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SPINAL FUSION AND INSTRUMENTATION

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Title: SPINAL FUSION AND INSTRUMENTATION


1
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

5
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

6
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

7
Types of Fusion
8
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

9
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
10
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

11
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
12
Cervical Spine Instrumentation
13
Cervical Spine Instrumentation
14
Thoracolumbar Spine Instrumentation
Z-plate (Danek)
Kaneda (AcroMed)
15
Thoracolumbar Spine Instrumentation
16
Thoracolumbar Spine Instrumentation
17
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

18
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.

19
Screw Hole Preparation
GSFS Implantation Procedure
Angle and depth of the screw holes?
20
Direction and Depth of the Screw
21
GSFS Implantation Procedure
Preparation of Fusion Bed and Grafting
  • Decortication
  • Marking screw holes
  • Grafting

22
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
23
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

24
Rod-Screw Assembly
25
Rod-Screw Assembly
26
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

27
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.
28
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.

29
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

30
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

31
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)

32
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

33
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.

34
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).

35
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
36
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.

37
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.

38
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.

39
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
40
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
41
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.

42
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)

43
MATERIALSandMETHODS
44
Flexibility tests Unstable Calf Spine
Model Finite element studies
45
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
46
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)

47
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

48
Finite Element Models
Moment
Moment
Vertebrae
Transfixators
(A) Horizontal transfixation (HTF)
(B) Diagonal transfixation (DTF)
49
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

50
RESULTS
51
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
52
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
53
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
54
Rate of Motion Change with respect to no TF
Case(FE Model Predictions)
55
Rate of Total Load (Stress) Changes in Pedicle
Screws(FE Model Predictions)
56
DISCUSSION
57
  • 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.

58
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

59
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.
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