentation
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Cervical Spine Instrumentation
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Cervical Spine Instrumentation
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Thoracolumbar Spine Instrumentation
Z-plate (Danek)
Kaneda (AcroMed)
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Thoracolumbar Spine Instrumentation
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Thoracolumbar Spine Instrumentation
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Operative Techniques

• Patient Positioning
• The intra-abdominal pressure must be minimized to
  avoid venous congestion and excess intraoperative
  bleeding, while allowing adequate ventilation of
  the anesthetized patient.
• Surgical exposure of the lumbar spine
• Midline incision extended to an additional level

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Screw Hole Preparation
GSFS Implantation Procedure

• Exposure of the junction between the pars
  interarticularis and transeverse processes
• 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
  fixation.

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Screw Hole Preparation
GSFS Implantation Procedure
Angle and depth of the screw holes?
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Direction and Depth of the Screw
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GSFS Implantation Procedure
Preparation of Fusion Bed and Grafting

• Decortication
• Marking screw holes
• Grafting

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GSFS Implantation Procedure
Screw Selection and Insertion

• Screw Diameter
• approx. 80 of the medial diameter of the pedicle
• 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

Screw Length For GSFS
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GSFS Implantation Procedure
Rod-Connector-Screw Assembly

• Rod Length
• - 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.
• Rod Bending
• Connector Selection
• Rod-Connector Assembly
• Screw-Connector-Rod Assembly
• Tightening the nuts and set screws

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Rod-Screw Assembly
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Rod-Screw Assembly
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Rod-Screw Assembly

• Medial-lateral adjustability can eliminate
• The use of additional components and
• Application of force in medial-lateral directions
  or additional rod bending
• In order to make the rod-screw connection

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GSFS Implantation Procedure
Rod-Connector-Screw Assembly
GSFS - Screw-Connector Polyaxial -
Connector Length M-L Adjustment No precise
rod-bending is required. Screw alignment is not
as critical.
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GSFS Implantation Procedure
Rod-Connector-Screw Assembly

• Rod-bending
• Insert the rod to the connectors
• Temporary tightening of set screws of the
  proximal and distal most connectors
• Place the rod-connector assembly on the screws
• Tightening the screw caps and set-screws in the
  proximal and distal most connectors while holding
  the rod in a desired shape and
• Fix the other screw caps and set-screws in the
  mid-portion.

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Ideal Features

• The use of connectors
• Polyaxial and medial-lateral adjustability
• No need for precise rod bending
• Easy screw-rod connection without a good
  alignment of screw heads
• Screw insertion according to the best possible
  anatomic conditions
• Rigid connection at rod-connector and screw-cap
  connection
• Strong maintenance of correction
• Better mechanical environment to enhance bone
  healing (fusion)
• Top-tightening
• Low Assembly profile

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Consideration Factors in Spinal Instrumentation

• Materials
• Bio-compatibility and Imaging compatibility
• Stiffness (or elasticity) and strength
• Corrosion
• Implant Strength
• Component (screw, rod, plate, wire, etc.)
  strength
• Metal-metal interface strength
• Construct strength
• Bone-metal interface strength Bonewire, -hook,
  and -screws
• Construct Stability
• Segmental stiffness or flexibility
• Profile
• Ease of Use

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Spinal Implant Materials

• 316L Stainless steel
• Biocompatible
• Strong and stiff
• Poor imaging compatibility artifact to CT and
  MRI
• Titanium Alloy (Ti6Al4V ELI)
• Biocompatible
• No artifacts during CT and MRI
• Excellent fatigue strength, high strength, high
  elasticity
• High resistance to fretting corrosion and wear
  (surface treatments)

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Spinal Implant Strength

• Static and Fatigue Strength of Components
• Depends on the material properties, size and
  shape of the components
• Metal-metal Interface Strength
• Rod-screw connections
• GSFS (rod-connector and screw-connector
  interfaces) excellent
• Construct Strength
• Excellent in GSFS

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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 (BMD)
  but not by minor diameter, thread type, and
  thread size.
• 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.

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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 levels).

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Surgical Construct Stability
EXT

• Ligamentous spines
• Pure moment
• in FLX, EXT, LB, and AR
• Maximum 8.2 Nm
• 3-D motion analysis system

AR
LB
FLX
L2
FLX
LB
AR
EXT
L5
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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.

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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
• Top-tightening
• All assembly procedures can be made from the top.

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BIOMECHANICAL EVALUATION OF DIAGONAL
TRANSFIXATION IN PEDICLE SCREW INSTRUMENTATION

• Tae-Hong Lim, Ph.D.
• Atsushi Fujiwara, M.D.
• Jesse Kim, B.S.
• Timothy T. Yoon
• Sung-Chul Lee, M.D.
• Howard S. An, M.D.

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Horizontal Transfixation (HTF)

• Construct stability
• No improvement in FLX and EXT
• Some improvement in LB
• Significant improvement in AR
• Increased AR stability when using 2 transfixators
• Optimum position for TF
• Proximal 1/4 points for 1 TF
• Proximal 1/8 and middle points for 2 TF
• Lim et al. 1995

Transfixator (TF)
VB
VB
Pedicle screw instrumentation
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Diagonal Transfixation (DTF)

• Construct stability
• No changes in FLX (Texada et al, 1999)
• Significant improvement in LB and AR (Texada et
  al., 1999 McLain et al. 1999)

Transfixator (TF)
VB
VB
Pedicle screw instrumentation
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Diagonal Transfixation (DTF)

• Clinical application of DTF using 2 TFs may not
  be practical.
• Limited space
• Higher construct profile
• DTF using 1 TF is feasible, but its effect has
  not been investigated yet.

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PURPOSE

• To evaluate the effect of diagonal transfixation
  (DTF) on the construct stability and the
  corresponding stress changes in the pedicle screw
  in comparison with the effect of horizontal
  transfixation (HTF)

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MATERIALSandMETHODS
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Flexibility tests Unstable Calf Spine
Model Finite element studies
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FLEXIBILITY TESTS
EXT

• 10 Ligamentous calf spines (L2-L5)
• Pure moment
• in FLX, EXT, LB, and AR
• Maximum 8.2 Nm
• 3-D motion analysis system

AR
LB
FLX
L2
FLX
LB
AR
EXT
L5
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Tested Constructs

• - Intact
• - Instrumentation without TF after total
  discectomy (no TF)
• - Instrumentation with HTF using 1 TF (HTF)
• - Instrumentation with DTF using 1 TF (DTF)
• Diapason Spinale Fixation System (Stryker,
  Allendale, NJ 6.5 mm screws and 6 mm rods and
  TF)

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Finite Element Studies

• To investigate the stress changes in the pedicle
  screws due to HTF and DTF.
• Boundary and Loading Conditions
• Nodes in lower vertebra were held fixed.
• FLX, EXT, LB, and AR Moments (8.2 Nm) at the
  middle point of the vertebra element
• ADINA Finite Element Analysis S/W

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Finite Element Models
Moment
Moment
Vertebrae
Transfixators
(A) Horizontal transfixation (HTF)
(B) Diagonal transfixation (DTF)
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Data Analysis

• Rotational motion of L3 with respect to L4 in
  response to 8.2 Nm
• Rate of motion change with respect to
• Intact case
• No TF case
• Total load Mx2 My2 Mz21/2
• Mx Torsional moment My Mz Bending moments
• Stress change ? changes in total load

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RESULTS
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Rotational Motions (deg) responding to Applied
Moments of 8.2 Nm
INT
no TF
HTF
DTF
9
8
7
6
5
Rotational Angle (deg)
4
3
2
1
0
Flexion
Extension
Lateral Bending
Axial Rotation
Loading Directions
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Mean Rate of Motion Change from Intact Case
0.4
no TF
HTF
DTF
0.2



0.0
-0.2
Rate of Motion Change
-0.4
-0.6
-0.8
-1.0
Axial Rotation
Flexion
Extension
Lateral Bending
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Mean Rate of Motion Change from no TF Case
0.2
HTF
DTF
0.1
0.0
Rate of Motion Change
-0.1
-0.2



-0.3

-0.4
Lateral Bending
Axial Rotation
Flexion
Extension
Loading Modes
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Rate of Motion Change with respect to no TF
Case(FE Model Predictions)
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Rate of Total Load (Stress) Changes in Pedicle
Screws(FE Model Predictions)
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DISCUSSION
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• The effect of DTF using 1 crosslinking device on
  the construct stability and the corresponding
  stress changes in the pedicle screws was
  investigated using flexibility tests and finite
  element techniques.
• In flexibility tests
• Calf spines were used to reduce inter-specimen
  variability.
• Most unstable model was made by performing total
  discectomy to highlight the stabilizing effect of
  pedicle screw instrumentation.
• Motion data were normalized by those of the
  intact and no TF case to emphasize the effect of
  TF.
• For FE studies
• Beam element was used for modeling for
  simplification.
• Predicted motion changes showed a good agreement
  with measured data.
• Stress changes were represented by the changes in
  total load in screws because of linear nature of
  the model.

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Summary of Findings in Comparison with no TF Case

• DTF
• Construct stability
• Significant improvement in FLX/EXT
• no improvement in LB and AR
• Stress in the screws
• 12 in left screw 11 in right screw in
  FLX/EXT
• 44 in left screw 7 in right screw in LB
• 8 in left screw 18 in right screw in AR

• HTF
• Construct stability
• no improvement in FLX/EXT
• Significant improvement in LB and AR
• Stress in the screws
• No increase in FLX/EXT
• 28 increase in LB
• 58 decrease in AR

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CONCLUSION

• DTF provides more rigid fixation in FLX and EXT
  but less in LB and AR as compared with HTF case.
• Pedicle screws may experience greater stresses in
  DTF than in HTF.
• These limitations of DTF should be considered for
  clinical application.